Lesson 16. Straw as a fuel

16.1. Introduction

In India, annual production of rice is about 136.5 million tonnes and about 136.5-150 million tonnes of paddy straw is estimated to be produced. Disposal of paddy straw has been a big problem since time immemorial. Paddy straw burning can be commonly seen during the harvesting season which causes soil erosion and emission of pollutants. Little proportion of paddy straw is also used for composting, surface mulching, in situ incorporation, mushroom production, animal feed and like. Direct combustion of biomass for heat and power generation, Biogas generated via anaerobic digestion, producer gas generated through gasification, ethanol production are the common ways of utilizing paddy straw for energy. 

16.2. Technologies for producing fuel from rice straw

In general, paddy straw can be converted into energy products via two processes:

1. Thermochemical processes

  • Combustion

  • Pyrolysis

  • Gasification

2. Biochemical processes

  • Anaerobic digestion

  • Fermentation

16.3. Pre-treatment prior to thermo-chemical conversion

The typical pre-treatment technologies include sizing, leaching (commonly known as washing) and pelletisation. Sizing refers to the process of reducing the size of rice straw with the aim to improve boiler efficiencies. In general, fuel with small-sized particles provides higher burning rates as well as ignition front speeds, leading to better combustion efficiency. Looking from the perspective of chemical properties, the high alkali content of rice straw can ultimately lead to slagging and fouling problems in combustion equipment. Hence, pre-treatment of these paddy straw becomes an integral part of utilizing these resources as fuel. Leaching process can remove unwanted substances in the rice straw. It can reduce slagging, fouling as well as corrosion problems in furnaces system and subsequently extend the operating life of a boiler. Studies have shown that distilled water or tap water can efficiently reduce the quantity of potassium, sodium and chlorine in the rice straw.  

16.4. Pre-treatment prior to bio-chemical conversion

In biochemical conversion, pretreatment of lignocellulosic biomass is an essential step that emphasizes on the removal of lignin network. The cellulose and hemicelluloses components of paddy straw are embedded within the lignin network consisting of polysaccharide layers that prevent the enzymatic hydrolysis. Hence, to expose the cellulose and hemicelluloses for enzymatic action, and subsequently increase the bioconversion efficiency, the lignin network must be removed with proper pretreatment.

Based on the application and type of pretreatment catalyst (liquid and steam water are not considered a catalyst in this paper), pretreatment techniques have generally been divided into three distinct categories, including physical, chemical, and biological pretreatment.

16.4.1. Physical pretreatment

Physical pretreatment does not use chemical agents, and typically includes uncatalyzed steam explosion, liquid hot water pretreatment (LHW), mechanical comminution, and high energy radiation. The former two pretreatment methods are more common than the later.

16.4.2. Chemical pretreatment

Chemical pretreatments were originally developed and have been extensively used in the paper industry for delignification of cellulosic materials to produce high quality paper products. The possibility of developing effective and inexpensive pretreatment techniques by modifying the pulping processes has been considered. Chemical pretreatments that have been studied to date have had the primary goal of improving the biodegradability of cellulose by removing lignin and/or hemicellulose, and to a lesser degree decreasing the degree of polymerization and crystallinity of the cellulose component. Chemical pretreatment is the most studied pretreatment technique among pretreatment categories. The seven common chemical pretreatment techniques include catalyzed steam-explosion, acid, alkaline, ammonia fiber/freeze explosion, organosolv, pH-controlled liquid hot water and ionic liquids pretreatments.

16.4.3. Biological pretreatment

Biological pretreatment employs wood degrading microorganisms, including white-, brown-, soft-rot fungi, and bacteria to modify the chemical composition and/or structure of the lignocellulosic biomass so that the modified biomass is more amenable to microbial action/enzyme digestion. Fungi have distinct degradation characteristics on lignocellulosic biomass. In general, brown and soft rots mainly attack cellulose while imparting minor modifications to lignin, and white-rot fungi are more actively degrade the lignin component.

16.5. Thermo-chemical process

Thermo-chemical processes can be divided into two categories. The first category involves direct utilisation of biomass as fuel for combustion, and subsequently for heat and electricity generation. The second category involves converting biomass into other useful forms of energy products prior to its utilisation as a source of energy.

16.5.1. Direct combustion

Rice straw could be used with the current heat and power technologies in many rice producing countries to replace fossil fuels to reduce sulphur dioxide and greenhouse emission.
In direct combustion, straw is utilised as a fuel in a combustion boiler to produce steam (a heat source) in the presence of sufficient air in the combustion chamber. Heat and electricity can be simultaneously generated (cogeneration) using turbines. Generally, biomass combustion technologies can be categorised into the fixed bed and fluidised bed combustion systems.

Co-firing is the simultaneous combustion of different fuels in the same boiler. Many coal and oil-fired boilers at power stations have been retrofitted to permit multi-fuel flexibility. Biomass such as paddy straw is a well-suited resource for co-firing with coal as an acid rain and greenhouse gas emission control strategy. Co-firing is a fuel-substitution option for existing capacity and is not a capacity expansion option.

Sizing is done to increase the energy conversion efficiency and combustion performance which involves the cutting of straw into smaller sizes to improve boiler efficiency. Rice straws are dried in the air for two weeks, the dried rice straws with length ranges from 70-140 cm are cut to 6.4 mm or smaller particle size. Equipment such as hoggers, hammer mills, spike rolls, and disc screens are required to properly size the feedstock. Unlike coal, paddy straw contains very small amounts of sulphur. Hence, substitution of paddy straw for coal can result in significant reductions in sulphur dioxide (SO2) emissions. Co-firing of biomass residues, rather than crops grown for energy, brings additional greenhouse gas mitigation by avoiding CH4 release from the otherwise landfilled biomass. It is believed that CH4 is 21 times more potent than CO2 in terms of global warming impact.

