Feasibility Study of Power Generation from Rice husk in Bangladesh
Background of the Study
In this modern world ‘Power’ and ‘Power Crisis’ are probably the most discussed issues. Along with the advancement of human civilization, the demand of power has increased with proportional quantity. Meeting the ever rising demand of energy is a challenging task for the nations to survive. In search of newer sources of power and for development of the existing systems, continuous research is going on in this field.
In a third world country like Bangladesh, power crisis has been an important issue since her birth. The recent discovery about the limited gas reserve in the country has worsened this condition as most of her present power plants use gas as fuel. So it is high time for the Govt. to look for other options like coal or renewable energy as sources of power. For an agricultural country like ours, rice husk is a probable source of fuel for generation of power. In this paper, we have tried to discover whether rice husk is a feasible option in respect of Bangladesh or not.
1.2 Renewable Energy
Renewable energy is energy generated from natural resources—such as sunlight, wind, rain, tides, and geothermal heat—which are renewable (naturally replenished) . In 2006, about 18% of global final energy consumption came from renewable sources, with 13% coming from traditional biomass (Global Status Report 2007).
Renewable energy sources are those energy sources which are not destroyed when their energy is harnessed . Renewable energy sources are distinct from fossil fuels, which must be consumed to release energy. Human use of renewable energy requires technologies that harness natural phenomena, such as sunlight, wind, waves, water flow, biological processes such as anaerobic digestion, biological hydrogen production and geothermal heat.
Traditional uses of wind, water, and solar power are already widespread; but the mass production of electricity using renewable energy sources has become popular only recently, reflecting the major threats of climate change, concerns about the exhaustion of fossil fuels and the environmental, social and political risks of extensive use of fossil fuels and nuclear power.
Renewable energies are sustainable energies. Renewable energy sources may be harnessed directly, such as in solar ovens, geothermal heating, watermills, and windmills. They may require energy harvesting through appropriate technologies such as: electricity generation through wind turbines or photo electrochemical cells (PEC)s, or photovoltaic cells, production of biofuels such as biogas from anaerobic digestion, or ethanol from biomass .
1.3 Bio Energy and Biomass
Biopower is the process of using biomass (plant and organic matter) to generate electricity. Biomass is organic material made from plants and animals. Biomass contains stored energy from the sun . The chemical energy in plants gets passed on to animals and people that eat them. When burned, the chemical energy in biomass is released as heat. Biofuels can be of three types- solid biomass, liquid Biofuel, biogas.
The term biomass is especially useful for plants, where some internal structures may not always be considered living tissue, such as the wood (secondary xylem) of a tree. Direct use is usually in the form of combustible solids, either wood, the biogenic portion of municipal solid waste or combustible field crops. Field crops may be grown specifically for combustion or may be used for other purposes and the processed plant waste then used for combustion .
Solid biomass is most commonly used directly as a combustible fuel, producing 10-20 MJ/kg of heat. Biomass can also be used to feed bacteria, which can transform it in another form of energy such as hydrogen, using a process called Fermentative hydrogen production. Its forms and sources include wood fuel, the biogenic portion of municipal solid waste, or the unused portion of field crops. Field crops may or may not be grown intentionally as an energy crop, and the remaining plant byproduct used as a fuel. Paddy is such a crop with the byproduct of rice husk. Rice husk can be used as the fuel for power generation.
1.3 Bangladesh Perspective
Power sector is one of the most important criterions in the development process of any country. In a third world country like Bangladesh, its condition has always been a very burning question. In spite of the prevailing problems, the power organizations are trying to maximize the output power.
The physical problems in the power sector consist of infrastructural & technological problems. Bangladesh has very limited resources to utilize in the power sector. Bangladesh does not have enough infrastructures to cope with the demand of the population of the country. Government of Bangladesh has not been consistent in financing the power sector. In previous national budgets, power sector was not given enough money. Also GOB does not have any particular policy on power sector. For the improvement of that sector, short term policies as well as long term policies are necessary. Poor policy making worsens the situation. The problem of corruption really has also crippled our power sector.
The huge shortage of electricity is hurting Bangladesh quite hard in each and every development sector. At present electricity coverage in Bangladesh is only 42% and per capita electricity consumption is about 140 kWh  which is one of the lowest in the World
For the solution of the problems renewable energy can be used as a weapon. Renewable resources offers many benefits compared to other conventional generation like they reduce reliance on fossil fuels, air emissions and other negative environmental impacts, protect against the exhaustion of non-renewable resources, provides provision of electricity in remote locations far from the main grid. Along with solar cell and biogas rice husk has also emerged as an alternative source of power in these days in Bangladesh.
