Nya systemlösningar i bioförgasningsanläggningar för att förbättra de ekonomiska förutsättningarna
Examensarbete för masterexamen
This diploma work studied the possibilities for improving the economic conditions for a biofuel-based CHP (combined heat and power) plant with an integrated gas turbine or combined cycle for electricity production. The CHP plant is assumed to deliver heat to both an industrial process and a district heat system. The plant is also assumed to be a joint venture between an industry and an energy utility company. Biofuel is an environmental friendly fuel, as it does not cause any net emissions of carbon dioxide. On the other hand, biofuel can cause serious problems for the equipment, e.g. corrosion and deactivated catalysts, which creates a need for extended cleaning of the product gas. The different technologies studied are all based on biomass gasification, either atmospheric or pressurized, followed by gas clean-up and combustion in a gas turbine. Electricity production is performed either with a single gas turbine or in combination with a steam turbine (combined cycle). The gas turbine sets the limit for how far the plant can operate at part-load. In this work, the limit is 60 % of full-load, based on heat delivery. During the period that the gas turbine is shut off, combustion of oil is used for the required heat production. The main aim of this work consists is to examine the possibilities for reduction of oil consumption during the period the biofuel-based CHP plant is not running. The reduction is assumed to be achieved by one of the following four design alternatives. 1. A gas duct burner is installed in the HRSG (heat recovery system generator). When the CHP load is lower than 60 % the product gas bypasses the gas turbine and is ducted directly to the burner where combustion occurs. During this period only heat is produced. The gas burner is designed for the required heat output during low load, i.e. 60 % of full load. 2. In this case a gas duct burner, designed for the conditions at low load, is installed in the HRSG. The difference from design alternative 1 is that the gas burner operates not only during low load, but also as supplementary firing during part of the peak load. To accomplish this the gasifier must be oversized to be able to produce the required amount of product gas. 3. A separate biofuel boiler is installed in the system. It is designed for the heat needed at low load, but is also used for part of the peak load. The biofuel boiler mainly replaces the oil boiler in the original system. 4. This design alternative gives priority to heat production rather than electricity production. A gas duct burner designed for combustion of all the product gas is installed in the HRSG. The gasifier is not oversized with respect to the gas turbine, which means that electricity is not produced when the burner is running. Thus, all heat content in the product gas can be used for heat production. The gas burner is used both at peak load and low load. This design alternative is not quite comparable with the other alternatives as electricity production is not given top priority. Therefore this alternative is, more or less, considered as a separate case. To be able to evaluate the economic performance of the design alternatives, the cost of electricity production was calculated. This cost is based on heat credit, which means that from the total cost the cost for heat production, by the cheapest technology, is subtracted and the balance is the cost for electricity production. The cheapest technology for heat production consists of a biofuel boiler and an oil boiler for peak loads. The cost of electricity production for the design alternatives is between 0,52-0,86 SEK/kWh. The result does not differ much from the original case, when oil is used both during peak and low load. The reason for this is that the main part of the cost represents the investment cost for the original CHP plant. The extra investment costs that are required for the design alternatives are relatively low. Furthermore, the oil that can be eliminated from the system is mainly used for industrial heat production and has a lower tax imposed on it than district heating has. The reduction in fuel cost (including tax) would have been greater if oil had been used for district heating. This work also studied how much emissions of carbon dioxide the different design alternatives cause. As all design alternatives means a transition from fossil fuel to biofuel the environmental effect will of course be positive. The total emissions of carbon dioxide for the different design alternatives were compared with the cost to introduce them in the original system. The comparison has been made with the heat producing reference, described above. Since the heat-producing alternative does not produce electric power, the calculations account for emissions associated with production of the same amount of electricity in a utility power plant fired with coal or, alternatively, natural gas. The results show that design alternative 3, a separate biofuel boiler, always has the largest reduction in carbon dioxide at the lowest specific cost. There is also a difference between atmospheric and pressurized gasification, as the reduction in carbon dioxide is lower for the atmospheric case. The reason for this is that atmospheric gasification produces less electricity than pressurized gasification. Design alternative 4, the gas burner is designed for combustion of all the product gas, has shown to be a worse alternative than the others. The cost for electricity production is of course higher for alternative 4, as this one gives priority to production of heat rather than electricity, but this alternative is also worse from an environmental perspective as the reduction in carbon dioxide is lower at a higher specific cost. For this kind of biofuel based CHP technology to be competitive it is necessary to introduce economic incentives that favor “green” electricity. These incentives could for instance be tax on fossil fuel which is used for electricity production, or that the electricity production sector is included in an international system of trading with greenhouse gas emissions rights.
Kemiteknik , Chemical Engineering