photo credits: NREL Biomass Pictures

When biomass is directly-fired, or burned it must first be dried, as dry wood burns more efficiently, sized into smaller pieces, and then briquetted. Briquetting " is a densification process of loose organic material, such as rice husk, sawdust and coffee husk, aiming to improve handling and combustion characteristics [2]." Briquetting can be done in a number of ways, with or without binders or with bio-coal technology [2]. Once preparation is complete, the biomass is added to a furnace or a boiler to generate heat which is then run through a turbine which drives an electrical generator. The heat generated by the exothermic process of combustion to power the generator can also be used to regulate temperature of the plant and other buildings, making the whole process much more efficient. A plant using this type of technology is called a combined heat and power (CHP) facility and as its name suggests, it uses both the heat and the steam, so that there is less potential energy wasted. For instance, wood waste is often used to produce both electricity and steam at paper mills [3]. If CHP is needed then the steam is condensed at a higher pressure in the water heater [4].

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Co-firing. Co-firing, combining biomass with coal to generate energy, is probably the most compatable way to use biomass with the current fossil fuel dependent system. Most of the electricity generated in the United States today is generated by coal-fired plants. With co-firing, woody and herbaceous biomass such as poplar, willow and switchgrass can fuel a small portion of an existing coal power plant. This process, entails biomass that represents between 1% and 15% of the energy of the coal plant, with the remainder consisting of coal. In these systems, biomass is placed into the boilers and burned, as coal would be. Often the only cost associated with upgrading the system to burn both fuels is to purchase a boiler capable of doing so, and retrofitting it into the system, which is a whole lot cheaper than building a whole other plant. Below is the Ottumwa Generating plant which uses co-firing to produce electricity.

http://www.nrel.gov/data/pix/searchpix.cgi

According to the National Renewable Energy Laboratory (NREL) Biopower Fact Sheet, co-firing has been "demonstrated, tested, an proved in all little boiler types commonly used by electric companies. . .with little or no loss in total boiler efficiency." In fact efficiency may even be increased by 33%-37% [3]. Using biomass with coal helps to bring down the highly damaging emissions which are given off by the coal plants. Large scale coal-powered boilers currently represent 310 GW of power in the United States (enough to power 14,000,000 homes). Seen below is a diagram of the natural gas co-firing process courtesy of www.cofire.com:

There are several environmental benefits of adding biomass to coal, including decreases in nitrogen and sulphur oxides, the causes of smog, acid rain and ozone pollution. Also, the amount of carbon dioxide released is also considerably less. Although other methods of bioenergy also have the same benefits with even lower (a theoretical net of zero) carbon dioxide emissions, the technology for co-firing is already installed in the Nation's infrastructure. In other words, co-firing appears to be a wonderful option for immediate use and a good stepping stone towards more viable and sustainable renewable energy practices. Additionally, the feedstock is currently available. The Oak Ridge National Laboratory (ORNL) recently released a biomass report for the DOE and the USDA which concluded that biomass production in the United States could consume over 1.3 billion dry tons a year, six times the amount that is currently used, causing the industry to grow substantially over the next 30 years [5].

http://www.nrel.gov/docs/fy00osti/28009.pdf

Biomass co-firing plants can also use fly ash (purchasing it from one company) as a source of the biomass [3], additionally providing another way to reuse the ash instead of wastefully discarding it.

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Pyrolysis. Pyrolysis is a process where biomass is combusted at high temperatures and decomposed in the absence of oxygen. However, some difficulties arises when trying to create a totally oxygen free atmosphere. Often a little oxyidation does occur which may create undesireable byproducts and also it is highly energy intensive and expensive at the moment. The burning creates pyrolysis oil, char or syngas [6] which can then be used like petroleum to generate electricity. It does not create ash or energy directly. Instead it morphs the biomass into higher quality fuel [6]. The process begins with a drying process in order to maximize burning potential from the biomass, similar to the direct combustion process above. When cooled, the brown liquidy pyrolysis oil can be used in a gasifier.

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FlowChart courtesy of http://www.eere.energy.gov/biomass/pyrolysis.html

When sped up, a process known as Fast Pyrolysis, up to 75% more bio-oil or pyrolysis oil is generated [7]. In fact, the European Biomass Technology Group has created bio-oil using the fast pyrolysis technique by combining wood residue with hot sand in a rotating cone. In a small scale experimental setting, the rotating pyrolysis cone technology uses 250 tons of wood/day and generates 50 tonnes of oil (the equivalent of .314 barrels of oil) [8]. Experimenters suggest that the cone can be modified to take on larger loads and if done, bio-oil is already at a competitive price on the market [8]. Some have suggested that pyrolysis even be used to generate hydrogen for use in fuel cells [9]. Below is a model of the proposed cone technology in a full scale electricity generation setting.

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Biomass gasification. Solid biomass can be converted into a gaseous form, known as syngas. The gas can then can run through “combined-cycle” gas turbine or another power conversion technology such as a coal power plant. Many experts hope that gasification can yield more efficient biomass power plants. At this stage, gasification is still in the “demonstration” phase as projects slowly come online. Biomass gasifiers operate by heating biomass in an environment where the solid biomass breaks down to form a flammable gas. This offers advantages over directly burning the biomass. The biogas can be cleaned and filtered to remove problem chemical compounds. The gas can be used in more efficient power generation systems called combined-cycles, which combine gas turbines and steam turbines to produce electricity. The efficiency of these systems can reach 60%.

