Optimization of Acid Hydrolysis in Ethanol Production from Prosopis juliflora

Contents

Acknowledgement

List of Tables

List of Figures

List of Acronyms

Abstract

1. Introduction
1.1 Background
1.2 Statement of the Problem
1.3 Objectives
1.3.1 General Objectives
1.3.2 Specific objective
1.4 Significance of the Research

2. Literature Review
2.1 Introduction
2.1.1 Biofuel Types
2.1.2 Ethanol as fuel
2.1.3 World market of ethanol
2.1.4. Ethanol production and supply in Ethiopia
2.2 Ethanol Feedstocks
2.2.1. Starch Feedstocks
2.2.2 Sugar Feedstocks
2.2.3 Lignocellulosic Feedstocks
2.3 Lignocellulosic Biomass as Ethanol Feedstock
2.3.1 Lignocellulosic Biomass
2.3.2 Composition of Lignocellulosic Materials
2.3.3 Prosopis juliflora as Ethanol feedstock
2.3.4 Prosopis juliflora in Ethiopia
2.4 Production Methods of Cellulosic Ethanol
2.4.1 Biochemical Conversion (sugar platform)
2.4.2Thermochemical Conversion (Syngas Platform)

3. Materials and Methods
3.1 Time and Place of the Study
3.2 Materials
3.3 Methods
3.3.1 Prosopis juliflora Sample collection area
3.3.2 Sample collection
3.3.3 Sample Preparation
3.3.4 Pretreatment of Prosopis Juliflora
3.3.5 Hydrolysis
3.3.6 pH adjustment
3.3.7 Fermentation
3.3.8. Distillation
3.4 Data Analysis

4. Results and Discussion
4.1Statistical Analysis of the Experimental Results
4.2 Interaction effects
4.3 Optimization
4.4 Model validation

5. Conclusion and Recommendation
5.1Conclusion
5.2 Recommendations

References

Appendices

Appendix A: Properties of Ethanol

Appendix B: The phenol-sulfuric acid method

Appendix C: Density Measurements using pycnometer

Appendix D: Density versus Percent Alcohol of Aqueous Ethanol Solutions at 200C (Robert H.Perry et.al)

Appendix E: Determination of Efficiency and Yield

Appendix F: Laboratory work pictures

List of Tables

Table 2.1The physicochemical properties of some oxygenated high-octane additives to gasoline

Table 2.2 Ethanol from Molasses Distillation (‘000 liters).

Table 2.3 Starch and Theoretical Ethanol Yield of Relevant Cereal Grains (Dry Basis)

Table 2.4 Cellulose, hemicellulose and lignin content in common agricultural residues and wastes..

Table 3.1 Maximum and minimum values of parameters of acid hydrolysis in ethanol production from Prosopis juliflora

Table 3.2 Codified (x1, x2, x3 and x4) and Respective no codified Levels (Acid%, Solid %, T, and Time) in experimental design for optimization of dilute-acid hydrolysis of the Prosopis juliflora by two level full factorial experimental design method

Table 4.1 Experimental results of acid hydrolysis in ethanol production from prosopis juliflora .

Table 4.2 Experimental results of Ethanol yields

Table 4.3Design summary

Table 4.4 Analysis of variance (ANOVA)

Table 4.5 Model adequacy measures

Table 4.6 Regression coefficients and the corresponding 95% CI Low and High

Table 4.7 Actual versus model predicted ethanol yields.

Table 4.8 Optimization criteria

Table 4.9 Optimum possible solutions.

List of Figures

Figure 1.1 Illustration of Projected World Energy Demand (a) projected world energy demand and (b) Increase in world primary energy demand by fuel.

Figure 2.1The biofuels ladder, road map of biofuels production from feedstocks and technologies

Figure 2.2 Ethanol potential production from different feedstocks.

Figure 2.3 Projected ethanol productions, 2007- 2012 (‘000 liters)

Figure 2.4 Chemical structure of cellulose

Figure 2.5 Schematic diagram of a representative section of the molecular structure of softwood lignin

Figure 2.6 Schematic Diagram of Ethanol productions from lignocellulosic feedstocks

Figure 2.7 Simplified Impact of Pretreatment on Biomass

Figure 3.1 Cutter mill for size reduction (a) and prosopis juliflora powder (b) (photo shot: Author).

Figure 3.2 Pretreated prosopis juliflora powder samples (photo shot: Author)

Figure 3.3 Samples acid hydrolysis inside autoclave (a) samples after hydrolysis (b)(photo shot: Author).

