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Biomass for Heat
Last updated: 08-24-2012
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Biomass is used for facility heating and, to a lesser extent, for 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. The focus of this section is limited to woody biomass for heat only. Biomass electricity, biogas from landfill gas, and anaerobic digestion, are covered in other technology resource pages in this guide:
Woody biomass is commonly used for facility heating in three forms: whole logs or firewood, wood chips, and wood pellets. Systems are available from 6,000 British thermal units (Btu)/hr to over 100 million Btu/hr. Small systems, particularly small- and mid-size pellet and log systems, are available off-the-shelf from numerous manufacturers. Larger pellet systems and wood chip-fired systems are commercially available from several companies. The larger systems typically require both facility modification and system customization, mainly for integration of the fuel storage and handling and conveying systems.
Biomass systems require more operator interaction than other renewable energy systems such as solar and wind. This includes ordering and delivering fuel, removing ash, and maintaining moving parts. Overall, however, biomass heating systems typically only require a few minutes of attention each day, plus a few hours per year for annual maintenance.
Compared to most other renewable energy options currently available, biomass has the advantage of dispatchability, meaning it is controllable and available when needed, similar to fossil fuel heating and electric generation systems. The disadvantage of biomass for facility heating is that the fuel needs to be purchased, procured, delivered, and stored. Biomass combustion also produces emissions, which must be monitored and controlled to comply with regulations.
This overview is intended to provide specific details for Federal agencies considering biomass heating technologies as part of a new construction project or major renovation. Further general information on biomass heating is available from the Biomass Energy Resource Center.
A biomass heating system is made up of several key components, which include some combination of the following items:
- Fuel storage and handling / conveying
- Fire suppression systems
- Exhaust / emissions controls
- System controls
- Automatic ash handling (optional)
- Backup boiler
- The building facility's heat distribution system.
For smaller systems, like pellet and wood stoves, these components are largely integrated into one packaged unit, and only require ducting of the exhaust through a wall or chimney. Mid-sized pellet and wood chip systems often use one or more silos for fuel storage. The silo will need to be located for convenient delivery access, usually outside, which will require some form of conveyor system to move the fuel to the combustion unit. Larger wood chip systems usually require more integration with the building in which it is housed, particularly if a chip bunker is part of the building structure.
Wood chip storage at the Boulder County Parks and Open Space campus district heating system. This system has three large bay doors through which chips are delivered into an underground pit where a series of augers and belts deliver the chips to the combustor.
All biomass systems require fuel storage space and usually some sort of fuel handling equipment and controls. A system using wood chips or pellets usually stores the fuel in a bunker or silo. An automated control system conveys the fuel from the storage area using a combination of belts, augers, or pneumatic transport. The fuel storage volume can be sized to supply a quantity of fuel that will last from one day up to several weeks.
It is generally recommended that this storage be sized to provide enough fuel for a minimum of three days of operation to get through a long, cold weekend without additional fuel deliveries. In addition, a larger supply of fuel is often located nearby to decrease the chances of running out of fuel during severe weather.
A metering bin or surge bin is often the last section of the fuel handling system, and controls the rate at which fuel is delivered to the combustor. The combustor is also called the firebox or reactor. Pellet and log systems can be used to directly heat air as the heat transfer medium, or they can heat water in a boiler or hot water system, known as a hydroponic heating system. Chip combustion systems almost always use water as a heat transfer medium.
The combustor and boiler can be configured end-to-end or the boiler can be mounted on top of the combustor. The first configuration requires more floor space and the second requires more vertical clearance. Some manufacturers of larger biomass heating equipment make all of the system components, including the boiler/heat exchanger and the combustor. Others specialize in specific components, like the combustor and fuel handling systems, and in integrating components into complete systems.
