Elsevier

Biomass and Bioenergy

Volume 33, Issue 3, March 2009, Pages 478-491
Biomass and Bioenergy

A techno-economic comparison between two technologies for bioethanol production from lignocellulose

https://doi.org/10.1016/j.biombioe.2008.08.008Get rights and content

Abstract

The conversion of biomass into biofuels can reduce the strategic vulnerability of petroleum-based transportation systems. Bioethanol has received considerable attention over the last years as a fuel extender or even as a neat liquid fuel. Lignocellulosic materials are very attractive substrates for the production of bioethanol because of their low cost and their great potential availability. Two different process alternatives (i.e. the enzymatic hydrolysis and fermentation process and the gasification and fermentation process) for the production of fuel ethanol from lignocellulosic feedstock are considered and analysed. After a rigorous mass and energy balance, design optimisation is carried out. Both processes are assessed in terms of ethanol yield and power generation as well as from a financial point of view. A sensitivity analysis on critical parameters of the processes' productivity and profitability is performed.

Introduction

The search for alternativ and sustainable energy sources has become more and more important due to the possible short-term shortage of fossil oil and the environmental threats that the exploitation of non-renewable sources is causing, particularly in terms of CO2 emissions. Energy for the transport sector represents a particularly critical area as it accounts for more than 30% of total energy demand in developed countries. Furthermore, it is 98% dependent on fossil fuel and is considered one of the main causes for CO2 increase [1], [2]. It is clear that a diversification of primary energy for fuel production will be necessary, for environmental and supply security concerns. The USA government has recently committed to increase bio-energy threefold in 10 years. The EU aims to replace diesel and gasoline in fuel to the level of 5.75% by 2010 and 10% by 2020. However, it is clear to anyone that such goals can be achieved only through further advancements in existing processes and new concept technologies.

Ethanol is one of the most promising biofuels, as in principle it could be derived from any material containing simple or complex sugars. Industrial ethanol production has been reported using sugar cane and various starchy materials (corn, wheat, potatoes). However, the most promising raw material is represented by lignocellulose: cellulose is the most common biopolymer on Earth (present in wood, organic industrial wastes, etc.) and is a polysaccharide, i.e. it can be converted into sugars and fermented. Although estimations from different sources may vary considerably, a general result is that resources of cellulose are usually abundant and locally available [3] and that its use for biofuels' production can play an important role in reducing greenhouse emissions [4]. Thus, ethanol produced from lignocellulosic materials has the potential to be a valuable substitute for, or complement to, gasoline.

A wide variety of processes for the production of ethanol from cellulosic materials have been studied and are currently under development. In fact, the large amount of technologies and processing options advocates for a more diffuse application of process engineering modelling, design and optimisation in order to help the research effort and guide investors and policy-makers towards the most effective technologies [5]. In this paper, two of the most promising processes will be analysed and assessed: these are the conversion of lignocellulosic biomass by hydrolysis and subsequent fermentation and the gasification of lignocellulose followed by syngas fermentation.

The first process (that here will be called the enzymatic hydrolysis and fermentation process or EHF process) is possibly the most mature process for the transformation of lignocellulosic materials into ethanol. It has been extensively described and studied (e.g. [6], [7], [8]), and pilot plants and pre-industrial facilities have recently being brought to operation. In the literature, several flowsheeting designs have been reported: for instance, Wooley et al. [7] describe the global process for ethanol production from wood chips and Cardona and Sànchez [9] use a process simulator to assess the energy consumption for several process configurations; other works have analysed the techno-economic performance of the production process [10], [11], [12].

On the other hand, the second process (that will be referred to as the gasification and fermentation process or GF process) has been somehow neglected in the scientific literature (at least when compared to the EHF process), notwithstanding the promising results demonstrated in the few works appeared so far (e.g. [13]). Although biomass gasification has long been studied [14], its integration with the fermentation process has been studied only in few reports [15]. To the authors' knowledge, no complete flowsheeting analysis and financial assessment has never been published in the scientific literature. However, the technology potential (which is already available as a commercial process) has nonetheless been widely recognised [16] and recently awarded through financing by the U.S.A. Department of Energy. Therefore, here the process has been chosen and assessed as a possible alternative to EHF for the production of bioethanol.

