Abstract
BackgroundFlow sheet options for integrating ethanol production from spent sulfite liquor (SSL) into the acid-based sulfite pulping process at the Sappi Saiccor mill (Umkomaas, South Africa) were investigated, including options for generation of thermal and electrical energy from onsite bio-wastes, such as bark. Processes were simulated with Aspen Plus® for mass- and energy-balances, followed by an estimation of the economic viability and environmental impacts. Various concentration levels of the total dissolved solids in magnesium oxide-based SSL, which currently fuels a recovery boiler, prior to fermentation was considered, together with return of the fermentation residues (distillation bottoms) to the recovery boiler after ethanol separation. The generation of renewable thermal and electrical energy from onsite bio-wastes were also included in the energy balance of the combined pulping-ethanol process, in order to partially replace coal consumption. The bio-energy supplementations included the combustion of bark for heat and electricity generation and the bio-digestion of the calcium oxide SSL to produce methane as additional energy source.ResultsEthanol production from SSL at the highest substrate concentration was the most economically feasible when coal was used for process energy. However this solution did not provide any savings in greenhouse gas (GHG) emissions for the concentration-fermentation-distillation process. Maximizing the use of renewable energy sources to partially replace coal consumption yielded a satisfactory economic performance, with a minimum ethanol selling price of 0.83 US$/l , and a drastic reduction in the overall greenhouse gas emissions for the entire facility.ConclusionHigh substrate concentrations and conventional distillation should be used when considering integrating ethanol production at sulfite pulping mills. Bio-wastes generated onsite should be utilized at their maximum potential for energy generation in order to maximize the GHG emissions reduction.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0169-8) contains supplementary material, which is available to authorized users.
Highlights
Ethanol production can be integrated with a paper and pulp facility, the energy balance needs to be optimized due to the energy-intensive nature of both processes [1,2,3]
These limitations imply that the spent sulfite liquor (SSL) fermentation products will typically contain 20 g/l of ethanol, which is significantly less than the minimum of 40 g/l required for energy- efficient distillation [10]
The present study investigated strategies for integrating ethanol production into an magnesium oxide (MgO) sulfite pulping plant, to achieve both the intended Net Greenhouse Gas Reduction (NGHGR) and economic viability, based on the flow rates of, and samples shipped from the Sappi Saiccor mill as industrial representatives
Summary
Ethanol production can be integrated with a paper and pulp facility, the energy balance needs to be optimized due to the energy-intensive nature of both processes [1,2,3]. Sugars represent 25 to 30% of the total dissolved solids (TDS), which implies that SSL sugars cannot be concentrated to more than 50 to 60 g/l At this point, the TDS of 30% will become inhibitory to fermentation due to osmotic stress and media viscosity [6,7,9], even for detoxified SSL. Flow sheet options for integrating ethanol production from spent sulfite liquor (SSL) into the acid-based sulfite pulping process at the Sappi Saiccor mill (Umkomaas, South Africa) were investigated, including options for generation of thermal and electrical energy from onsite bio-wastes, such as bark. The bio-energy supplementations included the combustion of bark for heat and electricity generation and the bio-digestion of the calcium oxide SSL to produce methane as additional energy source
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