Alkali Metals in Circulating Fluidized Bed Combustion of Biomass and Coal: Measurements and Chemical Equilibrium Analysis

Abstract

Combustion and co-combustion experiments with four kinds of straw, specially selected for their different alkali, Cl, and Si contents, and Columbian black coal were carried out in a circulating fluidized bed (CFB) reactor at Delft University of Technology. The influence of operating conditions and fuel composition on the release of the alkali compounds to the gas phase was investigated. The amount of the total gas-phase sodium and potassium compounds in the flue gases was measured with excimer laser induced fluorescence (ELIF). The results show that the release of gaseous alkali species depends on fuel composition, in particular the K/Cl and K/Si ratios in the fuel. The fuels with high K and Cl values show higher concentrations of the gaseous alkalis. A synergetic effect of the co-combustion with coal was observed, which led to a strong decrease in gaseous alkali concentrations. Together with experiments, chemical equilibrium modeling was performed to help in interpreting the experimental data. The calculations confirmed that the equilibrium is very strongly influenced by the composition of the fuel blend. Moreover, the simulations provided more information on sequestering of alkali species.

Fluidized Bed Gasification as a Mature And Reliable Technology for the Production of Bio-Syngas and Applied in the Production of Liquid Transportation Fuels—A Review

“…This has also been observed and investigated by other researchers [36,[101][102][103][104]. From the above it can be concluded that silica-containing bed materials should be avoided when operation with "difficult" biomass fuels is intended.…”

“…These issues, depending on the type of the gasifier, are related to: 3. Operability of the gasifier with fuels containing large amounts of ash, especially if the fraction of alkali, chlorine and sulfur is high [36];…”

A thermodynamic study of the gaseous potassium chemistry in the convection sections of biomass fired boilers

“…The thermodynamic dew points of KOH(g), KCl(g), and K 2 CO 3 (g) for concentrations typically measured in real boilers [23,24] are presented in Fig. 6.…”

“…In this calculation, the N 2 (g) partial pressure was still kept at 70 mol% and the change in H 2 O(g) partial pressure effected the CO 2 (g) level accordingly. The potassium and chlorine concentrations were varied between 1 and 100 ppm representing typical levels of these components measured in real boilers at the inlet of the convection section [23,24]. Pressure was fixed in all calculations to 1 atm and the studied temperature range was 400-1000 8C, simulating the temperature range typical in the convection section of a biomass boiler.…”

“…Negative values indicate that all released alkali compounds form sulfates due to an excess of sulfur. With fuels where sodium concentration was not reported in the reference, zero concentration for sodium was assumed [23,28] Fuel K, ppm Na, ppm Cl, ppm S, ppm K, % in ash Si, % in ash forming alloys should provide better protection in these service conditions. Direct experimental evidence to support the presented corrosion mechanism in biomass-fired boilers is not available.…”