Rice husk presently has been identified as a highly potential source of renewable energy. Based on the experience of other rice growing countries like China, Vietnam and Thailand, we can also opt for husk based cogeneration plant if proper study is carried out before the venture.
In the next few chapters, we will try to depict a true picture of the prevailing condition of Bangladesh in this field and try to analyze the condition from various perspectives.
1.4 Structure of the Thesis
This thesis paper has been divided into 6 chapters including this one. In Chapter 2 we include the different aspects of rice husk. This chapter gives us a brief idea about the definition of husk, sources and uses of it and the role of rice mills in this field. Our visit to a automatic rice mill has been included as a case study in this chapter.
In chapter 3, the conditions for a plant site and power generation process from husk have been mentioned. Also we include our findings from the visit to ‘Dreams Power Ltd’. In chapter 4, we enter the main section of the thesis and study the feasibility of plants in respect of Bangladesh. In chapter 5, we have mentioned some recommendations based on our study that we felt would be helpful if were ventured by the Govt. in this regard.
Finally in chapter 6, we have mentioned the conclusive remarks about the total study. We also mentioned here our shortcomings and the problems we had to face during the study, the contribution of this dissertation and the scope of future work.
We have also included a list of references and an appendix with our work. Interested readers may consult these references in case of any obscurity and verification.
Rice Husk and Different Aspects
To understand the uses and applications of rice husk in power generation discussed in later chapters, we need a clear perception of different aspects of rice husk. Accordingly, we are now going to give some basic information about rice husk, its types, chemical component analysis and its application in different fields.
We have also included a brief description of paddy, the source of husk and current situation of paddy production in Bangladesh as usable amount of husk largely depends upon it. Also a brief overview of the rice mills all over the country is included here along with their paddy processing capacity and operations.
To get along with the topics, we visited an automatic rice mill where husk is produced. In this chapter we have included our findings from the visit which will be helpful to be familiar with the topics.
2.2 Rice Husk
Rice, a cereal food plant, Oryza sativa, of the grass family Gramineae, is extensively cultivated in warm climates, especially in East Asia, producing seeds that are cooked and used as food . About 40 percent of the world’s population derives most of their calories from rice. Almost 90 percent of the population of Bangladesh, Myanmar, Sri Lanka, Vietnam and Kampuchea are rice eaters. Rice husk is the main by product of rice production. In this section an introduction to rice husk is included.
2.2.1 What is Rice Husk?
The rice husk is the hard, protective shell on the grain . To protect the seed during the growing season, the husk is made of hard materials, including opaline silica and lignin. The husk is mostly indigestible to humans. During the milling process, the husks are removed from the grain to create white rice. The very high content in amorphous silica of the husk confer to them and to their ash (SiO2 ~ 20 wt. %) after combustion very valuable properties .
2.2.2 Types of Rice Husk
During rice milling process three types of rice husk are produced. They are –
Pure ground rice husk: It is a coarse component and larger than bran. It is produced separately in automatic rice mills and used as the source of thermal energy needed both for parboiling and drying. The average production of pure rice husk is about 206 kg/ton of paddy and on average 92.7kg/ton is consumed for parboiling and 100 kg/ton is used for drying. The rest amount of husk is sold at a price about 2.78tk/kg .
Pure bran:It can be produced separately from engleberg huller mill with simple modification and mainly sold for pet food at high price .
Mixed:In engleberg huller mills, a mix of ground rice husk and bran is produced. The average production of this type of mixed byproduct is about 290kg/ton of paddy from which 131kg/ton is consumed for steam production and rest amount is mainly sold for poultry feed and fish farm at about 3.54tk/kg .
2.2.3 Byproducts of Paddy
By–products from the growing and processing of rice create many valuable new products. Rice husks, rice stubble, rice bran, broken rice and rice straw are used as common ingredients in horticultural, livestock, industrial, household, building and food product.
Rice stubble is the stalks and roots of the rice plant left in the ground after it has been harvested. Rice stubble is very thick and difficult to deal with. Livestock graze on recently harvested paddocks and eat some of the rice stubble. A portion of the remaining stubble is usually burnt off and a winter cereal crop, such as wheat, is planted. On some rice farms, rice stubble is left to break down naturally and is incorporated into the soil, to improve the soil structure .
Rice bran is the outer layer of the brown rice grain. The rice bran is removed during the milling process if white rice is to be produced. Stabilized rice bran is sold as a health food in supermarkets and health food shops, or to food manufacturers who use it as an ingredient in foods such as crisp breads and breakfast cereals . Unsterilized rice bran is used in stock feed and for other animal and industrial products.
Broken rice grains:
Unfortunately, during the rice milling processes some of the rice grains break. They are removed from the milling process. The larger broken rice grains are used in pet foods and stock feed, or breakfast cereals. The smaller broken rice grains are ground into rice flour which is used in baby foods, snack foods, including muesli bars, or as a baking ingredient. Ground broken rice grains are also used in manufactured foods, such as sausages and milk powder drinks.
