Opportunity Fuels

Opportunity Fuels are fuels which have some value for power generation, but are not traditionally used for this purpose. By displacing expensive natural gas, opportunity fuels have the potential to make combined heat and power economically feasible in otherwise unfavorable situations. Opportunity Fuels can be divided into three categories: Biomass, Industrial Byproducts, and Commercial/Industrial Waste. Click on the fuel below for more information.

Resources Available

Opportunity Fuels are resources that often are wasted but can be recovered and used to generate usable energy. These resources include:

Biomass:

Crop Residue

Ethanol

Animal Waste

Food Processing Waste

Wood Waste

Sewage Sludge

Industrial Byproducts

Commercial / Industrial Waste

Conversion Technologies

Opportunity Fuels are resources that often are wasted but can be recovered and used to generate usable energy. These resources include:

Direct-burn to generate process heat, steam, and/or electricity using conventional equipment is an option with some opportunity fuels, but many require additional processing to convert into a usable form. Leading conversion technologies are:

Much of the background information included below is adapted from the 2004 Resource Dynamics Report: 'Combined Heat and Power Market Potential for Opportunity Fuels'. See the library for additional reports and presentations

Resources Available (Fuels)

Crop Residue

Crop residues are materials that remain after crops have been harvested and/or processed. Bagasse (sugar cane residue), rice hulls, rice straw, wheat straw, nutshells, and prunings from orchards and vineyards are all considered crop residues. Crop residues are produced in abundance on nearly every United States farm.

 Fuel Availability

Crop Residues provide only five percent (575 MW) of all biomass electricity generated in the United States. Bagasse accounts for nearly half of this number (255 MW). Crop residue fuels are generally only favorable is when the prime mover is located reasonably close to the site of crop production, and when the collection of residues can be incorporated into farm operation. Otherwise, the cost of collecting and transporting the residues can be too high. Crop residue market prices vary considerably depending on crop availability and region, but are often considerably higher than fossil fuels. Most areas do not have an infrastructure for gathering, brokering and shipping crop residues. However, the Federal government has programs such as REPI that provide financial incentives and operating cost reductions to crop residue users. State loans, grants, credits and tax exemptions are also available in some areas.

At the present time, there is no market for trading crop residues for use as a fuel. The availability and quality of the residues are highly regional, and depend on which crops are grown locally and the quantities produced. Some contractual relationships exist to purchase crop wastes for power, but they are very limited. Seasonality, including possible floods and droughts, is another issue that can affect availability and quality. The lack of a market infrastructure along with high collection and transportation costs limit the use of crop residues to cofiring applications and regional use.

Fuel Characteristics

They all have the potential to generate power, with an energy content ranging from 2,500 to 4,000 Btu per pound when the crop is wet (6,000-9,000 Btu per pound, dry). Due to high moisture content, varying ability, and relatively high costs, crop residues are not a viable fuel alternative for most DG/CHP applications.

Biomass gas is just as clean, and sometimes cleaner than natural gas, so emission controls are less of an issue. Directly burning the feedstocks used in digesters typically results in a high level of particulate and unburned hydrocarbon emissions. Converting these materials into a clean-burning biogas can dramatically reduce emissions of harmful pollutants. The particulates and contaminants of the gas will change depending on the quality and type of gasifier used, and the feedstock utilized, but filters will usually suffice for gas cleaning gas prior to combustion (except in the case of sewage sludge were siloxanes can be a problem). However, supplemental emissions control equipment may be required in areas with stringent NOx emissions requirements.

Conversion / Utilization Options

When used as a fuel, crop residues are most often burned in boilers to create steam, although sometimes the residues are gasified. Existing coal boilers can be converted to burn solid crop-residue fuel in cofiring blends with few necessary modifications. Cofiring with coal is a common practice that typically decreases SOx and NOx emissions, but may increases the plant’s net heat rate. Crop residues can also be burned on their own, but a coal-fired boiler would require many modifications and adjustments. As with most steam turbine applications, crop residues are better suited for large industrial or utility operations .

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Ethanol

Ethanol is a liquid fuel produced from the fermentation of wood waste, crop residues, farm wastes, and other biomass fuels. While ethanol’s most common use is in alcoholic beverages and cleaning solutions, it has also been used to power various vehicles, modified diesel gensets, and steam turbine systems. In addition, it has recently been used extensively an additive for gasoline in vehicles, making them burn at a higher octane with fewer emissions. Ethanol is also being considered for powering fuel cells in future designs and applications.

Fuel Availability

Ethanol is mostly produced from corn crop residues in the Midwest, but it can come from a number of different sources. The overall cost to produce ethanol is not very high, since fermentation is a relatively simple process and feedstocks are abundant, although a good amount of energy is required. The cost to transport ethanol is much lower than solid fuels, but pipelines may have to be constructed or modified. Compared to solid biomass fuels, emissions are lower and efficiency is higher, and both of these are money-saving characteristics. Major equipment modifications may be required, however, for existing prime movers to run on liquid ethanol fuels. Maintenance costs, on the other hand, should not significantly increase.

The largest market for ethanol power production is likely farms and wood processing facilities that could produce the fuel for free (minus the cost of fermentation equipment and operation).Currently the market for ethanol fuels is highly focused on the transportation industry, and this is the only area for which the government has provided support. Until more attention is focused on ethanol as a stand-alone fuel for stationary power generators, its market potential as an opportunity fuel will be limited.

Ethanol could have potential as an opportunity fuel, but there are three things holding it back: 1) The cost of biomass fuels, 2) The energy and costs associated with fermentation, and 3) The focus on mixed ethanol-gasoline and ethanol-diesel blends for automotive purposes. Aside from these drawbacks, ethanol makes a promising opportunity fuel for fuel cells and certain steam turbine and reciprocating engine applications. However, the cost to obtain ethanol varies greatly depending on application and location, and not much research has been accomplished using 100 percent ethanol fuel for stationary power generation.

Availability: While ethanol fuel is not readily available for immediate use, almost any type of biomass can be converted into the liquid fuel through fermentation. There are over 500 million wet tons of biomass available each year, although only a fraction can be obtained at a market-clearing price.
Cost: Biomass fuels can be expensive to obtain (typically ranging from $20-$50 per ton and $1.25-$4.00 per MMBtu), so ethanol is best produced on-site or nearby farms and processing facilities, where biomass can be obtained at a relatively cheap price. The cost to obtain and maintain fermentation equipment can also be high.

Fuel Characteristics

Ethanol has a heating value of around 12,800 BTU/lb, somewhat lower than comparable transportation fuels like Diesel and gasoline. Because it is a liquid fuel, it is easily transported, and power generation with ethanol is more environment-friendly than combusting solid biomass fuels. Ethanol is a renewable source of energy. When burned for fuel, ethanol produces fewer emissions than fossil fuels in every significant category (NOx, SOx, CO2, CO, VOCs, particulates). No emission controls should be required for ethanol-powered gensets.

Ethanol, which is the same chemical as the alcohol in alcoholic beverages, can reach 96% purity by volume by distillation, and is as clear as water. This is enough for straight-ethanol combustion. For blending with gasoline, purities of 99.5 to 99.9% are required, depending on temperature, to avoid separation. These purities are produced using additional industrial processes. Assuming it is derived from biomass, the combustion of ethanol produces no net carbon dixide. When fully combusted, its combustion products are only carbon dioxide and water which are also the by-products of regular cellulose waste decomposition. For this reason, it is favoured for environmentally conscious transport schemes and has been used to fuel public buses. However, pure ethanol reacts with or dissolves certain rubber and plastic materials and cannot be used in unmodified engines. Additionally, pure ethanol has a much higher octane rating has 113, than ordinary gasoline, requiring changes to the compression ratio or spark timing to obtain maximum benefit.

Conversion / Utilization Options

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Animal Waste

Animal waste is a class of fuels consisting of animal excrement, or manure, and animal byproducts from processing. Manure is organic matter used as fertilizer in agriculture. Manures contribute to the fertility of the soil by adding organic matter and nutrients, such as nitrogen that is trapped by bacteria in the soil. Higher organisms then feed on the fungi and bacteria in a chain of life that comprises the soil food web.

