Effect of Wood Fuel on SO_2 Emissions in Co-firing with Coal in Fluidized Bed

摘要

The efficiency of sulfur reduction in combustion processes depends on many variables like combustion temperature, excess of air, air staging, fly ash recirculation, fuel type, limestone characteristics, limestone and fuel feed distribution and Ca/S ratio. This study examines how some process variables and fuel characteristics affect sulfur reduction efficiency. CFB combustion tests were performed in order to measure and characterize related effects and interactions. Coal and bark co-firing tests were performed with a laboratory-scale CFB combustor. The objective of the tests was to find out the effect of the following factors and parameters on sulfur removal: bark fraction in fuel feed, riser temperature, air excess and Ca/S ratio in the fuel blend. The coal/bark blends tested contained 0, 30,40 or 50% of bark on an energy basis. Corresponding sulfur removal efficiencies were from around 25% (no bark) up to 60% (50% bark). The riser bottom temperature also had a very strong effect on sulfur removal. A sharp decrease in SO_2 emission was observed when the riser bottom temperature was decreased from 900 to 870°C. Limestone was added as a sorbent in some co-firing tests corresponding Ca/S ratios between 1 and 2. Resulting sulfur reduction varied from 60 to 75%, respectively.