Overall Work-plan

The overall aims of this project were to develop cost effective processes for second generation biofuel production in South Africa, through integration into the Sugar Cane Crushing and Pulp & Paper Industries (i.e. SSCI and P&PI). In South Africa, the SSCI is represented by the RSI and the PPI by Sulphite Processes. This would be achieved by simulation of industrial processes, flow sheet analysis, economic assessment and determination of environmental impacts. The work-plan, which basically amounted to flow-sheet development, was designed to encompass the technical aspects to ensure a self-sustaining process; economic aspects to ensure that the production of biofuels is viable from an investment point of view; and environmental aspects to ensure that the anticipated environmental benefits of second generation biofuels are maintained. These objectives were designed to address shortcomings identified in scientific literature on the topic of integration of second generation biofuels production into these industrial facilities.

1.1 GENERAL METHODOLOGY AND ENVISAGEMENT

The underlying methodology to be employed in the flow-sheet analysis is process simulation in Aspen Plus®[1], based on experimental data and design elements found in verified literature. The results will be used to characterise the technical performances indicators, such as fuel output, electricity output, utility demands and availability. Results from the process simulations will be used to determine the capital investment costs and key economic indicators (KEI), such as the Internal Rate of Return (IRR) and minimum fuel selling price (MFSP), both of which are measures of the attractiveness of biofuels production from an investment (returns on invested capital) perspective. Economic modelling will be carried out with Risk Based Economic Assessments, which are based on

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Monte Carlo Methods in order to establish the risk associated with the KEIs [2–4] and there viabilities to private investors. Where processes involve a significant use of energy from fossil sources, results from process simulation will be used to calculate the greenhouse gas reductions associated with the life cycle of the fuel product, using documented methods[5,6] with supporting databases and software (GREET[7] and SimaPro[8]). Improvements to flow-sheets are incorporated into the simulations, to ensure that the desired environmental benefits of second generation biofuels production are preserved.

In this dissertation, it is envisaged that flow-sheet analysis be used to establish the relationship between technological processing options used in various processing stages, in terms of the energy- efficiency and economic characteristics of the flow-sheet. Furthermore, the necessity of process intensification measures such as Pinch Point heat integration[9–11] in the context of integrating biofuel production into an energy intensive industrial process will be determined. It is also planned to combine flow-sheet analysis with statistical methods to arrive at process conditions that are optimised economically and technically, and also subjected to the constraints that the process imposes. Thus, from such analysis, deductions can be made on the cause of viability (or un-viability) of individual process technologies and/or the combinations there-of.

1.2 SECOND GENERATION BIOFUEL INTEGRATED INTO THE RAW SUGAR MILLS

Integration of second generation biofuels into existing sugar mills (i.e. the RSI) in South Africa will require modernisation of the processing equipment and unit operations with more energy efficient technologies, so that steam demands of the mill are reduced to 0.40 tons of steam per ton of cane processed [12]. The capital costs of such modernisation should therefore be reflected in the costs of integrating second generation biofuels or advanced electricity generation into a sugar mill. Furthermore, the older sugar mills in South Africa have traditionally used low pressure boilers that

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are inefficient at steam and electricity generation, and thus modernisation for the purpose of exporting bio-energetic products should include the replacement of the existing low pressure boiler with higher pressure boilers[12–14]. The combination of the biofuel production process with the new boilers and gas/steam turbines for electricity generation, would therefore supply all of the process energy requirements (steam, electricity) of the modernised sugarcane mill[15,16]. Optimisation of energy efficiency in sugarcane processing will maximise the conversion of lignocellulose to biofuels, while energy optimisation of the combined sugar-mill and energy production processes will maximise the amounts of biofuels and/or surplus electricity for export to the grid, produced from these integrated facilities. Thus, several aspects of flow-sheet development and technology selection have to be addressed in the development of integrated scenarios, with co- production of biofuels and/or electricity by modernised, existing sugar mills.