The dried manure of animals has been used as fuel throughout history. Dried manure of camels and other animals (usually known as dung) was, and in some places still is, an important fuel source in treeless regions such as deserts. On the Oregon Trail, pioneering families collected large quantities of "buffalo chips" in lieu of scarce firewood. It has been used for many purposes, in cooking fires and to combat the cold desert nights. Manure, wood, and agriculture waste account for the bulk of cooking fuel in the developing world. Manure, while having some heating content, does not burn efficiently. A recent estimate by the world health organization stated that half a million women and children die in India each year from respiratory diseases caused by poor air quality due to cooking with manure and similar fuels.

Animal byproducts are created during the processing of animals for food. Whether poultry, beef, or lamb, animal processing generates a large quanitity of varied material byproducts: feathers, bones, and other materials with little economic value to the processor. Typrically, these materials are commingled during the facility cleanup, often contained in water, and eventually are converted to anaerobic digester gas at a wastewater treatment facility. The variety and degree of mixing makes the waste difficult to exclusively separate and harvest as fuel.

Fuel Availability

Manure, poultry litter, and animal waste in general is quite obviously available in areas of high animal concentration. Agricultural and animal production operations, such as feed lots or poultry growing facilities, are the best candidates due to the consistent production of animal waste in quantity. These operations are dispersed across the nation, and vary greatly in size and scope. Many types of large animal operations can be located with the help of data provided by the US Dept. of Agriculture.

Fuel Characteristics

Most animal manure is feces — excrement (variously called "droppings" or "dung" etc) of plant-eating mammals ( herbivores) and poultry — or plant material (often straw) which has been used as bedding for animals and thus is heavily contaminated with their feces and urine.

Often, animal waste is found in liquid form, due to a washing process to transport the waste. The constituents of animal wastewater typically contain: Strong organic content—much stronger than human sewage, High solids concentration, High nitrate and phosphorus content, Antibiotics, Synthetic hormones, high concentrations of parasites and their eggs, spore of cryptosporidum - a bacterium resistant to drinking water treatment processes, spore of Giardia, and Human pathogenic bacteria such as Brucella and Salmonella.

Conversion / Utilization Options

Manure has been used for centuries as a fertilizer for farming, as it is rich in nitrogen and other nutrients which facilitate the growth of plants. Liquid manure from pig/hog operations is usually knifed (injected) directly into the soil to reduce the unpleasant odors. Manure from cattle is spread on fields using a spreader. Due to the relatively lower level of proteins in grasses, which herbivores eat, cattle manure has a milder smell than the dung of carnivores — for example, elephant dung is practically odorless. However, due to the quantity of manure applied to fields, odor can be a problem in some agricultural regions. Poultry droppings are harmful to plants when fresh but after a period of composting are valuable fertilizers. Whilst solid animal waste heaps outdoors can give rise to polluting wastewaters from rain washing, this type of waste is usually relatively easy to treat by containment and/or covering of the heap.

Animal wastes from cattle can be as produced as solid or semisolid manure or as liquid slurry. The production of slurry is especially common in housed dairy cattle. Animal slurries require special handling and are usually treated by containment in lagoons before disposal by spray or trickle application to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal wastes, as are anaerobic lagoons. Excessive application or application to sodden land or insufficient land area can result in direct runoff to watercourses with the potential for causing severe pollution. Application of slurries to land overlying aquifers can result in direct contamination or, more commonly, elevation of nitrogen levels as nitrite or nitrate.

The disposal of any wastewater containing animal waste upstream of a drinking water intake can pose serious health problems to those drinking the water because of the highly resistant spores present in many animals that are capable of causing disabling in humans. This risk exists even for very low level seepage via shallow surface drains or from rainfall run-off. Some animal slurries are treated by mixing with straws and composted at high temperature to produce a bacteriologically sterile and friable manure for soil improvement.

Manure generates heat as it decomposes, and it is not unheard of for manure to ignite spontaneously should it be stored in a massive pile. Once such a large pile of manure is burning, it will foul the air over a very large area and require considerable effort to extinguish. Therefore, it is not recommended that manure be burned in such an uncontrolled fashion. Large feedlots must therefore take care to ensure that piles of fresh manure do not get excessively large. There is no serious risk of spontaneous combustion in smaller operations.

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Food Processing Waste

Food processing waste (FPW) consists of any waste generated in the food processing industry that can be used for fuel. Potato waste, cheese whey wastes, fruit pits, leftover sludge, and other energy-rich FPW can all be converted into a solid biomass fuel.

 Fuel Availability

Aside from a handful of food processing facilities and certain research projects, FPW is not currently used as a fuel for DER/CHP projects. Currently, most FPW is disposed as industrial wastewater and discharged to the local treatment plant. The varying characteristics and properties of different types of FPW make it hard to consolidate into a consistent source of fuel. Still, certain waste streams would make ideal fuel sources for the plants that produce them, and there could be a good amount of potential in the large industry of food processing.

Utilization of FPW can significantly reduce fuel costs for food processing facilities. While some processing costs may be incurred in drying and cutting the waste into chips, FPW is essentially a free fuel source for the food processing industry. Federal and state government incentives may be offered to users of the fuel, and cofiring is a cost-saving option for those already utilizing a coal-fired boiler.

There is virtually no market for food processing wastes as a fuel, except for in the food processing industry. It is environment-friendly and performs fairly well when processed, but due to the large variations in the types of waste and fuels produced, and the lack of a distribution infrastructure, it would be difficult to produce a consistent quality product on a large scale. It is possible that nearby plants may want to purchase the waste for cofiring in a coal-fired boiler or some other application. If so, the waste would sell for about the same rate as coal on a Btu-basis. However, the wide variety in the waste and fuel types and the lack of a market infrastructure prevents its widespread use, and potential candidates are hard to generalize and must be evaluated on a case-by-case basis.

Fuel Characteristics

Food processing wastes can produce a high quality and clean-burning fuel for a relatively low price. The variety of processing options for FPW - digestion, gasification, fermentation, and cofiring, make it difficult to generalize about the characteristics of the fuel. Depending on the feedstock and conversion equipment used, however, it is expected that FPW fuel would be substantially the same as other converted biofuels (AD gas, biogas, ethanol) in terms of its heat content and pollutants.

Conversion / Utilization Options

FPW can be dried and cut into chips to be fired in a boiler (similar to coal). Cofiring is usually preferred, as it reduces the emissions in a coal-fired plant and no boiler modifications are necessary. To create a gaseous fuel, anaerobic digestion can be used – the food waste is stored in an oxygen-deprived tank, where anaerobic bacteria consume it and release a methane gas. Gasification can also be utilized, but only with dry FPW. To create a liquid fuel, certain food wastes can be fermented and turned into ethanol. Some new technologies are capable of extracting the ethanol from the waste and using the liquid fuel to generate power. Different types of wastes will produce different types of fuel, and even the same food waste can be used in very different ways, which makes it hard to categorize certain characteristics of food processing waste. In this section, only solid food processing waste is considered (see ADG, Biomass Gas, and Ethanol for information on its gaseous and liquid forms).

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Wood Waste

Wood waste, refers to any type of wood or wood-based product that can be burned to generate power. There are four categories that wood and wood waste fall into: dedicated energy crops (not yet produced in the United States), harvested wood (wood chips), mill residue (bark, sawdust and planer shavings), and urban wood waste (treated/painted wood, yard trimmings, etc.).

 Fuel Availability

Wood collection and transportation can be labor intensive and expensive, although wood can usually be hauled up to 75 miles for $8 to $15 per ton. In the end, the cost of delivered wood fuel ranges from $15 to $45 per dry ton. Forest residues, or harvested wood, average about $30 per dry ton to obtain ($2.00 per MMBtu, more expensive than most coals), while urban wood wastes average about $18 per dry ton of fuel ($1.20 per MMBtu, cheaper than most coals). In addition, through the REPI, the Federal government may offer a 1.5 cents per kWh incentive to users of wood fuels, and most states offer some type of incentive.

The availability of wood and wood waste is highly regional – users must be close to the source. Wood and wood waste are promising biomass-based opportunity fuels. Although more costly than coal, wood waste burns cleaner and can easily be co-fired. While solid wood fuels are best suited for industrial applications, they can also be a fuel source for steam-powered DER and CHP, especially coal-fired units in the 10-50 MW range. With wood fuels produced from forest residues, or urban wood waste, the consumer must pay for the fuel, and usually the cost is only beneficial when the user is close to the source, since transportation costs can quickly make wood fuels uneconomical. In general, transportation of 25-50 miles produces marginal results, and any transportation over 50 miles will not be economical.

