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The various methods used for converting biomass into electricity are as follows: 1. Direct-Combustion 2. Co-Firing 3. Biomass Gasification 4. Anaerobic Digestors 5. Pyrolysis.
Method # 1. Direct-Combustion:
The oldest and most common way of converting biomass to electricity is to burn it to produce steam, which turns a turbine that produces electricity. The problems with direct combustion of biomass are that much of the energy is wasted and that it can cause some pollution if it is not carefully controlled. Direct combustion can be done in a plant using solely biomass (a ‘dedicated plant’) or in a plant made to burn another fuel, usually coal.
Method # 2. Co-Firing:
An approach that may increase the use of biomass energy in the short term is to mix it with coal and burn it at a power plant designed for coal—a process known as “co-fiding”. Through gasification, biomass can also be co-fired at natural gas-powered plants.
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The benefits associated with biomass co-firing can include lower operating costs, reductions of harmful emissions like sulfur and mercury, greater energy security and with the use of beneficial biomass, lower carbon emissions. Co-firing is also one of the more economically viable ways to increase biomass power generation today, since it can be done with modifications to existing facilities.
Method # 3. Biomass Gasification:
By heating biomass in the presence of a carefully controlled amount of oxygen and under pressure, it can be converted into a mixture of hydrogen and carbon monoxide called syngas. This syngas is often refined to remove contaminants. Equipment can also be added to separate and remove the carbon dioxide in a concentrated form.
The syngas can then be run directly through a gas turbine or burned and run through a steam turbine to produce electricity. Biomass gasification is generally cleaner and more efficient that direct combustion of biomass. Syngas can also be further processed to make liquid bio fuels or other useful chemicals. The electricity generation using gasifier is explained in detail in this section.
Method # 4. Anaerobic Digestors:
Micro-organisms break down biomass to produce methane and carbon dioxide. This can occur in a carefully controlled way in anaerobic digesters used to process sewage or animal manure. Related processes happen in a less-contolled manner in landfills, as biomass in the garbage breaks down.
A portion of this methane can be captured and burned for heat and power. In addition to generating biogas, which displaces natural gas from fossil fuel sources, such collection processes keep the methane from escaping to the atmosphere, reducing emissions of a powerful global warming gas.
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Methane can be used as an energy source in many ways. Most facilities burn it in a boiler to produce steam for electricity generation or for industrial processes. Two new ways include the use of microturbines and fuel cells. Micro-turbines have outputs of 25 to 500 kilowatts.
About the size of a refrigerator, they can be used where there are space limitations for power production. Methane can also be used as the “fuel” in a fuel cell. Fuel cells work much like batteries, but never need recharging, producing electricity as long as there is fuel.
Method # 5. Pyrolysis:
In addition to gas, liquid fuels can be produced from biomass through a process called pyrolysis. Pyrolysis occurs when biomass is heated in the absence of oxygen. The biomass then turns into liquid called pyrolysis oil, which can be burned like petroleum to generate electricity. A bio-power system that uses pyrolysis oil is being commercialized.
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In addition to above mentioned processes, several bio-power technologies can be used in small, modular systems. A small, modular system generates electricity at a capacity of 5 megawatts or less. This system is designed for use at the small town level or even at the consumer level.
For example, some farmers use the waste from their livestock to provide their farms with electricity. Not only do these systems provide renewable energy, they also help farmers meet environmental regulations.