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How Biomass Energy Works

To many people, the most familiar forms of renewable energy are the wind and the sun. But biomass (plant material and animal waste) is the oldest source of renewable energy, used since our ancestors learned the secret of fire.

Until recently, biomass supplied far more renewable electricity—or “biopower”—than wind and solar power combined.[1] 

If developed properly, biomass can and should supply increasing amounts of biopower. In fact, in numerous analyses of how America can transition to a clean energy future, sustainable biomass is a critical renewable resource.[2] 

Contents 

  1. Types of Beneficial Biomass
  2. Converting Biomass to Biopower
  3. Potential for Biopower
  4. Environmental Risks and Benefits
  5. Carbon Emissions
  6. Conclusions

Sustainable, low-carbon biomass can provide a significant fraction of the new renewable energy we need to reduce our emissions of heat-trapping gases like carbon dioxide to levels that scientists say will avoid the worst impacts of global warming. Without sustainable, low-carbon biopower, it will likely be more expensive and take longer to transform to a clean energy economy. 

But like all our energy sources, biopower has environmental risks that need to be mitigated. If not managed carefully, biomass for energy can be harvested at unsustainable rates, damage ecosystems, produce harmful air pollution, consume large amounts of water, and produce net greenhouse emissions.

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However, most scientists believe there is a wide range of biomass resources that can be produced sustainably and with minimal harm, while reducing the overall impacts and risks of our current energy system. Implementing proper policy is essential to securing the benefits of biomass and avoiding its risks.

Based on our bioenergy principles, UCS’ work on biopower is dedicated to distinguishing between beneficial biomass resources and those that are questionable or harmful—in a practical and efficient manner—so that beneficial resources can make a significant contribution to our clean energy future.

Note: This page addresses using biomass to generate biopower. For more information on biofuels, go to the UCS Clean Vehicles Program’s biofuels pages.

Biomass is a renewable energy source not only because the energy it comes from the sun, but also because biomass can re-grow over a relatively short period of time. Through the process of photosynthesis, chlorophyll in plants captures the sun's energy by converting carbon dioxide from the air and water from the ground into carbohydrates—complex compounds composed of carbon, hydrogen, and oxygen. 

When these carbohydrates are burned, they turn back into carbon dioxide and water and release the energy they captured from the sun. In this way, biomass functions as a sort of natural battery for storing solar energy. As long as biomass is produced sustainably—meeting current needs without diminishing resources or the land’s capacity to re-grow biomass and recapture carbon—the battery will last indefinitely and provide sources of low-carbon energy.

Types of Beneficial Biomass

Most scientists believe that a wide range of biomass resources are “beneficial” because their use will clearly reduce overall carbon emissions and provide other benefits. Among other resources, beneficial biomass includes

  1. energy crops that don’t compete with food crops for land
  2. portions of crop residues such as wheat straw or corn stover
  3. sustainably-harvested wood and forest residues, and
  4. clean municipal and industrial wastes.[3]

Beneficial biomass use can be considered part of the terrestrial carbon cycle—the balanced cycling of carbon from the atmosphere into plants and then into soils and the atmosphere during plant decay. When biopower is developed properly, emissions of biomass carbon are taken up or recycled by subsequent plant growth within a relatively short time, resulting in low net carbon emissions.

Beneficial biomass sources generally maintain or even increase the stocks of carbon stored in soil or plants. Beneficial biomass also displaces carbon emissions from fossil fuels, such as coal, oil or natural gas, the burning of which adds new and additional carbon to the atmosphere and causes global warming.

Among beneficial resources, the most effective and sustainable biomass resources will vary from region to region and also depend on the efficiency of converting biomass to its final application, be it for biopower, biofuels, bioproducts, or heat.

Energy Crops
Energy crops can be grown on farms in potentially large quantities and in ways that don’t displace or otherwise reduce food production, such as by growing them on marginal lands or pastures or as double crops that fit into rotations with food crops. Trees and grasses that are native to a region often require fewer synthetic inputs and pose less risk of disruption to agro-ecosystems.


Switchgrass

Grasses 
Thin-stemmed perennial grasses used to blanket the prairies of the United States before the settlers replaced them with annual food crops. Switchgrass, big bluestem, and other native varieties grow quickly in many parts of the country, and can be harvested for up to 10 years before replanting. Thick-stemmed perennials like sugar cane and elephant grass can be grown in hot and wet climates like those of Florida and Hawaii.