Fuel Characteristics

Wood and wood waste are considered renewable resources. Although carbon dioxide is produced in burning wood fuels, if new trees are planted, the net carbon dioxide emissions will approach zero. Urban wood waste may contain components and pollutants that need to be removed prior to burning, or else hazardous emissions and increased fouling will occur. SOx and NOx emissions, as well as the ash content, are much less than coal so co-firing will help reduce emissions. Wood ash is non-toxic and does not contain pollutants or heavy metals, but some states still consider it hazardous waste. The heating value of wood varies greatly depending on the type of wood and the moisture content. For instance, a green wood with a 50% moisture content my have a heating value of 8.6 MMBTU/ton, compared to heating values in around 16 MMBTU/ton for dried hardwoods and premium wood pellets.

Conversion / Utilization Options

Burning wood is one of the oldest methods of generating both thermal and electric energy. Wood fuels account for over two-thirds of all biomass electric generation capacity. Nearly 1,000 wood-fired plants exist in the U.S., generally ranging from 10 to 25 MW. There are at least 75 wood-fueled CHP units that qualify as distributed energy resources. In most wood and wood waste applications, the wood is dried, cut into chips, and transported to a boiler, where it is burned to produce steam that powers a steam turbine/generator. Cofiring with coal is sometimes used to increase the net heat rate of a coal-fired plant, but its effectiveness is limited due to wood’s poor grindability. Pulverizers for coal are unable to handle high quantities of wood. Stokers and cyclone boilers are the most suited to co- firing wood and wood waste fuels as they require the least modifications. In some cases, wood is liquefied into an ethanol fuel (see Ethanol) or gasified (see Biomass Gas). For best results with solid wood fuels, a boiler system made specifically for wood fuels should be used.

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Sewage Sludge

Sludge waste is sewage sludge from wastewater treatment plants. The sludge can be dried and burned as a fuel to generate steam and power. This same wastewater sludge is often converted into anaerobic digester gas for waste treatment and fuel use. Burning the solid sludge, however, is another power-producing alternative that eliminates most of the harmful constituents.

Fuel Availability

Sludge waste is not a strong potential energy source for outside markets. However, it is a free source of fuel that can be used by wastewater treatment plants in combined heat and power applications. If excess power is produced, it may be sold to local utilities or consumers. Anaerobic digester gas is almost invariably a more efficient and smarter choice for CHP projects at wastewater treatment plants.

Fuel Characteristics

The heat content of sludge waste is only about 3,500 Btu/lb (25-30 percent that of coal), its moisture content is very high, and sludge-fired boilers require additional maintenance. Sludge waste is not a particularly good fuel. It can be useful to waste water treatment plants, but even then its usefulness as a solid fuel is questionable. Except for small treatment facilities with boilers where no digester is installed, anaerobic digester gas is generally a better option.

Conversion / Utilization Options

For solid-firing, the sludge must be dried thoroughly prior to combustion. Once this occurs, it can be used in existing boilers in place of coal, or it can be co-fired. Some modifications to existing boilers will be necessary to accommodate the low combustibility of the fuel and increased cleaning and maintenance will be required. Stokers are preferred for firing the sludge waste since fewer modifications are necessary. Not many wastewater treatment plants use their sludge to generate electricity, but the technology exists and solid sludge waste can be used as a source of power. It is generally more effective to use an anaerobic digester to convert the organic portion of the waste to a more flexible, gaseous fuel. However, burning sludge waste directly is also an option.

Sludge is a generic term for solids separated from suspension in a liquid by a variety of processes. Most commonly sludge refers to solid waste extracted in the process of sewage treatment; the term sewage sludge is used commonly. When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter will settle out in about an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be "fresh" before anaerobic processes become active. Once anaerobic bacteria take over, the sludge will become putrescent in a short time and must be removed from the sedimentation tank before this happens.

This is commonly accomplished by two different ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where decomposition by anaerobic bacteria takes place resulting in liquefaction and a reduction in the volume of the sludge. After digesting for an extended period of time, the result is called "digested" sludge and may be disposed of by drying and then landfilling. It has value as a soil conditioner and has some nutrients found in fertilizer, being similar to humus. Alternately, the fresh sludge may be continuously extracted from the tank by mechanical means and passed on to separate sludge digestion tanks which operate at higher temperatures than the lower story of the Imhoff tank and as a result digest much more rapidly and efficiently.

Excess solids from biological processes such as Activated sludge can also be referred to as sludge, although more often called Biosolids among water professionals in the United States.

Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.

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Black Liquor

Black liquor is a byproduct of the pulping process. During pulping, wood fibers are separated and treated to produce a pulp, which is then converted into paper. With chemical pulping, the lignin in wood is dissolved in a digester, which separates the fibers and creates black liquor, a tar-like substance, as a waste product.

Fuel Availability

Most pulp and paper mills use all of their black liquor to provide for onsite heat and power needs.Black liquor is a proven opportunity fuel, already extensively used by pulp and paper mills, especially for steam generation. If a market were to develop, it could potentially be sold as an alternative boiler fuel. However, its scarcity and the lack of a supporting distribution infrastructure, keep the fuel from being a serious candidate for outside markets.

Fuel Characteristics

Black liquor, which comes from the pulp and paper derived from trees, can be considered a renewable resource. Black liquor contains some sulfur and small amounts of nitrogen, so SO x and NO x production are potential problems. Emission control technologies may be needed in some areas.

Conversion / Utilization Options

Black liquor is usually incinerated in special recovery boilers that recover any remaining chemicals and generate heat, steam, and electricity for the pulp or paper mill. Boilers designed for fuel oil and coal can be modified to accommodate black liquor. Gasification is another option, which produces a fuel gas that can power a gas turbine with a relatively high efficiency. Although gasification-systems burn cleaner and achieve higher efficiencies, their capital cost is also much higher.

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Coke Oven Gas

Coke oven gas refers to the gas and vapors generated during the production of coal and petroleum coke. Coke oven gas is currently used only in mills and refineries as an additional source of heat, and sometimes electricity. It is not produced in great quantities, and its production is limited by the use of petroleum and coal. Its inferiority to natural gas and its limited availability prevent it from being a serious contender in outside markets.

Fuel Availability

At steel mills and petroleum refineries, using coke oven gas to produce heat or electricity can be a good economic decision. The gas could also be sold to nearby power producers, transported through a pipeline and sold for roughly the same price as natural gas ($5-$6 per MMBtu). As with black liquor, most of the mills and refineries that can make use of their coke oven gas already do so, so the market that is leftover is relatively small.

Fuel Characteristics

Coke oven gas can be collected and burned as a fuel similar to natural gas, although the quality is not nearly as high (coke oven gas is only 35 percent methane and almost 50 percent hydrogen). Coke oven gas burns readily because of its high free-hydrogen content, which also makes it an ideal candidate for fuel cells. Its Btu content is around 550 Btu/ft 3 (about half that of natural gas).

Conversion / Utilization Options

Most gensets will require some modifications and additional maintenance to accommodate the lower heating value. The fuel can be used in place of natural gas in boilers, but larger burner-gas port openings may be required due to the higher flow rate, impurities, and the resulting deposit build-up. Coke oven gas can also be used to power modified engines and gas turbines, but the fuel’s variable supply and low methane content limit its energy producing capabilities.

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Industrial VOCs

Volatile organic compounds (VOC’s) evaporate easily during many industrial processes, and they are an ever-increasing threat to the environment. Typically, industrial VOC’s are removed in industrial sites by using thermal or catalytic oxidizers. Alternatively, the VOC’s can be used as a fuel to help supply power for the industrial operation, while at the same time eliminating environmental threats.

Fuel Availability

The market for industrial VOC’s as a fuel is limited to industrial plants that produces the volatile compounds. Many of these plants already use oxidizers to eliminate their VOC’s, and are unlikely to abandon them and switch to this gas turbine technology, unless a significant decrease in operating costs is incurred. The main market for the fuel is new or expanding industrial facilities, or plants located in areas with increasingly strict emissions regulations. As with most DER/CHP projects, the market also depends on the local price of electricity and natural gas. Areas with high electricity prices are more likely to benefit from distributed power, though in the case of VOC’s, high natural gas prices have a negative effect on the market for Industrial VOC’s, since they must be cofired with the fuel.

The fuel’s use is limited to VOC-laden air injection into a natural gas combustion stream. Most of the energy produced by the turbine comes from the natural gas, not from the dilute VOC-air mixture. While this practice is certainly worthy of consideration as a competitive treatment option, industrial VOC’s do not qualify as a standalone fuel.

