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Biomass for Electricity Generation
Last updated: 08-04-2011
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Biomass is used for facility heating, electric power generation, and combined heat and power. The term biomass encompasses a large variety of materials, including wood from various sources, agricultural residues, and animal and human waste.
Biomass can be converted into electric power through several methods. The most common is direct combustion of biomass material, such as agricultural waste or woody materials. Other options include gasification, pyrolysis, and anaerobic digestion. Gasification produces a synthesis gas with usable energy content by heating the biomass with less oxygen than needed for complete combustion. Pyrolysis yields bio-oil by rapidly heating the biomass in the absence of oxygen. Anaerobic digestion produces a renewable natural gas when organic matter is decomposed by bacteria in the absence of oxygen.
In Woodland, California, a generation station uses wood from the agricultural industry.
Different methods work bet with different types of biomass. Typically, woody biomass such as wood chips, pellets, and sawdust are combusted or gasified to generate electricity. Corn stover and wheat straw residues are baled for combustion or converted into a gas using an anaerobic digester. Very wet wastes, like animal and human wastes, are converted into a medium-energy content gas in an anaerobic digester. In addition, most other types of biomass can be converted into bio-oil through pyrolysis, which can then be used in boilers and furnaces.
This overview focuses on woody biomass used for generating electricity at a commercial-scale facility rather than a utility-scale project. Biomass heat and biogas, including anaerobic digestion and landfill gas, are covered in other technology resource pages in this guide:
Compared to many other renewable energy options, biomass has the advantage of dispatchability, meaning it is controllable and available when needed, similar to fossil fuel electric generation systems. The disadvantage of biomass for electricity generation, however, is that the fuel needs to be procured, delivered, stored, and paid for. Also, biomass combustion produces emissions, which must be carefully monitored and controlled to comply with regulations.
This overview provides specific details for those considering biomass electric generation systems as part of a major construction project. Further general information is available on the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Biomass Energy Basics website. Details on biomass use for combined heat and power is available at the U.S. Environmental Protection Agency's (EPA) Combined Heat and Power Partnership website.
Most biopower plants use direct-fired combustion systems. They burn biomass directly to produce high-pressure steam that drives a turbine generator to make electricity. In some biomass industries, the extracted or spent steam from the power plant is also used for manufacturing processes or to heat buildings. These combined heat and power (CHP) systems greatly increase overall energy efficiency to approximately 80%, from the standard biomass electricity-only systems with efficiencies of approximately 20%. Seasonal heating requirements will impact the CHP system efficiency.
A simple biomass electric generation system is made up of several key components. For a steam cycle, this includes some combination of the following items:
- Fuel storage and handling equipment
- Combustor / furnace
- Steam turbine
- Cooling tower
- Exhaust / emissions controls
- System controls (automated).
Direct combustion systems feed a biomass feedstock into a combustor or furnace, where the biomass is burned with excess air to heat water in a boiler to create steam. Instead of direct combustion, some developing technologies gasify the biomass to produce a combustible gas, and others produce pyrolysis oils that can be used to replace liquid fuels. Boiler fuel can include wood chips, pellets, sawdust, or bio-oil. Steam from the boiler is then expanded through a steam turbine, which spins to run a generator and produce electricity.
In general, all biomass systems require fuel storage space and some type of fuel handling equipment and controls. A system using wood chips, sawdust, or pellets typically use a bunker or silo for short-term storage and an outside fuel yard for larger storage. An automated control system conveys the fuel from the outside storage area using some combination of cranes, stackers, reclaimers, front-end loaders, belts, augers, and pneumatic transport. Manual equipment, like front loaders, can be used to transfer biomass from the piles to the bunkers, but this method will incur significant cost in labor and equipment operations and maintenance (O&M). A less labor-intensive option is to use automated stackers to build the piles and reclaimers to move chips from the piles to the chip bunker or silo.
Wood chip-fired electric power systems typically use one dry ton per megawatt-hour of electricity production. This approximation is typical of wet wood systems and is useful for a first approximation of fuel use and storage requirements but the actual value will vary with system efficiency. For comparison, this is equivalent to 20% HHV efficiency with 17 MMBtu/ton wood.
Most wood chips produced from green lumber will have a moisture content of 40% to 55%, wet basis, which means that a ton of green fuel will contain 800 to 1,100 pounds of water. This water will reduce the recoverable energy content of the material, and reduce the efficiency of the boiler, as the water must be evaporated in the first stages of combustion.
