Model performance of a biomass-fueled power station with variable furnace exit gas temperature to control fouling deposition

A major problem associated with the utilization of any biomass fuel in direct-combustion energy production is fouling (ash deposition on boiler surfaces) and the related issue of slagging, resulting from transformations among the inorganic constituents of the fuel. These deposits reduce heat transfer from the fire- to water-side, reducing power plant efficiency and necessitating the design of more tolerant heat exchange equipment. Wood: currently serves as the major source of fuel in biomass conversion to energy because of its more general availability, and it suffers less from fouling and slagging than many other biomass fuels such as rice straw. To reduce fouling severity, furnace exit gas temperature (FEGT) may be decreased to solidify ash ahead of superheaters and other heat exchanger equipment. Thermal and economic computer models of a direct-combustion Rankine cycle power plant were developed to predict the impact of variable FEGT and overall heat transfer coefficient on power plant efficiency and economy. No attempt was made to model the fire-side processes leading to the formation of fouling deposits. A base case: operational and economic profile of a biomass power plant was established, and models, were executed using these parameters, approximating a power plant efficiency of 19.9% and a cost of electricity (COE) of {dollar}0.0636 kWh–1 (including capital costs). If no capital, costs are included, then COE is {dollar}0.0205 kWh–1. Sensitivity analyses were performed on power plant efficiency and COE. Changes in FEGT through variable excess air resulted in substantial sensitivity in power plant efficiency (plant efficiency of 21.4% for FEGT of 1030°C (5% excess air) and 18.7% for 924°C (55% excess air)). Plant efficiency was determined to be moderately sensitive to changes in overall heat transfer coefficient on the secondary superheater (18.7% for no heat transfer through secondary superheater and 19.9% for base case heat transfer). Fouling scenarios showed that FEGT may be reduced by reducing steaming rate (20% reduction in steaming rate, FEGT of 939°C), but the reduction in steaming rate increased COE if the COE included capital costs (11.5% increase). However, if capital costs were not included, then COE may decrease (6.8% reduction).