Figure 3.4 pH adjustment (photo shot: Author)

Figure 3.5 Auto clave reactor (a), fermentation product (b) (photo shot: Author)

Figure 3.6 Distillation using Rotary evaporator

Figure 4.1 Ethanol yields and factors of acid hydrolysis in ethanol production

Figure 4.2 Normal plot of residuals

Figure 4.3 Plot of residuals versus model predicted values.

Figure 4.6 Response surface plot( a), contour plot (b) and interaction plot (c) and (d) of ethanol yield as a function of acid concentration and solid fraction

Figure 4.7 Response surface plot (a), contour plot (b) and interaction plot (c) and (d) of ethanol yield as a function of acid concentration and time

Figure 4.8 Response surface plot (a), contour plot (b) and interaction plot (c) and (d) of ethanol yield as a function of temperature and solid fraction

Figure 4.9 Response surface plot (a), contour plot (b) and interaction plot (c)and (d) of ethanol yield as a function of time and solid fraction

Figure 4.10 Response surface plot (a), contour plot (b) and interaction plot (c) and (d) of ethanol yield as a function of time and temperature

Figure 4.11Optimization contours on ethanol yield

Figure 4.12 Surfaces of possible optimum solutions

List of Acronyms

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Abstract

Lignocellulosic materials (eg.Prosopis juliflora) can be utilized to produce ethanol, a promising alternative energy source for the limited crude oil. This study involved optimisation of acid hydrolysis in ethanol production from prosopis juliflora. The conversion of prosopis juliflora to ethanol can be achieved mainly by three process steps: pretreatment of prosopis juliflora wood to remove lignin and hemicellulose, acid hydrolysis of pretreated prosopis juliflora to convert cellulose into reducing sugar (glucose) and fermentation of the sugars to ethanol using Saccharomyces cerevisiae in anaerobic condition. A two level full factorial design with four factors, two levels and two replicas (24*2=32 experimental runs) was applied to optimize acid hydrolysis and study the interaction effects of acid hydrolysis factors, namely, acid concentration, solid fraction, temperature, and time. An optimisation was carried out to optimise acid hydrolysis process variables so as to determine the best acid concentration, solid fraction, temperature, and contact time that resulted maximum ethanol yield. The screening of significant acid hydrolysis factors were done by using the two-level full factorial design using design expert® 7 software. The statistical analysis showed that the ethanol yield of (40.91% (g/g)) was obtained at optimised acid hydrolysis variables of 0.5%v/v acid concentration, 5%w/w solid fraction,105.01°C temperature, and 10 minutes hydrolysis time.

Keywords:Prosopis juliflora, pretreatment, hydrolysis, fermentation, 2 level factorial, optimization.

1. Introduction

1.1 Background

Oil prices are at all times high and there is growing zest to reduce our dependence on oil. It is finite resource, gas supplies and oil reserves are shrinking, will definitely run out in the future. World energy demand is expected to double by 2050 as it is shown in figure 1.1 below. The demand of energy is currently exponentially exceeding the rate of local supply sources, a look beyond the fossils is crucial for long term economic growth and energy security purpose. The volatile situations in the Middle East, where vast reservoirs are, are also creating uncertainties about the availability of the supply. There is also the greater environmental risks associated with exploitation of crude oil (IEA world energy outlook, 2004).

[illustration not visible in this excerpt] Year

(a)

[illustration not visible in this excerpt] Year

(b)

Fig.1.1 Illustration of Projected World Energy Demand (a) projected world energy demand and

(b) Increase in world primary energy demand by fuel (IEA world energy outlook, 2004).

MTOE= Million Tones oil equivalent. Method of assessing calorific value of different sources of energy in terms of one tone of oil

With the diminishing supply of petroleum oil and the political instability in countries where much of the world’s oil reserves are found, the prices of petroleum-based fuels are irreversibly going up. As a result of concerns of sustainability, environmental protection, and national energy security, more and more countries have prioritized the importance of renewable energy sources. Ethanol has once again become attractive in the energy marketplace and, in fact, the demand for ethanol has been increasing in recent years (Lin and Tanaka, 2006; Ford, 2004).

Ethanol as well as other bio-fuels produced from plant biomass are alternative to fossil fuels. Ethanol does not add to a net carbon dioxide atmospheric increase thus there is in theory no contribution to global warming. Combustion of ethanol results in relatively low emissions of volatile organic compounds, carbon monoxide and nitrogen oxides (Bailey, 1990 ). Ethanol was used as transportation fuel at the beginning of 20th century in the U.S., but it was abandoned for fuels processed from petroleum (oil) after World War II because these were cheaper and had higher energy values (Lin and Tanaka, 2006; Ford, 2004). During the last two decades, technology for ethanol production from non-food-plant sources has been developed to the point at which large-scale production could be a reality in the next few years (Montesinos and Navarro, 2000). Moreover, agronomic residues such as corn stover (corn cobs and stalks), sugar cane waste, wheat or rice straw, forestry and paper mill discards, the paper portion of municipal waste, and mainly dedicated energy crops collectively termed ‘biomass’ can be converted to fuel ethanol (Datar et al.,2004).