Rice straw is the stalks left over after the grains of rice have all been removed in the milling process. Rice straw is used as a building material because it is easy to work with, inexpensive and good for the environment. Some dairy farmers use rice straw as fiber for grain–fed stock. It can also be used to make paper .
2.2.4 Components Analysis of Rice Husk
To know about the gases that are produced as the result of burning rice husk, a complete analysis of rice husk is necessary. In the following figure we have shown the main components of husk that is calculated by chemical analysis . For the full chart we refer the reader to have a look at the Table A.1 and A.2 in appendix.
Figure 2.1: Component analysis of rice husk sample
From the Figure, we see that rice husk mainly contains carbon and oxygen. Methane gas is produced during combustion and separated for power generation . The sulphur content in rice husk is very small and the small amount sulphur is emitted as SO2.
2.2.5 Uses of Rice Husk
Rice husks are mainly used in the following sectors-
Building material: Rice husk is a popular building material  because of its excellent insulation property. It is difficult to burn and protects moisture and mold to propagate through it. It is mainly used to build mud house in Chittagong region.
Fertilizer: Rice husk are low cost material and available for farmers. It can be composted by vermicomposting techniques . Earthworm is used for the process because its high lignin contents can make the process slower.
Industry: Rice husk, as a low cost material, contains silicon carbide which can be used to reinforce ceramic cutting tools, increasing their strength tenfold. Cement industry can use rice husk to add silica in the product itself because rice husk content high silica .
Fuel: Methane gas can be extracted by gasification process which can be used to run gas engines for power generation. It is also used as a fuel in road construction nearly about 40% .
Briquette: Rice husk made briquette is widely used for cooking in urban area where gas is not available. Efficiency is enhanced than normal use in this process.
Poultry bed: It is also used to prepare poultry bed to protect chickens from moisture which is used as fish feed later.
2.3 Source of Rice Husk
Paddy is the only source of rice husk. As a cereal grain, rice is the most important staple food for a large part of the world’s human population, especially in tropical Latin America, the West Indies, East, South and Southeast Asia. Rice is our main food and different kind of rice is cultivated in our country but mainly there types are named.
Different kinds of Paddy
Rice is cultivated in Bangladesh throughout the year as 
· Aman: (i) Transplanted
· Boro: (i) High yielding
About 40 percent of the world’s population derives most of their calories from rice. Almost 90 percent of the population of Bangladesh, Myanmar, Sri Lanka, Vietnam and Kampuchea are rice eaters .
Different Aspects of Paddy
In Table 2.1, the relative per cent age of land occupied by different kinds of paddy along with their period of cultivation & harvesting time is pointed out . With the help of this table we can easily calculate the time during when most amount of husk is produced in Bangladesh.
Table 2.1: Land occupation & Cultivation time of Different kinds of Paddy 
|Paddy||Land occupied by this kind (%))||Planting period||Harvesting period||Cultivation Area|
|Aush||17.59||April-May||July- August||Scattered in most of the districts|
|Aman (Transplanted)||46.30||August-September||November-December||Throughout Bangladesh|
|Aman (Broadcast)||9.26||August-September||November- January||South & Sothern East part of Bangladesh|
|26.85||(a ) December- February||(a ) May -June||In various districts of Bangladesh specially in low-lying lands|
|(b)Local Boro||(b) December- February||(b) April-May|
From the above data, we observe that about 50% of the cultivating land is occupied by the ‘Aman’ as it grows throughout Bangladesh. Also Aman is regarded as the high quality paddy. Though Boro covers almost 25% of the total production, it is actually of low quality .
Gross production of rice from year to year basis
Rice husk production can be estimated from rice production for a country. So to find out available quantity of rice husk first we have to consider rice production quantity of our country. Rice production data are provided in Table B.2 in appendix.
Figure 2.2: Rice production in Bangladesh
2.4 Rice Mills and Operation
2.4.1 Rice milling processes
Three types of rice milling processes  are found
1. Puffed rice processing
2. Parboiled rice processing
3. Un-parboiled rice processing.
In first two processes supply of steam is needed. On the other hand there is no need of steam in the last process. In this processes two steps are followed and those are,
Step one (Removal of the outer hard protective layer)
The rice husk is the protective layer surrounding the grain. Once removed, the rice grain can be packaged, sold and eaten as brown rice. Brown rice still contains the rice germ and outer bran layers . If the rice is to be sold as white rice, the grain continues through further milling in Step Two.
Step two (Removal of the germ and brown layers)
The germ and bran layers from the rice grain are then removed to expose a white starch centre. This polished white starch centre is what is known as white rice .
