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BioEnergy Technology
Industrial Biomass Combustion
 
Industrial biomass combustion facilities can burn many types of biomass fuel, including wood, agricultural residues, wood pulping liquor, municipal solid waste (MSW) and refuse-derived fuel. Combustion technologies convert biomass fuels into several forms of useful energy for commercial or industrial uses such as hot air, hot water, steam and electricity. 
 
A furnace is the simplest combustion technology. In a furnace, biomass fuel burns in a combustion chamber, converting biomass into heat energy. As the biomass burns, hot gases are released. Commercial and industrial facilities use this heat directly or indirectly through a heat exchanger in the form of hot air or water.
 
A biomass-fired boiler is a more adaptable direct combustion technology because a boiler transfers the heat of combustion into steam. Steam can be used for electricity, mechanical (process) energy or heat. Biomass boilers supply energy at low cost for many industrial and commercial uses.
 
The direct-fired gas turbine is another combustion technology for converting biomass to electricity. In this technology, fuel pretreatment reduces biomass to a particle size of less than 2 millimeters and a moisture content of less than 25 percent. Then the fuel is burned with compressed air. Cleanup of the combustion gas reduces particulate matter before the gas expands through the turbine stage. The turbine drives a generator to produce electricity.
 
 
 

Cogeneration
 
Biomass combustion facilities that produce electricity from steam-driven turbine-generators have a conversion efficiency of 17 to 25 percent. Using a boiler to produce both heat and electricity (cogeneration) improves overall system efficiency to as much as 85 percent. That is, cogeneration converts 85 percent of the fuel´s potential energy into useful energy in two forms: electricity and steam heat.

 
Two cogeneration arrangements, or cycles, are possible for combining electric power generation with industrial steam production. Steam can be used in an industrial process first and then routed through a turbine to generate electricity. This arrangement is called a bottoming cycle. In the alternate arrangement, steam from the boiler passes first through a turbine to produce electric power. The steam exhaust from the turbine is then used for industrial processes or for space and water heating. This arrangement is called a topping cycle. Of the two cogeneration arrangements, the topping cycle is more common.
 
 

Co-Firing
 
Biomass can be co-fired as a secondary fuel in a coal-burning power plant.  If the plant is using high-sulfur coal co-firing biomass could help reduce sulfur dioxide and nitrogen oxide emissions. CO-firing with biomass decreases net carbon dioxide emissions from the power plant (if the biomass fuel comes from a sustainable source).

Gasification
 
Gasification is a thermochemical process that converts biomass into a combustible gas called producer gas. Producer gas contains carbon monoxide, hydrogen, water vapor, carbon dioxide, tar vapor and ash particles.
 
Producer gas contains about 70% to 80% of the energy originally present in the biomass feedstock. The gas can be burned directly for space or process heat or it can be burned in a boiler to produce steam. Filters and gas-scrubbers can remove tars and particulate matter from producer gas. The clean gas is suitable for use in an internal combustion engine, gas turbine or other application requiring a high-quality gas.
 
 

Types of Gasifiers
 
 
There are three principal types of gasification systems: updraft, downdraft and fluidized-bed.
 
In an updraft (or "counterflow") gasifier, the biomass fuel enters the top of the reaction chamber while steam and air (or oxygen) enter from below a grate. The fuel flows downward, and upflowing hot gases pyrolyze it. Some of the resulting charcoal residue falls to the grate, where it burns, producing heat and giving off carbon dioxide (CO2) and water vapor (H2O). The CO2 and H2O react with other charcoal particles, producing carbon monoxide (CO) and hydrogen (H2) gases. The gases exit from the top of the chamber. Ashes fall through the grate.
 
The updraft design is relatively simple and can handle biomass fuels with high ash and moisture content. However, the producer gas contains 10 percent to 20 percent volatile oils (tar), making the gas unsuitable for use in engines or gas turbines.
 