Production of ethanol is historically a well known process and is carried out by fermentation of plant sugars into ethanol using strains of yeast. However, plant biomass is made of sugar polymers, ordered in a matrix called lignocelluloses, which is not as easily fermented. In order to produce ethanol, this material must undergo degradation for the yeast more accessible components for example mono and dimers of sugars. This degradation can be made by hydrolysis of biomass using enzymes called enzymatic hydrolysis (J. Caraballo, August 2005).

1.2 Statement of the Problem

Ethiopia is currently looking at growing high-yielding crops for the production of bio-fuels as alternatives to traditional fuels (petrol and diesel) to address imminent shortages and reduce impacts of climate change. Owing to such phenomenon, and indeed in view of the recent trends in the escalating price of the traditional petro-fuel, biofuel has been gaining greater attention by the Ethiopian government. But due to the increased cost of food crops, producing ethanol using Prosopis juliflora wood is an alternative feed stock: for one thing, Prosopis juliflora is a fast growing tree species and grows in Ethiopia mainly in arid and semi-arid areas of the Rift Valley. And the other reason is it is a highly invasive exotic tree that is spreading in the pastoralist areas of Ethiopia making vast areas of land unavailable for grazing and it is becoming difficult to remove it.Thirdly, when the plant is cut, new off springs is grown from the root in a short period (Hailu Shiferaw et al., 2004). Invasion of rangelands by Prosopis juliflora also caused shortage of grazing land for livestock, which resulted in drastic reduction of livestock number as well as product; thorns damage eyes and hooves of camels, donkeys, and cattle then by poisoning eventually lead to death (Senayit et al., 2004). Prosopis juliflora is invading potential croplands forcing local farmers with less capital and machinery to abandon their farmland and settlement (Senayit et al., 2004). In general, this is a matter of serious concern for the life of the local people as pastoralists depending on livestock for their livelihood (Senayit et al., 2004). Due to the above reasons and as Prosopis juliflora is widely available in Ethiopia; we can use Prosopis juliflora as Ethanol feed stock.

1.3 Objectives

1.3.1 General Objectives

The main objective of this research work is to optimize the acid hydrolysis so that to produce high yield ethanol from Prosopis juliflora.

1.3.2 Specific objective

The specific objectives are:

- To determine the optimum acid hydrolysis process variables (acid concentration, total solid fraction, temperature and reaction time).
- To ferment acid hydrolysis products.

1.4 Significance of the Research

Oil reserves are quickly being depleted due to extensive and continuing over-utilization. If consumption goes in this rate the fossil fuel reserve will be depleted completely within short period. In addition to this, continuous burning of fossil fuel increase emission of green house gasses to the atmosphere and causes global warming. So to sustain the fast depletion of fossil fuel reserves and to solve environmental concerns, the search of an alternative fuel is crucial, and one of the alternatives is ethanol. Therefore, a renewable and non-food competitive feedstock is desirable for the production of alternative fuel such as bioethanol. This study used Prosopis juliflora as a feed stock to produce ethanol.

The use of ethanol in a conventional petrol engine can also greatly reduce emissions of unburned hydrocarbons, carbon dioxide, carbon monoxide, sulfates, polycyclic aromatic hydrocarbons, ozone-forming hydrocarbons, and particulate matter by increasing oxygen number.

In general, this study is important as Prosopis juliflora is a widely available plant and is an alternative feedstock for ethanol production and address problems related with energy security, promote rural development through job creation, promote environmental conservation and decrease greenhouse gas emission.

2. Literature Review

2.1 Introduction

2.1.1 Biofuel Types

Biofuels are a source of energy derived from biomasses, such as corn, grains, and other plant life.

There is currently a large push in research and development of these energy sources, as they are renewable as opposed to the currently used fossil fuels.

illustration not visible in this excerpt Fig 2.1The biofuels ladder, road map of biofuels production from feedstocks and technologies (Rafael Luqu, et al, 2008)

These biofuels are derivatives of food crops, agricultural residues, waste from municipalities and industrial waste. It is generally accepted that there are two different generations of biofuels that have been classified starting from the conventional technologies and feedstocks (1st generation) to the latest advances in the field (2nd and potential 3rd generation). Figure 2.1 above summarises a very simplified classification of biofuels (Rafael Luqu, et al, 2008).