Fuel Characteristics

To combust VOCs, high temperatures are used in order to eliminate all of the dangerous compounds, and this can only be achieved with a secondary fuel. In addition, the VOC-air mixture is simply too dilute to be used on its own. The VOC fuel is treated like an air injection into the gas combustor, and it is essentially just that, since the concentration of VOC’s is so low. However, the highly reactive VOC’s will provide additional energy to the natural gas stream as it enters the turbine, which can be used as a DER/CHP unit to power the entire facility.

Conversion / Utilization Options

Currently, the use of industrial VOC’s is limited to cofiring with natural gas turbines. Advanced in gas turbine technology that increase efficiency and reduce energy costs will help bolster utilization of this technology. While the fuel efficiency of the gas turbine is enhanced by a limited amount of VOC-air injection, the concentration of VOC’s is so low that there is no noticeable degradation in performance, and no additional maintenance is required.

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Petroleum Coke

Petroleum coke (pet coke), is a carbon-rich black solid byproduct of coking conversion processes, which separate light and heavy crude oil products. Petroleum coke is in abundant supply and its price is always less than that of coal. There are three types of pet coke produced in the coking process – sponge, shot, and needle. Only sponge and shot coke are used as a fuel. Customers are generally not willing to purchase pet coke if they can get coal for the same price. The production of coal coke has been on the decline, and it is almost always used up by iron and steel mills for additional heat.

Fuel Availability

Petroleum coke is a cheap and readily available energy source. Although it contains many contaminants and more emission controls are required than for coal, pet coke’s lower price can make it economically beneficial for consumers. Because of its impurities and contaminants, pet coke is only suitable for large-scale, high temperature industrial applications. Although petroleum coke could potentially power 25-50 MW steam turbines, DER and CHP petroleum coke-fired units likely will not become popular until a cleaner, more efficient method of burning the fuel is developed.

In the United States, large independent power producers and refineries are the main users of pet coke who often fire 100 percent coke, not a coke-coal blend, in boiler/steam turbine systems over 50 MW in size.– utilities only use it sparingly as an alternative boiler fuel. Worldwide, petroleum coke is most often used in cement kilns and calcining operations. The world production of petroleum coke in 1995 totaled over 50 million tons (Mt), with 80 percent coming from U.S. refineries. Accordingly, the majority of petroleum coke produced in the U.S. is exported to foreign markets, where it is used primarily as a fuel.

Fuel Characteristics

Some drawbacks of petroleum coke include a low volatility, a high sulfur content, and high nickel and vanadium contents in the ash. However, the fuel offers a high heat content (14,000 Btu/lb), a low ash content and easy grindability at a very low cost. The fuel contains many harmful contaminants and a high sulfur content so extensive emission controls are required. The price for petroleum coke is usually much less than that of coal, although it contains higher amounts of sulfur, as well as some heavy metals.

Conversion / Utilization Options

Coke can be used in place of coal or fuel oil in conventional boilers, with only a few modifications. For this reason, pet coke is often blended and co-fired with sub-bituminous coal in large-scale industrial applications. If not, several cleaning devices and emission control technologies must be put in place. The high Btu content of petroleum coke makes it attractive from a cost-benefit standpoint, however it has a low volatility and more emission control technologies are required.

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Waste Heat

Waste heat, while not a fuel in and of itself, is a common byproduct of many operations. Heat is generated as a byproduct of processes as diverse as the baterial processes found in anaerobic digestion, and the metalworking processes found in manufacturing plants. Some facilities, such as wastewater treatment plants, routinely use byproduct waste heat to maintain the optimum temperature of their process reactors. In other operations, waste heat can be largely unutilized.

Fuel Availability

Waste heat is commonly genereated as a byproduct of many industries, from chemical manufacturing and processing to fuel refining, to equipment fabricaiton and metalworking. Heat is also generated as a byproduct of many biological processes which have been adapted to production facilities. Generally, any mechanized or motive process generates heat, as do exothermic chemical processes. If the heat is contained within a medium which can be harvested - flue gas for instance - it can be used as a resource to generate work, electricity, or steam.

Fuel Characteristics

The characteristics of waste heat are as varied as the industrial processes which generate it as a byproduct. Waste heat is useful only if it is concentrated - in combustion flue gases for instance; The feasibility of heat capture or transfer is a function of the characteristics of the medium of transfer - the elements contained in the flue gas, for instance, or the species dissolved in the heated liquid. As such, waste heat necessitates individual evaluation prior to application.

Conversion / Utilization Options

Waste heat is typically utilized to generate electricity. While some processes require heat to maintain opertion (wastewater treatment plants were already mentioned), it is generally easier to convert the heat to electricity. Heat Recovery Steam Generators (HRSGs), essentially large heat exchangers, are designed to do this by transfering the waste heat found in boiler flue gas to circulating hot water - resulting in steam that then can generate electricity in a turbine, or be used for space heating. Steam and hot water are commonly used in many industrial and commerical applications, and provide a simple use for waste heat. While many possible options for the utilization of waste heat exist, the proximity of the heat source and load, the electrical supply and demand, and the economics of the situation generally dictate the possibilities.

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Landfill Gas

Landfill gas (LFG) is gas created by the decomposition of landfill waste, which is essentially an anaerobic digestion process. In the past, LFG was simply collected and flared, but now many landfills are taking advantage of their waste gas, using it to produce heat and power. In general, 1 million tons of municipal solid waste produces 300 cubic foot per minute of landfill gas that could generate 7,000,000 kWh of electricity per year, enough to power 700 homes. Most of the candidates for LFG projects have more than 1 million tons of waste in place.

 Fuel Availability

Most often, developers purchase the rights a landfill’s gas, transport it to a spot where a genset can interconnect with the power grid, and sell the electricity to a third party or utility. Landfills can also use their gas to meet their own heat and power needs, selling any excess electricity to the local utility. Landfill gas is a good energy source for landfills and the facilities immediately surrounding them. .

Of the estimated 2500 active landfills (6,000 total) in the United States, about 340 currently utilize their LFG for power. Many more landfills are in the planning process for LFG-to-energy projects, and at least 600 have been identified to have strong project potential. EPA’s Landfill Methane Outreach Program (LMOP) provides assistance and incentives to LFG-to-energy projects. With many of these projects, a third party developer pays for the rights to the landfill gas. They have the choice of maintaining a genset at the landfill site (and transporting the electricity to their facility) or pipelining the gas to their facility and using it in a DER/CHP application. For facilities within a 2-mile radius of the landfill site, the latter option is usually chosen. The market for LFG is generally limited to either the areas immediately surrounding landfills, or facilities that are interconnected to the power grid. Landfills are typically built far from commercial and residential locations. In addition, when the gas is pipelined, odor can be a concern. As such, landfill gas CHP units are usually limited to nearby industrial operations.

 Fuel Characteristics

LFG is similar to ADG, containing about 50 percent methane and just under 50 percent carbon dioxide; similar to natural gas, but with a smaller percentage of methane and much more carbon dioxide. The Btu content of landfill gas (500 MMBtu/ft 3) is about half that of natural gas, but it can still generate a substantial amount of power, and only minor modifications and increased maintenance are required for existing equipment. A potential problem with LFG is the presence of siloxanes, a silicon-oxide compound found in personal care products. This must be removed prior to combustion.

LFG sells for roughly the same price as natural gas on a per Btu basis ($5-$6 per MMBtu), although the Federal government (through REPI) sometimes offers a tax credit of approximately $1.00 for every MMBtu of energy produced, and will help finance nearly any LFG-to-energy project. State governments often provide financial incentives as well. Despite the high initial cost, some LFG-to-energy projects with pipelines as long as ten miles have become profitable DER/CHP operations, thanks mostly to government incentives and financing.

Conversion / Utilization Options

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Municipal Solid Waste

Municipal solid waste (MSW) is commonly referred to as trash or garbage. It is collected at landfills and can consist of any type of refuse. The section on landfill gas describes how MSW is naturally converted into a gaseous fuel. In some areas, however, MSW is dried and burned in high temperature boilers to generate steam and electricity.

 Fuel Availability

Recently, some collection sites have begun producing Refuse derived fuel (RDF), which has been thoroughly sorted so that only energy-producing components remain. This fuel can either be burned in boilers or gasified, and it performs better than MSW, but it costs money to produce. In the United States, over 200 million tons of municipal solid waste is produced each year. MSW is the second largest biomass fuel source in the United States, behind wood-based fuels, producing 2.6 GW of power each year. Most of this energy comes from projects started in the 1970’s, because of the oil embargo and worries about environmental pollutants from dumps and landfills. Baltimore and Montgomery County’s 60 MW waste-to-energy facilities in Maryland are examples of MSW projects still going strong.