Switchgrass is a perennial grass that grows throughout the Great Plains, the Midwest and the South. Switchgrass is a hardy species—resistant to floods, droughts, nutrient poor soils, and pests—and does not require much fertilizer to produce consistent high yields.[4] Today, switchgrass is primarily cultivated either as feed for livestock or, due to its deep root structure, as ground cover to prevent soil erosion. However, this prairie grass also has promise for biopower and biofuel production (see profile of Show-Me Energy below).  If demand for switchgrass outstrips the capacity of marginal lands, it could, however, compete with other crops for more productive land.[5]

Beneficial biomass: rice hulls in Arkansas
Riceland Foods and Riviana Foods built gasification facilities in Stuttgart and Jonesboro, which together process 650 tons of rice hulls per day to produce biogas for energy.  Rice hulls, which make up about 20% of the whole grain, are rubbed off the grain in processing. Due to their high silica content, rice hulls should not be burned and cannot be fed to cattle, so gasification is a cleaner way to produce energy from something that would otherwise be a waste product. The gas produced at the Arkansas facilities is used to replace natural gas and to generate biopower.[6]

Crop Residues
Depending on soils and slope, a certain fraction of crop residues should be left in the field to maintain cover against erosion and to recycle nutrients, but in most cases some fraction of crop residues can be collected for renewable energy in a sustainable manner. Food processing also produces many usable residues.

Manure
Manure from livestock and poultry contains valuable nutrients and, with appropriate management, should be an integral part of soil fertility management. Where appropriate, some manure can be converted to renewable energy through anaerobic digesters, combustion or gasification. The anaerobic digesters produce biogas which can either directly displace natural gas or propane, or be burned to generate biopower. For instance, dairy farms that convert cow manure with methane digesters to produce biogas can use the biogas in three ways (or in some combination of these end uses).

They can use the biogas on-site as a replacement for the farm’s own natural gas or propane use, clean up the biogas and pressurize and inject into nearby natural gas pipelines, or burn it to produce steam that is run through a turbine to generate renewable electricity for use on-site and/or fed into the local energy grid. The best application of biogas from manure will be determined by the type of manure, opportunity to displace natural gas or propane use, local energy markets and state and federal incentives.

Beneficial biomass: food waste, forest residues and perennial grasses in Minnesota
In Minnesota, food industry and other byproducts are feeding a new combined heat and power (CHP) plant that generates renewable electricity and efficiently uses waste heat from the boiler. Rahr Malting Company and the Shakopee Mdewakanton Sioux partnered to form Koda Energy, which in 2009 began generating up to 22 megawatts of renewable electricity with oat hulls, wood chips, prairie grasses, and barley malt dust from Rahr Malting.

The Koda plant burns about 170,000 tons of these agricultural waste products a year, and is able to operate at over 70 percent efficiency because Rahr Malting also uses the waste heat from the boiler in their operations, displacing the need for additional natural gas. About half of the plant’s renewable electricity is used to powering Rahr Malting, with the remainder purchased by Xcel Energy to supply to their customers. In the future, the Shakopee Mdewakanton Sioux Community hopes to use switchgrass grown on restored prairies to provide some of the biomass for Koda Energy.[7]

Poultry litter can be digested to produce biogas, or combusted to produce renewable electricity, either directly or through gasification, which improves efficiency and reduces emissions.

Woody biomass
Bark, sawdust and other byproducts of milling timber and making paper are currently the largest source of biomass-based heat and renewable electricity; commonly, lumber, pulp, and paper mills use them for both heat and power. In addition, shavings produced during the manufacture of wood products and organic sludge (or "liquor") from pulp and paper mills are biomass resources.  Some of these “mill residues” could be available for additional generation of renewable electricity.

Beyond these conventional types of woody biomass, there are additional sources of woody biomass that could be used for renewable energy. With the proper policy (see below), these additional sources could be sustainably harvested and make a significant contribution to renewable energy generation.

Forest residues
It is important to leave some tree tops and branches, and even dead standing trees, on-site after forest harvests. Coarse woody debris left on the soil surface cycles nutrients, especially from leaves, limbs and tops, reduces erosion and provides habitat for invertebrates.