First generation biofuels : The first generation biofuels referred to biofuels manufactured from readily available energy crops including sugar, starch and oil crops (edible feedstocks) using conventional technologies. The most common first generation biofuels are biodiesel and bioethanol (L. Plass and S. Reimelt, 2007).

Second generation biofuels: Alternative feedstocks, generally non-edible feedstocks including waste vegetable oils and fats, non-food crops and biomass sources, and/or technologies are starting to be developed in an attempt to overcome the major shortcomings of the production of first generation biofuels. The biofuels obtained from such technologies have been denoted as second generation biofuels (L. Plass and S. Reimelt, 2007.)

Ethanol: a distilled colorless liquid fuel obtained from numerous potential feedstock varieties such as sugar beet, wheat, corn, cassava, fruits, baggasse, barley, molasses, skim milk (whey),potatoes, sorghum, switch grass and cellulose biomass such as wood, paper, straw and other cellulose wastes such as grasses, others includes municipal solid wastes. These various waste streams for Ethanol production have their peculiar properties and generally differ. Feedstocks prices and price of natural gas are predominant influential factors that determine the cost of Ethanol Production. Ethanol as an alternative fuel, offers a sustainable economy by reducing the use of imported petroleum, emitting neutral CO2 (g), boost economy providing value added market opportunities for the Agricultural sector (Shell Global, 2001).

2.1.2 Ethanol as fuel

The use of ethanol as fuel goes back to the origin of the use of vehicles itself. For example, Henry Ford’s Model T, built in 1908, ran on ethanol. It was continued until the availability of cheap petrol effectively killed off ethanol as a major transport fuel in the early part of the 20th century. The energy crisis of the 1970s renewed interest in ethanol production for fuels and chemicals. Although the interest waned in the following decade due to oil price abatement, the environmental issue of reducing greenhouse gas, rising vehicle fuel demand, and the security of energy supply sustain the development of ethanol production from renewable resources. Ethanol is used in vehicles either as a sole fuel or blended with gasoline. As an oxygenated compound, ethanol provides additional oxygen in combustion, and hence obtains better combustion efficiency. Since the completeness of combustion is increased by the present of oxygenated fuels, the emission of carbon monoxide is reduced by 32.5% while the emission of hydrocarbons is decreased by 14.5% (Rasskazchikova et al., 2004).

2.1.3 World market of ethanol

World production and consumption of Ethanol is dominated by Brazil and the USA, which are responsible for 70% of world production with 15.3 and 12.9 billion liters, respectively (Rosillo-Calle and Walter, 2006). In addition, more than 30 countries have already introduced, or are interested in introducing, programs for fuel ethanol (e.g. Australia, Canada, Colombia, China, India, Mexico and Thailand). Brazil produces ethanol from sugar cane and has used it as a fuel since 1975, when Brazilian Alcohol Program (PROALCOOL) started with the purpose of using ethanol for blending with gasoline. Furthermore, after the second oil crisis, the production was expanded to include hydrated ethanol to be used as neat fuel in modified engines. It resulted in a rapid expansion of sugar cane production from 50 Mt/year in 1970 to 380 Mt/year in 2004 along with improving productivity from 4,200 L/ha/year to 6,350 L/ha/year respectively (Rosillo-Calle and Walter, 2006).

The USA is the world’s fastest growing fuel ethanol market, representing a twofold increase in the last 4 years. The main producers are the states of Iowa, Illinois, Nebraska, South Dakota and Minnesota with 82% of the national production in 2004. The main feedstock for ethanol production is corn and ethanol is sold as octane enhancer or oxygenated blended with gasoline, and currently represents about 2% of the US fuel market.

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Fig.2.2 Ethanol potential production from different feedstocks (Rosillo-Calle and Walter, 2006)

Within the European Union (EU) countries, the community produces more than 2 billion liters of ethanol annually, dominated by France, Germany and United Kingdom, but less than 5 % is used as fuel. In Sweden, cellulosic ethanol is produced in connection with a sulfite pulp mill at Domsjö, Örnsköldsvik, with a capacity of 13,000 cubic meters, while the Agro ethanol company at Norrköping produces about 50,000 cubic meters from barley and wheat. Ethanol is used in Sweden as an oxygenated high-octane additive (up to 5%) or as E85 (85% ethanol and 15% gasoline).

The market for ethanol is predicted to increase greatly in the future due to the rise of energy demand in transportation; the rising price of oil due to its slower production; and global concerns about environmental and energy security issues. Fulton (2004) (cited by Rosillo-Calle, (2006)) predicted a doubled demand in 2010 and an increase of 44-fold in 2050, where the share of ethanol in fuel increases from 5% to 54% respectively (Fig.2.2). Two important markets standing out for their potential impacts on fuel ethanol demand are the EU and China. In the EU recently, only a small fraction of ethanol is used as fuel and the demand is growing rapidly. Moreover, Chinese automobile industries are fast-growing and hence it is quite certain that China will become a big player in the fuel ethanol market.

