Gasification reactivity and ash behaviour of biomass fuels

Measuring data required for describing the reactivity and ash sintering behaviour of biomass-based solid fuels for atmospheric and pressurised gasification are discussed. The measurements were carried out in a pressurisable thermobalance and bench-scale fluidised bed reactors. The main variables were temperature and partial pressures of H₂O and H₂, and CO₂ and CO. The reaction rates were determined in binary gas mixtures H₂O-H₂, and CO₂-CO as well as in product gas mixtures H₂O-H₂-CO₂-CO. The total pressure range was 1-30 bar and the temperature range 650-950 °C. For wood, kinetic parameters were determined using the Langmuir-Hinshelwood kinetics, which takes into account the effect of the product gases on the reaction rate. The results indicated that the H₂O-H₂ system was well in conformity with this kinetics, while the reaction rates measured for the CO₂-CO system required the description of the catalytic effects of the ash-forming material. In the gasification of wood as well as of other biomasses, the behaviour of catalytically active ash components is more complex during char gasification. Experimental set-ups used for gasification reactivity measurements were compared in Aabo Akademi University and in VTT. The set-ups were pressurised thermogravimetric reactor (PTG), pressurised grid heater (PGH), pressurised bench-scale fluidised bed reactor (PFB), and pressurised entrained flow reactor (PEF). In addition, the effect of stabilising heat treatment was tested. The samples used in the study were wood, black liquor, peat, and coal. The gasification reactivity measurements were carried out in the temperature range of 700-950 °C, and in the pressure range of 1-10 bar. The gasification agent was either CO₂ in N₂ or CO-CO₂ mixture in N₂. In the pyrolysis carried out in different devices, mainly the same pressure and temperature were used as in the char gasification. According to the results, the heat treatment seemed to have an effect on char gasification when the gasification rate was low and pressure was high. The gasification rates of the peat chars obtained from PGH and PTG were quite similar. The peat chars produced in PEF had a surprisingly low gasification rate despite the high heating rate used as compared with that in PTG. No difference was observed when the black liquor chars produced in PGH at AaA and VTT were gasified at 700 °C and 800 °C in PTG. No distinct difference could be seen when coal was gasified in PFG and PTG, not even when CO was used in the gas. For wood, however, a clear difference in reactivity between PTG and PFB was observed, which was, however, independent of temperature. Ash sintering behaviour in biomass gasification was studied in the thermobalance and in the atmospheric fluidised bed reactor. Various biomasses such as straws and woody biomasses were tested. The effect of gasification variables like temperature, pressure, and gasification agent as well as reactivity on ash behaviour was characterised in the thermobalance. Test conditions were as follows: temperature range 700-850 °C, pressure 1-30 bar, gasification agent H₂O, CO₂, and their mixture. The appearance of the ash residues after the gasification was examined by microscopy. Ash behaviour in fluidised bed gasification was studied in a bench-scale atmospheric fluidised bed reactor, to find the conditions under which difficult biomass could be gasified. Bed agglomeration and deposit formation in the freeboard were followed by collecting samples after one-day test runs. The test conditions were relevant to those in the fluidised bed air-gasification process. The tests were carried out with the most difficult biomass types: wheat straw, willow, and spruce. In the test series, the main gasification variables were temperature (700-850 °C) and bed material (alumina, limestone, and dolomite). The thermobalance measurements gave results comparable to the ash behaviour in the fluidised bed reactor. The strongest ash sintering was observed for wheat straw both in the thermobalance and in the fluidised bed reactor. For willow, spruce bark, and alfalfa straw, the thermobalance tests showed that ash sintering and melting were much stronger under increased steam pressure than under 1 bar steam pressure. These ashes differed from those of wheat straw in that they contained much less silicon. Wheat straw ash was not affected by pressure. The fluidised bed tests carried out for wheat straw showed that the bed agglomeration was influenced by the bed material. It was stronger in the alumina bed than in the limestone or dolomite bed. The agglomeration in limestone and dolomite beds was observed to be dependent on the calcination of these materials. After each test run, different amounts of ash deposit were collected from the freeboard. They consisted of fused ash particles stuck slightly to the reactor wall.