2.4.2 Types of rice mills
There are two types of rice mills .
Steel shaft huller mill (Engleberg):
This is a traditional rice mill of low capacity
about 1-13 metric ton per day. In these mills mixed type rice husk is produced and extra machineries are needed to separate husk and bran. Here sun drying is followed so huge space and time is needed for a single cycle .
Automatic rubber roll mill with stone publisher:
This is a modern rice mill and are being popular for high capacity, 11.3-48metric ton per day, and high speed of operation.Here Louisiana State University type modern mechanical drying is followed for time and cost optimization .
2.4.3 Rice Mill Clusters in Bangladesh
According to directorate of food there are 25600 number of rice mills in our country which are located in different cluster. Size and location of a rice mill cluster depends on rice production and transportation condition of that area. There are some major clusters in the survey made by the GTZ.
The rice mills of small capacities were found in the BSCIC area at Pulhat. About 300 rice mills are situated in the cluster within 2km radius. There are about 50 automatic large rice mills in this district .
About 775 rice mills are situated in three different sub-clusters which are Naogaon sadar, Mohadebpur and Raninagar. All rice mills in a cluster are situated within 2km radius .
It is spitted into two sub-clusters are named as Sherpur and Santahar. Distance between these sub-clusters is 55km.There is about 110 rice mills in these cluster .
In this cluster rice mills are larger in capacity and situated separately in a group. There are about 16 large capacity rice mills few of those have more than one unit .
Rice mills are situated in a line at a road side and distributed within 2km radius. There are about 50 rice mills in this cluster .
Other major clusters are Jamalpur, Mymensingh, Ashugonj, Kustia, Chuadanga, Jessore, Khulna, Sylhet and Chittagong. A details map of Bangladesh is shown in Appendix C.
2.4.4 Paddy Processing Capacity of Rice Mills
To know the available quantity of rice husk in a cluster we have to know the cluster size as well as paddy processing capacity of rice mills in that cluster. We have mentioned few major clusters and their size in previous subsection and now average capacity of the mills of different clusters are tabulated as follow.
Table 2.2: Total amount of paddy processed (MT/year) in different cluster 
|Cluster||Annual paddy processed
|Number of rice mills (surveyed)||Average paddy processed (MT)||Total number of rice mills||Total amount of paddy MT/year|
Here we find out average paddy processed in metric ton by a mill from surveyed data and from this estimate the total amount of paddy processed in each cluster. Quantity of rice husk is assumed as 20% of the amount of paddy and so annual husk production of these clusters would be like this, (Refer to Table A.3 in appendix)
Figure 2.3: Available rice husks for commercial processing in different cluster
2.4.5 Parboiling and Drying Operations of Paddy
Amount of rice husk consumption during parboiling process shows a great variation due to use of traditional parboiling system with low efficiency. This variation indicates that there is a scope of saving husk if improved and efficient boiler could be used. Most of the mills consume from 100~150kg of rice husk per ton of paddy parboiling. On average, we assume 125kg of rice husk is used for per ton of paddy parboiling .
There are two types of drying methods which are sun drying and mechanical drying. For mechanical drying LSU type dryers are normally used in our country. Most of the mechanical dryer at rice mills use 80-115 kg husks per ton of paddy drying. On average we can assume 97kg husk is required for per ton paddy drying. We found that almost all (100%) rice mills in Naogaon and Ishwardi use sun drying . On the other hand 54%, 3% and 99% rice mills use mechanical drying in Dinajpur, Bogra, and Nawabganj respectively .
For mechanical drying thermal energy is produced in a furnace fired by rice husk. So amount of surplus rice husk after parboiling and drying can be calculated as follow.
Table 2.3: Use of rice husk for parboiling and drying 
|Cluster||Available rice husk (MT/year)||Total number of rice mills||Kg husk/ton paddy for parboiling||Kg husk/ton paddy for drying||Rice mills use mechanical drying (%)|
Amount of surplus rice husk= A – B×C – B×D×E/100.
Here A is total rice husk output from rice mills. But rice mills use rice husk for own parboiling and drying purpose which should be deducted. Amount of rice husk for parboiling is calculated by multiply total number of rice mills with average amount of rice husk required for parboiling (refer to Table A.4 in appendix). For drying requirement use of mechanical drying percentage should be consider.
By this equation the amount of surplus rice husks are shown in Figure 2.4
Figure 2.4: Surplus rice husk or rice husk avilable for sell.
2.5 Case Study: Visit to an Automatic Rice Mill
We visited an auto rice mill to get practical knowledge about the milling procedure, different equipments and machineries used in the procedure, different kinds of husk and also the stages of production of rice.