Combustor and boiler delivery, Boulder County, Colorado
One potential area for confusion when specifying biomass heating systems is the variety of unit systems used in specifying system capacity. Heating systems are described in terms of Btu per hour (frequently incorrectly stated as simply Btu), or million Btu per hour (abbreviated variously as MMBtu/hr, MBtu/hr, or MMBtuh), watts (W), kilowatts (kW), or megawatts (MW), boiler horsepower (1 horsepower is roughly equal to 9,810 W or 33,479 Btu/hr), pounds of steam per hour, equivalent direct radiation (EDR), and others. Some vendors specify system size in terms of fuel input, though most system size designations are in terms of boiler output capacity. Similar confusion is possible in stating heating system efficiency, particularly with a high-moisture content fuel like wood chips. This makes calculating the fuel input requirements for a given space's heating load particularly challenging.
A fire suppression system is sometimes used to prevent fire from spreading from the combustor back up through the conveyor system where the wood chips are held in the metering bin. For example, this system might include a temperature sensor mounted on the feed system just upstream of the combustor as well as a water-delivery and control system to quench any fire before it spreads through the feed system.
In a hydronic system, which uses steam or hot water, pumps are used to circulate water from the hot water tank to spaces or buildings being heated by the system, through flow valves controlled by a thermostat or other temperature controller.
Exhaust stack on a biomass system at a California greenhouse
Biomass heating systems generally use a combination of induced-draft and forced-draft fans to control combustion air into the firebox and to force air through any emissions controls systems.
Exhaust systems are used to vent combustion by-products, primary carbon-dioxide, and water to the atmosphere. Emissions controls might include a cyclone or multi-cyclone, a baghouse, or an electrostatic precipitator (listed in order of increasing capital cost and effectiveness) to capture particulate matter. Cyclones and multi-cyclones can be used as pre-collectors to remove larger particles upstream of a baghouse (fabric filters) or electrostatic precipitator.
System controls generally coordinate the functioning of the conveyor systems, combustion fans, igniters, soot blowers, automatic ash handling equipment, and backup boilers, and often include diagnostic systems and alarm systems to notify operators of system problems. Some systems can include phone dialers that are programmed to send text or e-mail messages and can be Web-enabled to allow remote viewing of system parameters.
Woody biomass typically contains 1% to 5% ash content, which is non-combustible material. To ensure that the system continues functioning properly, this ash must be removed periodically from the system. In some systems, ash is removed manually, using a special rake or shovel. In other systems, augers automatically remove the ash from the firebox and deliver it to a barrel or other container for disposal.
Inclusion of the automated system will increase capital cost, but will decrease operation and maintenance costs.
Left: Automated ash-removal system on a wood-fired boiler. The system uses an auger to remove the ash from the boiler (lower right) and delivers it to the blue barrel on the left.
Right: A biomass-fired boiler (right foreground) and a Hurst natural gas-fired backup boiler (left background)
Biomass-fired heating systems often include a secondary fossil fuel-fired heating unit. This offers several advantages, including:
- Reduced capital costs and increased operating efficiency by under-sizing the biomass system
- Reduced uncertainty, as the buildings will still have a heat source even if the biomass system is down for any reason
- Increased operational flexibility
- Increased fuel options.
Many facilities include a backup boiler even with a fossil-fuel system. In that case, the backup boiler cost would not count against the biomass financial analysis.
Smaller biomass-fired systems are often located in the space to be heated, which allows for space heating through radiation and natural and forced convection. Larger systems are usually tied into hydronic distribution systems, which provide space heating through radiant floor or baseboard systems. In the photo of the biomass-fired boiler, some of the distribution piping is visible. This particular system uses the insulated, underground piping to carry the hot water to several buildings at the campus.
How Does it Work?
Direct combustion is the most common method of producing heat from biomass. In a direct combustion system, the biomass is burned to generate hot gas, which is either used directly to provide heat or fed into a boiler to generate hot water or steam. In a boiler system, the steam can be used to provide heat for process or space heating. The hot water or steam from the boiler can be used to transfer heat to a facility through typical space heating methods.