However, recently many MSW power projects have been losing steam and shutting down. Large new landfills and the EPA’s backing of LFG have slowed down new solid waste to energy projects. The use of MSW as a fuel will likely decrease in the near future, as RDF gasification and LFG provide cleaner and more efficient alternatives for turning waste to energy. Because of this, MSW and RDF projects are best implemented at garbage collection sites, or at nearby facilities. Emission control technologies can be costly, but Federal and state government agencies offer various incentives for using MSW as a fuel. However, landfill gas-to-energy projects are usually more attractive, and new RDF gasification systems may make burning solid waste obsolete. MSW and RDF are not recommended as solid fuels, but biomass gas from RDF is potentially promising, and warrants further investigation.

Fuel Characteristics

MSW and RDF are not ideal fuels for a number of reasons - their quality varies, they are not easily transported, and emissions can be high because of various components found in the waste. Large emission control technologies and cleaning devices must be implemented when burning the solid fuels. In general, municipal solid waste is an inferior fuel to landfill gas, which has become the preferred method of burning waste methane.

The heating value of MSW averages less than 5,000 Btu/lb so much more ash and residue are left behind than coal, whose heating value is more than three times as high. Because MSW is a solid fuel, and the heat content is extremely low, transportation can be very expensive. Additionally, a great deal of drying, cleaning, and emission controls must be applied to the waste before it is ready to incinerate.

Conversion / Utilization Options

Major modifications must be made to existing coal-fired boilers if MSW is to be used as a substitute. A stoker-type boiler to incinerate the waste is usually the best choice, since they can burn MSW with the fewest modifications. Pollution control technologies, such as scrubbers, reduce toxic waste in the combustion smoke by neutralizing acid gases. Filters are also employed to remove certain objects and magnets are used to remove metal from the waste. Refuse derived fuel is handled more easily since most of the undesirable components have been removed. RDF, has had positive sucesses. REcently, UTRC published a paper describing their experience using low-cost garbage collection, preparation, and gasification integrated with an advanced combined cycle gas turbine. .

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Construction Waste

Construction waste consists of all manner of wood, plastic, and metal debris,and assorted fluids that are generated as byproducts of construction projects. These wastes vary greatly in composition, and location, and are typically disposed of in landfills. In a large scale construction project, however, these waste materials may be available in sufficient quanitity to provide some benefit as fuel.

 Fuel Availability

The market for construction waste is varied. While some materials can be reused according to their original design (wood scraps of sufficient size, for example, may be used in construction), generally construction waste is discarded. Enterprising persons searching for specific types of construction waste should be able to obtain it at very low cost (prior to transport), if an applciable construction site can be located.

Fuel Characteristics

The characteristics of construction waste vary with the material under consideration. Some fuels, like wood scrap, are readily combusted, but others may only be of use in cofiring applications, or may have no heating value at all. Careful identification and sorting of construction waste is necessary prior to utilization. For this reason, and additionally due to the limited, temporal nature of the supply and transport costs, construction waste is not widely utilized as a fuel.

Conversion / Utilization Options

Solid fuel (coal-fired) boilers can handle a blend of construction waste (if properly sorted) with few modifications. Aside from certain types of plastic or wood, construction waste cannot be widely used for construction.

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Combustible Production Waste

Combustible waste consists of textile waste, wood scrap/trimmings, plastic scrap, and non-reusable solvents. Textile waste can consist of excess yarn, thread, cloth, carpet, or any other fabric. The excess material waste can be utilized as an energy source with about the same heat content as biomass. Although the waste contains many more pollutants and contaminants than biomass fuels, it can still be cofired with coal to produce heat and power for textile mills. Currently, most textile waste is recycled, although some textile mills utilize their waste in cofiring applications to produce their own heat and power. For most textile mills, the benefit of utilizing their waste comes from saving on coal costs. Usually, textile waste is only a practical fuel for mills that already contain a coal-fired boiler. Textile waste is not promising as an opportunity fuel. Its heating value is lower than biomass, it contains more pollutants, and it must be cofired with coal to be effective. Furthermore, the market for textile waste as a fuel is generally limited to textile mills.

Wood scrap and wood trimmings, derived from furniture and cabinet making, is composed of variable sizes and compositions, depending on the exact manufacturing process and wood type. While difficult to recycle, wood scrap is easily pelletized and repackaged for burning, or it can be burned wholesale. Thermoplastic plastic scrap can often be recycled if not contaminated. Thermosetting plastics, however, must be disposed of or burned for energy. The combustion of plastics or solvents can be done readily, but emissions considerations necessitate careful consideration of gas cleanup.

 Fuel Availability

The market for combustible byproduct waste is varied. Textile waste as a fuel is generally limited to textile mills, due to its low value, and even then it is limited to coal cofiring applications. Mills already using coal-fired boilers are the best potential market. Wood scrap and trimmings from wood manufacturing operations (either milling or finished goods, such as furniture) and plastic scrap, are generally disposed of locally. If the scope of the operation is sufficiently large, wood and plastic scraps are often pelletized. Plastic pellets can generally be reused as feedstocks (depending on the composition), and wood pellets are gaining increasing popularity and demand as fuel for pellet stoves. At present there is no other identifiable place in the DER/CHP market for combustible product waste as a fuel, although a variety of local niche opportunities do exist.

Fuel Characteristics

The characteristics of combustible byproduct fuels vary with the fuel under consideration. Wood scrap and trimmings, for example, offer a clean, often pelletized fuel supply for wood-burning apparatus such as stoves and boilers. Plastics and solvents generally contain toxic components, and must be evaluated on a case-by-case basis to determine whether or not it is practical to burn them alone, or to cofire with another fuel. Although gasification systems exist for textile waste (to be cofired with natural gas instead of coal), these systems’ high capital cost-to-benefit ratio make them impractical for most textile mills. The fuel’s quality is generally too low to be fired by itself, so cofiring with coal is the only practical option.

Conversion / Utilization Options

Most coal-fired boilers can handle a 5-10 percent blend of textile waste with little, if any, modifications required. Subject to limitations on wood type, coal-fired boilers can also be expected to handle a comparable blend of wood waste. The question of cofiring with solvents or plastics is extremely composition-dependant and not recommended without advance research. In cases where on-site power generation could seriously reduce electricity costs (i.e. locations where the cost of electricity is high), installing a coal-fired boiler and using textile waste as a blended fuel is an option.

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Tire-Derived Fuel

Tire-derived fuel (TDF) is a solid fuel derived from scrap rubber tires. The fuel’s properties are similar to coal and it can be burned in most coal-fired boilers without modifications. According to the EIA 860-B database, there are over 300 coal-fired CHP units in the United States under 50 MW in size (totaling over 4 GW) that could potentially utilize tire-derived fuel.

Fuel Availability

In the United States, between 250 and 350 million tires are discarded each year. Tires are now banned from most landfills and must be disposed of at dedicated sites. TDF producers use specialized machinery to shred, screen, and remove metal from the tires before they sell the fuel to local consumers. TDF has not yet caught on in the DER and CHP industries, but it can replace or supplement coal in nearly any application. In the year 2000, over 4 GW of electricity and 300 trillion Btu’s of thermal output were produced by coal-fired CHP units under 50 MW. In many of these cases, cofiring with or switching to tire-derived fuel could be beneficial. Most TDF processing plants are located close to large tire piles, located most prominently in the Midwest and Northeast regions.

The processing costs for tire-derived fuel generally fall between $15 and $19 per ton, and the fuel sells for about 5 dollars more ($20-$24 per ton). Assuming these prices and a fifty-mile trip, TDF would cost about $1.00 per MMBtu to obtain. With average market conditions, the price of TDF is slightly less than the price of coal on a Btu basis, and contains less sulfur than coal. Cofiring with coal is the most popular method of TDF energy production because coal-fired boilers already exist and TDF can be easily co-fired with no modifications. Cofiring saves money since TDF is less expensive .