Given such huge demand in the future, the main ethanol feedstock will be shifted from crops to lignocellulosic materials. These materials are largely abundant resources such as (a) special production plants: timber, switchgrass; (b) agricultural and forestry residues; (c) industrial and municipal wastes. While sugar cane and corn are dominant sources for ethanol production at present, ethanol production from lignocellulosic material has not been proven and is still under development. Even though ethanol has not been largely produced from lignocellulosic material due to its high cost, it is predicted that the use of this feedstock will increase dramatically in the near future and became the main resource for ethanol production, occupying as much as two thirds of total ethanol production in 2050 (Rosillo-Calle and Walter, 2006).

2.1.4. Ethanol production and supply in Ethiopia

The worldwide recent awareness for the use of ethanol to replace petroleum and generation of power along with sugar mill plants should have led to setting up of number of ethanol plants and co-generations. Ethiopia has several sugar real estate (Fincha, Metehara and Wonji Shoa) industries which are run and administered by Sugar Development Agency. Among molasses derived products ethanol takes the largest part, but its utilization must attract the attention of the government policy makers in order to utilize as a bioethanol. Bioethanol or biofuel is ethanol based products that can process into liquid fuels for transport purposes (ESDA, 2005) as cited by Nigus Worku, 2010.

Table 2.2 Ethanol from Molasses Distillation (‘000 liters) (Ethiopian Sugar Agency, July 1999 EFY)

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2.2 Ethanol Feedstocks

Almost any plant- based material can be an ethanol feedstock. All plants contain sugars, and these sugars can be fermented to make ethanol in a process called biochemical conversion. Plant material also can be converted to ethanol using heat and chemicals in a process called thermochemical conversion. Some plants are easier to process into ethanol than others. Some require few resources to grow, while others need intensive care. Some are used for food as well as fuel, while others are cultivated exclusively for ethanol; even plant-based wastes can become ethanol. Climate and soil type determine the types and amounts of plants that can be grown in different geographic areas. Another important consideration is feedstock logistic is the steps necessary to move feedstocks from fields or collection areas to ethanol production plants. For agricultural and forestry feedstocks, these steps include harvesting, transportation, storage, and preprocessing (Cellulosic Ethanol, 2010) as cited by Nigus Worku, 2010.

2.2.1. Starch Feedstocks

Cereal Grain: Cereal grains are used mostly for food and feed. However, because of their high starch contents they are also good feedstocks for conversion to biofuels and other bio based products. Ethanol is the only biofuel that has been produced commercially from these feedstocks in large quantities (Co-products for fuel ethanol production reference database, 2003).Starch contents and theoretical ethanol yield of cereal grains which are relevant to ethanol production are summarized in Table 2.3.

Table 2.3 Starch content and Theoretical Ethanol Yield of Relevant Cereal Grains (Dry Basis) (http://www.afns.ualberta.ca/ hosted/DRTC/Articles/Barley Dairy.asp) and Juliano, 1993)

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*Theoretical yield of ethanol is 0.51 kg ethanol/kg glucose. Upon hydrolysis, 1 kg starch

produces 1.11 kg glucose.

**MT: metric ton.

2.2.2 Sugar Feedstocks

Sugarcane: is the primary feedstock for ethanol production in Brazil, the world’s largest biofuel ethanol producer. In 1970 only 20 percent of total sugarcane produced in Brazil was used for ethanol production while the rest was used to produce sugar. Production of ethanol in Brazil started to increase in the late 1970s and early 1980s. Typically sugar cane contains 12 to 17 percent total sugars on a wet weight basis, with 68 to 72 percent moisture. The sugars comprise about 90 percent sucrose and 10 percent glucose plus fructose. All these three sugars are readily fermented by the yeast Saccharomyces cerevisiae to produce ethanol. The typical efficiency of extraction of the juice by crushing is very high, about 95 percent. The solid residues are called bagasse, which normally are burned to satisfy part of the energy requirements in the ethanol plant. An important by-product of sugarcane processing is molasses, which contains up to 65 percent wet weight sugars. Molasses also can be used for ethanol production after adjustment of the sugar concentrations. Removal of suspended solids prior to fermentation may also be needed. Both sugarcane juice and molasses normally have sufficient nutrients to support ethanol fermentation (Wheals, A. E. et al., 1999).