Successful operation of a downdraft (or "co-flow") gasifier requires drying the biomass fuel to a moisture content of less than 20 percent. Fuel and air (or oxygen) enter the top of the reaction chamber. Downflowing fuel particles ignite, burning intensely and leaving a charcoal residue. The charcoal (which is about 5 to 15 percent of the original fuel mass) then reacts with the combustion gases, producing CO and H2 gases. These gases flow down and exit from the chamber below a grate. The producer gas leaving the gasifier is at a high temperature (around 700° C). Combustion ash falls through the grate. The advantage of the downdraft design is the very low tar content of the producer gas.
 
A fluidized-bed gasifier typically contains a bed of inert granular particles (usually silica or ceramic). Biomass fuel, reduced to particle size, enters at the bottom of the gasification chamber. A high velocity flow of air from below forces the fuel upward through the bed of heated particles. The heated bed is at a temperature sufficient to partially burn and gasify the fuel. The processes of pyrolysis and char conversion occur throughout the bed. Although fluidized-bed gasifiers can handle a wider range of biomass fuels, the fuel particles must be less than 10 centimeters in length and must have no more than 65-percent moisture content. The fluidized-bed design produces a gas with low tar content but a higher level of particulate compared with fixed-bed designs.
 
If the gasifier is pressurized, it produces gas at a pressure suitable for electric power generation using a gas turbine. High-pressure fuel-feed systems are in the development stage. Hot gas cleanup technology is also under development. Hot gas cleanup removes tars, chars and volatile alkalis to improve system efficiency.

Pyrolysis
 
Pyrolysis is a process that uses heat to breaks down organic matter in the absence of oxygen.  The process results in a variety of end products, namely, biochar, syngas, and bio-oil.  Pyrolysis is used in the industrial sector to produce charcoal, carbon, methanol and a variety of substances used in the chemical industry.  

Biomass Pellets and Bricks
 
Many Oregonians convert biomass to useful energy in their homes by burning wood in a fireplace or woodstove.   More and more residents, businesses, and government buildings are converting their boiler and heatings systems to utilize a more advance, efficient and cleaner fuels such as biomass pellets and bricks.
 
Pellets and bricks are different forms of densified fuels. These biomass fuels are made from wood wastes, waste paper, cardboard, or agricultural residues. and sizes, ranging from pea-sized pellets to logs 12 inches long and 6 inches in diameter.
 
Modern pellet stoves are efficient home or business heating appliances. A conventional fireplace is less than 10-percent efficient at delivering heat to a home. In contrast, average pellet stove efficiency is better than 55-percent.
 
The use of pellet stoves in place of conventional wood stoves reduces the amount of particulate matter in the air. This is especially beneficial in areas where wood smoke from home wood-heating is a major component of local air pollution. In pellet stoves, particulate matter emissions are as much as 90-percent lower than emissions from conventional wood stoves.
 
 
 

Anaerobic Digestion
 
Anaerobic digestion is a biochemical process in which particular kinds of bacteria digest biomass in an oxygen-free environment. Several different types of bacteria work together to break down complex organic wastes in stages, resulting in the production of "biogas."
 
Anaerobic digesters can help control the disposal and odor of plant and animal waste. Dairy farmers faced with increasing federal and state regulation of the waste their animals produce have found new ways to reduce the impacts and cost of manure management.  
              
Anaerobic digestion requires an airtight chamber, called a digester. To promote bacterial activity, the digester must maintain a temperature of at least 68° F.  Using higher temperatures, up to 150° F, can shorten the amount of time it takes to digest the material.  The temperature of the digester will depend on the species of bacteria contained inside
 
The biogas produced in a digester (also known as "digester gas") is actually a mixture of gases, with methane and carbon dioxide making up more than 90 percent of the total. Biogas typically contains smaller amounts of hydrogen sulfide, nitrogen, hydrogen, methylmercaptans and oxygen.
 
For individual farms, small-scale plug-flow or covered lagoon digesters of simple design can produce biogas for on-site electricity and heat generation. For example, a plug-flow digester could process 8,000 gallons of manure per day, the amount produced by a herd of 500 dairy cows. By using digester gas to fuel an engine-generator, a digester of this size would produce more electricity and hot water than the dairy consumes.
 
