Ethanol from Sugarcane Lignocellulosic Residues - Opportunities for Process Improvement and Production Cost Reduction

Abstract

Bioethanol from sugarcane is a sustainable alternative to fossil fuels, and the increasing demand for fuel ethanol has prompted studies on the use of the lignocellulosic residues of sugarcane, namely bagasse and leaves, as new feedstock. This thesis describes various process designs and the economic feasibility of producing second generation (2G) ethanol from bagasse and leaves via the enzymatic route in an integrated sugarcane biorefinery, where first-generation (1G) ethanol is produced from sugarcane sugar. Techno-economic analysis have been performed for the Brazilian context to evaluate the influence of several process designs and the main production factors on the 2G ethanol process, in terms of energy efficiency, 2G ethanol production cost(2G MESP) and profitability.

The study of process design focused on ways to integrate the 1G and 2G ethanol processes, and on configurations to hydrolyse and ferment bagasse. The existing 1G ethanol process and the proposed 2G ethanol process were combined in a single plant by integration of thermal and material streams. The resulting synergies could improve the use of feedstock and reduce the 2G ethanol production cost. Simultaneous saccharification and fermentation (SSF)and time-separated hydrolysis and fermentation (tSHF) were the configurations investigated experimentally for the production of 2G ethanol from bagasse. In an attempt to increase the ethanol concentration before distillation, the fermented liquid of tSHF was also recirculated back to tSHF. The tSHF configurations showed a lower 2G MESP than SSF.

Process options were also investigated considering the pentose use and the addition of leaves to the 1G+2G process. Pentoses can either be fermented to ethanol or anaerobically digested to produce electricity from biogas combustion, and in the former case the highest potential reduction in 2G MESP could be achieved. The addition of leaves could improve the overall profitability of the 1G+2G process. Residence time and water-insoluble solids (WIS) loading in hydrolysis were the main process conditions considered together with costing factors, such as enzyme, sugarcane and leaves costs. The selling price of electricity and ethanol were found to have relevant impacts on the profitability of the 1G+2G ethanol process. Among the numerous operating conditions studied for the 2G ethanol process, the cases showing the best trade-off between technical and economic feasibility were also tested experimentally on laboratory scale obtaining promising results. In fact, it was possible to achieve high concentrations of 2G ethanol (47 g/L) in short time (60 hours), overcoming the mixing problems by

feeding repeatedly the pretreated bagasse up to 20% WIS.

Abstract (Swedish)

Popular Abstract in English

Modern society is still largely based on fossil resources. However, there are growing concerns about the security and cost of these resources, as well as climate change resulting from the combustion of fossil fuels. Biofuels represent renewable fuels with the potential to mitigate the adverse effects of fossil fuels, providing a more sustainable alternative.

The transport sector in Brazil currently relies mainly on first generation (1G) fuel ethanol produced from the fermentation of the sugar fraction of sugarcane, and is considered a successful example of biofuel penetration and replacement of fossil fuels. Theoretically, almost double the amount of ethanol could be produced if the residues from the sugarcane industry, namely the bagasse and leaves, were also used as feedstock. Bagasse and leaves constitute the lignocellulosic fraction of sugarcane, and the ethanol obtained from these materials is known as second generation (2G) ethanol or lignocellulosic ethanol. However, lignocellulose is made up of a complex matrix containing the three constituents, cellulose, hemicellulose and lignin, which are strongly bound to each other. For this reason, it is more difficult to break down lignocellulose to fermentable sugars, than sucrose or starch, and the production process is thus more complex and costly. High production costs are

the major drawback of the 2G ethanol production process, delaying the

deployment of commercial-scale facilities. Many production parameters and economic factors influence the final cost of 2G ethanol, and several technological options and trade-offs can be explored and analysed in order to improve the competitiveness of 2G ethanol.

The aim of the work presented in this thesis was to analyse the production of 2G ethanol from sugarcane bagasse and leaves in Brazil, and to identify opportunities for reducing the production cost by considering process designs and factors affecting the cost. Simulation of the ethanol production process was the major tool used, together with laboratory experiments for interesting cases.

The availability of bagasse at 1G ethanol production sites and local

availability of leaves makes it favourable to co-locate 2G ethanol plants with existing 1G ethanol plants. The two processes can be combined in a plant where process and energy streams are integrated. Producing 1G and 2G ethanol in an integrated plant can provide a reduction in the cost of 2G ethanol, and more efficient energy use. Such an integrated ethanol production plant can be regarded as an ethanol-oriented biorefinery, where electricity is also produced from the combustion of residual streams, namely biogas and unreacted lignocellulose. Maximizing 2G ethanol production was found to be

more profitable than selling electricity. However, the additional cost of achieving a high conversion of bagasse and leaves to 2G ethanol depends on the strategies employed as well as the degree of conversion. A range of options with different technical feasibility and the most relevant production factors were studied. The process configuration can be designed to tackle specific issues, but experimental data are necessary to prove the feasibility of the concept both technically and economically. Experiments were performed to collect data for a few process configurations, and simulations showed that in some cases the production cost involved in improving the ethanol production was too high to be commercially feasible. External economic factors, such as the selling prices of electricity and ethanol,and the cost of feedstock and biocatalyst also appeared to have a considerable effect on the profitability of 2G ethanol.

In none of the cases investigated the combination of technical options and economic factors was found that could reduce the production cost of 2G ethanol from bagasse and leaves to that of 1G ethanol without subsidies. However, 2G ethanol could be cheaper than 1G ethanol if selling prices were lower for electricity and higher for ethanol. Moreover, 2G ethanol could also contribute to achieve better profitability than producing 1G ethanol and electricity. Finally, there is considerable scope for further reductions in cost

that could improve the competitiveness of 2G ethanol on the fuel market.

• author: Macrelli, Stefano
• supervisor: Ola Wallberg; Mats Galbe
• opponent: Dr Bonomi, Antonio, Brazilian Bioethanol Science and Technology Laboratory (CTBE), Campinas, SP, Brazil
• organization: Division of Chemical Engineering
• publishing date: 2014
• type: Thesis
• publication status: published
• subject: Chemical Engineering
• pages: 88 pages
• defense location: Lecture hall B at the Center of Chemistry and Chemical Engineering, Getingevägen 60, Lund University Faculty of Engineering, Lund
• defense date: 2014-10-03 10:30:00
• ISBN: 978-91-7623-087-9 (print)
• ISBN: 978-91-7623-088-6 (pdf)
• language: English
• LU publication?: yes
• id: f4ee80a4-2f55-433a-b396-7da6817513bb (old id 4628701)
• date added to LUP: 2016-04-04 10:23:51
• date last changed: 2023-04-18 18:41:09