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.

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Background: The economics of producing only electricity from residues, which comprise of surplus bagasse and 50% post-harvest residues, at an existing sugar mill in South Africa was compared to the coproduction of ethanol from the hemicelluloses and electricity from the remaining solid fractions. Six different energy schemes were evaluated. They include: (1) exclusive electricity generation by combustion with high pressure steam cycles (CHPSC-EE), (2) biomass integrated gasification with combined cycles (BIGCC-EE), (3) coproduction of ethanol (using conventional distillation (CD)) and electricity (using BIGCC), (4) coproduction of ethanol (using CD) and electricity (using CHPSC), (5) coproduction of ethanol (using vacuum distillation (VD)) and electricity (using BIGCC), and (6) coproduction of ethanol (using VD) and electricity (using CHPSC). The pricing strategies in the economic analysis considered an upper and lower premium for electricity, on the standard price of the South African Energy Provider Eskom' of 31 and 103% respectively and ethanol prices were projected from two sets of historical prices.Results: From an energy balance perspective, ethanol coproduction with electricity was superior to electricity production alone. The VD/BIGCC combination had the highest process energy efficiency of 32.91% while the CHPSC-EE has the lowest energy efficiency of 15.44%. Regarding the economic comparison, it was seen that at the most conservative and optimistic pricing strategies, the ethanol production using VD/BIGCC had the highest internal rate of returns at 29.42 and 40.74% respectively. Conclusions:It was shown that bioethanol coproduction from the hemicellulose fractions of sugarcane residues, with electricity cogeneration from cellulose and lignin, is more efficient and economically viable than the exclusive electricity generation technologies considered, under the constraints in a South African context.

BACKGROUND Brewers spent grains (BSG) represents ≤85% of a brewery's solid waste and common disposal to landfill is increasingly more difficult. Yet BSG is a food‐grade by‐product with potential economic valorization that can significantly improve resource efficiency and reduction in carbon emissions. This study investigated valorization of BSG through the application of novel high solids hydrothermal processing technology in a small‐scale biorefinery, annexed to a brewery. It focused on three scenarios for the production of: (A) the sugar replacement xylitol; (B) prebiotic xylo‐oligosaccharide (XOS); and (C) co‐production of xylitol and XOS. Economic assessment was conducted by comparing the capital and operating expenditure from process simulations created in Aspen Plus®. The process models developed were supplemented with experimental data to improve accuracy. RESULTS Internal rate of return (IRR) values obtained were greater than the hurdle rate of 9.7% for all scenarios when considering a conservative market price for xylitol and XOS as US$4500 t–1, yet dedicated production of XOS was economically more favourable with a minimum required selling price (MRSP) of US$2509 t–1 compared to US$4153 t–1 for xylitol. Additionally, the scenario for co‐production of xylitol and XOS achieved the lowest MRSP of US$2182 t–1. By‐products significantly contributed to 32.7%, 14.2% and 27.5% of the revenue generated in scenarios A, B and C, respectively. CONCLUSION These results provide a good platform from which to develop the cost‐effective commercial production of XOS and xylitol from BSG. © 2021 Society of Chemical Industry

Six conceptual process scenarios for the production of biobutanol from lignocellulosic biomass through acetone‐butanol‐ethanol (ABE) fermentation, using reported data on process performances, were developed with ASPEN Plus® V8.2 software. The six scenarios covered three fermentation strategies, i.e. batch separate hydrolysis and fermentation (SHF), continuous SHF, and batch simultaneous saccharification and fermentation (SSF) integrated with gas stripping (GS). The two downstream processing options considered were double‐effect distillation (DD) and liquid‐liquid extraction and distillation (LLE&D). It was found that the SSF‐GS/DD scenario was the most energy efficient with a liquid fuel efficiency of 24% and an overall efficiency of 31%. This was also the scenario with the best economic outcome, with an internal rate of return (IRR) of 15% and net present value (NPV) of US$387 million. The SSF‐GS/DD scenario was compared to a similar molasses process, based on the product flow rates, and it was found that the molasses process was more energy efficient with a gross energy value (GEV) of 23 MJ kg1 butanol compared to −117 MJ kg1 butanol for the lignocellulosic process. In addition, the molasses‐based process was more profitable with an IRR of 36% compared to 21%. However, the energy requirements for the molasses process were supplied from fossil fuels, whereas for the lignocellulose processes a portion of the feedstock was diverted to provide process energy. Improved environmental performance is therefore associated with the lignocellulosic process. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd

This study explored the potential of triticale feedstock for biofuel biorefining scenarios that included (i) producing ethanol, (ii) biochemically producing acetone–butanol–ethanol (ABE), and annexing catalytic processes to ethanol production for (iii) butanol and (iv) hydrocarbons (liquefied petroleum gas, gasoline, jet fuel, diesel). Based on simulations in Aspen Plus, the net greenhouse gas reduction (GHG) and minimum fuel energy price (MFEP) were determined. The maximum contracting prices of triticale (MCP) were then determined and benchmarked against historic animal feed prices to ascertain market competition. Thus, producing ethanol had the lowest MFEP of 18.3 US$/GJ and highest GHG reduction of 81%, while ABE had the highest MFEP of 27.3 GJ/kg and lowest GHG reduction of 63%; therefore, catalytically producing butanol was preferable (MFEP = 22 US$/GJ). If a CAPEX rebate of 30% is applied to the ethanol and butanol scenarios, the chances that existing animal feed prices outcompeted the MCPs were 50–98%. For the hydrocarbon fuels, the MFEPs were 60% higher than the fossil equivalent, though the GHG reduction was 75%. If the jet fuel product received a carbon tax rebate, the chances of the animal feed market outcompeting the MCP were only 22%. Thus, the need for incentivizing mechanisms for biofuels was further emphasized.

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