Dead standing trees provide bird habitat. Provided that appropriate amounts of residues are left in the forest, the remaining amounts of limbs and tops, which are normally left behind in the forest after timber-harvesting operations, can be sustainably collected for energy use. Often, limbs and tops are already piled at the “landing”—where loggers haul trees to load them unto trucks. Using these residues for biomass can be cheaper than making additional trips into the woods—and reduce impacts on forest stands, wildlife and soils.

Forest treatments
Many forest managers see new biomass markets providing opportunities to improve forest stands.[9] Where traditional paper and timber markets require trees to meet diameter and quality specifications, biomass markets will pay for otherwise unmarketable materials, including dead, damaged and small-diameter trees. Income from selling biomass can pay for or partially offset the cost of forest management treatments needed to remove invasive species, release valuable understory trees, or reduce the threat of fires, though the science behind fire reduction is very complex and site specific.[10] 

Removing undesired, early-succession or understory species can play an important role in restoring native forest types and improving habitat for threatened or endangered species, such as longleaf forests in the Southeast.[11]

Thinned trees
Thinning plantations of smaller-diameter trees before final harvest can also provide a source of biomass. In addition, thinning naturally regenerating stands of smaller-diameter trees can also improve the health and growth of the remaining trees. With the decline in paper mills, some areas of the country no longer have markets for smaller-diameter trees. Under the right conditions, biomass markets could become a sustainable market for smaller-diameter trees that could help improve forest health and reduce carbon emissions.

Beneficial biomass: bagasse in Florida
At its plant in South Bay, Florida Crystals burns 1 million tons of sugar cane stalks per year to produce up to 140 MW of electricity—enough to power the mill, refinery and 60,000 homes.  Florida Crystals sells the surplus energy to Florida Power & Light and other utilities.[8]

Short-rotation trees
Under the right circumstances, there may be a role for short-rotation tree plantations dedicated to energy production. Such plantations could either be re-planted or “coppiced.” (Coppicing is the practice of cutting certain species close to the ground and letting them re-grow.) Coppicing allows trees to be harvested every three to eight years for 20 or 30 years before replanting. 

Short-rotation management, either through coppicing or replanting, is best suited to existing plantations—not longer-rotation naturally-regenerating forests, which tend to have greater biodiversity and store more carbon than plantations.

Policy is needed to ensure that the growing biomass industry will use these beneficial resources, and use them on a sustainable basis. See below for more on the policy needed to guide the biomass industry toward sustainable, beneficial resources.

Urban wastes
People generate biomass wastes in many forms, including "urban wood waste" (such as tree trimmings, shipping pallets and clean, untreated leftover construction wood), the clean, biodegradable portion of garbage (paper that wouldn’t be recycled, food, yard waste, etc.).  In addition, methane can be captured from landfills or produced in the operation of sewage treatment plants and used for heat and power, reducing air pollution and emissions of global warming gases.

Converting Biomass to Biopower

From the time of Prometheus to the present, the most common way to capture the energy from biomass was to burn it to make heat. Since the industrial revolution this biomass fired heat has produced steam power, and more recently this biomass fired steam power has been used to generate electricity. Burning biomass in conventional boilers can have numerous environmental and air-quality advantages over burning fossil fuels.

Advances in recent years have shown that there are even more efficient and cleaner ways to use biomass. It can be converted into liquid fuels, for example, or “cooked” in a process called "gasification" to produce combustible gases, which reduces various kinds of emissions from biomass combustion, especially particulates

In 1998, the first U.S. commercial scale biomass gasification demonstration plant based on the SilvaGas process began at the McNeil Power Station in Burlington, Vermont.

The SilvaGas process, a particular form of biomass gasification, indirectly heats the biomass using heated sand in order to produce a medium Btu gas.

The McNeil power station is capable of generating 50 MW of power from local wood waste products.

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.

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-firing.” Through gasification, biomass can also be co-fired at natural gas-powered plants.

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.

Repowering
Coal plants can also be converted to run entirely on biomass, known as “re-powering.” (Similarly, natural gas plants could also be converted to run on biogas made from biomass; see below.)

Combined heat and power (CHP)
Direct combustion of biomass produces heat that can also be used to heat buildings or for industrial processes (for example, see textbox on Koda Energy above). Because they use heat energy that would otherwise be wasted, CHP facilities can be significantly more efficient than direct combustion systems. However, it is not always possible or economical to find customers in need of heat in close proximity to power plants.

