Co-gasification of black liquor and pyrolysis oil: Fuel conversion and activity of alkali compounds

Sammanfattning: Gasification using entrained flow reactors generates syngas that can be upgraded tochemicals with little gas cleaning. Black liquor (BL) is a by-product from pulping industrythat consists of residual wood constituents and spent pulping chemicals. Currently, it iscombusted to recycle the pulping chemicals and at the same time generate steam and power.Alternatively, BL is one of the most attractive fuels for entrained flow gasification due to thecatalytic activity of alkali compounds inherent in BL, possibility for pressurized feeding andthe shared logistics with the pulping plant. However, the high content of ash in BL is also anenergy penalty. Therefore the efficiency of BL gasification can be improved by co-gasifyingit with more energy rich fuels.The current work investigates the gasification characteristics of BL and pyrolysis oil(PO) blends by means of laboratory experiments. Experiments with varying BL/PO blendingratios were conducted using three different devices. An isothermal thermogravimetric reactorwas used to measure the reactivity of char under varying temperature and gas compositions. Asingle particle reactor was used to investigate the conversion of single droplets when exposedto high temperature reactive gas flow using lean, stoichiometric and rich CH4-air flames.Finally, a drop tube furnace was used to study the effect of temperature, gas composition andparticle size on gas, tar, and gasification residues at high temperature (800-1400 °C).Char reactivity of mixture samples was more than 30 times that of PO and comparableto that of pure BL, thereby indicating that catalytic activity was still very high after theaddition of PO. High temperatures enhanced alkali release in the gas phase; however, theconcentration of alkali left in the particles remained high at any temperature and for anymixing ratio. Additionally the blends showed better carbon conversion than pure BL. Theconversion rate of large particles (500-630μm) was controlled by mass diffusion and completecarbon conversion was never reached even at T =1400 °C. In comparison with pine-wood thatwas used as a reference, BL-based samples showed much lower tar concentrations in thesyngas. The difference was attributed to alkali elements. Remarkably, the addition of PO toBL further promoted tar reforming in the presence of CO2. The addition of PO alsosignificantly increased the yields of CH4 and CO.