Bioethanol from Cellulosic Materials

Cellulosic resources such as paper, cardboard, wood, agricultural residues and other fibrous plant material are in general very wide spread and abundant. For example, forests comprise about 80% of the world's biomass. Ethanol can be produced from different kinds of raw materials. The raw materials are classified into three categories of agricultural raw materials: simple sugars, starch and cellulose. Cellulosic biomass materials can be supplied from a variety of resources at a low price. They can be classified in four groups based on type of resource: wood,municipal solid waste, waste-paper and crop residue resources.

Being abundant and outside the human food chain, makes cellulosic materials relatively inexpensive inputs for ethanol production. Cellulosic materials are comprised of lignin, hemicelluloses, and cellulose and are thus sometimes called lignocellulosic materials. Cellulose molecules consist of long chains of glucose molecules (6-carbon sugars) as do starch molecules, but have a different structural configuration. These structural characteristics plus the encapsulation by lignin makes cellulosic materials more difficult to hydrolyze than starchy materials. Hemicelluloses are also comprised of long chains of sugar molecules, but contain pentoses in addition to glucose.

Based on average 42% cellulose and 21% hemicelluloses in wood, the maximum theoretical yield of ethanol can be calculated to be 0.32 grams of ethanol per gram of wood. This calculation is based on a full conversion of cellulose and hemicelluloses to sugars, and conversion of sugars to ethanol at the theoretical yield of 0.51 g/g.

Since pentose molecules (5-carbon sugars) comprise a high percentage of the available sugars, the ability to recover and ferment them into ethanol is important for the efficiency and economics of the process. Recently, special microorganisms have been genetically engineered which can ferment 5-carbon sugars into ethanol with relatively high efficiency. Bacteria have drawn special attention from researchers because of their speed of fermentation.

Cellulose hydrolysis produces glucose, which is readily fermented with existing organisms in much the same way as has been done for centuries. Hemicelluloses hydrolysis produces both hexose and pentose sugars: mannose, galactose, xylose and arabinose that are not all fermented with existing strains. The hemicelluloses fraction typically produces a mixture of sugars including xylose,arabinose, galactose and mannose. These are both pentosans: xylose and arabinose, andhexosans: galactose and mannose. The quantities are dependent on the material and alsothe growing environment and storage history of the material.

Pretreatment methods refer to the solubilization and separation of one or more of the four major components of biomass (hemicellulose, cellulose, lignin, and extractives) to make the remaining solid biomass more accessible to further chemical or biological treatment.Hydrolysis (saccharification) breaks down the hydrogen bonds in the hemicellulose andcellulose fractions into their sugar components: pentoses and hexoses. These sugars can then be fermented into bioethanol. After the pretreatment process, there are two types of processes to hydrolyze the cel-lulosic biomass for fermentation into bioethanol. The most commonly applied methodscan be classified in two groups: chemical hydrolysis (dilute and concentrated acid hydro-lysis) and enzymatic hydrolysis. In addition, there are some other hydrolysis methods in which no chemicals or enzymes are applied. For instance, lignocellulose may be hydrolyzed by gamma ray or electron beam irradiation, or microwave irradiation. However,those processes are commercially unimportant. Both enzymatic and chemical hydrolyses require a pretreatment to increase the sus-ceptibility of cellulosic materials. In the chemical hydrolysis, the pretreatment and thehydrolysis may be carried out in a single step. There are two basic types of acid hydrolysis processes commonly used: dilute acid and concentrated acid, each with variations.

Dilute Acid Hydrolysis

The dilute acid process is conducted under high temperature and pressure, and has a reaction time in the range of seconds or minutes, which facilitates continuous processing. As an example, using a dilute acid process with 1% sulfuric acid in a continuous flow reactor at a residence time of 0.22 minutes and a temperature of 510 K with pure cellulose provided a yield over 50% sugars. In this case, 1000 kg of dry wood would yield about 164 kg of pure ethanol. The combination of acid and high temperature and pressure dictate special reactor materials, which can make the reactor expensive. The first reaction converts the cellulosic materials to sugar and the second reaction converts the sugars too their chemicals. Unfortunately, the conditions that cause the first reaction to occur also are the right conditions for the second to occur.The biggest advantage of dilute acid processes is their fast rate of reaction, which fa-\cilitates continuous processing. Since 5-carbon sugars degrade more rapidly than 6-carbon sugars, one way to decrease sugar degradation is to have a two-stage process. The first stage is conducted under mild process conditions to recover the 5-carbon sugars while the second stage is conducted under harsher conditions to recover the 6-carbon sugars. 