The growing demand for TDF has begun to create a supply infrastructure with manufacturers and brokers. In the US, current TDF users are: Cement Kilns (30%), Pulp & Paper Mills (23%), Utility Boilers (19%), Industrial Boilers (13%), and Dedicated Tire to Energy (10%). Most of these facilities utilize TDF strictly for heat. The vast majority of TDF operations are industrial applications larger than 50 MW. Although many DER/CHP opportunities are available, TDF is best suited for large utility or industrial applications, and the market so far has consisted of cement kilns, utilities, dedicated facilities, industrial cofiring operations, or any sizeable energy user with coal generation on-site.

Fuel Characteristics

There are 20 different grades of ground and shredded rubber from discarded tires, based on the size and consistency of the rubber chips. Typical TDF grades are 0.25 to 3 inches in size with varying degrees of wire removal. An average tire contains 280,000 Btu – the equivalent of 2.5 gallons of oil or 20 pounds of coal. TDF-coal cofiring blends are common. TDF performs similarly to coal, and has a heating value of about 16,000 Btu per pound. While TDF contains more carbon than coal, it contains less nitrogen, sulfur and oxygen, which will result in fewer SO x and NO x emissions. Tire-derived fuel also has less ash, less moisture, and a higher heating value than coal. TDF does not require any special handling, and since the Btu content is so high, transportation is not as costly as for biomass and other opportunity fuels. Tire-derived fuel is an ideal opportunity fuel that can replace or be cofired with coal in nearly any application. A supply infrastructure has already been created, the fuel is usually available at a lower price (or at least competitive with coal), and fewer emissions are produced.

Conversion / Utilization Options

There are four steps that go into processing TDF: Primary Shred – Double rotor shear shredder – strips 2 to 4 inches wide. Secondary Shred – Second shredder/granulator makes the finished size chips. Screening – Chips are screened with trommel or disc screens – oversize chips returned to #2 . Metal Removal – Metal bead and wire is removed with magnets.

TDF is most often burned in boilers designed for coal. Minimal modifications are necessary, with only a slight increase in maintenance costs. For 100 percent tire-derived fuel, boilers specifically designed for TDF are recommended. If TDF is burned in a high-percent blend, higher boiler temperatures are preferred in order to completely burn the fuel. Although the high flame temperature will slightly increase NO x emissions, the emissions from coal are higher and control technologies are already in place. Cofiring tire-derived fuel almost always enhances boiler performance due to its high heating value and lower emissions. Fluidized bed, cyclone, and stoker-fed boilers are all options for TDF combustion.

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Conversion Technologies

Pelletize

Technology Description

Pellets can be generally described as small particles of identical combustible material and approximately equal size. Pellets are typically created out of loose fuels or scrap, such as sawdust or process scrap. To pelletize, the fuels are compressed, occasionally with a combustible binder such as heavy oil to insure cohesion, into pieces of approximately equal size – pellets. The physical size of pellets varies depending on feedstock and preparation method, but they typically have dimensions on the order of 1 inch.

Pellets are useful in that they can be handled like bulk material, and, due to their increased surface to volume ratio (relative to their bulk feedstock) they combust easily and efficiently. Pellets also facilitate easy subdivision of an otherwise unwieldy large load – by breaking into discrete measurable increments (e.g., a kg or a truckload of pellets). As a solid substance, pellets are most always consumed by combustion in a boiler or used as a feedstock to be converted to gas or liquid. The primary advantages of pellets are their low cost and ease of transport.

Technology Status / Development Issues

Pellets and pelletizing technology are well established methods of material organization. In industries like wood processing, pellets serve as simple means to generate revenue from (otherwise wasted) sawdust and miscellaneous scraps. Current state of the art home heating stoves are designed to burn wood or plant pellets. There are no development issues to pellets, but there are, at present, limited supply. If the demand for pellets grows as an alternative to natural gas or electric heating, the supply for pellet will likely pickup with novel feedstocks and techniques, as well as an expansion in existing pellet suppliers.

Suitable Feedstocks

Any combustible solid can be pelletized. The main technical challenge is the obtaining of high fuel quality (low dust component and no impurities or dangerous additives). The substances which are favored are combustible materials that have reasonable energy density, and (usually) have been reduced to scrap. Sawdust, for example, can be compressed with a binder to make pellets, as can food or refuse waste. Anaerobic sludge can be made to pellets. While typically created from wood or plant matter, pellets are relatively material-independent. As long as a feedstock material can be rendered sufficiently small in size, and a suitable binder can be found, almost any combustible material can be used for pellets (there are obvious limitations here associated with dangerous emissions).

Wood pellets have enormous market potential as a biomass fuel although their current share of the heat energy market in general and the biomass fuel market is very small at present. The potential exists because of the availability of the raw material, the comparatively high energy density, the possibility of their use in automatic firing systems with a high level of user convenience and, not least - provided that certain energy policy conditions are fulfilled - their competitive price. For small systems in particular this opens up a new dimension in practical biomass heating, which, under certain circumstances, could represent a genuine alternative in this area to oil or electric heating.

Switchgrass, often discussed as a crop for ethanol production, can also be pelletized for solid fuel. The grass is harvested, then chopped up and fed into a machine that compresses it and extrudes pellets. The grass pellets burn clean, minimizing pollutants and greenhouse gases. Additionally, it doesn't require as much drying as wood, so it takes less energy to produce.

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Fermentation

Fermentation is used to convert biomass to liquid fuel alcohol. Fermentation is a biological process whereby enzymes and bacteria, given proper temperature and acidity, convert biomass fuels to alcohol which can be combusted for heating content.

Technology Description

Ethanol for use in alcoholic beverages, and the vast majority of ethanol for use as fuel, is produced by fermentation: when certain species of yeast (most importantly, Saccharomyces cerevisiae) metabolize sugar in the absence of oxygen, they produce ethanol and carbon dioxide. The overall chemical reaction conducted by the yeast may be represented by the chemical equation

C6H12O6 → 2 CH3CH2OH + 2 CO2

The process of culturing yeast under conditions to produce alcohol is referred to as brewing. Yeasts can grow in the presence of up to about 20% alcohol, due to toxicity to yeast at concentrations higher than 20%, but the concentration of alcohol in the final product can be increased by distillation.

In order to produce ethanol from starchy materials such as cereal grains, the starch must first be broken down into sugars. In brewing beer, this has traditionally been accomplished allowing the grain to germinate, or malt. In the process of germination, the seed produces enzymes that can break its starches into sugars. For fuel ethanol, this hydrolysis of starch into glucose is accomplished more rapidly by treatment with dilute sulfuric acid, fungal amylase enzymes, or some combination of the two.

(Celluslosic Ethanol)

Potentially, glucose for fermentation into ethanol could also be obtained from cellulose. Cellulosic ethanol is a blend of normal ethanol that can be produced from a great diversity of biomass including waste from urban, agricultural, and forestry sources. There are at least two methods of production of cellulosic ethanol - enzymatic hydrolysis and synthesis gas fermentation. Neither process generates toxic emissions when it produces ethanol. The technology is very new and is still being tested.

Realization of this enzymatic hydrolysis would turn a number of cellulose-containing agricultural byproducts, such as corncobs, straw, and sawdust, into renewable energy resources. Until recently, the cost of the cellulase enzymes that could hydrolyse cellulose was prohibitive. The Canadian firm Iogen brought the first cellulose-based ethanol plant on-stream in 2004. In April 2004, Iogen became the first business to commercially sell cellulosic ethanol. The primary consumer thus far has been the Canadian government, which, along with the United States government (particularly the Department of Energy's National Renewable Energy Laboratory), has invested millions of dollars into assisting the commercialization of cellulosic ethanol.

Genencor and Novozymes are two other companies that have received United States government Department of Energy funding for research into reducing the cost of cellulase, a key enzyme in the production cellulosic ethanol by enzymatic hydrolysis.

To achieve the maximum ethanol yield for conversion of cellulose-containing materials, the five-carbon sugar xylose must also be converted to ethanol. The yeast commonly used to convert glucose to ethanol (Saccharomyces cerevisiae) does not metabolize xylose. Other yeasts and bacteria have been studied to convert xylose to ethanol.

(Syn-gas)

Some companies are currently using synthesis gas fermentation to convert a broad variety of urban, agricultural, and forestry waste into ethanol. Fossil fuels and virtually any blend of biomass could also be used as feedstock. After gasification, anaerobic bacteria (Clostridium ljungdahlii) are used to convert the syngas (CO, CO2, and H2) into ethanol. The heat generated by gasification is also used to co-generate excess electricity.