The rice mill that we visited is situated at a river bank near Muktarpur in Munshiganj district. In Muktarpur there is a small rice mill cluster which consists of 5 auto rice mills and a number of traditional rice mills.
Branches of Rice Processing Mill:
Dry raw paddy (Figure 2.4) is supplied by farmers of mainly the neighboring areas. This raw paddy consists of impurities and undesired wastes like grass, stubble, soil etc which have to be separated in the first step. The machine for this process is like a filter and uses simple techniques. This machine is shown in Figure 2.6.
Paddy is boiled and dried out by steam produces from the huge boiler (Figure 2.7). Rice husk is used to fire the furnace (Figure 2.8) (The included photo figures were taken by the writers) which is carried to the boiler room from the mill site by a conveyor belt. Husk consumption can be reduced if efficient furnace is employed. New paddy contains more wet so more steam is required for parboiling process. Required quantity of steam is reduced for older paddy.
Figure 2.5: Raw paddy from the farmers Figure 2.6: Paddy Cleaner Machine
By products of Paddy and different stages of processing
Clean paddy is stored in a large reservoir then small cylinders under the reservoir are filled through a duct. Steam and water are supplied to the cylinders when those are filled with clean paddy.
Lower portion of cylinders are shown in Figure 2.9. On the other hand, upper portion (Figure 2.10) of cylinders is connected with water and steam supply pipe where pressure gauge and water level indicator are employed to monitor pressure and water level respectively. Parboiled paddy are stored in new medium size reservoir then rotated
Figure 2.7: Boiler used in the rice mill Figure 2.8: Furnace of the boiler
Respectively. Parboiled paddy are stored in new medium size reservoir then rotated between upper and lower reservoir and dried out by steam with high pressure at the flowing path.
Figure 2.9: Lower portion of cylinders Figure 2.10: Upper portion of cylinders
The whole process needs ten hours so consistent power supply is mandatory after the process is began. This mill uses diesel engine generator. Diesel cost is a great headache for the owners so they appreciated the plan of power generation from rice husk to fulfill their own demand.
Rice husk (Figure 2.11) and rice bran (Figure 2.12) are produced separately in automatic rice mill. Initially those are in mixed form (Figure 2.11). Rice husk is conveyed to the boiler room and rice bran is packed up for sell. In the following pages we include the photos of the husk, bran and mixed husk that was just produced by the mill.
Figure 2.11: Rice husk
Figure 2.12: Rice bran
Figure 2.13: Mixed form of rice husk
After separating the mixed rice husk from the rice we get coarse rice which is full of dirt and impurity. In this stage produced rice is brownish and large.
Figure 2.14: Rice after first step (Coarse rice)
This coarse rice is then passed to the next stage for polishing. In the polisher machine, outside layers of rice are cut down to make it smooth and small. The polished rice looks like the following figure.
Figure 2.15: Rice after second step
This rice is passed to again to another polisher and same process is applied. The rice that we get from this stage is fine rice that is ready for sale.
Figure 2.16: Rice after third step
Vitamin density is higher in those layers so the process reduces food value of the rice. Waste of the process contains outside layers of rice which are sold at high price to biscuit companies.
From the visit, we came to know that about 30-40% of the produced husk is used by the rice mill itself as a fuel for the boiler that is used for parboiling. The rest of the total husks were sold at a low price by them which according to them were bought by Indian buyers who take husk to India to produce oil.
We also found out that the rice mill had to pay about BDT 0.1 to 0.11 million per month for the electric bill. The load shedding condition was really severe in the area and to operate during load shedding they had to spend about BDT 80,000 to 90,000 apart from their electric bill for the cost of diesel to run the generator.
Power Generation from Rice Husk
Now that we have the basic information about rice husk and current situation of husk production in Bangladesh, we can proceed to the process of its use as fuel in power generation process in details. In this chapter, we have first discussed the necessary considerable factors for power station siting and layout in brief to have a perception which will help us determine the condition of different locations in Bangladesh in the next chapter. After this technological procedures of generation process have been described step by step. Then a segment showing current technologies used for Power Generation has been included.
The first husk based dual-fuel plant in South Asia has been recently established in Bangladesh. For the thesis purpose, we visited the plant and collected information about it. We have given a long and short of our findings about this visit hereby.
3.2 Factors for Power Station Siting and Site Layout
It is important to identify and investigate a number of potential sites for constructing a power plant. The following considerations  can be made in this regard:
Whether the existing stations capable of further improvements.
Pieces of land already purchased by the Government for future development and pieces of land not owned by the Government, but identified as potential sites.
In this section we have described the important factors considered for the power station siting and layout.
(i) Transmission of Energy
It Energy transmission the plant area should be very easy. We have to consider several points  in this regard. They are-
Source of energy supply must be in the close proximity to the load centre.