Larger scale digesters are suitable for manure volumes of 25,000 to 100,000 gallons per day. In Denmark and in several other European countries, central digester facilities use manure and other organic wastes collected from individual farms and transported to the facility.
              
 

Types of Anaerobic Digesters
 
 
There are three basic digester designs. All of them can trap methane and reduce fecal coliform bacteria, but they differ in cost, climate suitability and the concentration of manure solids they can digest.
 
A covered lagoon digester, as the name suggests, consists of a manure storage lagoon with a cover. The cover traps gas produced during decomposition of the manure. This type of digester is the least expensive of the three.
 
Covering a manure storage lagoon is a simple form of digester technology suitable for liquid manure with less than 3-percent solids. For this type of digester, an impermeable floating cover of industrial fabric covers all or part of the lagoon. A concrete footing along the edge of the lagoon holds the cover in place with an airtight seal. Methane produced in the lagoon collects under the cover. A suction pipe extracts the gas for use. Covered lagoon digesters require large lagoon volumes and a warm climate. Covered lagoons have low capital cost, but these systems are not suitable for locations in cooler climates or locations where a high water table exists.
 
A complete mix digester converts organic waste to biogas in a heated tank above or below ground. A mechanical or gas mixer keeps the solids in suspension. Complete mix digesters are expensive to construct and cost more than plug-flow digesters to operate and maintain.
 
Complete mix digesters are suitable for larger manure volumes having solids concentration of 3 percent to 10 percent. The reactor is a circular steel or poured concrete container. During the digestion process, the manure slurry is continuously mixed to keep the solids in suspension. Biogas accumulates at the top of the digester. The biogas can be used as fuel for an engine-generator to produce electricity or as boiler fuel to produce steam. Using waste heat from the engine or boiler to warm the slurry in the digester reduces retention time to less than 20 days.
 
Plug-flow digesters are suitable for ruminant animal manure that has a solids concentration of 11 percent to 13 percent. A typical design for a plug-flow system includes a manure collection system, a mixing pit and the digester itself. In the mixing pit, the addition of water adjusts the proportion of solids in the manure slurry to the optimal consistency. The digester is a long, rectangular container, usually built below-grade, with an airtight, expandable cover.
 
New material added to the tank at one end pushes older material to the opposite end. Coarse solids in ruminant manure form a viscous material as they are digested, limiting solids separation in the digester tank. As a result, the material flows through the tank in a "plug." Average retention time (the time a manure "plug" remains in the digester) is 20 to 30 days.
Anaerobic digestion of the manure slurry releases biogas as the material flows through the digester. A flexible, impermeable cover on the digester traps the gas. Pipes beneath the cover carry the biogas from the digester to an engine-generator set.
 
A plug-flow digester requires minimal maintenance. Waste heat from the engine-generator can be used to heat the digester. Inside the digester, suspended heating pipes allow hot water to circulate. The hot water heats the digester to keep the slurry at 25°C to 40°C (77°F to 104°F), a temperature range suitable for methane-producing bacteria. The hot water can come from recovered waste heat from an engine generator fueled with digester gas or from burning digester gas directly in a boiler.
 
 

The Process of Anaerobic Digestion
 
The process of anaerobic digestion occurs in a sequence of stages involving distinct types of bacteria. Hydrolytic and fermentative bacteria first break down the carbohydrates, proteins and fats present in biomass feedstock into fatty acids, alcohol, carbon dioxide, hydrogen, ammonia and sulfides. This stage is called "hydrolysis" (or "liquefaction").
 
Next, acetogenic (acid-forming) bacteria further digest the products of hydrolysis into acetic acid, hydrogen and carbon dioxide. Methanogenic (methane-forming) bacteria then convert these products into biogas.
 
The combustion of digester gas can supply useful energy in the form of hot air, hot water or steam. After filtering and drying, digester gas is suitable as fuel for an internal combustion engine, which, combined with a generator, can produce electricity. Future applications of digester gas may include electric power production from gas turbines or fuel cells. Digester gas can substitute for natural gas or propane in space heaters, refrigeration equipment, cooking stoves or other equipment. Compressed digester gas can be used as an alternative transportation fuel.