The biggest problems with biomass-fired plants are in handling and pre-processing the fuel. This is the case with both small grate-fired plants and large suspension-fired plants. Drying the biomass before combusting or gasifying it improves the overall process efficiency, but may not be economically viable in many cases.
Exhaust systems are used to vent combustion by-products to the environment. Emission controls might include a cyclone or multi-cyclone, a baghouse, or an electrostatic precipitator. The primary function of all of the equipment listed is particulate matter control, and is listed in order of increasing capital cost and effectiveness. Cyclones and multi-cyclones can be used as pre-collectors to remove larger particles upstream of a baghouse (fabric filter) or electrostatic precipitator.
In addition, emission controls for unburned hydrocarbons, oxides of nitrogen, and sulfur might be required, depending on fuel properties and local, state, and Federal regulations.
How Does it Work?
In a direct combustion system, biomass is burned in a combustor or furnace to generate hot gas, which is fed into a boiler to generate steam, which is expanded through a steam turbine or steam engine to produce mechanical or electrical energy.
In a direct combustion system, processed biomass is the boiler fuel that produces steam to operate a steam turbine and generator to make electricity.
Types and Costs of Technology
There are numerous companies, primarily in Europe, that sell small-scale engines and combined heat and power systems that can run on biogas, natural gas, or propane. Some of these systems are available in the United States, with outputs from about 2 kilowatts (kW), and approximately 20,000 British thermal units (Btu) per hour of heat, to several megawatts (MW). In addition, small-scale (100 to 1,500 kW) steam engine/gen-sets and steam turbines (100 to 5,000 kW) that are fueled by solid biomass are currently available in Europe.
In the United States, direct combustion is the most common method of producing heat from biomass. Small-scale biomass electric plants have installed costs of $3,000 to $4,000 per kW, and a levelized cost of energy of $0.8 to $0.15 per kilowatt hour (kWh).
The two principal types of chip-fired direct combustion systems are stationary- and traveling-grate combustors, otherwise known as fixed-bed stokers and atmospheric fluidized-bed combustors.
There are various configurations of fixed-bed systems, but the common characteristic is that fuel is delivered in some manner onto a grate where it reacts with oxygen in the air. This is an exothermic reaction that produces very hot gases and generates steam in the heat exchanger section of the boiler.
In either a circulating fluidized-bed or bubbling fluidized-bed system, the biomass is burned in a hot bed of suspended, incombustible particles, such as sand. Compared to grate combustors, fluidized-bed systems generally produce more complete carbon conversion, resulting in reduced emissions and improved system efficiency. In addition, fluidized-bed boilers can use a wider range of feedstocks. Furthermore, fluidized-bed systems have a higher parasitic electric load than fixed-bed systems due to increased fan power requirements.
Biomass gasification systems
Small, modular biopower system by Community Power Corporation
Although less common, biomass gasification systems are similar to combustion systems, except that the quantity of air is limited, and thus produce a clean fuel gas with a usable heating value in contrast to combustion, in which the off gas does not have a usable heating value. Clean fuel gas provides the ability to power many different kinds of gas-based prime movers, such as internal combustion engines, Stirling engines, thermo electric generators, solid oxide fuel cells, and micro-turbines.
The efficiency of a direct combustion or biomass gasification system is influenced by a number of factors, including biomass moisture content, combustion air distribution and amounts (excess air), operating temperature and pressure, and flue gas (exhaust) temperature.
The type of system best suited to a particular application depends on many factors, including availability and cost of each type of biomass (e.g. chip, pellet, or logs), competing fuel cost (e.g. fuel oil and natural gas), peak and annual electrical loads and costs, building size and type, space availability, operation and maintenance staff availability, and local emissions regulations.
Projects that can make use of both electricity production and thermal energy from biomass energy systems are often the most cost effective. If a location has predictable access to year-round, affordable biomass resources, then some combination of biomass heat and electricity production may be a good option. Transportation of fuel accounts for a significant amount of its cost, so resources should ideally be available from local sources. In addition, a facility will typically need to store biomass feedstocks on-site, so site access and storage are factors to consider.
As with any on-site electricity technology, the electricity generating system will need to be interconnected to the utility grid. The rules for interconnection may be different if the system is a combined heat and power system instead of only for electricity production. The ability to take advantage of net metering may also be crucial to system economics.
The Federal Energy Management Program (FEMP)'s Integrating Renewable Energy into Federal Construction Guide has more information on interconnection requirements and net metering.