If the system is used as a combined heat and power system, the boiler can produce steam to run a turbine and power a generator, and remaining steam and hot water can then be used for heating.
Types and Costs of Technology
The two principal types of chip-fired direct combustion systems are stationary- and traveling-grate combustors, otherwise known as fixed-bed and atmospheric fluidized-bed systems. Biomass heating plants have installed costs that typically average between $500 to $1500 per kW-thermal of installed heating rate capacity. As these involve mature technologies, costs are not expected to drop significantly in the short term.
There are various configurations of fixed-bed systems, but the common characteristic is that fuel is delivered onto a grate where it reacts with oxygen in the air blown through the firebox. This is an exothermic reaction that produces very hot gases and generates steam in the heat exchanger section of the boiler.
Atmospheric fluidized-bed systems
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. Fluidized-bed systems generally achieve more complete carbon conversion, resulting in reduced emissions and improved system efficiency. In addition, compared to fixed-bed systems, fluidized-bed boilers can use a wider range of feedstocks. Fluidized-bed systems also have a higher electric load than fixed-bed systems due to increased fan power requirements.
Biomass gasification systems
Biomass gasification systems are similar to combustion systems, except that the quantity of air is limited. This process converts the biomass to a hot gas, which can be combusted in a boiler.
The efficiency of a direct combustion 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. A typical biomass system operating on fuel with a moisture content of 40% has a net efficiency of about 60% to 65%. Although the wood can be dried before combustion to increase efficiency, this is not typically done in practice because of the need for additional equipment and higher initial cost.
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), thermal peak and annual load, building size and type, space availability, operation and maintenance (O&M) staff availability, and local emissions regulations.
For buildings or campuses with more than 100,000 ft² to heat in a moderately cold climate, a system fuelled with wood chips will probably be the most economical, assuming that there is a stable local chip supply. The economics are even better for buildings with a year-round hot water or steam load or when competing against high-priced fossil fuels.
For buildings less than 10,000 ft² in a moderately cold climate, a wood pellet system might be the best option. These systems can be manually loaded with 40-lb bags of pellets. For larger systems, it is usually best to have pellets delivered in bulk (not bagged), where bulk delivery is available. These systems usually use a pellet silo or bunker to store large quantities of pellets. As a result, the pellets can be automatically conveyed from the silo to the pellet stove, pellet furnace, or pellet boiler.
Another option for smaller buildings is a cordwood system. The best of these systems have a burner surrounded by a large water jacket. The cordwood is loaded in batches and burned at full fire to heat the water. The hot water is a thermal energy storage medium that is circulated through the building's heaters as controlled by thermostats.
Some cordwood systems reduce burn rate by throttling combustion air, but this results in low efficiency and very high emissions of particulate matter and unburned hydrocarbons. As a result, these systems are not recommended, and are illegal in many jurisdictions.
Facilities with a high capacity factor (i.e., high average annual thermal load) are often good candidates for biomass heating. The capital costs for biomass heating systems are significantly higher than the costs for fossil-fuel plants, but savings on fuel use reduce the levelized cost of energy over time. The types of facilities for which biomass is usually a good investment include hospitals, prisons, school campuses, or other institutions where hot water use results in high load for all seasons. Additionally, biomass heating will not be cost effective if the competing heating fuel is significantly more expensive than the biomass feedstock.
A variety of incentives exist for biomass power, but vary with Federal and state legislation and policies. The Database of State Incentives for Renewables & Efficiency (DSIRE) lists incentives for biomass. The timing of incentive programs often allows less construction time than needed for biomass projects.
Federal agencies often cannot take direct advantage of financial incentives for renewable energy unless they use a different ownership structure. The Federal Energy Management Program (FEMP)'s Integrating Renewable Energy into Federal Construction Guide has more information on renewable energy project funding.
