QUALITY CONTROLÂŚ
whether they are the simple home heating appliances or the multimillion dollar power plant consuming hundreds of thousands of tons of material annually—are designed and built to exacting specifications which require a consistent fuel source. Users must know basic information about the quality in order to operate the combustion system at peak efficiency. Plant engineers require material quality information to accurately predict the energy content that will be yielded, maintain material handling systems needed for both the raw materials and the ash produced after combustion, and to determine the economics of using biomass. Biomass also may be used as a blend component with a primary fuel source for economic reasons, to comply with government mandates, and potentially receive tax credits.
Testing Procedures Samples for quality control are collected in much the same fashion as other solid fuels. Regular and systematic increments of the material to be tested are collected over an agreed-upon lot size. This may be an hourly, daily, or weekly sample or over a fixed tonnage such as a barge or vessel load. Global standards organizations, such as the American Society for Testing and Materials, International Organization for Standardization and European Committee for Standardization (CEN), have developed exact procedures specifying the size and number of increments to be collected, the sample handling procedures and the sample preparation techniques. Prior to laboratory analysis, samples require special preparation. The first step is to dry the samples before further processing can take place. Raw wood chips, for example, can contain up to 50 percent moisture content; whereas, a pelletized product may have only 1 to 3 percent moisture. Samples are generally dried at just above ambient temperatures. After drying, the biomass sample requires preparation or grinding to a consistent size, creating a homogeneous product to be tested. This is accomplished by specialized mills, usually a
knife mill consisting of fixed and rotating blades. The material is literally cut by the knives, creating a product which will have a nominal top size of approximately 20 mesh or roughly 1 millimeter. After grinding, the sample is mixed and then reduced in size by riffle dividers, much the same as coal or coke samples. Depending upon the tests required, 200 pounds of biomass may be prepared down to a simple, dry powder of a 100 gram size, which is delivered to the laboratory. The laboratory test equipment used for biomass is much the same as those used for other solid fuels analysis. The typical suite of testing includes moisture, ash, sulfur, calorific value and physical characteristics such as size distribution and pellet strength. Samples also may be tested for additional parameters such as ash fusion, temperature of ash, chlorine, fluorine and elemental oxides in the ash. Moisture content is a critical value for obvious reasons. Most buyers typically will not pay to buy the water. Contract pricing can be affected by water content. Premium penalties can be written into agreements. Material handling can also be affected by water content. Calorific value is probably the most important result. It gives the heat content of the biomass as a fuel and allows the user to affix a cost per Btu. Ash content of the material allows the user to anticipate the amount of inorganic material left over after the biomass is combusted. Ash fusion temperature is not a major issue in pure biomass as it is generally much lower than coal and other solid fuels. There are typically no fouling or slagging issues. Ash elemental oxides composition is determined from the material’s ash and is important to know for disposal purposes. Sulfur content, while generally very low in biomass, is a critical emissions value. Chloride and fluorine content are regulated by the clean air initiative. High values can cause fouling and corrosion in boilers.
Size distribution and pellet strength are critical traits for material handling systems. Pellet strength can predict material degradation during transportation and handling.
Potential Problems Biomass, much like coal, will see consistency within a particular product family. Wood chips created from pine will all have similar ash, sulfur, and calorific value. Biomass in pellet form can have variations in product quality. This can be due to the manufacturer potentially blending in other forms or grades of biomass such as refuse-derived fuel. Often times the use of a binder is required to help the pellets retain their size and shape throughout manufacturing, transportation and handling phases. Depending on the binder used, this can have dramatic effects on the overall quality of the end product. Global standards organizations have developed specific procedures for the analysis of biomass. Most of these are simple variations of coal and petroleum coke standards that have been in use for many years; however, these address only the most basic analysis. Where no definitive biomass test procedures exist, it may be required to utilize a specific coal or coke test method, such as in the case of the fusion temperature of ash or trace metals content. As industry and homeowners increase the use of biomass as a primary or secondary fuel source, it is expected that ASTM, ISO, CEN and other governing standards agencies will introduce more specific procedures for testing biomass in its individual forms. These will all most certainly be fashioned after coal and coke standard methodologies in use today. Authors: Stan Houser Minerals and Biomass Specialist, Intertek (815) 221-5002 stan.houser@intertek.com Scott Blakely Laboratory Technical Services Manager, Intertek (815) 221-5002 scott.blakely@intertek.com
AUGUST 2011 | BIOMASS POWER & THERMAL 51