The major capital cost items for a biomass power system include the fuel storage and fuel handling equipment, the combustor, boiler, prime mover (e.g. turbine or engine), generator, controls, stack, and emissions control equipment.
System cost intensity tends to decrease as the system size increases. For a power-only (not combined heat and power) steam system in the 5 to 25 MW range, costs generally range between $3,000 and $5,000 per kilowatt of electricity. Levelized cost of energy for this system would be $0.08 to $0.15 per kWh, but this could increase significantly with fuel costs. Large systems require significant amounts of material, which leads to increasing haul distances and material costs. Small systems have higher O&M costs per unit of energy generated and lower efficiencies than large systems. Therefore, determining the optimal system size for a particular application is an iterative process.
A variety of incentives exist for biomass power, but vary with Federal and state legislation policies. The Database of State Incentives for Renewables & Efficiency lists incentives for biomass. The timing of incentive programs often allows less construction time than needed for biomass projects. Also, Federal agencies often cannot take direct advantage of financial incentives for renewable energy unless they use a different ownership structure.
FEMP's Integrating Renewable Energy into Federal Construction Guide has more information on renewable energy project funding.
Of interest, the State of Massachusetts recently removed biomass-fired electricity from its Renewable Portfolio Standard, because state officials did not believe that biomass provided a clear reduction in greenhouse gases. As such, biomass projects no longer qualify for renewable energy certificates that count toward Massachusetts renewable energy goals or funding.
Assessing Resource Availability
The most important factors in planning for a biomass energy system are resource assessment, planning, and procurement. As part of the screening and feasibility analysis processes, it is critical to identify potential sources of biomass and to estimate the fuel quantities needed.
If possible, determine, in detail, the capability of potential suppliers to produce and deliver a fuel that meets the requirements of the biomass equipment. This can be a bit of an intensive process as it involves determining the load to be served, identifying possible equipment manufacturers or vendors, working with those vendors to determine a fuel specification, and contacting suppliers to see if they can meet the specification—and at what price. It is also necessary to estimate the monthly and annual fuel requirement, as well as peak fuel use, to help with fuel handling and fuel storage equipment sizing.
Since there is no established wood chip distribution system in most of the United States, it is sometimes difficult to find suppliers. One suggestion is to contact the regional U.S. Forest Service and state forest service offices. Other resources to contact include landscape companies, lumber mills, and other wood processors, landfills, arborists, and wood furniture manufacturers.
County-level biomass resource estimates are also available online through an interactive mapping and analysis tool. The Biomass Assessment Tool was developed by the National Renewable Energy Laboratory (NREL) using funding from EPA. Previously, resource assessment efforts were usually static and did not allow user analysis or manipulation of the data. This new tool enables users to select a location on the map, quantify the biomass resources available within a user-defined radius, and estimate the total thermal energy or power that could be generated by recovering a portion of that biomass. The tool acts as a preliminary source of biomass feedstock information; however, it cannot take the place of an on-the-ground feedstock assessment.
Available biomass resources in the United States.
A process must be developed to receive biomass deliveries and to assess the fuel properties. As of July 2011, there are no national wood chip specifications, but regional specifications are being developed. Having a specification helps to communicate and enforce chip requirements. The specification should include physical dimensions, fuel moisture content range, energy content, ash and mineral content, and other factors that affect fuel handling or combustion. To ensure fair value, fuel procurement contracts should scale purchase price inversely with moisture content, as higher moisture content significantly decreases combustion efficiency and increases the weight of material to be transported.
The following recommendations are critical to the success of any biomass energy project.
- Fully involve decision-makers and the general public during the planning stages and as progress is made, particularly if the system will be installed in a public building.
- Work closely with a biomass equipment manufacturer or vendor to collaborate on building design and equipment requirements.
- Coordinate building scheduling with the equipment delivery. For example, it is easier to deliver and install the equipment if a crane can access the installation site.
- Identify a fuel delivery route, to ensure that trucks can reach the storage area easily and turn around, if necessary.
Operation and Maintenance
O&M costs of biomass energy systems are predominately the costs of fuel and labor. In other respects, these systems are similar to other boiler-based electricity production systems. Operation is continual, so costs for operation and for the purchase and storage of fuel need to be assessed with the overall project costs.
The following are important special considerations for biomass electric systems.
Environmental Review / Permitting
The primary NEPA and permitting issue with a biomass energy system is the combustion emissions. Therefore, local requirements should be reviewed. Air emissions from a biomass system depend on the system design and fuel characteristics. If necessary, emissions controls systems can be used to reduce particulate matter and oxides of nitrogen emissions. Sulfur emissions are completely dependent on the sulfur content of the biomass, which is usually very low.
