Ethanol from Cellulose Waste

Published on 24/10/2011

Waste can be defined as rubbishtrashrefusegarbagejunklitter, and ort which are unwanted or useless materials for us. If we talk about the biological definition of waste then we can say that wastes are unwanted substances, metabolic refuse or toxins which are expelled from living organisms such as urea, sweat or feces. The quantity of waste generation is directly proportional to human development which can be either technological or social.

As soon as the development occurs, deforestation also takes place. When we cut forests & trees it disturbs the ecological balance in that area and this process leads to the generation of a special type of waste called ‘cellulose waste’. Cellulose is found in outer surface of the plant cell which gives them a tough and rigid structure.

What is cellulose?

Cellulose is the most common organic compound on Earth which is the structural component of the primary cell wall of green plants. It is also found in many forms of algae and oomycetes. It has been observed that some species of bacteria secrete it to form their bio films. Biochemistry says that cellulose is an organic compound having its empirical formula (C6H10O5)n which is nothing but a polysaccharide that consists of a linear chain of several hundred to over ten thousand ?(1?4) linked D-glucose units. The structure of the cellulose compound is:

Structure of Cellulose
Structure of Cellulose

Cellulose contributes of about 33% of all plant matter. The cotton which we use in our day today practices contains cellulose content of about 90% than that of wood is 40–50%. For paper and pulp industry cellulose is mainly obtained from chips and pulp of wood and cotton. In this industry cellulose is mainly used to produce paperboard and paper. At very small scale it can also be converted into a wide variety of derivative products such as cellophane and rayon. It is the commonly found natural substance which has a rigid structure that is why it has got many applications.

Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms that live in their guts. Humans can digest cellulose to some extent; however it is often referred to as “dietary fiber” or “roughage” (e.g. outer shell of maize) and acts as a hydrophilic bulking agent for feces.

How is cellulose waste Generated?

Waste containing cellulose in high concentration is generated from the various industries. In the paper and pulp industry for the production of the paper the raw material is denaturized using the chemicals to make the pulp and to extract the cellulose from the raw material thus making it cellulose free. It is then brought to the paper making section. Waste is also generated in the pulp making section which is recycled to the pulp making plant. Apart from this cellulose waste is also generated from the various sections of textile industry and sugar industry. In sugar industry when sugarcane stalks are pressed in the mills for juice the solid refuse left behind is called Bagasse which contains cellulose fibers in a high concentration. The disposal of this solid waste bagasse is the major problem for the establishment of sugar industry. Nowadays there are technologies which different companies are practicing to convert this bagasse into paper. Some industries which are not aware of this fundamental, they burn this bagaase to convert it into ash or in rural areas they use it as a fuel for making food or in domestic use.

Utilization of Cellulose waste

In present time for the management of cellulose waste technologies are still under development. Some of the ways to dispose of the cellulose waste is incineration. Incineration causes air pollution which can be fatal for the health of human being.

Researchers have found alternative to this problem by making possible the conversion of cellulose waste into ethanol rich biofuels. Cellulosic ethanol is a biofuel produced from wood, grasses, or the non-edible parts of plants. It is a kind of biofuel which is generally produced from lignocellulose which is a structural material and it comprises much of the mass of the plants. The building block units of lignocellulose are cellulose, hemicellulose and lignin. Lignocelluloses have increasingly favoured as most suitable raw material for the next generation of bio-ethanol around the world, as it’s a renewable energy source that, unlike most, is not based on a food crop. By products of lawn and tree maintenance waste, switchgrass, wood chips, miscanthus, solid refuse from paper & pulp industry, sugar industry ant textile industry are some of the cellulosic material for the production of ethanol. Switchgrass and Miscanthus are the major biomass materials being studied today due to their high productivity per acre.  The technology has advantage of diverse and abundant raw materials which are of no use and are easily available but it requires a greater amount of processing to make the monomeric units of sugar for the fermentation of ethanol. These monomeric units of sugar are consumed by the microorganisms in their metabolism as a substrate to convert them into biofuel, ethanol.

Switchgrass and Miscanthus are the major biomass materials being studied today, due to their high productivity per acre. Cellulose, however, is contained in nearly every natural, free-growing plant, tree, and bush, in meadows, forests, and fields all over the world without agricultural effort or cost needed to make it grow.

Production Methodology

Ethanol from cellulose can be produced by two processes:

  1. A.    Cellulolysis: This is also called the Biological process. In this process hydrolysis of pretreated lignocellulosic materials is done. Hydrolysis involves use of enzymes to break complex cellulose into simple sugars such as glucose. After hydrolysis fermentation and distillation is done to get pure form of ethanol. Cellulolysis is a step by step process which converts the cellulose waste into ethanol as follows:
  2. The very first step which is generally practiced in production of ethanol is the “pretreatment” phase. This step involves making the lignocellulosic material such as wood or straw amenable to hydrolysis.
    1. Once pretreatment is done hydrolysis of cellulose (cellulolysis) is performed in order to break down the molecules into the monomeric units of sugars.
    2. After that sugar solution is separated out from the residual materials.
    3. In the next step microbial fermentation of the sugar solution takes place.
    4. Fermented product contains many impurities which are not suitable for Ideal fuel so these are removed through distillation. Distillation produces roughly 95% pure alcohol.
    5. Then after distillation Dehydration is done by molecular sieves to bring the ethanol concentration to over 99.5%

However, in 2010, a genetically engineered yeast strain has been developed that produces its own cellulose-digesting enzymes. Assuming this technology can be scaled to industrial levels, it would eliminate one or more steps of cellulolysis, reducing both the time required and costs of production.

  1. B.     Gasification: In this process the transformation of lignocellulosic raw material takes place to convert them into gaseous carbon monoxide and hydrogen. After that these gases can be converted to ethanol by microbial fermentation or chemical catalysis. This process neither rely on chemical decomposition of the cellulose chain nor any kind of pretreatment is required. In this technique the carbon in the raw material is converted into synthesis gas instead of breaking the cellulose into sugar molecules, by partial combustion. The carbon monoxide, carbon dioxide and hydrogen may then be fed into a special kind of fermenter. Instead of sugar fermentation with yeast, this process uses a microorganism named Clostridium ljungdahlii. In this process microorganism ingests (eat) carbon monoxide, carbon dioxide and hydrogen and produce ethanol and water. The process can thus be broken into three steps:
    1. Gasification — Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
    2. Fermentation — Convert the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
    3. Distillation — Ethanol is separated from water

A recent study has found another Clostridium bacterium that seems to be twice as efficient in making ethanol from carbon monoxide as the one mentioned above.

Advantages & Environmental Effects

According to U.S. Department of Energy studies conducted by Argonne National Laboratory of the University of Chicago, one of the benefits of cellulosic ethanol is that it reduces greenhouse gas emissions (GHG) by 85% over reformulated gasoline. By contrast, starch ethanol (e.g., from corn), which most frequently uses natural gas to provide energy for the process, may not reduce GHG emissions at all depending on how the starch-based feedstock is produced. In comparison to gasoline, ethanol burns cleaner, thus produces less carbon dioxide and overall pollution in the air. Additionally, only low levels of smog are produced from combustion. Carbon dioxide gas emissions are shown to be 85% lower than those from gasoline. Cellulosic ethanol contributes little to the greenhouse effect and has a five times better net energy balance than corn-based ethanol. When used as a fuel, cellulosic ethanol releases less sulfur, carbon monoxide, particulates, and greenhouse gases. Cellulosic ethanol should earn producers carbon reduction credits, higher than those given to producers who grow corn for ethanol, which is about 3 to 20 cents per gallon.

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