We must consider the cost of copper and aluminium wires and make sure that total cost of transmission structure is reduced.
Long transmission line incurs large line drop. Hence high Voltage transmission might be required. So large insulation may be necessary.
(ii) Supply of Raw Material
The raw material from which electricity is made (in this case mainly rice husk) must be available in the region of the site.
The near the plant to the source of fuel, the less cost will be incurred.
In case of rice husk cogeneration plant the best sites are the places around the rice mills.
(iii) Land Requirements
Sufficient land will be required not only for the station when it is in operation, but also to provide adequate areas during its construction period. Areas should be provided for adequate working and storage areas for the contractors and for the construction car and bus parks. In addition, areas will be required for topsoil removed during excavations. Subsoil investigation , permeability test and groundwater tests are often performed to design proper cooling system.
(iv) Access to a Power Station
Access to a power station is required for construction materials and plant, fuel supplies and employees. Good road is essential for construction, and rail and sea facilities are useful advantages. Direct access to a main trunk road to bring in heavy loads is desirable. Road traffic can be reduced by delivering the loads through the sea .
While a power station is being built, traffic is greatly increased and so local roads adjacent to the site are often reconstructed and re-routed to avoid undue inconvenience or risk to other road users. The site must also be conveniently situated either close to a main railway line to accept rail-borne fuel or in areas remote from coal fields or refinery, on an estuary or the sea coast to enable it to take fuel from the colliers or tankers .
(v) Water Supplies
Water supply must be provided from a suitable source like a treatment plant or from a river or borehole. Where water for firefighting is to be taken from the town mains, allowance should be made either to duplicate the supply or to provide adequate storage capacity to ensure 100% availability; this is the most important factor during the commissioning period of a boiler  when the demands on the supply are heavy. During the construction period, water consumption depends on the size of the labor force, the nature of the civil engineering works e.g. aggregate washing, water jetting of piles, concreting etc and plant testing.
(vi) Safety Considerations
Historical data are used to assess and identify the fault location on the earth surface. Proper care must be taken while constructing a nuclear plant so that it is not located on the fault line.
Other Natural Hazards:
Studies are carried out to investigate the weather pattern of the site. If there is any possibility of cyclone or tidal wave or anything like that is investigated.
The industrial or manmade hazards are also important to deal with. Stations must not be positioned where there is regular traveling route of fuel or flammable liquid carrying transports.
Sites should not be constructed in the area where there is a dense population so that it doesn’t cause any major damage in case of any incident .
(vii) Other factors
Site and Station Levels:
A site should be reasonable level, not liable to flooding and not so high above the source of cooling water that excessive pumping power is required to supply water for cooling purposes.
Ash and Dust Disposal:
When selecting site for a rice husk cogeneration power plant, very careful consideration must be given to the provision of suitable economic ash disposal, either on low-lying ground or worked out mineral workings which can be filled by the creation of landscaped hills, or by the sale of pulverized fuel ash to the construction industry .
Labor Force Supplies:
For successful and efficient operation and construction process of a power plant, adequate size of labor force is required. Not only the size matters, but also the quality and efficiency of the labor force are a matter of significant concern.
3.3 Technological Procedures of Generation Process
3.3.1 Layout Area and Expected Capacity of Power Station
The power station is expected to be built on the area where rice husk are available which also depend on the production of paddy. In Bangladesh, the main production areas are mentioned in the previous chapter. We also consider the area near river side because transportation and collection of rice husks from neighboring rice mills will be easy by water way. The capacity of the plant will be variable depending on the coverage of the area. We will try to cover the rice mill and around the mill in plant located area.
3.3.2 Base for Selection of Technology for Cogeneration
The selection of rice husk combustion technology for producing energy (heat and power) is based on the following criteria  –
1. Production cost
2. Recovery of capital and financial benefit
3. Rice husk availability and fuel characteristics
4. Overall efficiency of the cycle (cogeneration plant)
5. Equipment manufacturing and supplying capability
6. Environmental impacts and measures for mitigation.
Some worldwide proven technologies are described below for analyzing and selecting the most appropriates to small-scale rice mills which are popular.
The existing six main biomass conversion technologies are:
1. Direct Combustion
At present, the most common technologies are direct combustion and gasification from rice husk to produce electricity . An analysis of these two technologies is carried out below in order to select the more appropriate in terms of capacity and practical application. Before selecting the technology, an analysis of fuel characteristics is needed.
Moisture content of biomass fuel is one of its important characteristics because after collection from the field it is not homogeneous. Thus, a careful consideration should be made in selecting the suitable mode for fuel feeding and combustion technology . The presence of water in biomass fuel will reduce the portion of combustible substances. Biomass having high moisture content should be dried naturally under the sun or in a dryer before being used as fuel.