This paper aims at achieving the following goals. The first one is to deliver a technical and economical comparison between two of the most important processes for the conversion of lignocellulose to bioethanol. In general, it is difficult to assess different processes when analysed by different research groups as preliminary assumptions, process design, financial modelling and data are rather “sensitive” to specific expertise, simplifying hypotheses and data availability. We believe that one strength in our technical and financial analyses is that the same methodology is carried out for both processes so that the final results are indeed comparable.

Secondly, this paper represents the first comprehensive analysis of the GF process (with the partial exception of Ref. [17]). Although the process is still in its early development (at least from what can be derived from published material) and some data definitely still exhibit a significant uncertainty, the work aims at assessing process design, potential optimisation directions and the effect of most important parameters on the overall yield and financial indexes.

Finally, a step forward is taken in the optimisation and analysis of the EHF process, too. The use of pinch analysis and a new design approach demonstrate that further reduction in the utilities' demand is still possible. The effect of the improved design is assessed in terms of energy efficiency, overall yield, product costs and financial profitability.

The paper is structured as follows: sections from 2 The EHF process: process overview, 3 The EHF process: modelling, 4 The EHF process: energy optimisation, 5 The EHF process: heat and power production, 6 The EHF process: process sensitivity analysis consider the EHF process in terms of modelling, process optimisation, heat and power generation and assess its performance when varying some critical parameters. Section 7 is dedicated to the financial assessment of the process. The GF process is then taken into account: sections from 8 The GF process: process overview, 9 The GF process: modelling, 10 The GF process: heat and power production, 11 The GF process: sensitivity analysis mirror the analysis and optimisation previously carried out for the EHF process. Similarly, Section 12 defines a financial evaluation for the process. The last section discusses and compares the main results concerning the two production processes and draws some conclusions. A processing capacity of 700,000 t/yr of dry biomass wood is assumed throughout the paper.

Section snippets

The EHF process: process overview

The EHF process is possibly the most mature technology for the conversion of lignocellulosic material into ethanol. As sketched in Fig. 1, the overall ethanol production process includes five main steps: biomass pre-treatment, cellulose hydrolysis, fermentation, separation and effluent treatment.

During biomass pre-treatment, the structure of cellulosic biomass is altered, lignin seal is broken, hemicellulose is reduced to sugar monomers (mainly xylose, a C5 sugar) and cellulose is made more

The EHF process: modelling

The process model was implemented on the Aspen Plus® v. 13.2 process simulator, a modelling tool performing rigorous material and energy balance calculations. The accuracy of the property data bank is of paramount importance for the reliability of the simulation results. The physical property database for biomass material, specifically developed by NREL [23], is used. The system thermodynamics are described by using an NRTL model (except for the CO2 solubility modelled in terms of Henry's law).

The EHF process: energy optimisation

The flowsheet previously described can be further optimised to reduce the energy consumption. A Pinch Technology Analysis (PTA) [29] approach has been carried out with concern to the recovery section. According to acknowledged practice, all the heat exchangers and the hot (i.e. to be cooled) and cold (i.e. to be heated) streams were identified. The ΔTmin representing the minimum temperature difference between the hot and the cold sides in all the process heat exchangers is set to 10 °C.

The pinch

The EHF process: heat and power production

The transformation of lignocellulose into ethanol is energy self-sufficient. In fact, the solid residues exhibit a heating value of about 29.54 MJ/kg [9]. The solid residues together with the concentrated syrup from the evaporators are burnt in a fluidized-bed combustor. The moisture of the combined feed is 55% and its average heating value is 9.54 MJ/kg. Additionally, through the anaerobic treatment of the columns' stillage biogas (60% methane; heating value: 20 MJ/m3) is obtained.

The solid

The EHF process: process sensitivity analysis

Current research focuses on different strategies for improving the performances and the economics of bioethanol production. In particular, a big effort is put on producing biotechnological advancements for high productivity enzymes (stable at higher temperature so as to increase the reaction rate, and tolerant to higher concentrations of products). A sensitivity analysis can be performed to evaluate the impact of a general progress on the process performances. Two potential future scenarios are

The EHF process: financial analysis

The process design study has been used to predict production cost and to assess its market potential. The process flow and balances were used to size equipments, and assess variable and fixed costs. The only products considered in this study are ethanol and electricity. For comparisons with gasoline a base of 70 $/bbl1 of oil is assumed; the gasoline production cost is roughly 0.50 €/L.

