Recovery Boiler or Fuel Route? Consequential Break-Even Conditions for the Use of Kraft Prehydrolysis Liquor

Abstract Kraft mills that produce dissolving-grade pulp burn their prehydrolysis liquor in the recovery boiler. We quantify when converting this liquor to hydrocarbon fuels, at constant pulp output, lowers the climate impact of the mill. A consequential ex-ante life cycle assessment compares combustion with two routes scaled from the laboratory to industrial operation: one-step hydrodeoxygenation of furfural to hydrocarbons (HC route) and five-step synthesis of aviation-range hydrocarbons (AF route). The difference between each route and combustion is a linear function of the carbon intensities of the marginal heat and electricity, the supplies that respond to a change in demand, so the break-even conditions form maps on which any mill can be located. In Sweden, with steam-reformed hydrogen, the HC route lowers climate impact by 0.0537 kgCO2-eq per kg of pulp, but only where the marginal heat emits less than 0.0198 kgCO2-eq per MJ, as biomass heat does and natural gas does not. The AF route has a higher climate impact than combustion at any marginal intensity, by 0.333 kgCO2-eq per kg of pulp in its demonstrated sequence and, at best, by less than the uncertainty of the background data. In twelve of fifteen other impact categories, combustion has the lower impact.