Sugar Beet: Sugar beet is an important potential feedstock for ethanol production in the EU. In 2004, the EU members produced 181 million MT of sugar beet; only 1 million tons, or 0.6 percent, were used for ethanol production. However, the use of sugar beet as feedstock for ethanol production in the EU is expected to increase significantly. Furthermore, sugar used for fuel ethanol production would be excluded from sugar production quota. Sugar beet normally contains 16 to 18 percent sugar, which is slightly higher than sugarcane. It is estimated that in the EU ethanol can be produced from sugar beet with a yield of 86 L/MT of feedstock. The potential ethanol yields in the EU vary widely from one region to another. They are estimated to range from a low of 2964 L/ha in Lithuania to a high of 7980 L/ha in France (Francis, M. K., 2006)

2.2.3 Lignocellulosic Feedstocks

The three main components of lignocellulosic biomass are cellulose, hemicellulose, and lignin. Cellulose and hemicellulose can be hydrolyzed with chemicals and/or enzymes to monomeric sugars, which can subsequently be converted biologically to biofuels. All three components also can be converted to synthesis gas or syngas by the gasification process. Syngas then can be converted to ethanol either biologically or catalytically. The first option, that is, hydrolysis of the two carbohydrate fractions followed by fermentation, has been investigated much more extensively and hence has much better chance of reaching commercialization first. The three main sources of lignocellulosic biomass are forest products and residues, agricultural residues, and dedicated energy crops (Perlack, R. D. et al., 2005).

Lignocellulosic materials such as agricultural and forest residues, crops and herbaceous materials in large quantities are available in many countries with various climatic conditions, making them suitable and potentially cheap feedstocks for sustainable production of fuel ethanol. The global production of plant biomass, with over 90% lignocellulose content, is estimated to be about 200×109 tons/year, where about 8-20×109 tons of primary biomass remain potentially accessible annually (Lin and Tanaka, 2006).

Over the last few decades, extensive attention has been devoted to research on the conversion of lignocellulosic materials to ethanol (Chandrakant and Bisaria, 1998; Prasad et al., 2007). Lignocelluloses are complex mixtures of carbohydrate polymers, namely cellulose, hemicellulose, lignin, and a small amount of compounds known as extractives. The compositional structure of common agricultural residues and wastes is shown in Table 2.4.

Table 2.4 Cellulose, hemicellulose and lignin content in common agricultural residues and wastes (McKendry, 2002; Prasad et al., 2007; Sun and Cheng, 2002)

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*Girmay Haile , 2011

2.3 Lignocellulosic Biomass as Ethanol Feedstock

2.3.1 Lignocellulosic Biomass

Lignocellulosic biomass refers to plant biomass that is composed of cellulose, hemicellulose, and lignin. The carbohydrate polymers (cellulose and hemicelluloses) are tightly bound to the lignin. Lignocellulosic biomass can be grouped into four main categories: agricultural residues (including corn stover and sugarcane bagasse), dedicated energy crops, wood residues (including sawmill and paper mill discards), and municipal paper waste( Wikipedia, the free encyclopedia). Lignocellulosic biomass, in the form of wood fuel, has a long history as a source of energy. Since the middle of the 20th century, the interest of biomass as a precursor to liquid fuels has increased.

To be specific, the fermentation of lignocellulosic biomass to ethanol (Andrew Carroll and Chris Somerville, 2006) is an attractive route to fuels that supplements the fossil fuels. Biomass is a carbon neutral source of energy since it comes from plants; the combustion of lignocellulosic ethanol produces no net carbon dioxide into the earth’s atmosphere. Aside from ethanol, many other lignocellulose derived fuels are of potential interest, including butanol, dimethylfuran, and gamma Valerolactone (Barbara A. Tokay, 1997).

One barrier to the production of ethanol from biomass is that the sugars necessary for fermentation are trapped inside the lignocellulose. Lignocellulose has evolved to resist degradation and to confer hydrolytic stability and structural robustness to the cell walls of the plants. This robustness or "recalcitrance" is attributable to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages (Barbara A. Tokay, 1997). Ester linkages arise between oxidized sugars, the uronic acids, and the phenols and phenyl propanols functionalities of the lignin. To extract the fermentable sugars, one must first disconnect the celluloses from the lignin, and then acid hydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge to biomass fermentation is the high percentage of pentoses in the hemicellulose, such as xylose, or wood sugar. Unlike hexoses, like glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.

2.3.2 Composition of Lignocellulosic Materials

Lignocellulosic materials do not contain monosaccharides that are readily available for bioconversion but polysaccharides such as cellulose and hemicellulose, lignin, extractives, and ashes. The polysaccharides need to be hydrolysed by means of either enzymes or acids to fermentable sugars (Kalman, 2006).

