November 22, 2006 | General

Woody Biomass As Renewable Energy Source

BioCycle November 2006, Vol. 47, No. 11, p. 29
In-depth analysis in Minnesota assesses the feasibility and cost-effectiveness of regional biomass generated electricity projects that tap into local feedstocks.

THE Center for Energy and Environment (CEE) in Minneapolis, Minnesota undertook a multiphase research project to create a framework that communities and legislators can use to determine the feasibility of biomass-generated electricity in their region and to develop cost-effective projects that use local biomass resources efficiently. The nonprofit organization received close to $400,000 in grant monies for the study from the Renewable Development Fund, which is funded by Xcel Energy.
“The first half of the study, which was initiated last year, focused on quantifying the potential of biomass,” says Keith Butcher of CEE. “We looked at feedstocks available and the state of the industry in terms of conversion technologies. The middle of the project involved putting together an Excel-based screening tool to compare all the diverse feedstocks and technologies. We now have inventories for 59 different feedstocks on a county-wide basis in Minnesota. Using that, policy makers and project analysts can run scenarios to identify the best opportunities for generating biomass-based electricity generation.”
The study, “Identifying Effective Biomass Strategies: Quantifying Minnesota’s Resources and Evaluating Future Opportunities,” is divided into six milestones to be reached over an 18-month period. (Summary reports of the first three milestones can be downloaded at Milestone 1 identified current trends in the field of electrical generation technologies and associated issues prevalent among the use of biomass fuels. Tasks in Milestone 2 involved putting together an overall view of biomass resources available in Minnesota. Feedstock energy contents and moisture levels were combined with inventory estimates at the county level. The result is an estimate for the amount of dry tons (and total million BTUs (MMBTUs)) of feedstock for each county in Minnesota. In addition, the project identified key characteristics of Minnesota’s infrastructure that would facilitate the development of biomass electric generating facilities. Finally, this part of the project developed a database of organizational contacts that have particular expertise in biomass generation projects, and a GIS map with information on the transmission network, roadways, railways, and waterways.
Milestone 3 discusses issues surrounding the processing of biomass feedstocks. The comprehensive summary report covers existing Minnesota biopower facilities and their current status, and presents an evaluation tool to assist in comparing different biopower projects. Notes the summary: “The choice of processing techniques of biomass fuels will have significant effects on the efficiency and long-term profitability of a biopower plant. In developing a fuel supply plan one must determine when, where and how processing will occur. The decision of when processing will occur will be influenced in part by the storage regime used by the plant as well as the stability of the biomass fuel. Where processing will occur will be influenced by the need for and location of storage facilities (are all fuels stored at the plant, in the field or somewhere in between?), the need to transport the fuel (processed fuels will often be drier and denser which reduces transportation costs), as well as the size of the plant itself (small facilities may not have sufficient space to process fuels onsite). The decision of how processing will occur will be determined in large part by the nature of the biomass fuel and the specifications of the power plant; however, the need for a facility to accept a wide variety of fuels may play an important role in this choice as well.”
The Milestone 3 report, “Biomass Fuel Processing and Current Minnesota Biomass Power Facilities,” was released in August 2006. A great deal of information in this report is pertinent to wood waste processors marketing a biomass fuel. The following are some highlights excerpted from the report.
“When designing a fuel supply plan, one must remember that money is being spent every time the fuel is handled or transported. Any opportunities to minimize the number of times biomass fuel must be loaded, unloaded, processed or stored should be taken advantage of. Processing costs will be minimized if processing can be achieved in a single step, either immediately upon harvest, upon delivery to a storage facility or to the plant itself … Biomass processing generally involves three steps – drying, sizing, and cleaning.”
Moisture Content/Drying: Generally speaking freshly harvested biomass has a higher moisture content than is desired. This will require that it be dried at some point before utilization. The moisture content of a fuel has a significant impact on transportation costs, suitability for long-term storage, and the efficiency with which it can be converted to useful energy. Each of these factors will play a role in the site and method of fuel processing.