16.5.2. Pyrolysis

Pyrolysis is a decomposition process of biomass at high temperature in the absence of air. Pyrolysis occurs under pressure and suitable typical operating temperature range between 350 oC and 550oC. The end products are in the form of gas and liquid as well as carbon-rich solid residue. The proportion of the products depends on the operating conditions.

16.5.3. Gasification

For electricity generation, two most competitive technologies are direct combustion and gasification. Gasification is the thermo-chemical process required to convert rice straw to producer gas, fuel that could replace natural gas and diesel. Fluidized bed gasification has been investigated since 1981 as a method to produce low Btu gas from rice straw. The system uses a bed of sand inside a refractory-lined cylinder reactor. The rice straw is fed into the sand bed which is fluidized by air supplied from below. The air provides only one-fifth to two-fifths of the amount needed for total combustion. Producer gas is a mixture of combustible gases carbon dioxide, hydrogen, methane, and a small amount of higher carbon gases. It also contains water vapor and nitrogen gas. The combustible gases range from 25 to 40 percent by volume of total gases. producer gas can be used in internal combustion (IC) engine to produce heat, or in a cogeneration system to produce heat and electricity.

In India gasifier technology has penetrated the applications such as village electrification, captive power generation and process heat generation in industries producing biomass waste such as rice straw, rice husk, bagasse, wood waste, wood, wild bushes and paper mill waste. Nearly 55 MW of grid connected biomass power capacity is commissioned and another 90 MW capacity is under construction.

16.6. Bio-chemical process

The bio-chemical process routes for biomass conversion into value-added products include the production of ethanol, hydrogen as well as methane.

16.6.1. Biogas generation via anaerobic digestion

Paddy straw has high content of cellulose (35-40%), hemi-cellulose (20%), lignin (12%) and silica (8%). But, the lignin complex and silica incrustation shields the microbial action and hence restricts paddy straw digestibility. So, the first step towards economical utilization of paddy straw is to remove/degrade lignin and silica to enable cellulose to be more accessible to the microbial/enzymatic attack. Pretreatment methods including alkali pretreatment, heat pretreatment, size reduction and seeding have been explored to increase the digestibility of straw. Among these methods alkali pretreatment is notably effective in treating straw for anaerobic digestion.

Biogas can be produced from paddy straw by anaerobic fermentation using cattle dung as a source of inoculum. Biogas generation involves consortium of microorganisms which is a group of hydrolytic, acidogenic and methanogenic bacteria. Hydrolytic bacteria degrade the complex organic matter (carbohydrates, proteins and fats) into simpler forms (sugars, amino acids, fatty acids and glycerol). Acidogenic bacteria breakdown these simpler forms (sugars, amino acids, fatty acids and glycerol) into CH3COOH, H2 and CO2 which is further utilized by methanogenic bacteria to produce biogas. Biogas is mixture of CH4 (50-60%), CO2 (30-40%),
H2 (1-5%), N2 (0.5%), CO, H2S and water vapors. The biogas is subsequently utilized as fuel to generate heat and energy.

16.6.2. Production of ethanol from fermentation of paddy straw

Production of ethanol from paddy straw contains three major processes, including pretreatment, enzymatic hydrolysis, and fermentation. Pretreatment is required to alter the biomass macroscopic and microscopic size and structure as well as its sub-microscopic structural and chemical composition to facilitate rapid and efficient hydrolysis of carbohydrates to fermentable sugars which has been discussed in the earlier section. Hydrolysis refers to the processes that convert the polysaccharides into monomeric sugars. The fermentable sugars obtained from hydrolysis process could be fermented into ethanol by ethanol producing microorganisms, which can be either naturally occurred or genetically modified.

After fermentation, ethanol can be recovered from the fermentation broth by distillation or distillation combined with adsorption or filtration, including drying using lime or a salt, addition of an entrainer, molecular sieves, membranes, and pressure reduction. The distillation residual solid, including lignin, ash, enzyme, organism debris, residue cellulose and hemicellulose, and other components may be recovered as solid fuel or converted to various value-added co-products.

16.6.3. Hydrogen production via fermentation

Hydrogen production from fermentation of agricultural wastes is a relatively new research area as compared to the well established anaerobic digestion. During fermentation, anaerobic bacteria ferment carbohydrates to produce hydrogen, volatile fatty acids and carbon dioxide. The fermentation process can be divided into photo-fermentation and dark fermentation where different types of bacteria function under different operating conditions. Biomass fermentation with carbohydrates such as rice or other agricultural wastes is a promising route to produce hydrogen. Further increase in the hydrogen production, yield to an economically feasible level, coupled with continuous development of industrial scale operations are however still needed.

In many countries paddy straw has great potential to be converted into energy in order to meet the countries’ energy demands. India, China, Indonesia and other rice producing countries can enjoy the environmental and economic benefits from utilization of rice straw as a source of renewable energy. Heat and electricity from cogeneration systems could be used to meet the energy demands of local rice mills. Alternatively, excess electricity can be exported to the national grid. Other potential sources of energy from rice straw that can be used for heating and power generation include methane and hydrogen generated via various biomass conversion processes. Ethanol is another important source of energy derived particularly from rice straw. It is typically used for public transportations and has potential to reduce dependency on fossil fuels. Despite all the potential benefits, further research is still required on optimal allocation of rice straw resources in rice mills as well as on industrial commercialization of these technologies.



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Last modified: Tuesday, 29 April 2014, 8:38 AM