The Renewable Electron Economy Part V.5: Generating Electricity from Biomass August 6, 2007Posted by Michael Hoexter in Renewable Energy, Sustainable Thinking.
Part V of the series on the Renewable Electron Economy continues with another renewable source for electricity, a consideration of one of the most complex topics in renewable energy, biomass. Though sometimes lumped together, biomass as a fuel is considered to be distinct from biogas and biofuels as well as the use of landfill gas and municipal solid waste, all of which can be used for electricity generation. All of these sources ultimately rely on plant or algal photosynthesis in part or full as a source for biomolecules that release their energy to generate electricity, heat a space or process, or do mechanical work in a heat engine.
Biomass is what it sounds like, the biologically produced mass of plants, animals, algae and is a precursor to finished energy and material products from that mass. In practice, biomass refers almost exclusively to plant matter, which means that biomass is mostly starch, cellulose and lignin. Plants, composed mostly of sugar building blocks, link sugars together into starches and even longer sugar polymers called cellulose, while woody lignin is a polymer of complex bioalcohols. Biofuels are derived from biomass or parts thereof: bio-ethanol and other bio-alcohols can be distilled using conventional processes from the sugar and starchy parts of plants, while biodiesel is derived from plant oils usually contained in plant seeds. Under the rubric “cellulosic biofuels” are experimental processes using enzymes or the Fischer-Tropsch reaction that may transform lignin and cellulose into simple liquid fuel alcohols or biodiesel but these have yet to be developed to the point that they are energetically and economically viable.
Biogas is a combination of methane and carbon dioxide produced when specific bacteria ferment biomass. Anaerobic digestion is one process by which biomass is fermented into biogas, in this case the feedstuff is often livestock manure though most biomass can be fermented by bacteria. Landfill gas is also biogas as are the naturally occurring swamp and marsh gases. Biogas can be used in the place of natural gas to run simple or combined cycle power plants to generate electricity or used as a heat source for home or industrial use. Biogas is largely methane so a much more potent greenhouse gas than carbon dioxide, therefore combusting it makes a great deal of sense. Biofuels and biogas can all be considered a renewable resource.
Municipal solid waste can also be burned to drive turbines and generate electricity but in this case it is not considered renewable even though a large portion of the combustible material may be plant-derived matter like wood, paper and cardboard. Municipal solid waste, unless it is painstakingly sorted, contains many petrochemical products, metals, and potential toxins, which may in the process of incineration become dioxins or other potent carcinogens. The burning of municipal solid waste then requires top-notch emission controls.
Biomass to Energy
The least wasteful, most energy efficient use of plant matter as non-food energy is to dry it and burn it in an efficient combined-cycle power plant. Rather than process it further into biogas or liquid biofuels, in which energy is required and therefore lost, if electric generation is the goal, combusting or gasifying and then combusting biomass is the most direct route. While photosynthesis is a fairly inefficient process by which sunlight is converted into energy as compared to contemporary solar panels, biomass power has the advantage of utilizing stored energy so biomass to energy power plants can operate according to the expectations of electric utilities, either functioning as baseline power or being fired on-demand, assuming a steady supply of feedstock, either from waste or from virgin fuel crops.
Far from being an exotic, newfangled process, biomass to electricity is already the second largest source of renewable electricity in the US with 11 GW capacity or 1.4% of total electricity. Wood and agricultural waste is burned in the lumber, paper, and food processing industries to produce power for industrial processes as well as the electric grid. Power stations that use biomass are often configured so that they have the capability to use natural gas or coal in addition to biomass to compensate for variability in fuel supply. The 50MW McNeil Generating Station in Burlington, Vermont can operate on wood waste as well as natural gas in variable amounts depending upon supply. Co-firing, the mixture of biomass with fossil fuels, is a common means by which a power plant can both maintain capacity and offer a somewhat greener emissions profile.
The most portable and common fuel type for biomass from trees is wood pellets that can be created from any woody waste after moisture is removed. Wood pellets are compact and allow for standardization, longer storage and easier handling. An analysis of European wood pellet to electricity systems including full lifecycle energy and carbon costs, indicates that substitution of wood pellets for fossil fuels in 55% efficient combined cycle power plants yields substantial reduction in carbon emissions: while coal yields 396 kgCo2/MWh, natural gas yields 251 kgCO2/MWh, wood pellets yield 55kgCO2/Mwh for the lifecycle.
Biomass can also be used as a fuel for combined heat and power (CHP) plants as well as, of course, direct heating applications like wood-pellet stoves, charcoal and cooking fires. In CHP, excess heat from electricity generation is used to heat a space or industrial process. A CHP system can have an efficiency as high as 90% if heat losses from plant to process or the heated space are minimal.
As with other high temperature, combustion-driven processes, emissions controls are key in biomass to energy, as particulate matter, sulphur and nitrogen oxides are formed along with carbon dioxide. Now-common emission scrubbers for SOx and NOx as found in coal or natural gas plants are also required in biomass power plants to maintain regional air quality. As we assume that carbon dioxide is re-absorbed by growing plants, the capture of carbon-dioxide (CCS) from biomass power plants is not a priority though on the other hand, CCS with biomass plants might function in the future as a carbon sink rather than simply neutralize the addition of new carbon to the atmosphere.