Levelized cost of energy for heating with biomass is typically $10 to $20 dollars per million Btu, but this is highly dependent on the feedstock cost and quality and on O&M costs. Federal agencies should consult local biomass experts for a better understanding of the available biomass resources and the available production and costs of energy from those resources.
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 subsequent feasibility analysis, it is critical to identify potential sources of biomass, and to estimate the fuel quantities needed.
Because of the complexity of determining available biomass resources and the usefulness of various types of biomass, Federal agencies should consult regional biomass experts to determine the available resources to a particular facility. They will need to determine, in detail, the capability of potential suppliers to produce and deliver a fuel that meets the requirements of the biomass heating system. For woody biomass, it is important to ask about:
- Whether forest products exist in the region or if they can be shipped in from another region
- Costs of the fuel, including transportation.
Since there is no established wood chip distribution system in most of the United States, it is sometimes difficult to find suppliers. One resource to contact is the regional U.S. Forest Service and state forest service offices. Other resources include landscape companies, lumber mills and other wood processors, landfills, arborists, and wood furniture manufacturers.
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. It is recommended that fuel procurement contracts scale purchase price inversely with moisture content, as higher moisture content significantly decreases combustion efficiency and increases the weight of material to be transported.
Design Considerations for Integrating
The following recommendations are critical to the success of any biomass energy project.
- Work closely with a biomass equipment manufacturer or vendor to coordinate 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.
- Ensure that doors are big enough for a truck to raise the dump bed (when designing a chip bunker).
Decision Steps to Biomass Heating
This section presents the steps involved in analyzing and installing a wood-fired heating system. Following is a brief outline of that process. The steps are not necessarily sequential; during the first steps, some iteration is expected as questions arise and knowledge is gained.
The purpose of the screening step is to see if a wood-fired heating system makes sense for a particular facility. A quick economic analysis should be performed to estimate capital costs relative to potential annual savings. A discussion should be held with O&M staff, management, and other affected parties to determine the organization's ability to support biomass heating.
If the concept of biomass heating passes the initial quick filter, it is time to learn more about biomass heating systems. Review manufacturers' websites and literature, talk to others with knowledge of biomass heating, and consider scheduling an appointment to visit similar regional installations.
Assess the Local Biomass Resource
Research the cost and availability of biomass fuel in the area and keep in mind that long haul distances increase chip cost. Search for potential suppliers that are able to deliver wood chips to the designated facility. Eventually there will be a need to select one or more suppliers whose delivery vehicles are compatible with the chosen system.
Conduct a Feasibility Study
At this stage, it would be beneficial to have a feasibility study performed by a firm with experience in biomass heating systems. The study will cover most of the topics in this document and provide a detailed economic analysis of heating and comparison of wood versus conventional fuel. The feasibility study can also determine how well a biomass heating system will help meet the facility's energy requirements and goals.
Determine Appropriate System Size
The size of the wood-fired system should be based on the peak load. Compared to an undersized system, an oversized system will not perform as well, will be inefficient, and will have higher emissions. A supplemental boiler, using a conventional fuel, will almost always be needed and will be sized to meet the peak load. The wood-fired boiler can be sized at 50% to 80% of the peak load, and still meet 90% to 95% of the annual load. The smaller system will have lower capital costs and will operate more efficiently than a large system. An economic analysis can be used to determine the optimal size of the wood-fired system.
Estimate Heating Energy Use
Estimate heating energy use and expenses for a wood-fired system and for a system using alternate fuel such as natural gas, propane, and heating oil. The annual cost of wood versus alternate fuel will be a factor in the economic analysis, and will also be used to estimate air emissions, ash production, fuel storage requirements, and wood delivery schedules.
Make the Decision
After sufficient information has been gathered, it is time to decide if proceeding with a biomass heating system fulfills the facility's specific needs, goals, and economic requirements.