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 biofuels or other useful chemicals.

Anaerobic digestion
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-controlled 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.

Beneficial biomass: crop residues, switchgrass, wood waste in Missouri
Among new biomass pelletizing facilities, Show Me Energy cooperative is pioneering a unique way to combine the community benefits of smaller-scale, locally owned biomass facilities with the efficiencies needed to serve the export market. Founded with the investment of its hundreds of farmer-members, Show Me is pelletizing crop residues, switchgrass and urban wood residues. In addition to selling pellets locally, Show Me is exporting pellets to Europe.

If successfully developed across the country, facilities like Show Me could create markets for farmers and jobs in rural communities, make biomass more economical to transport and easier for utilities to use and reduce carbon emissions by displacing coal and other fossil fuels with a variety of locally-available beneficial biomass resources.[13]

Energy density
Another important consideration with biomass energy systems is that unprocessed biomass contains less energy per pound than fossil fuels—it has less “energy density.” Green woody biomass contains as much as 50% water by weight. This means that unprocessed biomass typically can't be cost-effectively shipped more than about 50-100 miles by truck before it is converted into fuel or energy.

It also means that biomass energy systems may be smaller scale and more distributed than their fossil fuel counterparts, because it is hard to sustainably gather and process more than a certain amount of in one place. This has the advantage that local, rural communities will be able to design energy systems that are self-sufficient, sustainable, and adapted to their own needs.

However, there are ways to increase the energy density of biomass and to decrease its shipping costs. Drying, grinding and pressing biomass into “pellets” increases its energy density. Compared to raw logs or wood chips, biomass pellets can also be more efficiently handled with augers and conveyers used in power plants. In addition, shipping biomass by water greatly reduces transportation costs compared to hauling it by truck.

Thus, hauling pelletized biomass by water has made it economical to transport biomass much greater distances—even thousands of miles, across the Atlantic and Pacific, to markets in Japan and Europe. In the last few years, the international trade in pelletized biomass has been growing rapidly, largely serving European utilities that need to meet renewable energy requirements and carbon-reduction mandates. Several large pellet manufacturers are locating in the Southern US, with its prodigious forest plantation resource, to serve such markets.[12]

Potential for Biopower

In the United States, we already get over 50 billion kilowatt-hours of electricity from biomass, providing nearly 1.5 percent of our nation's total electric sales. Biomass was the largest source of renewable electricity in the U.S. until 2009, when it was overtaken by wind energy.  Biopower accounted for more than 35 percent of total net renewable generation in 2009, excluding conventional hydroelectric generation.[14] The contribution for heat is also substantial. But with better conversion technology and more attention paid to energy crops, we could produce much more.

Technical resource potential for developing biopower from beneficial biomass:

 Renewable Resource  Electric Generation Capacity Potential (in gigawatts) Electric Generation (billion kilowatt-hours)  Renewable Electricity Generation as % of 2007 Electricity Use
Energy Crops   83  584  14%
Agricultural Residues  114  801  19%
Forest Residues  33  231  6%
Urban Residues  15  104  3%
Landfill Gas  2.6  19  0.4%
Total  248  1,739  42%

(Source: DOE, 2005 [15])

The growth of biopower will depend on the availability of resources, land-use and harvesting practices, and the amount of biomass used to make fuel for transportation and other uses. Analysts have produced widely varying estimates of the potential for electricity from biomass. For example, a 2005 DOE study found that the nation has the technical potential to produce more than a billion tons of biomass for energy use (Perlack et al. 2005).

If all of that was used to produce electricity, it could have met more than 40 percent of our electricity needs in 2007 (see Table above). In a study of the implementation of a 25 percent renewable electricity standard by 2025, the Energy Information Administration (EIA) assumed that 598 million tons of biomass would be available, and that it could meet 12 percent of the nation’s electricity needs by 2025 (EIA 2007). In another study, NREL estimated that more than 423 million metric tons of biomass would be available each year (ASES 2007).

In UCS’ Climate 2030 analysis, we assumed that only 367 million tons of biomass would be available to produce both electricity and biofuels. That conservative estimate accounts for potential land-use conflicts, and tries to ensure the sustainable production and use of the biomass. To minimize the impact of growing energy crops on land now used to grow food crops, we excluded 50 percent of the switchgrass supply assumed by the EIA.