According to US Department of Energy studies conducted by the Argonne Laboratories of the University of Chicago, one of the benefits of cellulosic ethanol is that it reduces [greenhouse gas emissions] (GHG) by 85% over reformulated gasoline. By contrast, sugar-fermented ethanol reduces GHG emissions by 18% to 29% over gasoline.

At petroleum prices like those that prevailed through much of the 1990s, ethylene hydration was a decidedly more economical process than fermentation for producing purified ethanol. Recent increases in petroleum prices, coupled with perennial uncertainty in agricultural prices, make forecasting the relative production costs of fermented versus petrochemical ethanol difficult at the present time.

Technology Status / Development Issues

For a mixture of ethanol and water, there is a maximum boiling azeotrope at 96% ethanol and 4% water. For this reason, fractional distillation of ethanol-water mixtures (of less than 96% ethanol) cannot yield ethanol purer than 96%. Therefore, 95% ethanol in water is a fairly common solvent.

Several competing approaches may be used to produce absolute ethanol. To break the azeotrope for performing distillation, a small amount of benzene can be added, and the mixture is again fractionally distilled. Benzene forms a tertiary azeotrope with water and ethanol to remove the last of the water, and a binary azeotrope with ethanol removes most of the benzene. The resulting ethanol is water free, for processes that require it. However, several parts per million of benzene remain, so consumption by humans leads to distinctive liver damage. Nowadays benzene as entrainer is replaced by cyclohexane to avoid the health hazards.

Alternatively, a molecular sieve can be used to selectively absorb the water from the 96% ethanol solution. Synthetic zeolite in pellet form can be used, as well as corn grits. The zeolite approach is especially of value, for it is possible to recycle the zeolite in a closed system essentially an unlimited number of times, through drying it with a blast of heated CO2. Absolute ethanol produced this way has no residual benzene, and can be used as fuel, or, when diluted, can even be used to fortify port and sherry in traditional winery operations.

Also possible is the method of pressure swing distillation, which is still a topic of current researches. The idea of pressure swing distillation is, to distillate at 2 different pressures, due to the pressure dependence of the azeotropic composition. For example the first distillation step would be preceded at a pressure at which the azeotropic composition is above 96% for example 97% afterwards a distillation at a pressure with a lower azeotropic composition would follow. By this it is possible to purify ethanol without the use of an entrainer.

Ethanol is also being mulled for a hydrogen feedstock and transport medium – it could be used to transport the hydrogen, and then the hydrogen could be liberated from its associated carbon in a hydrogen reformer, and fed into a fuel cell. Alternatively, some fuel cells can be directly fed by ethanol or methanol. As of 2005, fuel cells are able to process methanol more efficiently than ethanol.

Suitable Feedstocks

Sugar cane grows in the extreme southern United States, but not in the cooler climates where corn is dominant. However, many regions that currently grow corn are also appropriate areas for growing other crops that can be used for energy production. These crops include corn stover, sugar beets, wheat straw, hybrid poplars, and dedicated herbaceous biomass feedstocks such as switchgrass or bermudagrass. Some studies indicate that using these sugar beets would be a much more efficient method for making ethanol in the U.S. than using corn. United States Department of Energy reports have shown that a minimum farmgate price, hybrid poplars and switchgrass would be economically advantageous over conventional crops in certain regions of the U.S.

There is also growing interest in the use of waste biomass as a source for alcohol other types of fuel. New technologies such as cellulose to ethanol production could provide much higher positive energy ratios of 2 to 3 times more energy in ethanol produced than input. Cellulose to ethanol production could also run on any cellulose and hemicellulose source from farm waste, hay/grass, basically any plant matter including wood, cardboard and paper. Theoretically farms could produce fuel without sacrificing food production, because all that is needed is the left over plant matter after harvesting.

Cellulose to ethanol production is still in development and has seen limited use in industrial ethanol production. Using current technologies, 1 ton of biomass (such as switchgrass) would be able to produce 80 gallons of ethanol using a conventional enzymatic fermentation process.

The biggest challenges in using cellulose as a feedstock is the treatment and disposal of process waste and the conversion of C5 sugars (hemicellulose). Lignin, a part of the cell wall that provides plant structure, does not readily break down to simple sugars but has a energy equivalent of soft coal. Lignin would be incinerated to produce energy for the ethanol plant and surrounding areas or gasified to produce a syngas (hydrogen and carbon dioxide). Unlike grain based processes which produce a by-product known as distillers grain with minimal waste treatment needs, cellulosic processes are typically effluent and waste treatment intensive.

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Anaerobic Digestion

Anaerobic Digestion, or AD, refers to a biological process whereby enzymes and anaerobic bacteria, given proper temperature and acidity, decompose organic material in the absence of oxygen. Anaerobic digester gas (ADG) is the gas recovered from the digestion process. The gas produced by the bacteria, about 50 percent methane and 30 percent carbon dioxide, is usually flared and/or used as a heat source for the digester tank. However, it has the potential to be a steady and reliable source of fuel, essentially free to those that produce it.

Technology Description

Anaerobic digestion is the breakdown of organic matter by bacteria in the absence of oxygen. Anaerobic digestors use the natural process of anaerobic digestion to treat waste, produce energy or both. The digester is a sealed, heated enclosure that provides a suitable environment for naturally occurring anaerobic bacteria to convert waste into methane gas. The source material can be wastewater (public sewage or industrial), animal manure, or other organic waste sludge.

Increasing environmental pressures on waste disposal has increased the use of digestion as a process for reducing waste volumes and generating useful byproducts. It is a fairly simple process that can greatly reduce the amount of organic matter which might otherwise end up in landfills or waste incinerators.

Almost any organic material can be processed in this manner. This includes materials such as waste paper, grass clippings, leftover food, sewage, and animal waste. After sorting or screening to remove inorganic or hazardous materials such as metals and plastics, the material to be processed is often shredded or minced to achieve a better reaction ( ultrasound has even been used in the process to aid in the break up of solids). Breaking the material into smaller pieces provides the bacteria with more surface area, allowing them to complete the process quicker. The material is then fed into a sealed digester. In the case of dry materials, water is added.

As of 2004, there were over 75,000 wastewater treatment plants (industrial and municipal) in the US, and only about 5,000 currently contain anaerobic digesters. Most industrial treatment plants use aerobic digestion, since it is the traditional method of treating organic wastewater streams, and most facilities already have these digesters in place. Many smaller industrial plants simply send their wastewater to local municipal facilities, which also mostly utilize aerobic digestion. While aerobic digesters are well established, anaerobic digesters offer many potential benefits to plant operators. With anaerobic digestion, less solid waste is left over, no power is required to aerate the wastewater, and recoverable energy is produced in the form of methane gas. However, the startup time for an anaerobic system is much greater, especially when the organic waste volume is low, so a steady, non-dilute stream of wastewater sludge is required for continuous operation. Because of this, anaerobic digesters are best suited for large facilities with a constant, high-volume organic waste stream.

The most common industries for anaerobic wastewater treatment are food and beverage processing, pulp and paper, and petrochemicals. However, only a small fraction of these treatment plants utilize their digester gas for energy. Aside from wastewater treatment plants, some large animal farms in the United States utilize anaerobic digestion to treat waste manure. Farms using anaerobic digesters to treat cow and pig waste produce less emissions and odors than conventional treatment methods, which usually let the waste decompose naturally and use the remains for fertilizer.

Technology Status / Development Issues

There are two conventional processes— mesophilic, which takes place at ambient temperatures typically between 20° and 40°C, and thermophilic, which takes place at elevated temperatures, typically up to 70°C. The residence time in a digester varies with the type and amount of feed material and the temperature. In the case of mesophilic digestion, residence time may be between 15 and 30 days; the thermophillic process is usually faster, requiring only about two weeks to complete. However, thermophilic digestion is more expensive, requires more energy, and is less stable than the mesophillic process. Therefore, the mesophillic process is still in use.

Many continuous digesters have mechanical devices to slowly mix the contents and to allow excess material to be continuously bled off to maintain a reasonably constant volume.

The digestion of the organic material is done by a range of many different species of different naturally occurring bacteria all doing a different job at a different step in the digestion process. Maintaining suitable conditions in the digester is essential in maintaining a healthy bacterial population.

Four stages of digestion have been recognized.

• The first is hydrolysis, in which complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups.
• The second stage is acidogenesis, where a further breakdown occurs producing ammonia, carbon dioxide and hydrogen sulfide.
• The third stage is acetogenesis where the products of acidogenesis are further digested to produce products such as carbon dioxide, hydrogen and acetates.
• The fourth stage is methanogenesis where methane, carbon dioxide and water are produced.