Promote Public Discussion of Biomass Heating
If it is in the best interest to pursue a biomass heating system, be sure to fully involve decision-makers and the general public during the planning stages and as progress is made, particularly if the system is to be installed in a public building.
Operation and Maintenance
O&M costs of biomass heating systems are predominately the costs of fuel and labor. A well-operated and maintained wood chip-fired heating system should require 2 to 5 hours of O&M per week during the heating season. This includes fuel ordering and a daily walk-through.
The following is a conservative initial maintenance plan. As the operators become familiar with the equipment, this plan can be modified as needed. Many installations can go significantly longer between maintenance routines than the amount of time indicated here.
Ash resulting from wood combustion
Grate maintenance consists of cooling the box down slightly, and scraping the grate with a rake or hoe. Ash removal can be automated or manual. Manual ash removal takes about 10 minutes and should initially be done about twice a week. If there has been light use, systems can go as long as two weeks between cleanings.
Boiler tube cleaning is also dependent on use. Most systems in schools clean these tubes twice a year—once during the summer and once during a mid-season shut down (during the holiday period). By monitoring the stack gas temperature, the frequency of required boiler tube cleaning can be determined.
Equipment maintenance consists of lubricating gear drives and occasionally replacing seals, bearings, and gaskets. Depending on the size of the facility, a $1,000 to $2,000 per year "sinking fund" will normally cover any equipment maintenance required.
Additional operating costs include the cost to move the wood around on-site, and to load the hopper from a larger storage area. This handling has been estimated to cost about $3,000 per year for a medium-sized chip system.
The following are important special considerations for biomass heating systems.
Environmental Review / Permitting
The primary NEPA and permitting issue with a biomass heating system is the combustion emissions. Therefore, local requirements should be consulted. Air emissions from a biomass system depend on the system design and fuel characteristics. The following table shows typical emissions for a biomass heating system (based on CHIPTEC gasifier data) operating on 40% moisture content pine. If necessary, emissions controls systems can be used to reduce particulate matter (PM) and oxides of nitrogen (NOx) emissions. Sulfur emissions are completely dependent on the sulfur content of the biomass, which is usually very low.
Air emissions for a typical biomass heating system (lb/green ton)
|Typical biomass system emissions||2.1||2.8||0.6||1.7|
The storage of wood chips also raises a few issues. When the chips are stored in a building, there is a potential for dust from the chips to build up on horizontal surfaces and to get inside the equipment. For this reason, many facilities either store the chips in silos or enclosed containers, or build a dust wall to limit this dust to a small area.
Another concern, though rare in occurrence, is the ability of wood chips to self-ignite, or spontaneously combust, when stored for long periods of time. 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 decreasing surface area-to-mass ratio.
Ontario, Canada, has experienced some wood chip fires. To help with this issue, the Office of the Fire Marshal fire code 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 in height, 90 m in width, and 150 m 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 noncombustible buildings and equipment.
- Smoking shall be prohibited in chip pile areas.
Wood chip fires can also 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 recommended 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 harmful air pollution, including mercury, cadmium, and particle pollution.
The EPA also enacted Clean Air Act Permitting for Greenhouse Gas Emissions on January 2, 2011. 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 the 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.
Biomass Heating Resources
- Biomass Energy Resource Center offers technical services for potential biomass projects and resources to encourage the use of community-scale biomass energy.
- Biomass Thermal Energy Council provides webinars, interviews and presentations from industry leaders designed to expand public knowledge of biomass.
- Colorado Renewable Energy Society gives a basic overview of biomass energy technology and resources about Colorado biomass power.
- Ontario Minister of Agriculture Biomass Page reviews biomass combustion: the technology, the industry, pros and cons, and system set up and operation.
- Ontario Sustainable Energy Association's Biomass page provides useful insight into how biomass is harnessed and how it can be used.
- RETSCREEN Biomass Heating Analysis Module software helps assess costs, savings, energy production and risks of biomass heating systems.