That allows for most switchgrass to grow on pasture and marginal agricultural lands—and also provides much greater cuts in carbon emissions (for more details, see Appendix G of Climate 2030: http://www.ucsusa.org/assets/documents/global_warming/climate2030-app-g-biomass.pdf). The potential contribution of biomass to electricity production in our analysis is therefore just one-third of that identified in the DOE study, and 60 percent of that in the EIA study.[16]

Distribution of biomass
Whether crop or forest residues, urban and mill wastes, or energy crops, biomass of one kind or another is available in most areas of the country. For information on the availability of various kinds of biomass resources in particular parts of the country, see the National Renewable Energy Labs’s searchable biomass databases.

Environmental Risks and Benefits

Like all energy sources, biomass has environmental impacts and risks. The main impacts and risks from biomass are sustainability of the resource use, air quality and carbon emissions.

Sustainability
Biomass energy production involves annual harvests or periodic removals of crops, residues, trees or other resources from the land. These harvests and removals need to be at levels that are sustainable, i.e., ensure that current use does not deplete the land’s ability to meet future needs, and also be done in ways that don’t degrade other important indicators of sustainability. Because biomass markets may involve new or additional removals of residues, crops, or trees, we should be careful to minimize impacts from whatever additional demands biomass growth or harvesting makes on the land.

Markets for corn stover, wheat straw and other crop residues are common and considerable research has been done on residue management. In addition, participation in some federal crop programs requires conservation plans. As a result of established science and policy, farmers generally leave a certain percentage of crop residues on fields, depending on soil and slope, to reduce erosion and maintain fertility. Additional harvests of crop residues or the growth of energy crops might require additional research and policy to minimize impacts.

In forestry, where residue or biomass markets are less common, new guidelines might need to be developed. Existing best management practices (BMPs) were developed to address forest management issues, especially water quality, related to traditional sawlog and pulpwood markets, with predictable harvest levels. But the development of new biomass markets will entail larger biomass removals from forests, especially forestry residues and small diameter trees. Current BMPs may not be sufficient under higher harvesting levels and new harvests of previously unmarketable materials.

However, because woody biomass is often a low-value product, sustainability standards must be relatively inexpensive to implement and verify. Thankfully, we can improve the sustainability of biomass harvests with little added cost to forest owners through the use of existing forest management programs, including 1) biomass BMPs, 2) certification or 3) forest management plans.

Working with forest owner associations, foresters, forest ecologists, wildlife conservation experts and biomass developers, UCS helped develop practical and effective sustainability provisions that can provide a measure of assurance that woody biomass harvests will be sustainable.

State-based biomass Best Management Practices (BMPs) or guidelines. Missouri, Minnesota, Pennsylvania, Maine and Wisconsin developed biomass harvesting guidelines to avoid negative impacts of biomass removals. Other states and regions, including Southern states, are also developing biomass guidelines. Developed through collaborative stakeholder processes, BMPs are practical enough to be used by foresters and loggers.

Third-party forest certification. Certification can also be used to verify the sustainability of biomass harvests. Between them, the Forest Stewardship Council, the Sustainable Forestry Initiative, and Tree Farm have certified nearly 275 millions of acres of industrial and private forestland in the U.S. Certification programs already address, or are being updated to address, many of the concerns related to biomass harvests.

Forest management plans written by professionally-accredited foresters. Foresters can help anticipate and therefore minimize impacts of additional biomass removals. Although a minority of smaller forest owners have management plans, forest owner associations have long recommended that more forest owners have them written to better achieve their financial and conservation objectives. Forest owners who have management plans stand to make more money than if they lacked such plans. To avoid out-of-pocket costs, proceeds from biomass sales could cover the cost of writing management plans.

Whether implemented through BMPs, certification or management plans, sustainability standards should minimize short-term impacts and avoid long-term degradation of water quality, soil productivity, wildlife habitat, and biodiversity—all key indicators of sustainability. Science and local conditions need to be used in determining the standards. For example, fire-adapted forests will likely require retention of less woody biomass than forests adapted to other disturbances such as hurricanes.

Sustainability standards should ensure nutrients removed in a biomass harvest are replenished and that removals do not damage long-term productivity, especially on sensitive soils. Coarse woody material that could be removed for biomass energy also provides crucial wildlife habitat; depending on a state’s wildlife, standards might protect snags, den trees, and large downed woody material. Biodiversity can be fostered through sustainability standards that encourage retention of existing native ecosystems and forest restoration. Lastly, sustainability standards should provide for the regrowth of the forest—surely a requirement for woody biomass to be truly renewable.

