Energy Optimization of Bioethanol Production Via Gasification of Switchgrass

The limited energy resources in the form of fossil fuels, the lack of security in their supply, and the increasing demand have led to consider alternative and renewable energy sources.  Among the different possibilities only biomass provides an alternative fuel that can be implemented in the short-term for the transporting sector, which is the most challenging due to the need for high density energy sources. Thus, ethanol from biomass has become one of the most important alternatives to gasoline due to its compatibility with current automobile engines (Cole 2007) and to the fact that it can take advantage of the existing supply chain of liquid fuels that is already well established and thus, its production has been supported by governmental policies in the US and the EU. However, the volume of production of ethanol that is required to meet these policies and the competence of the first generation ethanol with the food supply chain has raised questions regarding its technological feasibility as an alternative fuel. Thus, the so called second generation of biofuels has received worldwide attention in order to improve the yield and reduce the consumption of utilities in the production process while using a raw material not related to food. However, currently no production process of ethanol from lignocellulosic raw materials has been implemented due to existing technical, economic and commercial barriers (Huang 2008).

Two types of process technologies can be used to transform lignocellulosic raw materials into ethanol. The first one is based on the hydrolysis of the raw material to break down the physical and chemical structure of the crops to expose the sugars that are fermented to ethanol. Due to its similarity with the current production of ethanol and the expected lower capital cost, this technology has received the attention of many researchers, e.g. Hamelinck et al. (2005); Cardona & Sánchez (2006); Zhang et al. (2009); Keshwani & Cheng (2009). The main disadvantage of the hydrolysis of the lignocellulosic material is the fact that lignin cannot be processed and thus a part of the carbon source of the raw material cannot be used to obtain ethanol. The second technology is based on the gasification of the raw material into syngas, which is used to obtain ethanol either via Fisher-Tropsch based catalytic reaction or via fermentation of the syngas (Phillips et al, 2007; Huhnke, 2008;  Piccolo and Bezzo, 2009; Zhu et al 2009).

In this paper we present the bioethanol production process from switchgrass that features optimal energy use. The process consists of three different parts. The first one is the gasification of the raw material. Two different technologies are considered: (1) indirect low pressure gasification with steam where the combustion of char in a parallel equipment (combustor) provides the energy for the gasification of the biomass by heating sand which is fed back to the gasifier; or (2) direct high pressure gasification of the raw material with steam and pure oxygen to avoid the dilution of the gas.

The second part comprises the technologies to clean up the gas from solids as well as other compounds like hydrocarbons, NH₃, CO₂ or H₂S and to adjust the gas composition. The hydrocarbons are partially removed in the tar where they are either reformed with steam, or partially oxidized. In the case of high pressure, the solids are removed in a ceramic filter and next the gas will be expanded generating energy. If the indirect lower pressure method of gasification is used, the solids are removed together with NH₃ in a wet scrubber and compressed. In both cases, the last traces of hydrocarbons are removed in a Pressure Swing Adsorption (PSA) system with a bed of silica gel. Then, the composition of the gas is adjusted to a molar ratio of CO:H₂ of 1. In order to accomplish this, three technologies such as water gas shift reactor, bypass and hybrid membrane / PSA for removal of H₂ (with a bed of oxides) are considered. The selection depends on the performance of the gasifier and the tar reformer. Sour gases such as CO₂ and H₂S are next removed. In case of using catalytic synthesis of ethanol, the H₂S must be completely removed from the gas due to its poisoning effect on the catalysts. In contrast, fermentation with bacteria can handle up to 2.5% in volume of H₂S. The three technologies considered for removing the sour gases are: (1) the absorption of the sour gases in Monoethylamine (MEA), (2) a PSA system with a bed of Zeolite 5A, and (3) the use of a membrane permeable to CO₂ and also using MEA as carrier. Selexol is another possible technology but the solvent is more expensive and more suitable for higher pressures than the ones used in the flowsheet.

 Finally, two synthetic paths are considered. The first option is the fermentation path where the syngas is fermented in a stirred tank reactor. The unreacted gases are recycled to the gas cleanup section of the process. The water must be removed from the ethanol-water solution to obtain fuel quality ethanol. A beer column greatly reduces the amount of water in the diluted solution of ethanol. In the next step four technologies are evaluated to dehydrate the ethanol: (1) distillation to the azeotropic point, (2) adsorption in corn grits, (3) the use of molecular sieves with a bed of Zeolite 13 X, and (4) pervaporation. The second path for the synthesis of ethanol is the high alcohols synthesis production .The light hydrocarbons and the unreacted gases are recycled back to the cleanup section of the process The purification of ethanol is carried out using a sequence of distillation columns. Two sequences are evaluated, direct and indirect.

 The optimization of the system is formulated as a Mixed-Integer Non-linear Programming problem where all the units are modeled using short cut methods, design equations and mass and energy balances. The problem is solved through a special decomposition scheme. We relax the binary variables related to gasification, reforming and synthetic path generating 8 subproblems that can be solve as NLP's.  For each of the subproblems we optimize the energy needed for the cleanup and purification stages. Once the structure of the process is determined, multieffect columns replace distillation columns and heat integration is performed. Finally an economic evaluation is carried out accounting for labor, maintenance, chemicals, raw material and utilities.

The optimal process consist of direct gasification followed by steam reforming, removal of the excess of hydrogen to be sold while the sour gases are removed in sequence using PSA and MEA  in this order . Finally the catalytic synthetic path is selected followed by direct distillation sequence to purify ethanol to fuel grade. This process reports a promising production cost of 0.42 $/gal when the excess of hydrogen in sold as a byproduct.

 

References:

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Cole, D. E. (2007)  Issues facing the Auto Industry: Alternative Fuels, Technologies, and Policies ACP Meeting Eagle Crest Conference Center June 20, 2007

Keshwani, D. R., Cheng, J.J. (2009) Switchgrass for bioethanol and other value-added applications: A review Bioresource Technology 100, 1515–1523

Hamelinck, C. N., Geertje van Hooijdonk, G., Faaij, A.P.C. (2005) Ethanol from lignocellulosic biomass: techno-economic performance in short-, middle- and long-termBiomass and Bioenergy 28, 384–410

Huang, J. Qiu, H. and Scott Rozelle, S., 2008, More pain ahead for China's food prices, Far Eastern Economic Review, 171, 5, 8–13.

Huhnke, R. L. (2008) Cellulosic ethanol using gasification-fermentation. Resource: Engineering & Technology for a Sustainable World

http://www.articlearchives.com/energy-utilities/renewable-energy-biomass/896186-1.html

Phillips, S., Aden, A., Jechura, J. and Dayton, D., Eggeman, T (2007) Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis of Lignocellulosic Biomass Technical Report, NREL/TP-510-41168, April 2007

Piccolo, C., Bezzo, F., (2009) A techno-economic comparison between two technologies for bioethanol production from lignocelluloses. Biomass and Bioenergy 33 (2009) 478 – 491

 

Zhu, Y., Gerber, M.A., Jones, S.B., Stevens, D.J (2009) Analysis of the effects of compositional and configurational assumptions on product costs for the thermochemical cornversion of lignocellulosic biomass to mixed alcohols  FY 2007 Progress report. DOE PNNL.17949 Revision 1.