Bioenergy Technologies
Bioenergy comes from organic matter (biomass) like timber, crops, woodchips, landfill gas, and other gases created through natural decomposition. This biomass can be converted into heat and power in many different ways. Here we look at the types of biomass fuels, how they are used, and how they are connected to the electric grid.
What are Biomass Fuels?
Bioenergy can be produced using a variety of materials that include wood, crops like corn and soy beans, and waste from consumer, municipal, industrial, and agricultural processes. Each of these materials are sources of fuels that can be burned to produce energy.
Useable fuel can be extracted from these materials in a number of ways:
Solid fuels: In the case of woody biomass (wood chips, construction waste, and other solid wood sources), the wood itself is burned to produce energy. This energy can not only be used for heat but can be used to generate electricity.
Liquid fuels: In the case of some crop-based fuels like ethanol (corn) and bio-oil (soy), a liquid fuel is extracted from the crop or its wastes. Like solid fuels, this can be used to produce electricity as well as heat.
Gas fuel: When organic material decomposes, its chemical structure breaks down and releases gases in the process. This not only happens in nature but in human-produced wastes such as those found in municipal landfills. Landfills in particular produce large quantities of methane gas as the organic materials in them decompose. This methane can be used as a fuel.
Bioenergy technology converts the chemical energy stored in organic matter into heat and power. It encompasses a broad range of solid, gaseous, and liquid fuels that result from living organisms or from the wastes and by-products of human activities. The sun is the root source of all biofuels, making the Earth inhabitable for life itself and fueling the photosynthetic processes that transform seeds into trees and plants.
Organic matter can be used directly or indirectly as a fuel:
• Primary bioenergy sources include harvested timber as well as trees and non-woody crops grown and processed specifically for energy production.
• Secondary bioenergy sources include wood residue such as trimmings and woodchips generated by logging and other industries, pulping (or black) liquor from pulp and paper facilities, and urban wood waste such as pallets and construction debris. They also include municipal solid waste (MSW), animal waste, agricultural residue, and food processing waste.
• Derivative bioenergy sources include landfill gas (LFG) resulting from the anaerobic decomposition of organic materials at MSW disposal sites, as well as digester gas resulting from similar processes at wastewater treatment plants and livestock operations. They also include gaseous and liquid biofuels produced by living organisms or derived from organic matter, such as methane, ethanol, biodiesel, and hydrogen.
Bioenergy Technologies
Many different approaches exist for converting biomass into heat and power. Currently, direct firing technology is used to generate nearly all of the energy in the United States produced from solid biomass fuels (trees, plants, wastes). In both electricity generation and combined heat-and-power (cogeneration) applications, the fuel is collected and processed via mechanical or other means, then burned in a conventional boiler or other combustion chamber to produce steam, spin a turbine, and supply energy. Like conventional power plants, large-scale bioenergy facilities are equipped with back-end environmental control systems to reduce pollutant emissions.
Cofiring
Cofiring represents an option for producing some green electricity at power plants designed to operate on coal or other fuels. Processed solid biomass is added to the boiler along with the fossil fuel to help reduce reliance on finite resources and decrease overall emissions of pollutants and greenhouse gases.
Landfill and Digester Gases
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| Methane gas from a landfill is burned in a flare.
This gas can be put to use as a fuel for generating electricity. |
Landfill gas is created when food, wood, and other organic waste in a landfill decomposes under anaerobic – or oxygen free – conditions. Because landfill gas is about 50 percent methane, it can be used as a source of energy similar to natural gas (which is about 90% methane). Since landfill gas is generated continuously, it provides a reliable fuel for a range of energy applications, including heating and electric power generation.
The landfill gas consists primarily of methane and carbon dioxide. If a landfill does not use the gas for electricity, it still needs to be managed because the gas is potentially explosive if it is allowed to accumulate. Many landfills simply collect the gas and flare it.
To instead use the gas for electricity, wells or horizontal trenches are drilled into the landfill. Through a vacuum system and pipes, the gas is then directed to a central location where it is cleaned to remove particulates and moisture before burning it to run a turbine. Once a collection system is in place, the gas flow will be fairly continuous, allowing round-the-clock electricity generation, until the organic matter in the landfill decomposes.
There are several landfill gas facilities in Massachusetts, including one which recently opened at the Chicopee landfill with partial funding from the Renewable Energy Trust.
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| The Deer Island treatment
plant in Boston uses an anaerobic digestor to treat sewage. This
digester creates a gas that can be used to generate electricity. . |
Landfill gas and digester gas are typically burned in conventional internal combustion engines or combustion turbines after being collected and treated. These biofuels arise when bacteria that thrive in oxygen-poor environments feed on organic materials buried in landfills or found in human and animal wastes. Landfill gas is collected by drilling wells or installing pipes in horizontal trenches within a landfill, while digester gas can be extracted directly from enclosed digester systems at wastewater treatment facilities and agricultural operations.