Cellulose: is a linear polymer of D-glucose units linked by β-1, 4-linked glucose. Cellulose molecules are completely linear and have a strong tendency to form intra and intermolecular hydrogen bonds (Figure 2.5).

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Figure 2.5 Chemical structure of cellulose (Fan L. T. et al., 1987)

Bundles of cellulose molecules are thus aggregated together in the form of micro-fibrils, in which highly ordered (crystalline) regions alternate with less ordered (amorphous) regions (Sjöström, E., 1981).The crystalline region in which the linear molecules of cellulose are bonded laterally by hydrogen bonds is characterized by the cellulose lattice which extends over the entire cross section of the micro fibrils. This crystalline region is bounded by a layer of cellulose molecules that exhibit various degrees of parallelism. The less ordered region is called the paracrystalline or amorphous region. The disordered region allows disintegration of the cellulose by hydrolysis into rod like particles with aqueous, non swelling, strong acid (Fan et al., 1987).Micro fibrils build up fibrils and finally cellulose fibers. As a consequence of its fibrous structure and strong hydrogen bonds cellulose has a high tensile strength and is insoluble in most solvents (Sjöström, E., 1981). Orientation of the linkages and additional hydrogen bonding makes the polymer rigid and difficult to break (Hamelinck et al., 2005). The molecular arrangement of this fibrillar bundle is sufficiently regular that cellulose exhibits a crystalline X-ray diffraction pattern (Fan et al., 1987). Typically, cellulose chains in primary plant cell walls have degrees of polymerization in the range of 5,000 to 7,500 glucose monomer units, with the degree of polymerization of cellulose from wood being around 10,000 and around 15,000 from cellulose cotton. The basic repeating unit of cellulose is cellobiose. Under normal conditions, cellulose is extremely insoluble in water, which is of course necessary for it to function properly as the structural framework in plant cell walls (Wyman et al., 2005).

Hemicellulose : were originally believed to be intermediates in the biosynthesis of cellulose. Today it is known, however, that hemicelluloses belong to a group of heterogeneous polysaccharides which are formed through biosynthetic routes different from that of cellulose. In contrast to cellulose which is a homopolysaccharide, hemicelluloses are heteropolysaccharides (Sjöström, E., 1981) (as cited by Nigus Worku, 2011). Hemicelluloses are heterogeneous polymers of pentoses (xylose, arabinose), hexoses (mannose, glucose, galactose), and sugar acids. They are generally cataloged according to the main sugar residue in the backbone, e.g., xylans, mannans, and glucans, with xylans and mannans being the most common (Wyman et al., 2005).Hemicellulose, because of its branched, amorphous nature, is relatively easy to hydrolyze (Hamelinck et al., 2005). Some hemicelluloses contain mostly xylan, whereas others contain mostly glucomannans.

Among softwood hemicelluloses there are galactoglucomannans, arabinoglucuronoxylan, and arabinogalactan, meanwhile hardwood hemicellulose comprises mainly glucuronoxylans and glucomannan (Sjöström, E., 1981). Besides xylose, xylans contain arabinose, glucuronic acid, 4-O methyleter, acetic, ferulic, and p-coumaric acids. For example, corn fiber xylan is one of the complex heteroxylans containing β – (1, 4) linked xylose residues. It contains 48-54% xylose, 33-35% arabinose, 5-11 % galactose, and 3-6% glucuronic acid (Saha, B.C. and Bothast, R.J., 1997).

Lignin: is a long-chain, heterogeneous polymer composed largely of phenyl propane units most commonly linked by ether bonds (Saha et al., 1997).

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Figure 2.6 Schematic diagram of a representative section of the molecular structure of softwood lignin ( Hamelinck et al., 2005)

It is presented in all lignocellulosic biomass; therefore, any ethanol production process will have lignin as a residue (Hamelinck et al., 2005). It is a large, complex polymer of the phenylpropane and methoxy groups, a non-carbohydrate polyphenolic substance that encrusts cell walls and reinforces cells together figure 2.6 (Hamelinck et al., 2005).

