Key issues in modeling and optimization of lignocellulosic biomass fermentative conversion to gaseous biofuels

Highlights

• Efficient production of biofuels requires the implementation of optimization procedures.

• High hydrolysis ratio is crucial for efficient consumption of monosugars in fermentation.

• Lignin is responsible for biomass recalcitrance and inhibitory compounds formation.

• ADM1 is the most advanced and relatively universal model for biogas production.

• Authors propose a classification of mathematical models for biofuels production.

Abstract

The industrial-scale production of lignocellulosic-based biofuels from biomass is expected to benefit society and the environment. The main pathways of residues processing include advanced hydrolysis and fermentation, pyrolysis, gasification, chemical synthesis and biological processes. The products of such treatment are second generation biofuels. The degree of fermentation of organic substances depends primarily on their composition and chemical structure. Optimization of fermentation conditions leads to better understanding of occurring processes. Therefore, an overview of recent developments in fermentation modeling is necessary to establish process parameters enabling high yields of biofuels production. Among process parameters affecting the yield and rate of biogas and biohydrogen, pH of the pulp, temperature, composition, biomass pre-treatment and digestion time are to be considered. The technology of anaerobic co-digestion has been intensively developed as a valuable solution for the disposal of organic wastes and sewage sludge. Modeling of biogas production from lignocellulosic biomass has been intensively investigated and is well described by adapted ADM1 model. Modeling of fermentative hydrogen production lacks a kinetic model incorporating process parameters with the view of pretreatment and fermentation. This paper presents the state-of-the-art on the problems related to lignocellulosic biomass pre-treatment and discusses the mechanisms of lignocellulosics conversion to gaseous biofuels.

Introduction

Large amounts of the biomass-originating energy come from processing of lignocellulosic biomass. Fuels generated from biomass include liquid and gaseous biofuels. Lignocellulosic materials consist of cellulose, hemicellulose, lignin and extractives. Cellulose and hemicellulose are a very good carbon source and may be potentially used in different biological processes after the pre-treatment step. This kind of biomass is typically inedible plant material, including crops of wood, grass, and agro-forest residues. Conversion of various types of biomass to useful products i.e. fuels has recently been an important topic both for scientific and industrial research.

The industrial-scale production of lignocellulosic-derived biofuels from plant biomass is expected to benefit society and the environment in numerous ways. The development of technologies for biomass processing focuses mainly on biorafination processes. Biogas and biohydrogen are the most important gaseous biofuels while the most popular liquid biofuels are bioethanol, biomethanol, biodiesel, bio-based methyl or ethyl tert-butyl ether and pure vegetable oil [1]. The main pathways of lignocellulosic biomass and residues processing are advanced hydrolysis and fermentation, pyrolysis, gasification, chemical synthesis and biological processes. The main products are second generation biofuels, as given in Fig. 1.

Biomass conversion through fermentation processes is crucial because it allows for production of various groups of substances under relatively mild conditions. The degree of fermentation of organic substances depends primarily on their composition and chemical structure. Only feedstocks not competing with the food request should be considered for biofuel production. Therefore, agricultural and forestry residues and wastes seem to be the most interesting sources of biomass, as their exploitation leads to energy recovery.

High hydrolysis ratio is needed for efficient utilization of monosugars present in lignocellulosic structures. From a biochemical point of view, organic substances present in the hydrolyzed solution can be divided into several groups of substances: simple and complex carbohydrates, proteins, lipids and heteropolymers. Different microorganisms are able to convert the cellulose and hemicellulose fraction of agricultural residues. Therefore, the potential of biogas and biohydrogen production from lignocellulosic biomass may be enormous. The efficiency of fermentation leading to biofuels, related with the type of pretreatment is widely discussed. The major problems related to biofuels production from lignocellulosic biomass lie basically in the conversion ratio of polymeric compounds into fermentable sugars such as hexoses and pentoses. This kind of processing must involve pretreatment steps such as physical, chemical and physicochemical pretreatment, biological or enzymatic treatment, fermentation and purification [2,3]. The recalcitrance of lignocellulosic materials requires pretreatment to facilitate enzymatic action [4]. During the hydrolysis inhibitors i.e. lignin derivatives affecting further conversion processes are formed. To maximize the fermentation of hexoses and pentoses and to minimize the presence of inhibitors during fermentation processes for cellulosic biofuels, application of microbial metabolism in the degradation and saccharification of the plant cell wall is considered [5].