On the other hand, too high moisture content always needs more time for heating biomass up to fire setting temperature. Now-a-days, new existing technologies and techniques allow burning the fuels having high moisture content up to 60% . Thus, we have to consider and choose the moisture content in a range suitable to the technology.
Heating Value of Fuel
It is the amount of heat liberated from the complete combustion of 1 unit of fuel. This is a basic feature, which will be used for calculating the parameters of combustion chamber like heat volume, surface of grates as well as combustion and mass/heat transfer processes in the furnace. In the technical documents on combustion of biomass in furnace / boiler from abroad, it was proved that the heat value of biomass having moisture content at 50% should be not less than 1850 kcal/kg .
Homogeneity of Fuel
If the homogeneity of fuel in terms of size and type is not ensured, the combustion process in the furnace could not be stable. It needs to select an appropriate combustion technology.
From the above analysis, ash content has important effects on fuel properties: reducing heating value, causing dust and corroding the material of boiler, leading to decreased heat transfer intensity. For biomass fuel, ash content is very low and the ratio between fly ash and slag depends a lot on the shape and size of fuel as well as selected combustion technology, size and form of boiler / furnace. For conventional combustion on grate, this ratio is of 60/40 and even 80/20 . During combustion process, the ash is usually entrained in the smoke stream due to suction effect of the fan. Consequently, in order to keep on the environmental allowable parameters it needs to use the ash traps, flue gas filters (dry, wet or bag).
3.3.3 Analysis and Selection of Technology Biomass Gasification
Biomass gasification is a process of converting solid biomass into a combustible gas by combustion with insufficient oxygen supply. There are 3 modes of biomass gasification , they are:
2. Updraft and
3. Gross draft.
The composition of produced gas (mainly volatile matter) depends on the factors like temperature, pressure, heat transfer process and type of gasifier. In gaseous mixture, beside combustible gases, there exist also other substances such as steam, and tar. This gaseous mixture should be cleaned (for removing tar and particles) and cooled before coming to the combusting appliance / furnace. For internal combustion engines, the content of tar in combustible gas should not be more than 50 ppm (part per million) while for gas turbine this feature should be well lower .
(i) In the case of down -draft gasifier, producer gas has to pass a zone with higher temperature so its temperature is rarely high, at 600-800° C
(ii) In the case of updraft gasifier, producer gas should pass a bed of raw biomass fuel, which has very low temperature. That’s why its outlet temperature is low, ranging from 100 to 300oC.
When using this type of gasifier for internal combustion engines (also for gas turbine), the produced gas needs to be cleaned due to higher content of tar. The up-draft gasifiers are suitable only for fuels having high moisture content. Both types of gasifier are designed with a “throat” to form a high temperature zone for cracking tar. However, this throat will restrict the biomass flow, especially for the biomass having very low bulk density (kg/m3).
In current development trends, Fluidized Bed Combustion (FBC) technology is used for combustion of solid fuels, including biomass, and particularly rice husk. FBC combustion is chosen when fuel particle size is less than 6 mm . The bed consists of inert particles, and commonly, sand is used. Two types of FBC, which could be used for combustion of rice husk fuel are Bubbling Fluidized Bed Combustion (BFBC) and Circulating Fluidized Bed Combustion (CFBC). They are described below:
(i) Bubbling Fluidized Bed Combustion (BFBC)
· Reducing NOx emission.
· High heat transfer effect due to the increment of contact surface when the fuel particles are sunk in the “boiling layer”.
It is important to note that biomass fuel has high volatile content (V ~ 70%); the heat liberated in the fire box is much higher than that on the grate as the volatile matter released from biomass fuel will burn in the space of combustion chamber . Based on this, FBC would be affected in the furnace with two combustion chambers. In the first chamber, fuel burns at low temperature. The generated volatile and unburned fuel particles are led to the second chamber, to which the secondary air is supplied sufficiently for complete combustion.
(ii) Circulating Fluidized Bed Combustion (CFBC)
A typical feature of FBC is the great quantity of fly ash, which contains a considerable amount of unburned carbon (only volatile matter was burnt out). Fly ash recycle system should be used for improving the furnace efficiency. Fly ash, after being separated from flue gas precipitators (cyclone type), is returned back to furnace.
Combustion on Grates
Based on the required capacity, the type of furnace and various fuels feeding mode van is selected. The main factors for this selection are:
• Fuel characteristics
• Plant’s capacity
For on-grate combustion furnace, there are some types of grate which might be chosen: fixed, flat grate, inclined step grate, moving grate (shocker grate) but only the inclined moving grates are in common use. The furnace may be divided into two separate parts: combusting and heating (pre – furnace) or direct . Fuel feeding could be done from the bottom or from the top, continuously or in batch. To facilitate the selection, two modes of fuel feeding are analyzed.