The GF process: process overview

A block diagram for the GF process [15] is sketched in Fig. 5. The first and most important step is the biomass gasification where the biomass is thermally cracked into a mixture of H2, CO (syngas), CO2 and other by-products [34]. Many gasification methods are available for syngas production [14]. However, in terms of throughput, cost, complexity, and efficiency issues, only circulated fluidized-bed gasifiers are suitable for large-scale fuel-gas production [35]. In this process, the

The GF process: modelling

Thermochemical gasification is the conversion by partial oxidation at elevated temperature of a carbonaceous feedstock into a gas product. The partial oxidation can be carried out using air, oxygen, steam, or a combination of these. Gasification occurs in sequential steps: drying (to evaporate feedstock's moisture), pyrolysis (to give gas, vaporized tars and a solid char residue), gasification (or partial oxidation of the solid char, pyrolysis tars and pyrolysis gases).

Numerous models (e.g. [37]

The GF process: heat and power production

The co-generation system consists of a combustor, a number of heat exchangers and a multistage steam turbine [45]. In the burner exhausted gases from the fermentation tank are burnt with preheated air (250 °C). The air flow is set in order to obtain a ratio of oxygen in the flue gases with respect to the inlet air equal to 0.6: this is done to limit the maximum temperature to 1110 °C.

Feed water (143 t/h) enters the steam boiler and in the economiser is heated to about 25 °C below saturation

The GF process: sensitivity analysis

A critical parameter in assessing the process performance concerns the conversions in the fermentation reactions. Other references suggest that sensibly higher conversions can be obtained in the fermentation reactor (for instance, Spath and Dayton [15] declare that 90% of CO and 70% of H2 can be converted into ethanol on lab-scale facilities). Such conversions, if confirmed, would obviously change the overall technology potential. In this study, it was decided to consider the effect of a 15%

The GF process: financial analysis

Similar to what was done for the EHF, process equipments' size and installation cost are assessed through direct factored estimation or from literature sources [35]. Table 18 summarises the estimated installation costs.

Final remarks

In this work two conversion technologies for the lignocellulose conversion into ethanol have been assessed in terms of yield and profitability.

The first technology is the enzymatic hydrolysis and fermentation (EHF) process. A further improvement in the recovery design in order to reduce the process energy consumption has been demonstrated. The improved design and a comprehensive use of information from previous works have allowed to better assess the present process profitability and the effect

Acknowledgements

The authors gratefully acknowledge Fondazione Cassa di Risparmio di Padova e Rovigo for Progetto Dottorati di Ricerca 2006 under whose framework this research has been carried out.

References (50)

  • A.A. Rostami et al.

    A biomass pyrolysis submodel for CFD applications

    Fuel

    (2004)
  • C. Gueret et al.

    Methane pyrolysis: thermodynamics

    Chem Eng Sci

    (1997)
  • Y.L. Lee et al.

    Theoretical study of thermodynamics relevant to tetramethylsilane pyrolysis

    J Cryst Growth

    (1997)
  • R. Zanzi et al.

    Rapid high-temperature pyrolysis of biomass in a free-fall reactor

    Fuel

    (1996)
  • M.J. Prins et al.

    From coal to biomass gasification: comparison of thermodynamic efficiency

    Energy

    (2007)
  • C. Piccolo et al.

    Ethanol from lignocellulosic biomass: a comparison between conversion technologies

  • International Energy Agency (IEA)

    Energy technology perspectives 2006 – in support of the G8 plan of action – scenarios and strategies to 2050

    (2006)
  • G. Campbell

    Panorama of transport

    (2007)
  • C.N. Hamelinck et al.

    Outlook for advanced biofuels

    Energy Policy

    (2005)
  • A.E. Farrell et al.

    Ethanol can contribute to energy and environmental goals

    Science

    (2006)
  • L.R. Lynd

    Overview and evaluation of fuel ethanol from cellulosic biomass: technology, economics, the environment, and policy

    Annu Rev Energy Environ

    (1996)
  • R. Wooley et al.

    Lignocellulosic biomass to ethanol process design and economics utilizing co-current acid prehydrolysis and enzymatic hydrolysis. Current and futuristic scenarios

    (1999)
  • C.A. Cardona et al.

    Energy consumption analysis of integrated flowsheet for production of fuel ethanol from lignocellulosic biomass

    Energy

    (2006)
  • L.R. Lynd et al.

    Likely features and costs of mature biomass ethanol technology

    Appl Biochem Biotechnol

    (1996)
  • A. McAloon et al.

    Determining the cost of producing ethanol from corn starch and lignocellulosic feedstocks

    (2000)
  • Cited by (0)

    View full text