Wastewater
 
Municipal sewage contains organic biomass solids, and many wastewater treatment plants use anaerobic digestion to reduce the volume of these solids. Anaerobic digestion stabilizes sewage sludge and destroys pathogens. Sludge digestion produces biogas containing 60-percent to 70-percent methane, with an energy content of about 600 Btu per cubic foot.
 
Most wastewater treatment plants that use anaerobic digesters burn the gas for heat to maintain digester temperatures and to heat building space. Unused gas is burned off as waste but could be used for fuel in an engine-generator or fuel cell to produce electric power.
 
A fuel cell at the Columbia Boulevard Wastewater Treatment Plant in Portland, Oregon, converts digester gas into electricity. The fuel cell began producing power in July 1999. The Columbia Boulevard fuel cell produces an estimated 1,500,000 kilowatt-hours of electricity each year
 
 

Landfill Gas
 
The same anaerobic digestion process that produces biogas from animal manure and wastewater occurs naturally underground in landfills. Most landfill gas results from the decomposition of cellulose contained in municipal and industrial solid waste. Unlike animal manure digesters, which control the anaerobic digestion process, the digestion occurring in landfills is an uncontrolled process of biomass decay.
 
The efficiency of the process depends on the waste composition and moisture content of the landfill, cover material, temperature and other factors. The biogas released from landfills, commonly called "landfill gas," is typically 50-percent methane, 45-percent carbon dioxide and 5-percent other gases. The energy content of landfill gas is 400 to 550 Btu per cubic foot.
 
Capturing landfill gas before it escapes to the atmosphere allows for conversion to useful energy. A landfill must be at least 40 feet deep and have at least one million tons of waste in place for landfill gas collection and power production to be technically feasible.
 
A landfill gas-to-energy system consists of a series of wells drilled into the landfill. A piping system connects the wells and collects the gas. Dryers remove moisture from the gas, and filters remove impurities. The gas typically fuels an engine-generator set or gas turbine to produce electricity. The gas also can fuel a boiler to produce heat or steam. Further gas cleanup improves biogas to pipeline quality, the equivalent of natural gas. Reforming the gas to hydrogen would make possible the production of electricity using fuel cell technology.
 

Ethanol
 
Fermentation is the biochemical process that converts sugars into ethanol (alcohol). In contrast to biogas production, fermentation takes place in the presence of air and is, therefore, a process of aerobic digestion. Ethanol producers use specific types of enzymes to convert starch crops such as corn, wheat and barley to fermentable sugars. Some crops, such as sugar-cane and sugar beets, naturally contain fermentable sugars.
 
Ethanol has a higher octane than gasoline, but its energy content is only about two-thirds the energy content of gasoline. Most new cars are designed to run on a blend of gasoline and ethanol. "Gasohol" is a mixture of 90-percent unleaded gasoline and 10-percent denatured ethanol. With modification, spark ignition engines can run on 100-percent ethanol. E-85 fuel consists of 85-percent ethanol and 15-percent gasoline. The major automobile manufacturers in the United States now produce flexible fuel vehicles that can use either E-85 fuel or gasoline.
 
Ethanol may also be used as a hydrogen source for fuel cells. A recent paper by the Renewable Fuels Association concludes that there are no technical barriers to the use of ethanol in fuel cells. Because ethanol is easier to transport and store than hydrogen, fuel reforming (using a chemical process to extract hydrogen from fuel) may be a practical way to provide hydrogen to fuel cells in vehicles or for remote stationary applications. Ethanol is easier to reform than gasoline and most alternative fuels because of its relatively simple molecular structure.
 

Grain to Ethanol
 
Most of the ethanol produced in the United States today comes from grain (predominantly corn). In the wet mill process, grain is steeped and separated into starch, germ and fiber components. In the dry mill process, grain is first ground into flour and then processed without separation of the starch.
 
Wet milling is more common. After the grain is cleaned, it is steeped and then ground to remove the germ. Further grinding, washing and filtering steps separate the fiber and gluten. The starch that remains after these separation steps is then broken down into fermentable sugars by the addition of enzymes in the liquefaction and saccharification stages.
 