Selection of Technology
Based on the above analysis, a conclusion is made on the possibility of using one of the following technological schemes for power generation from rice husk :
1. Rice husk ? downdraft gasifier ? internal combustion engine or small scale gas turbine ? generator
2. Rice husk ? Combined cycle (gas – steam) ? gas and steam turbines ? alternator
3. Rice husk ? furnace / boiler ? steam turbine ? alternator
Cleaned produced gas of biomass is preheated and led to gas turbine/I.C engine for combustion. Low investment cost and simple operation (few of facilities required) are the advantages of this scheme. However, it can be used for small scale power generation (up to 1000 kW) and it need tar removing process  since along with operation, the dust / tar will accumulate on heat exchange surfaces.
Rice husk is gasified in a gasifier. Produced gas is led to gas turbine for combustion and power generation. The temperature of gas exhausted from gas turbine is still high enough to produce steam. This superheated steam will be led to the steam turbine to drive the generator producing electricity . This scheme has some advantages like high overall efficiency and high electric capacity.
Biomass is burnt in a furnace (fluidized bed / grate type) for preheating water and producing steam, which will be used in a steam turbine for driving the generator. This scheme has higher efficiency compared to the first one and easy to apply for cogeneration . It requires, therefore, higher investment cost and skilled operators.
Conclusions on selection of technology
Cogeneration plant consists of rice husk storehouse, conveying and automatic boiler feeding systems, and furnace/boiler. The boiler is equipped with automatic ash removal system, heat exchangers and turbo-generator. The turbine used here is a backpressure.
3.4 Current Technologies used for Power Generation
3.4.1 TORBED process reactor technology
Rice husk is currently being used for energy production through direct combustion or gasification in many areas of the world. Unfortunately, in almost all of these installations, the ash produced is not suitable for use as a silica fume substitute . Generally there are two shortcomings in the ash by-product from current rice husk to energy technology: first, they can contain unacceptably high concentrations of residual carbon; and second a portion of the amorphous silica has been transformed into crystalline silica, cristobalite. The second of these two problems is the more serious; cristobalite does not have the same pozzolanic (cementitious) properties, as the amorphous form , and in the particle size range at which it would be used in concrete, it is recognized as a potential human carcinogen. The transformation to the crystalline state takes place if the ash is exposed to high temperatures and becomes even more likely if it is exposed to these high temperatures for extended time periods. Most of the current energy generation technologies do not control temperatures well and most allow the ash to remain at high temperatures for a relatively long residence time .
TORBED Process Reactors applied to rice husk combustion and gasification technology utilize a unique reactor configuration that completes the combustion or gasification of husk in a short residence time at precisely controlled temperatures . It has been shown that, using the TORBED reactor technology, an ash can be produced at a moderate temperature that has zero or at most trace quantities of cristobalite and a residual carbon content of 1-4%. The first commercial TORBED rice husk combustor was installed and successfully started up in India during September and October 2003 .
Because of the moderate temperatures used in the TORBED reactor there is a slight reduction in the usable energy that can be recovered from a TORBED reactor used as a rice husk combustor. However, in some instances this may be compensated for by achieving a much more complete combustion of the available fuel. The TORBED reactor can be designed into a new facility to combust rice husk for energy production, or this combustor can be retrofitted into an existing facility to replace a current combustor that is producing an unusable ash waste.
Figure 3.1: A typical TORBED reactor
The capital investment in a replacement combustor will generate an attractive Return On Investment (‘ROI’) based on the benefits of turning a waste disposal cost into a by-product credit. Similarly, installation of new plant for energy generation will produce an attractive ROI based on both energy and ash values .
3.4.2 Firing Process
(i) Process Description
First of all, the system boundary of the study is set as shown in Figure 3.2, for doing the environmental assessment using the Life Cycle approach. Raw material, electricity and resource consumption, electricity generated, waste and emissions generated have been considered within the boundary . This figure also shows list of all flows – water, flue gas, steam, electricity and ash. The process started at, water from Shi River is treated before using for steam production. Particulates and ions in water must be removed to protect erosion of boiler. This is accomplished by coagulation by PACl. Sludge is removed as solid waste and is sent to the landfill .
Figure 3.2: System boundary for power generation scheme.
Then water is passed through filter tank and demineralization tank to remove ions by ion exchange resin. After that, water is heated in the economizer using the waste energy from the hot flue gas released after steam production in the boiler . The heated water is then sent to the boiler. Rice husk is the fuel source for boiler and is ignited by burning paper during startup. Steam is produced at 300°C and is further heated by super heater to produce higher energy steam at approximately 400°C . This superheated steam is then used for steam turbine for generating electricity. The exhaust steam is then condensed to water by the cooling tower.