Abstract
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. 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. 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.
Highlights
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
If the facility utilizes the Combustion with High Pressure Steam Cycles (CHPSC) technology, the gross steam generation refers to gross amount of steam generated by the biomass-fired boiler
The steam contingency refers to the amount of steam that is reserved once all the demands of the sugar mill and ethanol plants are met, and is essentially an indication of the operating leeway the scenario offers in terms of meeting steam when fluctuations in the plant occur
Summary
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. South African sugar mills (from crushing to raw sugar production) typically have poor efficiency and the average steam demand is 0.58 tons per ton of sugarcane processed [3] (58% on cane) When such process designs are coupled with low efficiency biomass-to-energy conversion systems, no surplus bagasse is generated by the sugar mill and no export of electricity occurs [4,5]. The low efficiency biomass-to-energy systems in older cane milling operations utilized combustion systems that had raised steam to pressures of between 15 and 22 bar [5,9] Such systems provided a low cost means of disposing of bagasse [1,9] at a time when exporting electricity was not economically interesting. A capital estimate based on a matured estimate could be significantly lower than the pioneer estimate [18]
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