From Waste to Energy
- By Kevin Rhodes
- April 1st, 2008
Academic institutions have been on the leading edge of sustainable design and construction, including their pursuit of LEED certification, because they have recognized the benefits associated with high-performance buildings: decreased environmental impacts, decreased life-cycle costs, and enhanced quality of life for building occupants. As part of this drive toward sustainability and high performance, many seek alternatives to nonrenewable fossil fuels to generate steam and/or electricity. However, they have been stymied by the limitations associated with the available technologies, including solar, wind, and geothermal energy — in particular, the limitations associated with the availability of these energy sources due to geographic location and local climate.
Biomass energy may offer a feasible alternative for academic institutions. Biomass currently supplies 14 percent of the world’s energy needs and accounts for 4.54 percent of total U.S. energy production, according to the U.S. Department of Energy. In fact, biomass supplies almost five times the energy output of solar, wind, and geothermal energy sources combined. Yet few institutions have explored this alternative.
From Wood Chips to Sludge
Biomass is an organic material, normally a waste or byproduct of an industrial or agricultural process, with little or no commercial value. Often biomass ends up in the industrial waste stream and is landfilled. Yet it is a potentially valuable source of energy for steam and/or electrical generation in a central plant. Some forms of biomass, such as wood chips, can be burned conventionally in a solid fuel boiler such as a coal burner. Other forms of biomass do not directly combust easily, but they can be converted to a fuel gas through the process of gasification. These include rice hulls, corn stover (leaves and stalks), chicken litter, and undigested sewage sludge from the wastewater treatment process.
Biomass gasification combines heat, oxygen, and the fuel source in a closed vessel. The products of gasification are fuel gas — which is composed primarily of carbon monoxide and hydrogen — and ash. The fuel gas can be used in a gas turbine generator to generate electricity and/or burned in a boiler to generate steam. Alternatively, human or animal waste can be converted to fuel gas using the process of digestion, a biological process using microorganisms to break down organic waste. Digestion can take place in the presence or absence of oxygen; i.e., aerobic and anaerobic, respectively. Larger wastewater treatment plants use anaerobic digestion, as do septic tanks. The primary constituents of digester gas are methane and carbon dioxide.
Pros and Cons for Academic Institutions
Renewable energy from biomass offers several advantages for academic institutions. The potential availability of biomass at little or no cost is a strong economic incentive for academic institutions. In fact, producers of waste streams that are inconvenient to dispose of, such as chicken litter or undigested sewage sludge, may pay the institution to accept it. For those campuses that currently use natural gas for power or steam generation, biomass also may be a method to cushion against swings in the pricing of other fuels, such as natural gas.
Moreover, use of an otherwise wasted energy source for heating and/or power generation may offer an academic institution a competitive edge with respect to student and faculty recruitment.
There are also disadvantages associated with use or production of biomass fuel. There may be challenges to the biomass supply chain, depending upon the biomass. These may occur during pick-up, transportation, off-loading, handling, storage, and processing. Certain biomass streams, such as undigested sewage sludge, may require specialized loading and transport equipment. Some are available in batches rather than continuously, and will need to be stored. As an example, chicken litter is only collected from the poultry barn every three to four months between flocks. And as one can imagine, some biomass streams may have an odor.
Strategic siting of the gasification plant itself may allay some of these concerns. Rather than locating it on campus, it can be constructed at a remote location and the gas can be piped to the boiler plant.
Determining the feasibility of using biomass on a particular campus requires an analysis of many technical, economic, and environmental factors. First and foremost, there must be local access to a reliable source of a sufficient quantity of biomass stream, and preferably, a homogeneous biomass stream.
Economic and technical feasibility also depend on the current technology employed by the central plant and the cost of equipment modifications to accept biomass fuel, if any. For example, a campus with a coal-fired plant can directly burn wood waste. However, a natural gas-fired plant must be adapted to burn gas produced from a biomass source, which has a heating value of one quarter of natural gas per cubic foot. Nozzles and other components of the system must be replaced with larger components to accommodate the higher gas flow. Add to that the cost to construct a gasification plant (or digester), if necessary, for conversion of the available waste stream into fuel gas, and pipe the fuel gas to campus.
These costs must be weighed against the relative costs of burning fossil fuel versus biomass. Currently, the cost of burning natural gas is in the range of $7 to $8 per one million input BTUs, and the cost of coal is approximately $3 to $4 per million BTUs. As an example, wood waste with a heating value of approximately 6,000 BTUs per pound mass and cost of $50 per ton is a little over $4 per MMBTU. However, cost and availability vary widely by region.
The decision also depends greatly on environmental factors, including state and local permitting regulations, as well as public opinion.
Clearly, implementation of an emerging technology, such as biomass energy, is not without risk. But those colleges and universities that carefully analyze the pros and cons of biomass may find that there are long-term benefits to turning waste into energy.
Kevin Rhodes, P.E., is project director of the Energy Utilities Group of Woolpert, Inc., based in the Cincinnati office. Rhodes has more than 20 years of experience in the energy and construction industry, including gas, oil, coal, and nuclear plants. A member of IDEA, Rhodes’s work includes extensive experience in planning, design, and implementation of district utility systems. The Energy Utilities Group’s clients include Duke University, Ohio State University, Penn State University, and Purdue University. Rhodes can be reached at 513/272-8300 or firstname.lastname@example.org.