It is the thermochemical conversion of organic matter at high temperatures with partial oxidation. Energy in organic matter produces combustible gases (CO, CH4 and H2) and minor products (char, water and condensable matter). The first step, called Pyrolysis, decomposes organic matter by heat to produce gaseous and volatile matter and char, which is non-volatile, containing high carbon content. The second step is when hot char reacts with the gasses (mainly CO2 and H2O) to form producer gas (CO, H2 and CH4). The produced gas is mixed with pollutants and thus has to be cleaned. This clean producer gas, when mixed with air, can be used in gas turbines, gas engines, gasoline or diesel engine. Its heating value is 4 â” 20 MJ/m3. The heating value depends on carbon and hydrogen content of biomass, properties of gasifier and type of gasifier agent or oxidant (for example, air, pure O2, steam or mixture of these gases) (Figure 6).
Figure 4 Schematic of a 1 MW rice husk gasification & power generation system (XiuLi Yin, et al., 2002)
This schematic is further explained into its small system components in figure down, till end process.
Figure 5 Basic process steps of a biomass gasification plant (NGUYEN, 2014)
Figure 6 Design and operation of a CFB gasication and power generation system for rice husk(XiuLi Yin, et al., 2002)
This figure above shows the compositions of different producer gases at different temperature and gas heating value during gasification of rice husk. The optimum gas composition in the sample lies in a temperature range of 750 and 800 Â°C. however increasing the temperature increases the efficiency but imposes a high risk of fire in the engine.
Out of all its system components Gas cleaner, gas engines and waste water treatment are most crucial for environment and health hazard because gas like carbon monoxide when exposed in the plant due to corrosion, erosion caused by particulate (ash, char), alkali metals, tars, H2S and HCl, etc. can cause fatality in the surrounding.
Waste water system components
Figure 7 Aeration tank where contaminated water with ash, char and tars is first discharged(XiuLi Yin, et al., 2002)
Aeration -tank where contaminated water with ash, char and tars is first discharged and COD of water is reduced from as mush as 3.6 g/m3to a range between 300 and 1800 mg /m3. Then it flows to precipitation tank where heavy particulates settle down. Absorbing tank has husk ash to absorbers contaminants and to bring down at 100 mg /m3. At the end filters remove remaining ash and now water can be discharged openly or can be further treated for safe drinking. The gas cleaning process requires 7 to 8 t hâ’1 of water if not treated properly would create many environmental and human health problems and the whole process will not be green.
2.2 Direct Combustion
A direct combustion system has two main components â” the biomass-fixed boiler (that produces steam) and the steam turbine (that generates electricity). The two common types of boilers are stoker and fluidized bed. A direct combustion system is where the fuel requires no refinement and thus fuels are burnt in its natural form. 97% of global bio-energy production is by direct combustion. It is the most common way of producing heat and/ or power from rice husk. In this process, biomass is burnt to produce hot flue gases. This is used to either provide heat or fed to a boiler system to generate steam, where the steam is used for industrial purposes, space heating or drive turbines to generate electricity (Figure 7). The efficiency of direct combustion system to produce electricity through steam turbine is 15 â” 35%. It depends on moisture content of biomass, combustion air distribution and amounts, operating temperatures and pressures, fuel feed handling, distribution and mixing, and furnace retention time.
Figure 8 The combustion process (NGUYEN, 2014)
2.3 Choice of Technology
The most apt conversion technology for rice husk – based power plants depends on the available rice husk, ratio between heat and power demand, and fuel characteristics. Figures 8 & 9 describes gasifiers range size on the basis of electricity capacity and gasifier capacity (or the amount of biomass input), respectively.
Figure 9Gasifier range size (NGUYEN, 2014)
Figure 10 Gasifier technology capacity range based on odt (Oven Dry Ton) of biomass (NGUYEN, 2014)
For rice husk three schemes fulfil the analysis of technology on the basis of its characteristics (moisture content, heating value, homogeneity and ash content) considered for a pre-feasibility study conducted in 2004 by the PREGA National Technical Experts from the Institute of Energy, Vietnam (http://download.docslide.us/uploads/check_up03/192015/546b2ccfb4af9fba128b4f63.pdf).
The process involves cleaned produced gas of biomass is preheated and led to gas turbine or the internal combustion engine for combustion. The advantages are its low investment cost and simple operation. Its disadvantages are that it is used for small scale power generation (up to 1000kW) and it needs a tar removing process, because during the operation, dust/ tar accumulates on heat exchanger surfaces.
The process involves rice husk being gasified in a gasifier. The produced gas is led to a gas turbine for combustion and power generation. Steam is produced using the gas exhausted from gas turbine, because its temperature is till high. The steam produced is superheated. The superheated steam is used to produce electricity by driving the generator in the steam turbine. The advantages of this scheme are its high overall efficiency and high electric capacity.
The process is that the biomass is burnt in furnace (fluidized bed/ grate type). This is for preheating water and producing steam. This is in turn used for driving the generator in a steam turbine. The advantages of this scheme are that it has higher efficiency as compared to the first scheme and also it can be easily applied for cogeneration. The disadvantages of this scheme are that it requires higher investment costs as compared to the first scheme and also requires skilled operators.
The choice of technology also depends on the third factor, that is the heat to power ratio (H:P). It is the ratio of thermal energy to electricity required by the facility consuming the energy. H:P = KWth/KWe, KWth = thermal load in KW and KWe = electrical load in KW. Table 4 is the conventional data for H:P and the expected overall efficiency of various cogeneration schemes. A gas turbine is suitable for process sites having H:P ratio ranging 2.0-4.5 (maximum), while steam turbine should be applied for the ones having H:P ratio at 4.5 upwards (maximum). Table 5 entails some typical features of several cogeneration equipments. While combined cycle is more suitable to district heating, gas and steam turbines are the best types of CHP for industrial processes because of their large capacity and ability to produce the temperature of steam most needed by these processes. Reciprocating internal combustion engines (ICEs), on the other hand, are widely used in small-to-medium applications (under 10 MW), so are well-suited for commercial as well as light-industrial situations.
