AN ASSESSMENT OF THE POTENTIAL OF INVASIVE WEEDS AS MULTIPLE FEEDSTOCKS FOR BIOFUEL
The quest for alternative and renewable liquid transportation fuel has intensified over the past few years. This is driven by fast and continuing depletion of fossil fuels threatening energy security, and increasing concerns about climatic change as a result of large emission of greenhouse gases through vehicular exhausts. Waste biomasses in the form of noxious invasive weeds (in addition to conventional lignocellulosic substrates of agro– and forestry residues) (Ranjan et al., 2013; Moholkar and Singh, 2013; Singh et al., 2014) could form a large potential feedstock source for economic biofuel production (Raspolli et al., 2013, Ekman et al., 2013, Van et al., 2014). Large numbers of such invasive species exist in India. These species infest millions of hectares of arable and non–arable (or infertile) land, leading to enormous monetary loss due to a reduction in crops and forage production. The actual biomass produced by these noxious weeds is in the range of 15–20 tons per hectare. Nonetheless, these biomasses can form a feedstock for liquid biofuels due to their significant content of holocellulose (in addition to lignin), which can be hydrolyzed to produce fermentable monomeric sugars. Hexose and pentose sugars produced from the hydrolysis of cellulose and hemicellulose can be used to produce alcoholic biofuels, such as ethanol and butanol. The pretreatment of biomass prior to fermentation encompasses the removal of lignin and the acid/enzymatic
hydrolysis of hemicellulose and cellulose fractions to pentose and hexose sugars.
Removal of the lignin matrix in biomass causes a better exposure of the cellulose and hemicellulose fraction to acid/ enzymatic action, which leads to enhancement of the sugar yield. The pretreatment of biomass is also aimed at a reduction in the crystallinity of cellulose and in increasing the biomass porosity and surface area, which all contribute to a faster and higher hydrolysis of the cellulose/hemicellulose, thus maximizing the yield of fermentable sugars.
2.1.1 Aim and approach
The use of a cheap feedstock and optimization of the cost–intensive pretreatment techniques are crucial aspects of the economic feasibility of a sustainable biofuels process. In the present study, we dealt with the pretreatment of multiple invasive species or weeds (which are essentially waste biomasses), and determined the yield of total (hexose + pentose) reducing (or fermentable) sugars, which is a measure of their potential as a feedstock for biofuels. The invasive weeds considered in this work are: (1) Arundo donax (AD), (2) Chromolena odorata (CO), (3) Eichhornia crassipes (EC), (4) Ipomea carnea (IC), (5) Lantana camara (LC), (6) Mikania micrantha (MM), (7) Parthenium hysterophorus (PH) and (8) Saccharum spontaneum (SS). A number of previous authors have also addressed the matter of the pretreatment of these waste biomasses and a summary of some representative studies in optimization of the pretreatment and fermentation of these invasive weeds is given in Table 1.5 of Chapter 1. Most of these studies include the following components: acid hydrolysis, delignification, enzymatic hydrolysis and fermentation of the acid/enzyme hydrolyzates to bioethanol. It could be inferred from Table 1.5 that the optimum pretreatment conditions differ significantly for each biomass. The approach in the present study is
somewhat different from the earlier studies listed in Table 1.5, which studied the pretreatment of individual biomasses. In a previous paper, (Singh et al., 2014) we presented an extensive study on the assessment and optimization of as many as 17 pretreatment techniques (physical/ chemical/physico–chemical) for the invasive species of Parthenium hysterophorus for the maximum production of reducible sugars that could be fermented to produce alcoholic fuels. In the present study, we carried out pretreatment of the eight invasive weeds mentioned above at optimized conditions determined for Parthenium hysterophorus, (Singh et al., 2014) and assessed the yield of reducible sugars. It could be expected that the optimum pretreatment conditions for the eight invasive weeds could be different than those for Parthenium hysterophorus. The major contemplation underlying the approach of pretreating the invasive weeds listed above at conditions optimized for Parthenium hysterophorus was to assess the output of a bioprocess with feedstock flexibility. This can be explained in greater details as follows: depending on the availability of biomasses in different parts of the year, the biofuel industry for the large–scale production of alcoholic biofuels may require to change the feedstock or to use a mixed feedstock comprising several biomasses as sufficiently large quantities of a single biomass may not be available throughout the year. In such a situation, it may not be feasible or practical to perform comprehensive optimization of the pretreatment conditions for each biomass used as feedstock.
Moreover, the optimum pretreatment conditions (such as acid/alkali concentrations or temperature/pressure of the autoclaving) may show significant variations for different biomasses. Thus, the specifications of processing equipment designed for the pretreatment of one biomass may not be suitable for other biomasses. Obviously, the replacement of process equipment for different biomasses is rather impractical and so under this limitation, it is inevitable to treat different biomasses at conditions optimized
for the representative biomass that was considered for the process design. In such a situation, it is necessary to make a preliminary estimate of the alterations in the quality of the hydrolyzates in terms of the concentrations of pentose and hexose sugars with changing feedstock. The present study essentially attempts to paint a picture of such variations by pretreatment of the eight selected invasive weeds at conditions optimized for the weed of Parthenium hysterophorus.