The storage of wood chips requires consideration, preparation, and attentiveness. When the chips are stored in a building, there is potential for dust from the chips to build up on horizontal surfaces and to get inside equipment. A concern, though rare in occurrence, is the wood chips' ability to self-ignite, or spontaneously combust, when stored for long periods of time. For more information see OSHA's safety and health information bulletin, Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions.
This is due to a chain of events, which starts with the biological breakdown of the organic matter and can lead to smoldering of the pile. The critical moisture range that supports spontaneous combustion is roughly 20% to 45%. The probability of spontaneous combustion also increases as pile size increases, due to the increasing depth.
To help with this issue, the Office of the Fire Marshal fire code in Ontario, Canada provides the following guidelines:
- The storage site shall be well-drained and level, solid ground or paved with asphalt, concrete, or other hard surface material. The ground surface between piles shall be kept free of combustible materials. Weeds, grass, and similar vegetation shall be removed from the yard. Portable open-flame weed burners shall not be used in chip storage yards. Piles shall not exceed 18 m (59 ft) in height, 90 m (295 ft) in width, and 150 m (492 ft) in length unless temporary water pipes with hose connections are laid on the top surface of the pile.
- Space shall be maintained between chip piles and exposing structures, yard equipment, or stock equal to (a) twice the pile height for combustible stock or buildings, or (b) the pile height for non-combustible buildings and equipment.
- Smoking shall be prohibited in chip pile areas.
Wood chip fires can be caused by other factors, such as lightning strikes, heat from equipment, sparks from welding activities, wildfires, and arson. These fires are sometimes called surface fires because they start, and spread, along the exterior of the pile.
For storage, it is critical to keep the wood chips clean. When chips are stored on dirt or gravel, some of this material will often get scooped up along with the chips and end up in the combustor.
Relevant Codes and Standards
On February 21, 2011, EPA established Clean Air Act emissions standards for large and small boilers and incinerators that burn solid waste and sewage sludge. These standards cover more than 200,000 boilers and incinerators that emit hazardous air pollutants (HAP), also known as air toxics. The new EPA standards should be followed as part of project planning for any combustion boiler.
EPA also enacted Clean Air Act Permitting for Greenhouse Gas Emissions on January 2, 2011. Also referred to as the "tailoring rule," this process requires permitting for greenhouse gas production, but exempts smaller facilities. Final rules are expected to be developed over a three-year study period, but Federal facilities using biomass electricity production as part of a new construction project may want to ensure the size of the biomass facility does not trigger these requirements.
In 2009, the State of Massachusetts issued a document titled Biomass Boiler & Furnace Emissions and Safety Regulations in the Northeast States (PDF 478 KB). Although this document provides a review of existing regulations in that region, it can be a useful reference for other parts of the country.
The following additional resources can provide further detail on biomass electricity generation.
Biomass Electric Resources
- Biomass Power Association is one of the nation's leading biomass organizations with information on biomass, policies and industry news.
- Biomass Thermal Energy Council offers webinars, interviews, and presentations from industry leaders designed to expand public knowledge of biomass.
- Coordinated Resource Offering Protocol reviews the Coordinated Resource Offering Protocol projects intended to facilitate biomass removal among public agencies.
- EPA Combined Heat and Power Policies and Incentives are available for biomass projects, including available state and Federal financial incentives.
- Woody Biomass Utilization Database provides information on the use of biomass resources for local economic development and the educational programs currently being offered by the Western Governors' Association.
- NREL Energy Technology Cost and Performance Data for Distributed Generation compares cost estimates for distributed generation (DG) renewable energy technologies across capital costs, operations and maintenance (O&M) costs, and levelized cost of energy (LCOE).
- Biomass Strategic Value Analysis (PDF 3.35 MB) by California: California Energy Commission.
- Cost and Performance Assumptions for Modeling Electricity Generation (PDF 1.66 MB) by NREL.
- Creating Markets for Renewable Energy Technologies EU RES Technology Marketing Campaign: Cogeneration at Small Scale (PDF 1.31 MB) by European Renewable Energy Council.
- Market Assessment of Biomass Gasification and Combustion Technology for Small- and Medium-Scale Applications (PDF 1.01 MB) by NREL.
- Woody Biomass Feedstock Yard Business Development Guide (PDF 4.8 MB) by Federal Woody Biomass Utilization Working Group, chartered under the Biomass Research and Development Board.