Techno-economic comparison of ethanol and electricity coproduction schemes from sugarcane residues at existing sugar mills in Southern Africa

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 Sugarcane processing industries in Southern Africa generate bagasse at a yield of 0.30 tons per ton of cane processed [1]. In most sugar mills in Southern Africa, the generated bagasse is mostly burnt to provide heat and electricity for the sugar milling operations [1,2]. 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, then no surplus bagasse is generated by the sugar mill and therefore no export of electricity occurs [4,5]. If efficient * Correspondence: [email protected] Department of Process Engineering, University of Stellenbosch, Cnr Banghoek Road & Joubert Street, Stellenbosch 7600, South Africa © 2014 Petersen et al.; licensee BioMed Centra Commons Attribution License (http://creativec reproduction in any medium, provided the or Dedication waiver (http://creativecommons.or unless otherwise stated. sugar mills that have steam demands below 40% [5,6] are coupled with efficient systems that convert biomass to energy [6], then excess bagasse becomes available. This excess, if combined with other post-harvest residues like sugarcane trash, could provide the feedstock for the production of bio-energetic products in an integrated facility. The costs associated with the utilization of such residues would include the cost of collection and transport, and the investment costs required to upgrade the energy efficiency of existing sugar mills to enable the liberation of surplus bagasse. These costs are significantly lower than the purchasing costs of biomass [7] that hinders the economic viability of ‘stand-alone’ facilities for biomass conversion to energy [8]. The low efficiency biomass-to-energy systems in older cane milling operations utilized combustion systems that l Ltd. This is an Open Access article distributed under the terms of the Creative ommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and iginal work is properly credited. The Creative Commons Public Domain g/publicdomain/zero/1.0/) applies to the data made available in this article, Petersen et al. Biotechnology for Biofuels 2014, 7:105 Page 2 of 19 http://www.biotechnologyforbiofuels.com/content/7/1/105 had raised steam to pressures of between 15 and 22 bar [5,9]. Such systems also provided a low cost means of disposing of bagasse [1,9] at a time when exporting electricity was not economically interesting. For that means, combustion with high pressure steam cycles allowed for greater turbine efficiency in the conversion of steam to electricity and thus, pressures of 82 to 85 bar [1,10] would have typically been preferred. At a pressure of 60 bar, it has been shown that a net electricity export of 72 kW (per ton of cane processed per hour) was possible for an efficient sugarcane mill, where a steam demand of 0.4 tons per ton of cane was required [5]. This amount of export electricity could have been increased substantially if the harvesting residues (trash) was also considered [5,7,11]. The electrical efficiencies resulting from biomass power plants utilizing combustion and high pressure steam cycles are reported to be between 23 and 26% on an HHV (Higher Heating Value) basis [12,13], while efficiencies reported for Biomass Integrated Gasification and Combined Cycles system (BIGCC) were at 34 to 40% [14]. The implementation of BIGCC in industry has been limited due to the reportedly high capital investment that is required [12,13,15]. The capital estimates of BIGCC systems in previous techno-economic assessments [12,13,16] however, were based on the estimates in a period where BIGCC technology was still new (1990 to the early 2000s) [17], and thus, capital estimates based on the vendor quotes in this period would have reflected the pioneer plant costs. A capital estimate based on a matured estimate could be significantly lower than the pioneer estimate [18]. As an alternative to the conversion of all of the available lignocellulose residues to electricity, a fraction of the bagasse and post-harvest residues could be used to produce ethanol, with co-generation of electricity. The hemicellulose, which makes up about 20 to 35% [19] of the biomass matrix, can be solubilized by steam explosion or dilute acid hydrolysis and converted to ethanol, while the remaining cellulose-lignin fractions are converted to heat and power [20,21]. This scenario for the coproduction of ethanol and electricity from lignocellulose has been proposed for the South African industry [20], but a detailed process flow sheet and technoeconomic investigation of such for existing sugar mills is not available. Of particular interest would be the techno-economic comparison of coproduction of ethanol and electricity against a scenario where the residues are used exclusively for electricity generation. Previous studies have compared electricity generation alone with the complete lignocellulose conversion to ethanol (hemicellulose and cellulose) as options for integration with