Co-firing of Lignite with Peat and White Pine in a Pilot Scale Bubbling Fluidized Bed -- Air Emissions and Feedstock Reactivity

The increasing awareness of the environmental impact of fossil fuels (mainly coal) combustion, which leads to high levels of CO2, NO x, SO2, mercury and particulate emissions, has motivated research for potential alternatives such as switching from fossil fuels to biomass, or co-firing of both fuels. Co-firing proved to be a promising technology for large scale use of biomass for energy production, as it makes use of the extensive infrastructure associated with the existing coal-based power systems, and requires only relatively modest additional capital investment to achieve a significant CO2 reduction. The research objectives of the present work were to: (1) Investigate on combustion/cofiring lignite and woodwaste/peat in a 16.19 kW pilot-scale fluidized-bed combustor, and effects of fuel-blending ratios, excess air, particle size and moisture contents on CO2, CO, SO2, and NOx emissions in the combustion/co-combustion; and (2) Study the combustion reactivity of lignite, woodwaste, peat, and the blended fuels using thermogravimetric analysis (TGA).;The combustion/co-combustion behaviour and kinetics of lignite, peat and woodwaste (white pine sawdust) and their blends were investigated using non-isothermal thermogravimetric analysis (TGA) technique. The TGA experiments were performed for pure fuels and compared to blended fuels with respect to their performance in air over a temperature range of 25-700 °c and at a heating rate of 20°C/min. The overall kinetic de-volatilization-combustion reactions for these fuels and their blends were evaluated using the power law model. Using the differential thermal analysis (DTA) data and applying the least square multi-linear regression method, kinetic parameters for the overall devolatilization/combustion reactions including the apparent activation energy (Ea), reaction order ( n) and the pre-exponential (A) factor were calculated for each homogeneous fuel and the lignite-peat or lignite-pine sawdust blended fuels (50 wt%-50 wt%%). The wood waste and peat demonstrated a higher reactivity when compared to lignite. The activation energies for lignite, peat, and white pine were determined to be 124.10 kJ/mol, 83.95 kJ/mol, and 98.23 kJ/mol, respectively. Compared with the devolatilization/combustion of homogenous solid fuels, blending peat/white pine with lignite resulted in synergistic effects, enhancing the combustion reactivity of each component fuel.;The effects of particle size (pellets or crushed), the fuel blending ratios (0, 20%, 50%, 80% and 100% on a heat input basis), moisture content and excess air ratio on combustion efficiency and air emissions (CO2 , CO, SO2 and NOx) from combustion/co-combustion of woodwaste or peat and lignite were examined in a pilot-scale bubbling fluidized bed combustor. The results showed that properly controlling the co-firing parameters could achieve an increase in combustion efficiency and a reduction of air emissions. Compared to solid fuels in fine particles (< 4mm), fluidized-bed combustion of solid fuels in the pellet form could obtain higher dense-phase temperatures and a more uniform temperature profile along the fluidized-bed column and achieve a much higher efficiency (>94%), while yielding minimal effects on the emissions of SO2 and NOx. Co-firing of lignite and white pine pellets at an increasing blending ratio led to a proportional reduction in both SO2 and NOx emissions. Co-firing of peat and lignite led to an increase in SO2 emission, but co-combustion of peat (0-100%) and lignite resulted in a reduction in NOx emission. The presence of moisture in the fuels promotes the combustion of solid fuels by the steam gasification/reforming and gas-water shift reactions, leading to increases in combustion efficiency and CO emissions, and the combustion of fuels of a higher moisture content led to a decrease in SO2 emission, but an increase in NOx emission. To achieve higher combustion efficiency and lower air emissions for combustion/co-combustion in a fluidized bed combustor, a too high excess air ratio (>40%) should be avoided.