Part 3 of Biomass
By: Hannah Jeon, Lance Penny, & Jaylon Wells
How Is Biomass Energy Used?
One of the advantages of biomass is its versatility. It can be burned directly, or converted into a gas or oil, to generate electricity (biopower) and heat. It can also be converted into liquid fuels (biofuels) for our transportation needs.
There are several ways of turning biomass into electricity and heat, including direct combustion, co-firing, gasification, pyrolysis and anaerobic digestion.
The simplest way, and oldest way, of generating electricity from biomass is to burn it. This is called direct combustion. Direct combustion (or “direct-fired”) systems burn biomass in boilers to produce high pressure steam. This steam turns a turbine connected to a generator. As the steam causes the turbine to rotate, the generator turns and electricity is produced. Most of the world’s biomass power plants use direct combustion. In some cases, the steam from the plants is also captured to heat water and buildings. These are known as cogeneration facilities.
Although this technology is dependable and proven, its efficiency is limited. Direct combustion systems typically have thermal efficiencies around 20 per cent. These efficiencies can be increased through cogeneration.
Co-firing involves burning biomass, along with coal, in traditional power plant boilers. This is considered to be one of the most economic ways to produce electricity from biomass, because existing power plant equipment can be used without major modifications. Some coal-fired power plants in North America use this technology to help reduce the use of coal and, thereby, lower emissions of carbon dioxide, sulphur dioxide and nitrogen oxides. Co-firing also allows biomass to be converted to electricity at a higher thermal efficiency in the 33 per cent to 37 per cent range.
New technologies have been developed to gasify biomass into a useful energy source. These operate by heating solid biomass at high temperatures in an oxygen-deprived environment to produce a fuel gas, which contains between one fifth and one half the heat content of natural gas. This gas can be used to drive highly efficient combined cycle systems to generate electricity. Gasification offers some advantages over direct combustion: for example, gasifying biomass to produce electricity is twice as efficient as burning it directly and results in lower emissions of particulate matter and greenhouse gases. Gasification systems can also be combined with fuel cell systems, which convert hydrogen gas to electricity and heat.
In pyrolysis, biomass is heated to high temperatures in an oxygen-free environment to produce a gas rich in hydrocarbons. This gas is quickly cooled to create an oil-like liquid (bio-oil) and the remaining solid is a charcoal-like residue (called “char”). The bio-oil can then be burned like petroleum to generate electricity while the char can be used for heating.
Certain kinds of bacteria break down or “digest” organic material in the absence of oxygen and produce biogas as a waste product. This biological process is called anaerobic digestion. This process occurs naturally in city dumps or landfill sites containing organic wastes. When these materials are buried, they are digested by bacteria, resulting in biogas (“landfill gas”) rich in methane. This gas can be collected and used to heat buildings, run engines or generate electricity. Biogas can also be produced by mixing plant and animal wastes with water in oxygen-free tanks.
By: Centre for Energy; 2002-2012
Biomass to Electricity
The term "biomass" encompasses diverse fuels derived from timber, agriculture and food processing wastes or from fuel crops that are specifically grown or reserved for electricity generation. Biomass fuel can also include sewage sludge and animal manure. Some biomass fuels are derived from trees. Given the capacity of trees to regenerate, these fuels are considered renewable. Burning crop residues, sewage or manure - all wastes that are continually generated by society -- to generate electricity may offer environmental benefits in the form of preserving precious landfill space OR may be grown and harvested in ways that cause environmental harm.
At present, most biomass power plants burn lumber, agricultural or construction/demolition wood wastes. Direct Combustion power plants burn the biomass fuel directly in boilers that supply steam for the same kind of steam-electric generators used to burn fossil fuels. With biomass gasification, biomass is converted into a gas - methane - that can then fuel steam generators, combustion turbines, combined cycle technologies or fuel cells. The primary benefit of biomass gasification, compared to direct combustion, is that extracted gasses can be used in a variety of power plant configurations.
In terms of capacity, biomass power plants represent the second largest amount of renewable energy in the nation.
Because biomass technologies use combustion processes to produce electricity, they can generate electricity at any time, unlike wind and most solar technologies, which only produce when the wind is blowing or sun is shining. Biomass power plants currently represent 11,000 MW - the second largest amount of renewable energy in the nation.
By: Power Scorecard; 2000-2012
Whether combusting directly or engaged in gasification, biomass resources do generate air emissions. These emissions vary depending upon the precise fuel and technology used. If wood is the primary biomass resource, very little SO2 comes out of the stack. NOx emissions vary significantly among combustion facilities depending on their design and controls. Some biomass power plants show a relatively high NOx emission rate per kilowatt hour generated if compared to other combustion technologies.
This high NOx rate, an effect of the high nitrogen content of many biomass fuels, is one of the top air quality concerns associated with biomass.
Carbon monoxide (CO) is also emitted - sometimes at levels higher than those for coal plants.
Biomass plants also release carbon dioxide (CO2), the primary greenhouse gas. However, the cycle of growing, processing and burning biomass recycles CO2 from the atmosphere. If this cycle is sustained, there is little or no net gain in atmospheric CO2. Given that short rotation woody crops (i.e., fast growing woody plant types) can be planted, matured and harvested in shorter periods of time than natural growth forests, the managed production of biomass fuels may recycle CO2 in one-third less time than natural processes.
Biomass power plants also divert wood waste from landfills, which reduces the productions and atmospheric release of methane, another potent greenhouse gas.
Another air quality concern associated with biomass plants is particulates. These emissions can be readily controlled through conventional technologies. To date, no biomass facilities have installed advanced particulate emission controls. Still, most particulate emissions are relatively large in size. Their impacts upon human health remain unclear.
The collection of biomass fuels can have significant environmental impacts. Harvesting timber and growing agricultural products for fuel requires large volumes to be collected, transported, processed and stored. Biomass fuels may be obtained from supplies of clean, uncontaminated wood that otherwise would be landfilled or from sustainable harvests. In both of these fuel collection examples, the net environmental plusses of biomass are significant when compared to fossil fuel collection alternatives. On the other hand, the collection, processing and combustion of biomass fuels may cause environmental problems if, for example, the fuel source contains toxic contaminants, agricultural waste handling pollutes local water resources, or burning biomass deprives local ecosystems of nutrients that forest or agricultural waste may otherwise provide.
By: Power Scorecard; 2000-2012