Deposit formation and emissions from co-firing miscanthus with Daw Mill coal: Pilot plant experiments

Abstract

An investigation has been carried out into the deposits generated on cooled probes and the gas compositions produced from co-firing miscanthus mixed with Daw Mill coal. The biomass additions to the coal were at 0, 20, 40, 60, 80, 100wt.% in a 100kWth pilot-scale pulverised fuel (PF) combustion test rig operated with a feed rate of ∼7–15kg/h. Online monitoring of the combustion gas stream was carried out for CO2, O2, H2O, SO2, CO, NO, NO2, N2O, HCl and HF using a high resolution multi-component Fourier Transform Infra-Red (FTIR) gas analyser. The deposits were collected from the upstream, side and downstream surfaces of three air-cooled probes that were exposed in the flue gas path and operated at surface temperatures of 500, 600 and 700°C (to simulate heat exchanger tubes). The compositions of these deposits were determined using a range of analytical techniques including Environmental Scanning Electron Microscopy (ESEM) with Energy Dispersive X-ray (EDX) and X-ray Diffraction (XRD). The composition of the combustion gas streams and deposits changed as a function of the fuel mixtures used in the combustion process. The increasing miscanthus share in combination with the Daw Mill coal resulted in a reduction of SOx, NOx and HCl levels. The deposition on the upstream surfaces of the probes decreased with increasing biomass percentage, but increased on the downstream surfaces. The concentration of K and S in the deposits increased with increasing miscanthus share in the fuel up to 80wt.%, particularly on the coolest probe (∼500°C surface temperature). In addition, Cl was detected in the deposit on the two lower temperature probes only when 100% miscanthus was used in the combustion process. Thermodynamic modelling of the combustion/deposition process carried out in parallel also suggested formation of KCl only at the highest levels of biomass co-firing for miscanthus/Daw Mill coal, with K2SO4 being predicted at lower co-firing levels for these specific fuels.