Transforming landfill gas and digester gas into useful energy has an important side benefit-these by-products of human activities contain methane and other substances that, if released to the air, can cause localized odor problems and contribute to air pollution and greenhouse warming.
Demonstration projects are under way in which landfill gas and digester gas are collected, treated, reformed, and fed to fuel cells. A variety of other emerging bioenergy technologies exist. Perhaps the most promising is biomass gasification, which offers the potential for much higher efficiencies and lower air emissions than conventional direct firing of solid fuels.
Biomass Gasification |
| Mount Wachusett Community College in Gardner, Massachusetts uses one of the first biomass gasification systems in the state. |
In gasification systems, biomass (wood or other solid plant matter) is heated to high temperatures (600-800 oC) in a gasifier. The solid biomass is converted to a gas primarily composed of hydrogen carbon monoxide, carbon dioxide, water vapor, and methane. The gas is then used in a variety of applications, including gas electricity-generating turbines and boilers.
Gasifiers have several advantages over systems that burn biomass. Most notably, they emit less air pollution. They are significantly more efficient than biomass combustion facilities, so they require less raw material and can potentially generate electricity more cheaply. They are used in an oxygen-starved environment to convert into a gas (a mixture of hydrogen, carbon monoxide, and methane). The technology is still being perfected and refined for use in large power plants, so it is not yet been used widely.
In biogasification systems, solid biomass is first broken down from complex hydrocarbons into simpler gaseous molecules. This is accomplished by heating it to a very high temperature or by "feeding" it to anaerobic bacteria in a process analogous to that occurring at landfills and in digesters. The by-product mixture includes desired constituents-hydrogen and carbon monoxide-as well as a variety of contaminants. The gaseous fuel is then burned in conventional boilers or gas turbines, either directly or after cleanup, to generate useful energy.Liquid Pyrolysis
Liquid pyrolysis technology is similar in concept. Solid biomass is heated rapidly in a high-temperature, oxygen-free environment, converting it into a liquid fuel (bio-oil) as well as other products. The bio-oil can then be converted into useful energy in conventional combustion systems.
In both biogasification and bioliquefaction systems, the process design controls the relative proportion of desired constituents and contaminants and, thus, the energy content of the by-product mixture and the degree of cleanup required prior to combustion. Process variations are being explored to enable the production of biofuels with high hydrogen concentration suitable for direct use in fuel cells, as well as the development of integrated systems-biorefineries-for coproduction of biofuels and high-value industrial chemicals.
Refineries already exist for ethanol and biodiesel. These biofuels are commercially available for energy production and transportation applications, either as additives or as direct substitutes for conventional fossil fuels in combustion-based systems.
The competitiveness of all bioenergy technologies depends strongly on the cost and the characteristics of the fuel source. Fuel collection, transport, storage, and processing requirements can be negligible, or they may represent a significant cost consideration. For solid biomass, genetic engineering is an active R&D area, with goals being to design dedicated feedstocks that have higher energy content, are easier to handle, are optimized for specific climate conditions, allow more complete resource utilization, or otherwise improve process economics. Genetic engineering techniques are also being applied to develop bacteria suitable for industrial-scale production of hydrogen fuel.
Biodiesel Cogeneration
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A block of the
Brian J. Honan Apartments, pictured here ,is powered by a biodiesel
generator The complex was completed in 2005. |
An emerging use of bioenergy is the use of bio-diesel, a liquid bio-based fuel, to power a combined heat and power generation system. These systems produce both power and heat and are typically run on a liquid or gas fuel like oil or natural gas.
The Allston-Brighton CDC, located in Boston, is developing the first system of this type for a residential application. At the Hano Apartments in Allston, a diesel generator is being installed with biodiesel as its fuel. Based on the projected economics of this project, it appears that this use of biofuel has significant promise for future applications.
How Bioenergy is Connected to the Electric Grid
Because most bioenergy options for generating electricity involve the use of conventional combustion systems, they employ the same grid interconnection technologies as large-scale power plants and smaller-scale distributed generators running on fossil fuels. In addition, today's bioenergy technologies face many of the same environmental permitting issues as fossil-fired systems.
Although the overwhelming majority of bio-power plants serve on-site needs for electricity, the number of grid-connected facilities in the United States is growing rapidly. Larger power plants typically burn solid fuels such as wood wastes, municipal solid wastes, and construction materials. Smaller installations may burn these fuels or emerging bioenergy sources such as landfill gas, digester gas, and biodiesel fuel. Bioenergy may also be used as a supplemental fuel at existing coal-fired plants.
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