Technical and Economic Analysis of a Power Plant Based on Rice Husk: Optimized generation of power and amorphous silica ash based on controlled rice husk combustion
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- Institutt for elkraftteknikk 
Elkem Materials is in the initial phase of developing rice husk combustion as a business strategy. By burning rice husk under controlled conditions the ash generated is amorphous silica in nature. This amorphous silica ash can be used as a substitute for micro silica in the concrete industry. Due to a growing demand for finely divided silica in the concrete industry, combustion of rice husk is an alternative method to generate this product. Elkem Materials and SN Power are considering to join forces in a power project based on rice husk. The idea is to install 20MW power plants in designated Asian countries. The power will be generated from a steam cycle which heat exchanges with the rice husk combustion products. More insight is needed in order to understand how a general power plant based on rice husk can be put together. Today, two companies have developed different combustion technologies that can control the strict combustion conditions required. Still, more experimental data is needed to investigate under which conditions the reactors have to operate in order to meet the strict ash quality required by Elkem Materials. This thesis has carried out an evaluation of the major components in a 20MW power plant facility based on rice husk. A central part has been to develop a model for how the energy content in rice husk can be utilized in a combustion process for further heat utilization. Another model is developed to estimate the surface area required to transfer a certain amount of heat from the combustion products. These models can be used for different rice husk samples and for different combustion conditions. All numbers calculated are guiding values. Lack of experiments and information has influenced their order of magnitude. Several assumptions and simplifications are carried out to be able to do some general estimations. The aim is to show the tendency, and more comprehensive calculations have to be done for further development of the project. The energy utilization of rice husk has been studied for four different combustion temperatures. In addition, five different samples of rice husk are analyzed in the combustion process to see how their characterizations influence the operation of the power plant. The results are also given an economical aspect. The greatest difference between each rice husk sample is the ash content. For the samples analyzed, this varies between 14-22% (mass basis). The moisture content is low for all samples, about 9%. Compared to other biomass fuels the carbon fraction is low and the oxygen fraction is high. This results in a low calorific value, between 12-14 MJ/kg. Lower ash content gives higher calorific value. Rice husk has a low bulk density, about 120 kg/m3, and enormous volumes are necessary for generation of 20MW electricity based on this fuel. The fuel feed varies between 480-580 ton/day, mainly depending on the calorific value. Different combustion temperatures have minor effect on the fuel feed. To generate a constant heat rate, a little increase in the fuel feed is necessary when the combustion temperature decreases. The ash product is a source for generating profit. Fuel with lower calorific value generates more ash. The fuel price is often decided by the calorific value, and in this case cheaper fuel will increase the profit. Between the two rice husk extremities analyzed, one sample generates 70 ton of ash per day, while the other 130 ton/day. A proximate and ultimate analysis of the different rice husk samples should be done prior to the decision of where to locate the power plant. The steam cycle is defined to operate with 55 bar and 530°C into the turbine, while the outlet values are 0,08 bar and 42°C. This will generate about 21MW. 1 MW is available for own internal demands, and 20 MW can be supplied to the local electricity grid. Operating under these conditions, the steam cycle requires 63,25 MW of heat. Approximately 83MW of thermal effect is needed to satisfy this need. Heat losses are not included. Depending on the size of the reactor, this amount requires several reactors in parallel where the generated flue gas is conveyed into a joint heat recovery steam generation system. The flue gas temperature has been taken down to 125°C. The limited outlet temperature is decided by the amount of sulfur in the fuel, and by the SO3 dew point temperature. To avoid degradation of the materials in lower temperature regions, the sulfur content and the corresponding dew point temperature have to be known. The most complex component in this power plant is the heat transfer unit. Because the flue gas heat transfer coefficient is low, large surface areas are needed to transfer the required amount of heat. In addition, since the majority of the ash is conveyed with the flue gas into the heat exchange units, several design restrictions have to be implemented in the boiler system. Extended surface area tubes (finned tubes) can not be used because of the impure flue gas. By installing smooth tubes, the required area will be enormous. Factors that have greater influence on the required surface area are the ash emissivity, the gas side fouling factor and the flue gas velocity. Heat transfer from radiation may contribute remarkably because of the high ash content in the flue gas. The flue gas velocity has to be strictly regulated because of the flue gas composition. This affects how efficient the heat transfer might be. Further investigations should focus on how to avoid too much reduction in the velocity. By implementing appropriate cleaning techniques, the velocity can probably be kept at higher levels. The fouling factor is the last critical parameter. This interacts with the velocity and can also be reduced by cleaning systems. Bullet cleaning is a suitable cleaning technique. When the combustion temperature decreases, the total surface area of the heat exchanger tubes will increase. Going from 850°C to 770°C the area increases about 1000m2. The areas calculated are instructive numbers. However, it is a clear relation between the combustion temperature and the required surface area. The evaluations are based on single units. Integration of the reactor and the boiler will probably increase the heat transfer efficiency and reduce the tube s surface area. To perfectly control the combustion temperatures, great amounts of oxygen are necessary. Seen from an energy utilization perspective, this is not a very efficient use of energy. A lot of air is used to reduce the temperature, and the flue gas energy content is low. Due to high excess of air in the flue gas, recycling the outlet oxygen is an important supplement for increasing the combustion efficiency. Depending on the rice husk sample and on the combustion temperature, the flue gas stream is approximately 80 kg/s. Secondary emission reduction measures are necessary to separate the fly ash from the flue gas stream. A recommended solution is to implement a cyclone to separate the larger particles (80-90%) in combination with bag house filters that separate the smaller once. For these quantities of flue gas streams, the magnitude of the ash collectors have to be extraordinary large. Because the ash has an economical value, investment in this technology is important. An alternative separation method could be to locate a cyclone prior to the heat recovery system. This would simplify several challenges linked to the impure flue gas entering the water-tubes. Yet, commercialized solutions for this practice are not developed. From an economic perspective the project has great potential, though all numbers depend on future pricing, supply and demand. Realistic price estimates based on today s market justify investment in this type of power plant. The sold electricity will constitute 75% of the income, while the amorphous silica ash constitutes the other 25%. Though this project seems beneficial, the size of the plant should be reconsidered. Due to low bulk density, the power plant has to be located close to the rice husk suppliers. Calculations show that 53 trucks are necessary to supply the power plant. In general, the rice milling industry is characterized by smaller mills, and rice husk collected from several milling plants is necessary. A perfect industrial symbiosis would be to locate the plant on site with the mill. This presumes smaller power plants, operating with the amount of rice husk generated by the mill. Experience with controlled rice husk combustion and power generation is scarce. By operating smaller power plants, further experience will be gained. This will improve the possibility for successful design of larger power plants.