Below is a diagram that depicts the newest conversion of biomass into heat and electricity using the gasification combined cycle technology, which is currently in the development stage for widespread use. First the fuel is placed into the gasifier, where it is turned into a hot pressurized combustion gas. Then it is fed thorough a gas cleaner to precipitate out elements that would corrode the system. These elements vary depending on the source burned. The cleaned gases are then combusted and used to spin a turbine, which generates electricity. Heat released from the gas in the turbine can be recovered using water in the heat exchanger. The hot water can be recycled through the system. The only other by-product is non-toxic ash, which could, for example, be mixed with compost to help grow more biomass fuel [10].

From www.bbc.co.uk

Gasification systems are also being developed for the fuel cell sector for future applications. Fuel cells convert hydrogen gas to electricity (and heat) using an electro-chemical process. There are very little air emissions and the primary exhaust is water vapor. "As the costs of fuel cells and biomass gasifiers come down, these systems will proliferate," reports the Energy Efficiency and Renewable Energy Office. [11]

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Anaerobic Digestion, or AD, is a biological process, where the methane released by the synergistic actions between bacteria and archaea are contained and used to create energy. Anaerobic Digestion uses biowaste such as manure and municipal solid waste (MSW) as a feedstock. The manure or waste is bagged and broken down by using bacteria and water. This process releases the methane in the bag, and it is siphoned off into another holding bag. From there, the gas is used to power turbines which generate electricity. Below is a picture of this bagging process:

http://www.uwec.edu/grossmzc/martycw.html

At the molecular level, hydrolysis converts a wide range of solid organic materials into sugars and amino acids. Fermenting these materials produces volatile fatty acids (VFAs). Then the VFAs form hydrogen, carbon dioxide, and acetate through Acidogenesis. Finally, methanogenesis produces biogas, a mixture of 55-70 % methane, 25%-35% carbon dioxide, and trace elements of nitrogen and hydrogen sulfide [12]. Conducted in an airless environment, the methane can be captured and used to power a gas turbine or even fuel cells [9].

Microbial growth and natural biogas production is very slow at ambient temperatures. It tends to occur naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen, most commonly in the bottom sediments of lakes and ponds, in swamps, peat bogs, intestines of animals, and in the anaerobic interiors of landfill sites. The ultimate yield of biogas depends on the composition and biodegradability of the waste feedstock but its rate of production will depend on the population of microorganisms, their growth conditions and the temperature of the fermentation [13]."

When used as a waste treatment process, the digestion rate is greatly increased by operating in the mesophilic temperature range (35-40 degrees C). For certain feedstocks such as MSW, it can be further increased by operating at thermophilic (50-60 degrees C) temperatures [11].

Anaerobic digesters are available at competitive rates and are currently in use on farms across the country, although on a small scale. Utilizing the methane in this manner also aids in odor control and prevents the methane from seeping dangerously into the atmosphere, thereby rising levels of greenhouse gases and smog. For more information on odor control, please visit the Research Website of Dr. Samir Khanal.

Landfill Gas

Landfill gas uses a similar technology to anaerobic digestion and it carries the same benefits. It occurs as a by-product of the decomposition of solid waste and consists of 50 percent methane (natural gas), 45 percent carbon dioxide and 4 percent nitrogen. Additionally, it helps to reduce landfill waste by using the waste stream for electricity generation.

The Trans-Jordan Landfill in West Jordan is even experimenting with capturing natural gas from the decomposing landfill. "Collecting the gas and selling it will be a lot better than letting the methane seep into the atmosphere," said Dwayne Wooley, Trans-Jordan's general manager. "And as long as there is garbage decaying, we will have natural gas [14]."

According to an article by Steven Oberbeck of the Salt Lake Tribune, "Operators of the dump, jointly owned by seven Salt Lake-area cities, expect their new $3 million "landfill gas project" to create an additional revenue source that will help reduce the cost of operating the site. They are teaming up with Michigan-based Granger Energy Co., a privately owned company with experience developing similar landfill projects, and hope to sell the gas to the nearby Interstate Brick Co. as a fuel source for their furnaces [15]."

There are two ways to collect landfill gas, the traditional method is called conventional drilling and the other is called push-in collection. Before gas can be collected a 3-D map of the landfill is often drawn up to categorize where the gas is, and how it can best be obtained [15]. The traditional drilling method works as if drilling for gas anywhere else, but with adjustments made for the landfill terrain. Often the pipes are laid through the landfill vertically to make collection easier. Then a blower and a flare pump the gas out of the landfill into collection areas. [16] The Push-In Method uses the 3-D map to find the gas wells and can individually be "pushed" into the landfill as needed. It is a technology of STI Engineering.

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Modular Systems

Modular systems employ some of the same technologies mentioned above, but on a smaller scale that is more applicable to villages, farms, and small industry. These systems are now under development and could be most useful in remote areas where biomass is abundant and electricity is scarce. There are many opportunities for these systems in developing countries [1]. Below is a small modular system courtesy of the NREL Photo Information Exchange (PIX) website.

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