There are three principal by-products of anaerobic digestion.

• Biogas, a gaseous mixture comprising mostly of methane and carbon dioxide, but also containing a small amount hydrogen. Biogas can be burned to produce electricity, usually with a reciprocating engine or microturbine. The gas is often used in a cogeneration arrangement, to generate electricity and use waste heat for thermophilic digesters or to heat buildings. Excess electricity can be sold to electricity suppliers. Since the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle, biogas does not contribute to increasing atmospheric carbon dioxide concentrations; because of this, it is considered to be an environmentally friendly energy source. The production of biogas is not a steady stream; it is highest during the middle of the reaction. In the early stages of the reaction, little gas is produced because the number of bacteria is still small in size. Toward the end of the reaction, only the hardest to digest materials remain, leading to a decrease in the amount of biogas produced.
• The second by-product is a liquid that is rich in nutrients and can be an excellent fertilizer dependant on the quality of the material being digested. If the digested materials include low levels of toxic heavy metals or synthetic organic materials such as pesticides or PCBs, the effect of digestion is to significantly concentrate such materials in the digester liquor in order to dispose of this liquid properly. In extreme cases, the disposal costs and the environmental risks posed by such materials can offset any environmental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialised catchments.
• The third by-product is a stable organic material comprised largely of lignin and chitin, but also of a variety of plastics and mineral components in a matrix of dead bacterial cells. This resembles domestic compost and can be used as compost or to make low grade building products such as fiberboard.

Nearly all digestion plants have ancillary processes to treat and manage all of the by-products. The gas stream is dried and sometimes sweetened before storage and use. The sludge liquor mixture has to be separated by one of a variety of ways, the most common of which is filtration.

Any DER/CHP technology normally powered by natural gas can be modified to run on anaerobic digester gas. The most ideal ADG-fueled DER/CHP technologies are reciprocating engines, microturbines and fuel cells. Combustion turbines require too many modifications. Boilers feeding steam turbines can be used with little modifications, but are usually used for larger applications.

No matter what the technology, however, the anaerobic digester gas is produced and treated in the same manner. First, the organic sludge is stored, thickened and heated before it enters the digester tank. In the tank, anaerobic bacteria consume the sludge and release a methane gas that is collected and treated to remove contaminants. The treated gas can be fed to a prime mover to produce heat and electricity. Some of the heat produced can be used to preheat the sludge.

Suitable Feedstocks

Digestion can be either wet or dry. Dry digestion refers to mixtures which have a solid content of 30% or greater, whereas wet digestion refers to mixtures of 15% or less.

The two main types of reactors are continuous and batch. Batch is the simplest, with the biomass added to the reactor at the beginning and sealed for the duration of the process. In the continuous process, which is the more common type, organic matter is constantly added to reactor and the end products constantly removed, resulting in a much more constant production of biogas.

Although there will always be a net loss in energy in the whole system (the energy to grow the biomass is more than the output of the reactor), for the processing of waste organic material, anaerobic digestion is the preferable choice because it is environmentally friendly. The biggest impacts on the environment include the energy and materials used to build the plant, transport costs and fuel use in transporting material to site and visual and audible impacts of the site operation. Odor can be a severe problem during emptying cycles. This is a particularly difficult issue for batch reactors.

To be economically viable, there must be a market for the end products. Biogas can be sold or used in almost all parts of the world, where it will offset demand on fossil fuel stocks. The digester liquor is suitable for use as a fertilizer, although frequently supplemental nutrients need to be added.

The sludge component, even when dried and available as a soil conditioner, is not easily disposed of. However, it has its uses in non-agricultural areas, such as golf courses, and as cover for landfills. In some localities, the sludge itself is used as a fuel in heating systems, and the residual ash is disposed of in a landfill.

Anaerobic digester gas has a Btu content of about 600 MMBtu/ft 3 (60 percent that of natural gas). Anaerobic digester gas could conceivably be sold at the same rate as natural gas on a Btu-basis (currently about $5.00-$6.00 per MMBtu), but facilities are much more likely to use the gas for their own heat and power needs. When a digester is already in place, ADG is a free fuel source to plant operators, and when one is not in place, many benefits other than power production can be seen. Installing an anaerobic digester typically costs between $900 and $1,500 per kW, depending on various factors, and about $0.001 to $0.003 per kWh to maintain. ADG performs better than landfill gas, coke oven gas, and the other low-Btu gases, and can replace natural gas in almost any prime mover technology, although some equipment modifications may be required.

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Gasification / Pyrolysis

Gasification refers to thermal processing to convert feedstock into a gaseous or liquid fuel, product composition varies over a wide range depending on feedstock combposition and conversion process characteristics.

Technology Description

Gasification is a process that converts carbonaceous materials, such as coal, petroleum, petroleum coke or biomass, into carbon monoxide and hydrogen. In a gasifier, the carbonaceous material undergoes three processes: pyrolysis, combustion, and gasification.

• The pyrolysis (or devolatilization) process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.
• The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. Pyrolysis and combustion are very rapid processes.
• The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen. The resulting gas is called producer gas or syngas (or wood gas when fueled by wood) and may be more efficiently converted to energy such as electricity than would be possible by direct combustion of the fuel, as the fuel is first combusted in a gas turbine and the heat is used to produce steam to drive a steam turbine. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing high temperature combustion of the gas from otherwise problematic fuels.

A gasifier is a special piece of equipment that extracts volatile fuel vapors from biomass and leaves only ash and small particulates behind. Gasifiers make use of a process called pyrolysis, which releases the volatile components of a fuel at around 600 oC via a series of complex reactions. In addition to pyrolysis, a second gasification process is often employed, converting the leftover char into a carbon gas using steam and/or combustion. With most gasifiers, about 80 percent of the volatile contents of a fuel are recovered, but new gasification systems have reached higher conversion efficiencies. The most efficient method of utilizing biomass gas is a combined cycle gasification system. More simple (but less efficient) gasifier systems have been developed for smaller DER/CHP applications with low-quality wood waste fuels, but their track record has not ben nearly as impressive as their large industrial counterparts.

Technology Status / Development Issues

A gasifier is an expensive piece of equipment. When all of the gas cleaning equipment and installation costs are considered, gasifiers cost about $800 per kW to obtain in the 5-50 MW range. In the near future, the cost is expected to lower to $400-$600 per kW, but this is still a significant hurdle. Usually a gasifier can only justify its cost is if a large amount of power is produced – small biomass gasification systems are typically not economical. While CHP systems under 20 MW can be beneficial in applications with a free fuel source, larger systems are have proven themselves to be more cost-effective. Biomass gas is not very suitable for small CHP applications. Because the cost of a gasifier is so high, the power output and efficiency must be high enough to cover that cost; either that or the biomass fuel must be obtained at a very cheap price.

Gasification relies on chemical processes at elevated temperatures >700°C, contrary to biological processes such as anaerobic fermentation ( digestion) which produces biogas. Breakdown of hydrocarbons into syngas is done by carefully controlling the amount of oxygen present while heating the hydrocarbons to extreme temperatures. Four types of gasifiers are currently available for commercial use: counter-current fixed bed, co-current fixed bed, fluid bed and entrained flow.

• The counter-current fixed bed ("up draft") gasifier consists of a fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the "gasification agent" (steam, oxygen and/or air) flows in counter-current configuration. The ash is either removed dry or as a slag. The slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent. The throughput for this type of gasifier is relatively low. Thermal efficiency is high as the gas exit temperatures are relatively low. However, this means that tar and methane production is significant at typical operation temperatures, so product gas must be extensively cleaned before use or recycled to the reactor.
• The co-current fixed bed ("down draft") gasifier is similar to the counter-current type, but the gasification agent gas flows in co-current configuration with the fuel (downwards, hence the name "down draft gasifier"). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources. The produced gas leaves the gasifier at a high temperature, and most of this heat is often transferred to the gasification agent added in the top of the bed, resulting in an energy efficiency on level with the counter-current type. Since all tars must pass through a hot bed of char in this configuration, tar levels are much lower than the counter-current type.
• In the fluid bed gasifier, the fuel is fluidised in oxygen (or air) and steam. The ash is removed dry or as heavy agglomerates that defluidise. The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly suitable. The agglomerating gasifiers have slightly higher temperatures,