Particle Size: Each biomass facility will have its own particle size specification that is determined by the power plant’s characteristics. Particle size is important for both the efficiency and safety of a facility’s operation. Particles that are too large will lead to incomplete combustion and increase the mass of the facility’s waste stream. At the opposite extreme, dust can be explosive, both in the boiler and in processing and handling equipment. Facilities often remove dust for these reasons.
Cleaning: Most biomass fuels have some contaminants in them. Agricultural crops and residues will often have rocks and dirt, pieces of metal from fences or machinery, and chunks of wood from fence posts. Wood wastes can have all of the above and more if the wood is sourced from urban environments or construction debris. Cleaning will often involve magnets and nonferrous metal detectors, screens and occasionally visual inspection to ensure that the materials are sufficiently cleaned.
Chemical Composition: The chemical composition of some biomass feedstocks also presents problems within the boiler or reactor. Biomass feedstocks with high nitrogen content can cause elevated levels of NOx in exhaust. The problem typically is solved by mixing with other low nitrogen feedstocks. Wood waste, for example, is very low in nitrogen content and can be mixed with high nitrogen biomass.
Chippers And Grinders: The report compares size reduction equipment: “Chippers usually make an angular shaped chip with approximately the same width, height, and depth. Tub grinders produce a long and stringy particle size, e.g., 1-inch by 2-inches by 6-inches. The longest dimension is called a ‘tail.’ Biomass products with tails may cause bridging and blockages in storage bins and problems with bucket elevators and auger type conveyors. While tub grinders are capable of handling 14-foot diameter trees, the largest chippers do not perform well handling trees over 2-feet in diameter. To chip logs larger than 2-feet in diameter, shears must first split the log into pieces with a diameter of less than 2-feet. For this reason, tub grinders reduce larger logs at a lower cost than chippers because only one step and one machine are needed…”
“It would be common in Minnesota to pay $8 to $15/ton for grinding or chipping logs and brush into a particle size of less than 2-inches. To grind or shred wood to one-quarter or one-half-inch would probably increase this cost by another $3 to $10/ton depending on many factors. Therefore, biomass facilities that accept larger particle sized materials can procure much cheaper feedstocks.”
Issues of Scale: A project’s scale has a significant influence on who processes the fuel. Notes the report, “Very small facilities are likely to require that fuel be delivered to the facility in a form that can be used without further processing. This will often arise from the need to minimize labor and capital costs as well as a lack of sufficient space for fuel processing operations in very small facilities. Small facilities will often need to pay a premium for fuels delivered to specification, unless they are providing suppliers with an opportunity to avoid high disposal costs for a waste product. Very large facilities in a buyer’s market may also be able to require that fuel be delivered to specification due to their market power. This will enable large facilities to minimize their processing costs, as well as enable them to accept the very large volumes of fuel necessary for powering a large facility. Facilities that must compete for biomass fuels with other power producers, or other industries, are less likely to have the market power to impose high standards on the state of the fuels delivered to them, and are less likely to be able to pay a premium for fuels delivered to high specifications. Facilities in this situation are likely to need to perform most of their fuel processing on site.”
Processing for Storage: Processing woody biomass for storage is similar to processing it for combustion. The materials must be screened for contaminants, sorted by size, with large pieces reduced to the appropriate size and then sent to the storage pile. Wood chips can be stored outside in a pile for up to a year before odors become a problem. Some facilities report that they will consciously spread new chips over the length of the pile to increase the heterogeneity of the pile, thus minimizing fluctuations in fuel quality. Chips made from brush that still has green leaves or green stems contain nitrogen in the form of proteins; the carbon is concentrated in the woody portion. “The mix of the two is optimal for rapid and high temperature composting,” explains the report. “Temperatures of 120° to 180° F are not uncommon in brush chip piles. The upper level of these temperatures can lead to spontaneous combustion. For this reason, long-term storage of brush chips is a serious fire hazard.” – N.G.

Sign up