Concentrated Acid Hydrolysis

Hydrolysis of cellulosic materials by concentrated sulfuric or hydrochloric acids is arelatively old process. The concentrated acid process uses relatively mild temperatures,and the only pressures involved are those created by pumping materials from vesselto vessel. Reaction times are typically much longer than for dilute acid. This method generally uses concentrated sulfuric acid followed by a dilution with water to dissolve and hydrolyze or convert the substrate into sugar. This process provides a complete and rapid conversion of cellulose to glucose and hemicelluloses to 5-carbon sugars with little degradation. The critical factors needed to make this process economically viable are to optimize sugar recovery and cost effectively recovers the acid for recycling. The solid residue from the first stage is dewatered and soaked in a 30 to 40% concentration ofsulfuric acid for 1 to 4 hours as a pre-cellulose hydrolysis step. The solution is again dewatered and dried, increasing the acid concentration to about 70%. After reacting in another vessel for 1 to 4 hours at low temperatures, the contents are separated to recover the sugar and acid. The sugar/acid solution from the second stage is recycled to the first stage to provide the acid for the first stage hydrolysis. The primary advantage of the concentrated acid process is the potential for high sugar recovery efficiency. 

The acid and sugar are separated via ion exchange and then acid is reconcentrated via multiple effect evaporators. The low temperatures and pressures employed allow the use of relatively low cost materials such as fiberglass tanks and piping. The low temperatures and pressures also minimize the degradation of sugars. Unfortunately, it is a relatively slow process and cost effective acid recovery systems have been difficult to develop. Without acid recovery, large quantities of lime must beused to neutralize the acid in the sugar solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and creates additional expense. 

Enzymatic Hydrolysis

Another basic method of hydrolysis is enzymatic hydrolysis. Enzymes are naturally occurring plant proteins that cause certain chemical reactions to occur. There are two technological developments: enzymatic and direct microbial conversion methods.

The chemical pretreatment of the cellulosic biomass is necessary before enzymatic hydrolysis. The first application of enzymatic hydrolysis was used in separate hydrolysis and fermentation steps. Enzymatic hydrolysis is accomplished by cellulolytic enzymes. Different kinds of "cellulases" may be used to cleave the cellulose and hemicelluloses. A mixture of endoglucanases, exoglucanases, glucosidases and cellobiohydrolases is commonly used. The endoglucanases randomly attack cellulose chains to produce polysaccharides of shorter length, whereas exoglucanases attach to the non reducing ends of these shorter chains and remove cellobiose moieties, glucosidases hydrolyze cellobiose and other oligosaccharides to glucose. 

For enzymes to work efficiently, they must obtain access to the molecules to behydrolyzed. This requires some kind of pretreatment process to remove hemicelluloses and break down the crystalline structure of the cellulose or removal of the lignin toexpose hemicelluloses and cellulose molecules.

Biomass residues available from agricultural and forest processing constitute a potential source for production of chemicals such as ethanol, reducing sugars and furfural,using enzyme or acid-catalysed hydrolysis. Bioethanol can be produced from plentiful,domestic, cellulosic biomass resources such as herbaceous and woody plants, agricultural and forestry residues, and a large portion of municipal solid waste and industrial waste streams. To ensure that a low cost energy feedstock is available, researchers are examining dedicated energy crops, wood and grass species that have been selected to produce high yields. To produce bioethanol from cellulosic biomass, a pretreatment process is used to reduce the sample size, break down the hemicellulose to sugars, and open up the structure of the cellulose component. The cellulose portion is hydrolyzed by enzymes into glucose sugar that is fermented to bioethanol. The sugars from the hemicellulose are also fermented to bioethanol. The worldwide desire to reduce greenhouse gas emission will lead to an increased interest in renewable resources for energy production. Cellulosic biomass materials are among the candidates to be used as a renewable resource. Ethanol has very good characteristics to be used as a fuel either in a neat form or in a mixture with gasoline. Bioethanol is a domestically produced liquid fuel from cellulosic biomass resources. It is a high octane fuel that can contribute substantially to the automotive fuel supply. Ethanol is a potentially clean burning fuel that reduces smog and emissions of carbon monoxide. The use of gasohol (ethanol and gasoline mixture) as an alternative motor fuel has been steadily increasing around the world for a number of reasons. Domestic production and use of ethanol for fuel can decrease dependence on foreign oil, reduce trade deficits, create jobs in rural areas, reduce air pollution, and reduce global climate change carbon dioxide build up. Ethanol, unlike gasoline, is an oxygenated fuel that contains 35% oxygen, which reduces particulate and NOx emissions from combustion.