Characterization of conversion zones in a reciprocating grate furnace firing wet woody biomass

Sammanfattning: Grate-firing systems are a common and popular combustion technology for burning biomass. However, combustion of biomass in these furnaces may emit a large amount of pollutants in the form of CO, CH4, PAH, NOx, and particles. These furnaces need to be further developed to increase their efficiency and improve the overall system performance while ensuring environmental compatibility. To improve the combustion of biomass in grate furnaces, it is vital to understand the processes occurring inside both the fuel bed and the gas phase above the bed (freeboard).The aim of this study was to characterize the conversion zones in two reciprocating-grate furnaces, with 4 and 12 MW maximum thermal outputs, firing wet woody biomass, mainly by measuring the temperature and gas concentration distributions. Measurements in the freeboard were conducted in both of the furnaces. However, measurements in the fuel bed were carried out only in the 4-MW furnace, in which the influence of two parameters, i.e., the fuel moisture and the primary airflow rate, on the bed conversion was investigated. Furthermore, a simplified model of the drying of wet biomass on a grate was developed and used to estimate the drying rate and to simulate the extent of the drying zone along the grate. The model was then validated against measurements of the fuel moisture and flue gas water vapor concentration made by NIR and FTIR, respectively.Measurements of temperature and gas concentration (i.e., CO, CO2, O2, CH4, and NO) profiles in the fuel bed were carried out through ports located in the wall of the furnace, by means of a stainless steel probe incorporating a K-type thermocouple. The temperature distribution along the height of the fuel bed was measured through a view-glass port, using a bent probe. High temperatures within a layer of about 0.1 m from the grate surface indicated the existence of a combustion layer at the bottom of the fuel bed (co-current combustion pattern). The drying rate of the moist fuel was calculated by the model to be about 0.0211–0.0235 kg water/m2s, under a certain condition. Therefore, the drying layer of the moist fuel was estimated to occupy almost two-thirds of the total grate length.Measurements of temperature and gas concentration profiles in the freeboard of the two furnaces were carried out by means of a water-cooled stainless-steel suction pyrometer. The pyrometer was introduced into the furnaces through the measuring ports, located along the pathways of the hot gas flow, and positioned at several locations between the wall and the furnace width center. For both of the furnaces, the temperature varied mainly in the 600–1000°C range in the primary combustion chamber, and reached about 1100–1200°C in the secondary chamber. A significant rise in NO concentration was observed in the transition sections between the primary and secondary combustion chambers of the furnaces. There was a positive correlation between NO and O2 at the ports in the secondary combustion chambers, which implied that in this high temperature region, the O2 concentration was the limiting factor for oxidation of N-volatiles to NO.