Biohydrogen and biogas can be produced via anaerobic fermentation from hydrolysates of lignocellulosic biomass. Due to the presence of inhibitory compounds from lignin derivatives, an efficient method for lignin bioconversion without detoxification is not defined. Therefore, it is crucial to consider an influence of the presence of different by-products on the fermentation process. Optimization of fermentation may lead to better understanding of occurring processes. Anaerobic digestion is a multi-step process carried out by highly differentiated microorganisms. The process requires strictly anaerobic conditions enabling the transformation of organic matter into carbon dioxide and methane or biohydrogen. Different types of microbial populations have specific optimal working conditions and are inhibited by various process parameters such as pH, temperature, alkalinity, concentration of free ammonia, hydrogen, sodium ions, potassium ions, volatile fatty acids (VFA) or heavy metals. An overview of recent developments in fermentation modeling is necessary to define process parameters ensuring high yields of biofuels production.

Anaerobic digestion of lignocellulosic biomass towards biogas production has been well described. The substrate characterization is ultimately the most influential model input on methane yield prediction. The development of methods for feedstock characterization and accurate calculations of kinetic factors to provide the required model inputs are still the supreme challenges. Lignocellulosic biomass may also be used for biogas production, either exclusively or mixed with other organic materials so as to obtain a feedstock with a convenient relation of carbon to nitrogen. Among different process parameters affecting the yield and rate of biogas generation, the pH of the pulp, temperature, substrate composition, biomass pre-treatment method and digestion time seem to be the most important. A kinetic model incorporating important parameters affecting fermentative hydrogen production is required. Modeldescribing a bioprocess should be a sum of biological, chemical and physical processes occurring in the bioreactor. Modeling of hydrogen production from complex organic substrates by dark fermentation requires the knowledge of other bioprocesses i.e. hydrolysis or acidogenesis. However, modeling of conversion towards biohydrogen is still developed.

It is assumed the future energy economy will be based on renewable sources. Biomass-based fermentative technology utilizing microorganisms capable of conversion of waste to valuable acids and alcohols with liberation of biogas or biohydrogen is tested for different types of biomass and process parameters. The possibility of predicting the fermentation process leading to biofuel production may allow saving time and increasing the efficiency of resources utilization, scaling up and the design of the system including appropriate operational factors. Possible problems occurring during biomass conversion stage are pointed in Fig. 2. Probable solutions and conclusions for the purposes of this review have been mentioned.

The industrial application of a given solution for the production of gaseous biofuels requires a comprehensive analysis of its costs. To select the optimal production method, biogas or bio-hydrogen yield and energy requirements, ease of production as well as different production costs including capital costs, operating costs, variable and fixed expenses, and replacement costs should be taken into account [[6], [7], [8], [9]]. Nevertheless, the commercialization of the proposed solution depends on a large extent on the prices of fossil fuels as well as legal rules and policy on biofuels established in a given country [8,9].

In the field of biogas production, technologies are currently successfully implemented. There are many installations producing biogas by anaerobic digestion and the improvement can be done on the basis of experience of existing plants [[10], [11], [12]]. The working installations for anaerobic digestion are usually integrated with heat or energy generation that can be used on-site and surplus can be an additional benefit to the total cost analysis [10,13]. Recently the new inexpensive solutions have been proposed to utilize local waste and integrate waste management with the energy generation [14,15]. Research is also carried out to optimize the key steps of anaerobic digestion process to improve both economic and environmental performance of AD plants [12].

In the case of biohydrogen production from lignocellulose biomass, high cost and low hydrogen yields as well as relatively low operating fermentation broth concentration are still major bottlenecks in the development of its production [7,16]. Even improving above mentioned parameters, it is projected that the cost of bio-hydrogen obtained via dark fermentation will still be too high to be economically viable. Therefore, integrated technologies for bio-hydrogen production are proposed, taking into account the use of added-value products and co-generation of energy [7,8] or combining solid state fermentation and dark fermentation for hydrogen production [17,18]. Because bio-hydrogen technologies are still at a laboratory scale, further and intense research is required to explore the potential, feasibility, and extent of the possible improvements [7].

This review is focused on the description of the key challenges in modeling and optimization of lignocellulosic biomass conversion processes. The main objective is to develop a framework and methodology presenting a holistic influence of a particular stage of the bioconversion process on the overall system performance and efficiency.