(704f) Torrefied Biomass: Superior Fuel for Co-Firing in Pulverized Coal Fired Power Generation Facilities
Approximately 54% of the United States power supply is being generated by coal fired electric generators. Due to the abundance of coal reserves in the U. S., the use of coal will be predominant. However, the environmental concerns including greenhouse gas emissions have prompted several agencies to regulate the gaseous emissions. Among the techniques that were proposed to reduce these gaseous emissions and the costs associated with these gaseous emissions, biomass co-firing with coal has drawn huge attention from electric utility producers. Biomass co-firing is considered as one of the most practical strategic options that has a short to medium term implementation potential for renewable power production. It is one of the most attractive options that will help utility companies in complying with the greenhouse gas reductions and more importantly the renewable energy portfolio standards.
Biomass is considered carbon neutral, and has lower sulfur, nitrogen, ash, and heavy metals than coal. However, several disadvantages exist compared with coal, such as lower heating value, lower density and higher moisture content. These properties pose several challenges related to fuel preparation, handling, storage, transportation, milling, and feeding in coal fired power plants. Torrefaction is a thermal pretreatment technique and has a potential to address the above mentioned issues. This process involves heating biomass to temperatures of 230-300oC in the absence of an oxidizing agent and under atmospheric pressure. During this process, 70% of mass is retained in the solid product which contains 90% of the initial energy. The remaining 30% of biomass is converted to volatiles and gases that contain 10% of the biomass energy. Thus, energy densification can be achieved via torrefaction by a factor of 1.3 on mass basis. The product, torrefied biomass, has improved fuel properties in terms of increased mass energy density, possesses hydrophobic nature, and is more uniform, in comparison with unprocessed biomass. Also, the grindability characteristics of the torrefied biomass are greatly improved and are comparable with coal and petroleum coke. This allows for effective utilization of torrefied biomass in coal fired power plants using the existing burners with minor modifications.
Research is being performed on both laboratory scale and pilot scale, using various lignocellulosic materials including pine, willow, arundo, bamboo, cotton waste, and pecan shells at UL, Lafayette. The effect of various parameters including temperature, residence times, particle size, moisture content on torrefaction process is being evaluated, the results of which will be presented. Also, work is being performed on a pilot scale reactor that was fabricated by an industrial counterpart, which has a capacity of up to 30 lbs/hour. Testing is being carried out on the pilot scale reactor to improve the overall process efficiency. Mass and energy balances on the pilot scale reactor will be performed the results of which will be presented. Also, the effect of parameters tested on the energy density, bulk density, hydrophobicity, and grindability using modified hard grove grindability index (ASTM D409) will be presented and compared with representative samples of coal and petroleum coke.