Biomass Energy from Purpose-grown Fuel Crops
There is still room for growth in converting waste streams to energy but aggressive strategies to grow biomass to energy will additionally include the cultivation and use of fuel crops, as has been the case with liquid fuels like ethanol and biodiesel. The much anticipated “cellulosic” fuels mean that the whole plant can be used not just the simple sugars, starch or oil that is currently used in biofuel processing. The fast-growing switchgrass or miscanthus would appear in some US climate zones to be suited to provide biomass for electricity generation, just as they have been considered as feedstocks for cellulosic ethanol production. Despite using most parts of fast growing plants, fuel crops to energy will continue to be more expensive than the use of waste biomass to energy: current pricing is $2.50 per Gigajoule vs. $.95 per Gigajoule for waste. While cellulosic ethanol is still a ways away, the firing of this type of biomass in power plants could proceed today if that is economically feasible and regionally appropriate.
Issues that arise with the use of purpose-grown crops however are issues that have already sprung up in relationship to biofuel production: competition with food crops for investment, land and labor; replacing high-carbon natural ecosystems with lower-carbon farmed landscapes; conservation of the organic matter content of the soil; water conservation; endangerment of tropical forests; and net energy yielded from the process. To consider biomass to energy as truly green when virgin feedstocks are used, an eco-certification program is required: such a program is the only way forward for those who support biomass/biofuels for environmental reasons. There already exists a certification system for wood products, the Forest Stewardship Council (FSC) and FSC processes can already be applied to waste and virgin wood used in all biomass to energy schemes. The use of non-woody fuel crops requires however a somewhat different certification system that would prevent a runaway market for biomass energy from doing unforeseen damage to the soil, society, and the atmosphere.
If we can establish sustainability standards for biomass farming for energy, there are still significant logistical and economic challenges associated with cultivating, preparing and transporting biomass fuel crops. Biomass by its nature is massive, especially when compared with the mass of photons of sunlight, the weight of air or steam from the earth. In order to reduce the energetic cost of transportation, biomass would need to be produced fairly close to the power generation facility. Like hay, biomass crops would need to be dried to help reduce mass and discourage fungal or bacterial attack on the plant fiber. The energy required to dry biomass may in drier climates come in part from the sun but usually some additional energy expenditure is required. Furthermore, biomass utilizing power plants would need to either coordinate the harvest of fuel crops around them and/ or invest in massive, dry but fire-safe storehouses for fuel which might need to use some form of pest or microbial agent control to preserve the feedstock. At some point in the storage process, the conversion of biomass into a liquid or compressed gaseous biofuel may be necessary to facilitate storage.
Potential for Biomass to Electricity
According to the International Energy Agency, currently 11% of the world’s energy, both heat and power, is derived from biomass, with the poorest nations deriving 90% of their energy from biomass. As societies become more technologically advanced, they have tended to turn towards fossil fuels, hydroelectric, nuclear, and now wind and solar energy to generate electric power. Compared to wind and solar technologies, biological plants are much more inefficient though as energy conversion “capital” they cost a lot less per unit than a hydro-plant, a wind turbine or a solar cell.
As the focus in cultivating biomass has been on the conversion of fuel crops to liquid biofuels, estimates for the use of these feedstocks to generate electricity are not to my knowledge available. The current estimates for US domestic production of liquid biofuels is that US cropland could cover 30% of petroleum energy demand for the US under the most favorable assumptions for biofuel production. Biomass to electricity could using the same aggressive assumptions probably generate a similar proportion or greater of electric demand for the US. As population density is greater in Europe and most other parts of the world, this proportion would be less for European economies.
As geothermal, hydroelectric, solar and wind technologies can generate a majority of our electric needs, the potential of biomass to electricity is most favorable as a regional alternative for areas with long growing seasons but with diffuse sunlight, low wind and no shallow geothermal resources. The US Southeast, many of the tropical and sub-tropical regions of the world with high humidity and intermittent cloud cover are where biomass will be most productive and competitive as a energy source.
Multiple Markets for Biomass
One of the difficulties of looking ahead to where biomass-to-electricity might grow is that biomass has many uses that in a sustainable economy have high value. In addition to food and feed, a non-toxic bioplastics industry and, of course, construction materials will use biomass as raw material. Even in the electron economy that recommends the use of electric motors for most terrestrial tasks, biofuels will probably have a leading role in aviation and marine fuels. In addition, biofuels may function as special use fuels on land. The degree to which one or the other of these demands will influence the production and distribution of fuel crops and forestry and agricultural wastes is a function of the ultimate demand for each use.
Scenario for Biomass-rich Areas
Given some of the limitations of biomass as an energy source, the following scenario may be one possibility to optimize the development of biomass in a sustainable direction.
- Biomass production might center on areas with long growing seasons though seasonal production can also work with sufficient storage of processed biomass or in-field/forest storage. In the United States, the Southeast, the Ohio River Valley, the Mid-Atlantic and the Pacific Northwest may be areas where biomass for materials production and biomass to energy would concentrate. Tropical areas with high humidity and rapid plant growth are also very favorable areas as would be the vast woodlands and agriculture areas of Eastern Europe and Russia.
- Biomass-to-electricity, biofuel refineries, and biomass materials/plastics processors might cluster at the center of a double cloverleaf of local freight rail lines that are patrolled by shorter freight trains powered by biofuels that collect regional farmers’ biomass production for processing as either electricity, biofuels or biomaterials. The cloverleaf may have a radius of 50-80 miles (80 to 130 km) with inner rings of track to cover a wide region yet increase the efficiency of moving goods to market. Farmers will have flexibility with regard to what crops and end uses are best suited for their businesses. Finished products could then be shipped from the hub by rail or water to end-use markets.
- Daytime electric demand could be partly met by thin-film solar panels while nighttime demand would be met by biomass power plants. Thin-film solar has great promise for increasing solar production in diffuse light areas of the world.
Biomass is a flexible way to meet partial energy demand in a modern economy as well as create non-toxic raw material for various industrial processes. Biomass also allows for energy production under conditions where there is no exceptional wind, sun, water, or geothermal heat.