International Journal of Environmental Science and Development, Vol. 2, No. 4, August 2011

An Improved Method to Generate Electricity and Precipitated Silica from Rice Husk: Perspective Bangladesh
Arif Md. Waliullah Bhuiyan, Md. Rejwanur Rashid Mojumdar and A K M Kamrul Hasan

Abstract— Electricity supply for rural areas, so long, has been a far cry in context of Bangladesh. Realizing this, many organizations along with Bangladesh government are trying to bring renewable energy in a competitive package to meet deficiency. But solar energy, mainly for its inefficiency and high cost, is not having much success in a broader aspect. SHSs are capable to maintain only household demand for 4-6 hours. But for the emergence of small enterprises and to boost the rural economy, energy security at countryside has indispensible role. This paper explores rice husk based biomass plant as an alternate energy source to meet the electricity demand of local enterprises, irrigation and rural households. Since the availability of rice husk is at stake, a mixture of biomass (rice husk & saw dust or rice husk & bamboo dust) instead of using only rice husk as fuel is also proposed for electricity generation. Ash generated from the husk is processed to produce silica and calcium carbonate through silica precipitation method. Demand and market analysis is also done to examine sales potential of products (electricity, silica and calcium carbonate). Index Terms—Biomass gasification, Electricity generation by rice husk, Silica By-product generation, Product utilization

residues and process residues where field residues are residues that are left in the field after harvesting and generally used as fertilizer. On the other hand, process residues are generated during crop processing, e.g. milling [1]. Between these two types, process residues are usually available at a central location that enables to collect raw materials efficiently. Out of various biomass energy technologies, biomass gasification process is yet to get recognition in Bangladesh. Other neighboring Asian countries especially India and China have already undertaken gasification programs on large scale. On the other hand, only a demonstration biomass gasification of 200 kW based on rice husk is being set up by Local Government Engineering Department (LGED) at Dinajpur. The first commercial rice husk based power plant is established by a private entrepreneur in Kapasia, near Dhaka, with financial assistance from Infrastructure Development Company Limited (IDCOL).

II. BIOMASS GASIFICATION Biomass gasification is the process of converting solid fuels (wood/ wood-waste, agricultural residues etc.) into a combustible gas mixture usually referred as producer gas or synthesis gas (syngas). The process involves partial combustion of such solid fuels that occurs when air supply (O2) is less than adequate for the complete combustion of biomass. During partial combustion solid biomass fuels are broken in an oxygen-starved environment by the application of heat to produce a mixture of combustible gas which is called producer gas. Fuels that are conducive to biomass gasification include dry materials such as dry wood, dried leaf, charcoal, rice husks, coconut shells etc. The main component of the biomass gasification system is primarily a reactor into which fuel/feed stock is fed with a limited (less than stoichiometric, that required for complete combustion) supply of air to craft chemical breakdown; eventually generate producer gas. The calorific value of this gas varies between 4.0 and 6.0 MJ/Nm3 or about 10 to 15 percent of the heating value of natural gas [2].

I. INTRODUCTION Expanding rural electrification is the key to the prosperity and development of rural areas as well as to fulfill Bangladesh Government’s vision of ensuring access to affordable and reliable electricity for all by 2020. It is well recognized that energy demand in our rural areas is increasing and supply of fossil fuel at subsidized prices is becoming an unattainable challenge for the government. Moreover, providing power without intensifying the effect of climate change is also a priority for Bangladesh. Eventually, emphasis on increasing the energy conversion efficiency and promoting the use of alternate energy sources has been increased lately. Approximate land use for agriculture is 54.5%, while the forest covers 17.6% of total land area of the country. About 46% of traditional biomass energy is supplied from agricultural residues such as rice straw and rice husk from rice plants, bagasse from sugarcane and jute stick. Some amount of residues produce from wheat, potato, oilseeds, spices, etc. Crop residues can be distinguished into field
Manuscript received July 13, 2011, revised August 9, 2011. Arif Md. Waliullah Bhuiyan is with the BRAC, Mohakhali, Dhaka, Bangladesh. (e-mail: bhuiyan.waliullah@gmail.com). Md. Rejwanur Rashid Mojumdar is now with HM Enviro Care, Dhaka-1000, Bangladesh. (e-mail:md.rejwanur.rashid.mojumdar@gmail.com). A K M Kamrul Hasan is with Islamic University of Technology, Dhaka, Bangladesh. (e-mail: kamrul_2075@yahoo.com ) Arif Md. Waliullah Bhuiyan and Md. Rejwanur Rashid Mojumdar both are members of IACSIT.

Fig. 1. Biomass conversion into energy [3]

The advantage of gasification technology is cost effectiveness and higher efficiency. In this process electrical
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and thermal energy can be generated simultaneously. Gasifiers offer a flexible option for thermal applications, as they can be fitted into existing gas fueled devices such as ovens, furnaces, boilers etc., where syngas may replace fossil fuels, lower the household demand for natural gas. Heating value of syngas is generally around 4-10 MJ/m3 [4].

III. TREND OF BIOMASS ENERGY USE IN BANGLADESH Over last 30 years, there has been an increasing trend of biomass fuel supply in Bangladesh. The total supply of biomass fuel was 236.08 PJ in 1980 and has increased over next 20 years to 356.66 PJ (1.73% growth). Traditional fuel supply usually comes from main three sources viz. crop residues, animal dung and trees [5].