Lignins can be divided into several classes according to their structural elements. So-called “guaiacyl lignin” which occurs in almost all soft woods is largely a polymerization product of coniferyl alcohol. The “guaiacyl-siryngyl”, typical of hardwoods, is a copolymer of coniferyl and sinapyl alcohols, the ratio varying from 4:1 to 1:2 for the two monomeric units (Sjöström, E., 1981). Guaiacyl-lignins have a methoxy-group in both the 3-carbon and 5-carbon positions (Palmqvist et al., 2000). Lignin effectively protects the plant against microbial attack and only a few organisms, including rot-fungi and some bacteria, can degrade it. Lignin restricts hydrolysis by shielding cellulose surfaces or by adsorbing and inactivating enzymes. It was understood that the close union between lignin and cellulose prevented swelling of the fibers, thereby affecting enzyme accessibility to the cellulose. To solve this problem, several studies have shown that taking away lignin enhances cellulose hydrolysis (Wyman et al., 2005). The conversion of cellulose and hemicellulose to fuels and chemicals generates lignin as a by-product. Such by-product can be burned to provide heat and electricity, or used to manufacture various polymeric materials (Saha et al., 1997). There are some publications on microbial breakdown of lignin; however, due to extreme complexity of the problem, a vast amount of research needs to be done (Saha et al., 1997).

Extractives: are woody compounds that are soluble in neutral organic solvents or water. The extractives usually represent a minor fraction (between 1-5%) of lignocellulosic materials. They contain a large number of both lipophilic and hydrophilic constituents. The extractives can be classified in four groups: (a) terpenoids and steroids, (b) fats and waxes, (c) phenolics constituents and, (d) inorganic components (Taherzadeh, 1999).

2.3.3 Prosopis juliflora as Ethanol feedstock

This plant was described by De Candolle under the name of Prosopis juliflora. The specific name juliflora, comes from julus meaning whip-like; referring to the long inflorescence, and flora being flower (Havard, 1884).The genus Prosopis was systematically described and organized by Burkart (1976) in to five sections that together contains 44 species and with many varieties (Pasiecznik et al., 2001). Prosopis juliflora belonging to the family Leguminaceae (Fabaceae) and subfamily Mimosoideae, section Algarobia that has six series; specifically it belongs to the series Chilensis that contain eleven species and many varieties. Prosopis juliflora is particularly closely connected to Prosopis pallida. It is a tree or shrub sized woody perennial plant found mainly in the arid and semi arid regions of the world (Geezing et al., 2004). The plant is predominantly xerophilous spiny and sometimes unarmed evergreen tree with height of 3-15 meters depending on genetic difference and other environmental factors, but under favorable environmental conditions some individuals may reach up to 20m (Pasiecznick et al., 2003).

Prosopis juliflora landraces often have multi-stemmed, coppiced and prostate shrub forms with long branches and a crown that even touches the ground and have erect, flat topped and decumbent tree forms. Prosopis juliflora produced coppices except those stumped at 10 cm below the ground (Hailu Shiferaw et al., 2004).

Mean monthly maximum temperature above 300C linked to the availability of soil moisture between 40-60% favored the weed germination and establishment. With increasing temperature and fluctuating precipitation, weeds may pose threats to the biodiversity. Prosopis juliflora can thrive in a wide range of rainfall. It extends from areas with an annual rainfall of only 50 mm in dry coastal zones to1500 mm mean annual precipitation of high altitude and grows well in areas receiving 250-600 mm annual rainfall (FAO, 2002; Pasiecznik et al., 2001). It can also survive areas where lowest rainfall recorded in Arabian and Atacama Desert of the world (Ibrahim et al., 1988). If the root system is able to find water during drought, Prosopis juliflora will stay in green leaf throughout the dry season. Altitude does not appear to be limiting factor for the distribution of the plant. It is generally well adapted to different altitudes ranging from 200 meters above sea level up to 1500 meters above sea levels (Pasiecznik et al., 2001).

2.3.4 Prosopis juliflora in Ethiopia

Amibara Woreda of Afar NRS is thought to be the assumed starting point for the spread of Prosopis juliflora in Ethiopia. The area represents degraded semi-arid ecosystem in the country. Contrary to the purposes of its introduction and Prosopis juliflora is rapidly invading the traditional agro- and silvo-pastoral land of the Afar and Isa ethnic groups in the Afar National Regional State and has encroached hundreds of kilometers away from the initial plantation area. Prosopis Juliflora now occupies about over 700,000 hectares of prime grazing land and cultivable land following the Awash River in the Afar Region. It was also pointed out that the species has spread rapidly in eastern Ethiopia, from the Middle Awash Valley in to the Upper Awash Valley and Eastern Hararghe and some localities of Raya Azebo plaines of South Tigray. It is now a common sighting from Awash Arba all the way to Dire Dawa and Harar (Senayit et al., 2004; Taye et al., 2004).

The naturalized extent of invasion is unknown at this stage, but is estimated to be in the order of 4000 ha, especially at Afar NRS. Invasion of Prosopis juliflora is also reported in the town of Arba Minch and neighboring localities in the Southern Region of the country. However, no systematic survey and monitoring have been undertaken to determine the distribution, area coverage and density of Prosopis juliflora in Ethiopia (Rezene et al., 2005).

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