sugar mills [22] and autonomous distilleries [7,11]. The ethanol generation scheme in this study builds on the concept of ‘value prior to combustion’ that has previously been evaluated as a green-field (stand-alone) scenario [21]. There has been a considerable success in developing microbial strains that efficiently converts pentose-rich hydrolysates to ethanol [23], which is the key area of importance if the proposed technology is to be feasible. Using adapted strains of a the native pentose fermenting yeast Pichia stipitis, Kurian et al. [24] converted 82.5% of the hemicellulose sugars in a hydrolysate derived from sorghum bagasse that contained 92 g/l of dissolved sugars, while Nigam [25] converted 80.0% in an acid hydrolysate from wheat straw, containing 80 g/l sugars. The development of robust recombinant strains, such as the Saccharomyces cerevisiae TMB400, have resulted in pentose conversions in excess of 85% in toxic environments in simultaneous saccharification and fermentation experiments [26]. More recently, the National Renewable Energy Laboratory (NREL) achieved an ethanol yield of 92% on hemicellulose sugars in a toxic enzymatic hydrolysate that contained a total of about 150 g/l of sugars, using the Zymomonas mobilis strain that was genetically engineered by Du Pont [27]. Thus, fermentation technology for converting pentose sugars in hydrolysates to ethanol has been successfully demonstrated on a laboratory scale. The present study provides a detailed techno-economic comparison of scenarios that entail ethanol coproduction with export electricity, produced either by combustion or BIGCC systems, against those that produce only export electricity using the same systems. For either scenario, the upgrading costs of the existing sugar mill to achieve an energy efficiency of 0.40 ton of steam per ton of cane, is included in the capital investments considered in the economic analysis. The development of process models for the ethanol coproduction scenario will be based on established flow sheets and process performances for lignocellulosic ethanol [28,29], and will also consider various processing options to ensure the most energy efficient and economical flow sheet. The projects are assumed to be in Kwa-Zulu Natal where the sugar cane crushing plants are concentrated. All South African legislations would apply. Energy efficiency for all of the scenarios will be maximized through pinch point analysis (PPA) for the heat integration of the processing streams [30-32]. This approach will ensure that the energy utilities for ethanol production are kept to a minimum [29,30], consequently maximizing the export electricity while still providing the energy requirements of the (energy efficient) mill [33]. From the process simulations (massand energy-balances) for the various scenarios economic evaluations, incorporating capital and operational costs as well as sales prices, will be performed from an economic risk perspective [34-37]. These methods are based on Monte Carlo simulations that are super-imposed on standard methods for process economic methods, in order to account for the risks associated with the fluctuations in economic variables, thereby Petersen et al. Biotechnology for Biofuels 2014, 7:105 Page 3 of 19 http://www.biotechnologyforbiofuels.com/content/7/1/105 providing not only the estimates for investment returns, but also the probability of achieving economic success. Results and discussion Technical evaluation Six scenarios for the production of electricity from sugarcane residues, either as the only energy product or with coproduction of ethanol from hemicellulose, were evaluated through process modelling to estimate process energy efficiency and economics. The results of the energy characteristics for the various process alternatives that have been optimized by pinch point analysis are presented in Table 1. Furthermore, the amount of steam generated by the heat and power facility in each scenario, whether this facility forms an exclusive electricity scenario or an energy generation section of an ethanol coproduction scenario, is presented. If the facility utilizes the Combustion with High Pressure Steam Cycles (CHPSC) technology, then the gross steam generation refers to gross amount of steam generated by the biomass-fired boiler. If the heat and power plant utilizes the BIGCC technology then the gross steam generation refers to the steam generated by the heat recovery steam generator (HRSG) that recovers heat from the gas turbine’s exhaust. The steam contingency refers to the amount of steam that is reserved once Table 1 Bio-energetic product yields, utility demands and ene CHPSC – Ethanol Production with Conventional Distillation w CHPSC; CD/BIGCC – Ethanol production with Conventional Di residues using BIGCC; VD/CHPSC – Ethanol Production with V pretreatment residues using CHPSC; VD/BIGCC – Ethanol pro from pretreatment residues using BIGCC; CHPSC-EE – Exclusiv CHPSC-EE (Dryer) – Exclusive electricity generation from resid Exclusive electricity generation from residues using BIGCC)