Figure 11 Specific environmental footprints and costs for different systems (Jittima Prasara-A*a, et al., 2012)
Table 10 Fiscal incentives available for biomass power generation(Indian Renewable development agency Ltd., 2015)
100 % depreciation in the first year can be claimed for the following power generation equipment
1. Fluidized Bed Boilers
2. Back pressure, pass-out, controlled extraction, extraction and condensing turbine for Power generation with boilers
3. High efficiency boilers
4. Waste heat recovery equipment
2. Tax Holiday
10 year tax holiday
Customs Duty Duty leviable for NRSE power projects of less than 50 MW capacity (under Project Import Category) is 20 % ad valorum. This covers machinery and equipment components required for generation of electric power.
Central Excise Duty Exempted for renewable energy devices, including raw materials, components and assemblies.
Central Sales Tax –
General Sales Tax Exemption is available in certain States.
3 Rice Husk Cogeneration
3.1 Current Scenario
The cogeneration method is sustainable, economical and environmentally safe. This technology is being successfully used in many Asian countries. Unfortunately, for India, there are local hindrances such as state governmentâs hold on electricity, inadequate knowledge and experience and very few schemes or policies.
India, being the largest producer of rice in the world, also produces huge quantities of rice husk. But it is underutilized. It is used ineffectually in furnaces and boilers for producing steam.
It has been reported that the electricity and thermal requirements of a rice milling plant take up almost half of the operating costs. It is essential to lower these costs. According to authors Lim Jeng Shiun, Haslenda Hashim, Zainuddin Abdul Manan and Sharifah Rafidah Wan Alwi, their paper from the Industrial & Engineering Chemistry Research journal researched numerically that an optimum way was the use of âa 15-tonne boiler for the co-generation system and eight units of cyclonic husk furnaces (CHF) to satisfy the rice mill heat and power requirements.â(Reidy, 2012)This would lower the yearly costs by 18.5%. (Reidy, 2012)
Rice husk is a waste produced from rice milling plants, but it is also an important biomass and energy efficient. Cogeneration technology helps tap this resource.
There has been notable development in technology with the power plants now being automated and having more efficient boilers.
In Asia, the most widely used methods are fluidized bed combustion (FBC) system, reciprocating step grate system and travelling grate system. Cyclonic combustion system and circulated fluidized bed combustion (CFBC) system are also used. In Southeast Asia, reciprocating step grate system and travelling grate system are widely used, whereas in India, it is the FBC system.
Cogeneration is still a new concept to the rice industry. The gestation period for cogeneration project is about 1.5 â” 2 years as compared to 5 â” 6 years for thermal project and still longer for hydro projects.
Cogeneration (Combined Heat and Power or CHP) is a technology that produces heat and electricity at the same time.
Figure 12CHP System (Jude, 2003)
Figure 13 Conventional System (Jude, 2003)
A cogeneration plant must be flexible to provide electricity for rice milling operations all through the year and also provide heat for dryers used during the rice harvesting season. A common mistake committed while designing a cogeneration plant is that it is constructed to be co-fired by any fossil fuel. Instead it should be constructed depending on the quantity of rice husk produced by a rice mill.
Also at certain times, when there is a scheduled or an unscheduled shutdown of the cogeneration plant, diesel generators might be used as a standby.
3.3 Economies of Scale
There is a growing competition in the rice husk industry. There is competition among the rice husk boiler suppliers, who are mastering from technological point of view. Due to the competition, the total investment cost is steadily reducing.
The cost of rice husk in Asia is very high. But in some parts of the world, such as Philippines, Laos, Cambodia, other Asian countries and Africa, it is still being dumped a waste. This is where the opportunity is to install rice-husk based power plants. Since the raw material cost would be very low.
The aim of any firm is to increase itâs the Internal Rate of Return (IRR). This is possible by proper project development, implementation and operation, which in turn would reduce the project cost and thus increase the IRR.
The financial advantages are savings made (electricity production), income generated (selling excess electricity and also ash), jobs generated, fuel saved and no need of foreign currency.
There are certain factors that need to be considered, while designing a rice mill utility system, that have financial effect. They are:
â¢ Different heat and power demands throughout the year
â¢ Various alternatives of energy supply
â¢ Limitation of supply, transportation and purchasing rice husk
It is to be noted that the cost of establishing a cogeneration plant is much less than thermal or hydro plants.
3.4 Case Study: Comparison
To reduce the effect of electricity tariff on a rice mill, co-generation is a favorable option to fulfill heat and power demands. As compared to cyclonic husk furnace (CHF), which uses rice husk to produce thermal energy, cogeneration uses rice husk to generate thermal and electrical energy. Because of this, cogeneration utilizes more rice husk than CHF to supply the same amount of thermal energy.
Planning and constructing a good rice mill utility system that uses rice husk as raw material is a complicated dilemma because it involves compromising between the capital and operating costs. But this can be solved by integrating financial and process data.
Bagasse is the largest mill-generated resource that can be used for energy generation. The second largest is rice husk. The reasons for choosing rice husk over bagasse or any other biomass are as follows:
â¢ Easily available
Rice husk is easily available to rice mills. Thus the supply to the power plant is stable and reliable.
â¢ Cost-effective, easy storage and handling
Because of easy storage and handling, due to requirement of no processing, it is cost-effective.
â¢ Low cost of transmission and distribution
Since rice husk is locally produced and used, the costs of transmission and distribution are reduced.