Air quality
Especially with the emissions from combustion systems, biomass can impact air quality. Emissions vary depending on the biomass resource, the conversion technology (type of power plant), and the pollution controls installed at the plant. The table below from the National Renewable Energy Laboratory and Oak Ridge National Laboratory compares air emissions from different biomass, coal and natural gas power plants with pollution control equipment.

Because most biomass resources and natural gas contain far less sulfur and mercury than coal, biomass and natural gas power plants typically emit far less of these pollutants than do coal-fired power plants.[17] Sulfur emissions are a key cause of smog and acid rain. Mercury is a known neurotoxin.

Direct Air Emissions from Biomass, Coal and Natural Gas Power Plants, by Boiler Type

 (Source: DOE, 2003 [18])

Similarly, biopower plants emit less nitrogen oxide (NOx) emissions than conventional coal plants.  NOx emissions create harmful particulate matter, smog and acid rain that results in billions of dollars of public health costs each year. Biopower systems that use either fluidized bed or gasification have NOx emissions that are comparable to new natural gas plants.

Biopower facilities with stoker boilers do emit significant quantities of particulates (PM 10) and carbon monoxide (CO), but these emissions can also be significantly reduced with fluidized bed and gasification systems.  Advanced coal gasification power plants also produce significantly lower air emissions than conventional coal plants.

Carbon Emissions

Burning or gasifying biomass does emit carbon into the atmosphere. With heightened interest in renewable energy and climate change, scientists have put biomass’ carbon emissions under additional scrutiny, and are making important distinctions between biomass resources that are beneficial in reducing net carbon emissions and biomass resources that would increase net emissions. While our understanding of specific biomass resources and applications will continue to evolve, we can group biomass resources into three general categories, based on their net carbon impacts.

Beneficial biomass
As mentioned previously, there is considerable consensus among leading scientists that there are biomass resources that are clearly beneficial in their potential to reduce net carbon emissions. These beneficial resources exist in substantial supplies and can form the basis of increasing production of biopower and biofuels.

Harmful biomass
In contrast to these beneficial biomass resources, scientists generally agree that harmful biomass resources and practices include clearing forests, savannas or grasslands to grow energy crops, and displacing food production for bioenergy production that ultimately leads to the clearing of carbon-rich ecosystems elsewhere to grow food.[19] Harmful biomass adds net carbon to the atmosphere by either directly or indirectly decreasing the overall amount of carbon stored in plants and soils.

Marginal biomass: resources that could be beneficial—or harmful
Scientists think the carbon benefits and risks of some biomass resource range widely, depending on how and where they are harvested, how efficiently they are converted to energy, and what fossil fuels they replace. In other words, these resources might be beneficial or harmful depending on specific situations. The use of trees harvested especially for energy use is a good example.

Using trees that will quickly and certainly re-grow to efficiently displace more carbon-intensive fossil fuels may be beneficial. On the other hand, using trees that will re-grow slowly or maybe not be fully replaced in an inefficient facility or to displace less carbon-intensive fuels may not be beneficial, or may be beneficial only over unacceptably long time frames in comparison to other available resources.[20] Marginal resources should only be used when their use can be demonstrated to reduce net emissions.

Navigating the path forward
We all should be concerned that biomass will be developed sustainably and beneficially—in ways that are cleaner and safer than our current energy mix, that are truly sustainable and that will reduce net carbon emissions. Beneficial biomass resources will in most cases be cleaner, sustainable and beneficial. Harmful biomass resources almost always will not. Marginal biomass resources may be cleaner, sustainable and beneficial—or not—depending on specific circumstances.

On the basis of the science, it would be unwarranted to support the use of all biomass resources, with any conversion technology and for any application. It would also be unwarranted to oppose all biomass on the basis that some biomass resources, conversion technologies or applications are not sustainable or beneficial.

Unfortunately, some biomass advocates and biomass opponents alike make just these mistakes—failing to distinguish beneficial from harmful biomass resources. Thus, all too often the debate about biomass is conducted in absolutist terms, either arguing that all biomass is “carbon neutral” or that “biomass” writ large will accelerate global warming, increase air pollution or lay waste to forests.

