Performance of a Pilot-Scale Entrained-Flow Black Liquor Gasifier

Pilot-scale entrained flow gasification experiments were carried out at the 3 MWₜₕ LTU Green Fuels black liquor gasification (BLG) plant, using ∼140 tons of Kraft black liquor (BL) with a dry solids content of ∼72.5%. Comprehensive mass and energy balances were performed to quantify process performance under varying pressure, load, and oxygen/fuel ratio. Carbon conversion efficiency of the BLG process was 98.3%–99.2% and did not vary systematically in response to process changes. The unconverted carbon is almost exclusively present as dissolved organic carbon in the green liquor (GL) stream. GL is an aqueous solution of sodium carbonate and sodium sulfide used to recover the inorganic pulping chemicals present in BL for reuse in the pulp mill. A small fraction of syngas CO is converted to formate ions dissolved in GL through reaction with hydroxide ions. Unconverted carbon present in GL solids is insignificant. Syngas produced is subsequently upgraded to methanol and dimethyl ether (DME) in an integrated fuel synthesis facility. Concentration of H₂ in syngas is not significantly affected by operating point changes in the domain investigated, while CO and CO₂ concentrations are. Syngas hydrocarbon concentration values are typically in the single-digit parts per million (ppm) with the exception of C₆H₆, which was present at 16–127 ppm. CH₄ is present at 0.5%–1.2%, with lower concentrations at higher temperatures, and shows good correlation with C₆H₆. A quantity of 24%–27% of BL sulfur ended up in the syngas as 1.5%–1.7% H₂S and 64–72 ppm COS. Cold gas efficiencies (CGEs) on a lower heating value (LHV) basis, when including syngas CH₄, were 52%–55% and decreased at higher temperature. CGEs on an LHV basis, when considering only H₂ and CO with a sulfur-free BL heating value relevant for catalytic syngas upgrading, were 58%–60% and showed the opposite temperature dependence. Good mass and energy balance closures show the figures presented to be reliable. The results obtained from this study demonstrate process stability at varying operating conditions and can be further used for techno-economic analysis and design purposes.