To produce ethanol, yeast is added to a slurry (or "mash"), which is a solution of fermentable sugars and water. The yeast ferments the sugars, producing a solution called beer. The beer solution contains about 10-percent to 12-percent ethanol. The rate of the conversion process depends on the amount of water in the slurry and its acidity, temperature and oxygen content. Up to a third of the original dry weight of the feedstock leaves the fermentation process as carbon dioxide. The solids that remain after the mash has fermented still contain nutrients suitable for use as livestock feed. Distilling the beer produces a solution of 80-percent to 95-percent ethanol.
 
Producers can use several methods of dehydration to purify the ethanol solution further to 100-percent (200-proof) alcohol for use as a motor fuel.
 
 

Lignocellulosic Biomass to Ethanol
 
The use of lower-cost feedstock is of particular interest in Oregon and the Pacific Northwest region due to the abundance of potential feedstock. This regionally available feedstock (called lignocellulosic biomass) includes waste paper, wood waste, pulp sludge and grass straw. Mechanical preparation steps include cleaning, drying and reducing the size of biomass feedstock.
 
Cellulose-to-ethanol technology converts lignocellulosic feedstock (LCF) into component sugars, which are then fermented to ethanol. This technology is currently in an early stage of commercial development. However, as early as 1945, Oregon pioneered cellulose-to-ethanol technology. At that time, Dr. Raphael Katzen designed, built and operated a 17 million-gallon-per-year ethanol plant in Springfield, Oregon, that used wood feedstock
 
All LCF materials are made of cellulose, hemicellulose and lignin. Lignin acts like glue in plant material. It holds the other components together and gives trees and plant stalks their strength. The lignin removed in pretreatment is itself a biomass fuel. Burning the lignin produces heat, useful for other steps in the cellulose-to-ethanol conversion process.
 
Several methods are available to breaking down the chemical bonds of cellulose and hemicellulose and to remove the lignin. Methods include dilute and concentrated acid hydrolysis and enzymatic hydrolysis. Hydrolysis releases fermentable sugars from cellulose and hemicellulose. This stage is sometimes called saccharification.
 
Fermentation, the next stage of the process, uses enzymes to convert the sugars into ethanol. As with the grain-to-ethanol process, the final stage is distillation of the fermented beer into ethanol that is about 95-percent pure.
 
 

Biodiesel
 
Biodiesel production is a chemical conversion process. The process converts oilseed crops into biodiesel fuel, a substitute for petroleum diesel. Demonstration projects in Idaho and at Yellowstone National Park use biodiesel produced from winter rapeseed grown in northern Idaho. Other oil seed crops can be used to make biodiesel, and it can be manufactured from waste vegetable oils or animal fats.
 
The two main processes for extracting oil from seed feedstock are mechanical press extraction and solvent extraction. In mechanical press extraction, the oil seed feedstock is first heated to about 110° F. The oil seed is then crushed in a screw press. After most of the oil is removed, the remaining seed meal can be used as an animal feed.
 
The solvent process extracts more of the oil contained in the oil seed feedstock but requires more costly equipment. The process uses a solvent to dissolve the oil. After extraction, a distillation process separates the oil from the solvent. The solvent condenses and can be recycled and reused in the process. Solvent extraction produces vegetable oil with a higher degree of purity than the mechanical press process.
 
Vegetable oils, such as rapeseed, corn or safflower, can be used as a diesel fuel without further processing. However, the process of transesterification reduces the high viscosity of vegetable oil, resulting in a higher-quality fuel. In the transesterification process, vegetable oil reacts with alcohol (methanol or ethanol) in the presence of a catalyst. When rapeseed oil is the feedstock, the products of the reaction are glycerol and rapeseed methyl or ethyl ester (RME or REE). As biodiesel fuels, RME or REE can be used straight or in a blend with petroleum diesel.
 
Safely producing biodiesel or ethanol from renewable resources can be done in Oregon. Facilities making biodiesel are already here and ethanol facilities are not far behind