These absolutist approaches to biomass have led to two pitfalls in developing biomass policy. Absolute advocates have supported policy that would let almost any kind of biomass resource be eligible for renewable energy and climate legislation. On the other extreme, absolutist opposition has led to proposals to effectively remove most kinds of biomass from policy, especially at the state level.

Both approaches pose challenges to the development of beneficial biopower generation. The “anything goes” approach risks the development of harmful biomass resources that will increase net carbon emissions and cause other harm. Such a path also risks undermining the confidence the public and policymakers can place in biomass as a legitimate climate solution—which could eventually threaten the inclusion of beneficial biomass as a renewable energy resource in policy.

In tarring biomass with too broad a brush, some biomass opposition lumps beneficial resources with harmful ones and risks not developing beneficial biomass at large enough scale to capture important benefits for the country and the planet. As a group of biomass experts, comprising both advocates and skeptics, noted in an article in Science, “society cannot afford to miss out on the global greenhouse gas reductions and the local environmental and societal benefits when biofuels are done right.”[21]

To capture the benefits of beneficial biomass and avoid the risks of harmful biomass, federal and state policies should distinguish between beneficial and harmful biomass resources. Most policy related to biomass-based energy, be it for fuels, electricity or thermal, includes a definition of eligible biomass resources.

This definition should make beneficial biomass resources eligible, exclude harmful biomass resources and practices, and include practical, reasonable sustainability standards to ensure that harvests of biomass do not degrade soils, wildlife habitat, biodiversity and water quality. UCS has developed practical, effective sustainability standards for inclusion in biomass definitions, especially at the federal level.

Conclusions

When done well, biomass energy brings numerous environmental benefits—particularly reducing many kinds of air pollution and net carbon emissions. Biomass can be grown and harvested in ways that protect soil quality, avoid erosion, and maintain wildlife habitat. However, the environmental benefits of biomass depend on developing beneficial biomass resources and avoiding harmful resources, which having policies that can distinguish between them.

In addition to its many environmental benefits, beneficial biomass offers economic and energy security benefits.[22] By growing our fuels at home, we reduce the need to import fossil fuels from other states and nations, and reduce our expenses and exposure to disruptions in that supply. Many states that import coal from other states or countries could instead use local biomass resources.[23] 

With increasing biomass development, farmers and forest owners gain valuable new markets for their crop residues, new energy crops and forest residues—and we could substantially reduce our global warming emissions. For instance, a 2009 UCS analysis found that beneficial biomass resources could provide one-fourth of the electricity needed to meet a 25 percent by 2025 RES, while generating $12 billion in new biomass income for farmers, ranchers, and forest owners and reducing power plant carbon emissions as much as taking 45 million cars off the road.[24]

Growing our use of beneficial biopower will require policy to guide industry to the right kinds of resources, public confidence that biomass can be a sustainable and beneficial climate solution, and the use of appropriate biomass conversion technologies and applications.


Endnotes

 1 Energy Information Administration (EIA). 2008. Renewable energy trends 2007. Washington, DC. Online at http://www.eia.doe.gov/cneaf/solar.renewables/page/trends/rentrends.html.
 2 Union of Concerned Scientists. 2009a. UCS Climate 2030: A national blueprint for a clean energy economy. Cambridge, MA.
 3 Tilman, David., et al. 2009. Beneficial biofuels—the food, energy and environment trilemma. Science, July 17, 270-271.
 4 Natural Resources Conservation Service (NRCS). 2006. Switchgrass burned for power. Washington, DC: U.S. Department of Agriculture. Online at http://www.ia.nrcs.usda.gov/News/newsreleases/2006/switchgrass.html
 5 Marshall, Liz., et al. 2010. Fields of Fuel World Resources Institute. Washington, DC. Available online at: http://pdf.wri.org/fields_of_fuel.pdf
 6 PRM Energy Systems. 1996. A case study of two biomass gasification systems converting 650 tons/day of rice hulls to PRME NaturallyGas. Hot Springs, AR. Online at: http://gasifiers.bioenergylists.org/files/PRM%20ENERGY%20Gasification%20Systems.pdf
 7 Schill, Susanne. 2008. Koda biomass energy facility to begin final testing. Available only online at: http://www.biomassmagazine.com/article.jsp?article_id=2237&q=koda
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