Integration of second generation bio-ethanol production into sugar-mills will explore an alternative to the traditional conversion of the cellulose components to bioethanol, which includes the costly process of enzymatic hydrolysis[16,17]. Instead, process flow-sheets for the conversion of only the hemicellulose sugars[18] that are obtained through conventional pre-treatment’s will be investigated. This avoids the need for enzymatic hydrolysis of cellulose, while also providing a combustible cellu-lignin residue fuel that is suitable for co-generation of electricity from sugarcane lignocelluloses. Flow-sheet and technological options for a combined process, producing both bio- ethanol and surplus electricity for export to the grid, will thus be investigated. The economic attractiveness of such a combined process would have to show an improvement compared to a process where the entire lignocellulose feedstock is converted to electricity, which is the conventional approach for electricity co-generation in sugar mills[16,17]. As various process technologies and process intensification measures are available for the combined ethanol-electricity process, systematic flow-sheet analysis of possible technology-combinations is required, to identify preferred and robust process options that fulfil the requirements of economic and environmental

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viability, as described in Chapter 4. Heat integration by Pinch Point Analysis is a key method in process intensification and ensuring that high conversion/energy efficiencies are achieved in the process integration scenarios.

An alternative method of second generation biofuels production in the sugar industry is the conversion of lignocellulose to methanol or FT syncrude, through gasification-synthesis technologies. Gasification-synthesis processes will therefore be compared to fermentation-based processes, in terms of energy yields, economics and environmental impacts, as described in Chapter 6. Given that the amount of bagasse and trash available for processing equates to 0.2 tons per ton of sugarcane processed[16,19], then an integrated FT synthesis plant could hypothetically provide the steam requirements of a modern sugarcane mill that is expected to operate with a steam demand of 0.4 tons of steam per ton of cane, since the total steam produced would equate to about 0.6 tons per ton of cane. A steam balance for methanol synthesis however, shows that the exhaust steam available from the turbines is about 0.7 ton per ton of biomass processed[20], without considering the waste heats that was dispensed to the environment. Thus, it would have to be ascertained whether integrating methanol synthesis into a sugar mill is capable of servicing the process energy requirements of the sugar-mill, which is an essential requirement for integrated production of biofuels. This will be addressed through adequate heat integration between the sugar mill and waste heats of the methanol synthesis process. Furthermore, as the major costs of a gasification- synthesis process lies in the production of clean, compressed syngas, it is envisaged that the sub- process for producing the conditioned syngas be optimised to meet the requirements for synthesis processes in an efficient and costs effective manner, as part of technology selection options to be investigated.

51 1.3 SECOND GENERATION BIOFUEL INTEGRATED INTO THE P&PI

In this dissertation, focus on the potential of SSL from an MgO-based sulphite pulping process as a feedstock for second generation bio-ethanol production will be explored, as this SSL contains mostly pentose sugars originating from hardwood-pulping that is implemented on a large-scale in South Africa. The conversion of pentose sugars in SSL to ethanol by robust strains has been efficiently done at high concentrations of dissolved solids (20-30%) on a laboratory scale[21]. However, the conversion of SSL to ethanol as an export product, will result in a deficit in the energy-balance of the sulphite pulping process, as SSL is presently burnt to provide some of the required process steam [22,23]. Flow-sheet analysis for ethanol production from SSL would thus also explore alternative sources of renewable fuel, to fulfil the process energy requirements of the sulphite mill. As an example, the Saiccor sulphite pulping process presently augments the supply of process energy from biomass sources with the combustion of coal, which is associated with significant greenhouse gas emissions. The deficit in process energy supply, caused by ethanol production from SSL, would thus be resolved either by (i) increasing the reliance of coal or (ii) installation of a bark boiler to take advantage of the bark that is currently disposed in a landfill, and/or (iii) incorporation of a biodigester to produce biogas as supplementary fuel from the organics in the liquid effluents that could supplement the recovery boiler. Flow-sheeting, economic and environmental assessment would identify the most appropriate process solutions for integration of ethanol production from SSL into a sulphite mill, such as the Saiccor facility, as described in Chapter 5.

The combustion of MgO-based SSL is presently done to both recover process energy and pulping chemicals. A possible alternative is the gasification of SSL, but gasification technology for MgO- based SSL has not shown to be technically viable yet, and thus, gasification at MgO-based Sulphite Mills would only focus on bark residues. Such a gasification process can be considered both for the provision of process energy requirements of the sulphite pulping process, and provision of syngas for

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production of synthetic fuels. An expansion to the integrated facility, producing ethanol from SSL, with a gasification-synthesis process for bark will thus be investigated through flow-sheet development and economic/environmental assessments. Such a facility would thus produce two biofuels (ethanol and synthetic fuel) using the residues from the existing pulping process, as described in Chapter 7.