MT/day of paddy processing capacity. The number of lower-midsized rice mills (with paddy processing capacity range : > 25 - 50 MT/day) are reported to be about 490 and that of mid to large sized rice mills (30- 120 MT/day) are about 50 units. These four distinct ‘cluster’ areas are distinguished as - (i) Dinajpur (North Bengal) (ii) Sherpur (near Bogra) (iii) Inshawerdi and (iv) Kaliakoir (near Dhaka), where most of the rice mills are located in close proximity to each other.

Fig. 2. Trends of traditional fuel supplied during 1980 to 2004

The percentages of different traditional energy were as follows: cow-dung 20.4%, jute stick 7.5%, rice straw 11.6%, rice husk 23.3%, bagasse 3.2%, fire wood 10.4%, twigs and leaves 12.5% and other wastes 11.1%. Rice husk contributes biggest share of biomass energy and it was 83.04 PJ in 2003-2004. Energy production from rice husk is steadily increasing. In 1991, the production of rice husk energy was 76.35 PJ and it increased to 106.1 PJ in 2004 (Figure 2) [5].

Fig. 4. Districts with rice mill clusters (red marked) in Bangladesh [6]

IDCOL already financed two such projects in Kapasia, near Dhaka and recently one in Thakurgaon sadar, Thakurgaon. Hence, based on preliminary surveys (Ref.: Bangladesh Rice Research Institute, BRRI and information from Rice Mill Owners’ Association), typically at least over 500 rice mills are located in these cluster areas. Taking an average (lower-mid) capacity range of about 100-200 kW, there is a 50-100 MW power market in these cluster areas [7]. Areas like Dinajpur, Sherpur etc are having intermittent power supply which often leads to less production for these rice mills and also other energy craving small enterprises and local dwellers are potential consumers for future rice husk based biomass power plant.

Fig. 3. Trends of rice husk energy production during last decade in Bangladesh [5]

V. ELECTRICITY GENERATION BY RICE HUSK GASIFICATION Gasification is the conversion of biomass to a gaseous fuel by heating in a gasification medium such as air, oxygen or steam. Unlike combustion where oxidation is substantially complete in one process, gasification converts the intrinsic chemical energy of the carbon in the biomass into a combustible gas in two stages [3]. If biomass is gasified efficiently, it can generate a high amount of clean product gas. The initial step to the process is a thermo-chemical decomposition of lignocellulosic compounds where char, tar and volatile compounds produce as output. By thermo-chemical gasification solid fuel is transformed into gaseous fuel. Through this process, the chemical energy of solid fuel is converted into both thermal and chemical energy. The chemical energy contained depends on its chemical composition that determines the quality of product gas. High concentration of combustible gases like H2, CO and CH4 increase the combustion energy of the product gas.

IV. RICE HUSK CLUSTERS IN BANGLADESH About 4 – 4.5 million metric tons (MT) of paddy is annually produced in the Bangladesh. A 20% yield of input paddy leads to a production of 8 - 9,000,000 metric tons rice husk annually. Using biomass gasifier technology and considering that about half of the husk is used for energy applications such as domestic cooking, steam production for rice parboiling etc., the theoretical potential (TP) of the remaining half i.e. 4 million MT / year of husk, is estimated at a little over about 400 MW of capacity. According to the Rice Mill Owners’ Association of Bangladesh there are over one hundred thousand rice mills located in a scattered manner all over the country with four ‘cluster’ areas. Over ninety percent of these rice mills are reported to be in smaller capacity ranges, i.e. < 20 – 25

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Several types of gasifiers e.g. fixed-bed updraft and downdraft gasifier, fluidized bed gasifier and bubbling bed gasifier are available in the existing market with different sets of pros and cons. However, the downdraft gasifier is a comparatively cheap and the gasification in this type of gasifier can produce a product gas with very low tar content [3]. Keeping the process in mind, fixed-bed downdraft gasifier is thus recommended for small-scale rice husk biomass plant. Here, biomass fuel is fed at the top of the reactor/gasifier. The fuel then slowly moves down and during this time, the fuel reacts with air (the gasification agent), which is supplied by the suction of a blower or an engine and is converted into combustible producer gas in a complex series of oxidation, reduction, and pyrolysis reactions [2]. Generated ash is then removed from the bottom of the reactor for silica production.

drawback as the generated ash would be reused to produce silica by precipitation process. The moisture content of the biomass has to be carefully monitored as on wet basis moisture content must be less than 25 % by mass [12]. The relative high temperature of the leaving flue gases result in lower gasification efficiency because these gases do not exchange heat with the wet biomass to be gasified [11].

Fig. 6. Schematic of downdraft gasifier Fig. 5. Electricity generation by rice husk gasification

A gasifier model is proposed below in table I:
TABLE I: FEATURES OF A PROPOSED GASIFIER (MODEL OI-405RHP) Particulars Biomass consumption Gas heat value Gas capacity (maximum) TAR content Gasification efficiency Self consumed elec. Power Water consumption (replace evaporation) Model OI – 405RHP 1.6-1.8 kg/kWh 4500-5500 kJ/m3 1200 m3/hr 70% 20-40 kW