Integrating Biomass in Existing Natural Gas-Fired Power Plants

Licentiate thesis, 2010

Using biomass for utility heat and electricity generation can supplement use of fossil fuels, whose emissions of carbon dioxide likely risk causing serious climate change and ocean acidification. Biomass as energy source is a limited resource and when used as fuel in thermal power plants this currently results in higher production costs than using fossil fuels such as natural gas and coal. It is, therefore, important to find highly efficient and reliable biomass conversion technologies. Reliability is important to minimize investment risk. Co-firing biomass with coal is well known as a potential low cost and high efficient way of introducing biomass in the stationary energy system. Yet, several regions around Europe and elsewhere are strongly dependent on natural gas for electricity generation, typically applying combined cycle gas turbine (CCGT) plants. Although CO2 emissions from natural gas is considerably lower than from coal fired plants, it is less known how to introduce biomass as fuel in natural gas fired plants than in coal. Therefore, the present thesis evaluates options for integration of thermal conversion of biomass with existing CCGT plants. The focus on existing gas fired plants is motivated by a) gas fired plants are becoming increasingly dominant in some regions and there should be sought integration options that are not only based on co-firing with coal, and b) if the existing power plant infrastructure can be used to introduce biomass this could facilitate a near term introduction of biomass, contributing to near term CO2 mitigation targets, as opposed to building biomass-only dedicated plants. The biomass-based options for CCGT integration investigated in this work are hybrid combined cycles (HCC) and biomass gasification. A HCC is a combined cycle firing various fuels; in the present case natural gas in gas turbines (GTs) as topping cycle, and biomass in a fluidized bed boiler as bottoming (steam) cycle. HCC options include fully-fired (hot windbox) schemes with uncooled flue gases from GTs used as fluidization media in boiler, and supplementary fired schemes with GT flue gas used for preheating air and economizing. Gasification means thermal conversion of biomass to producer gas, that is cleaned and upgraded to medium-value syngas or high-value methane, which can be used in GT-based plants. The gasification schemes considered in this work also include minor steam cycles. The above options are compared to power (and heat) generation in commercial state-of-the-art fluidized bed boiler steam plants, as reference. All are applied to two basic case studies (three CCGT plant concepts, based on actual/typical plants) with varying gas plant sizes and configurations with different gas turbine types and steam data, as well as in condensing or combined heat and power schemes. The results show that increased thermal and cost efficiency can be attained by some of the proposed schemes for biomass/CCGT-integration. Integrated fully-fired hybrid schemes can give an increase of the specific biomass efficiency related to electricity production in the range 4-9 %-points, and for the gasification options up to 14 %-points, compared with a stand-alone state of the art biomass boiler, which has an electrical efficiency in the order of 35 %. Gasification schemes for methane were found to be less efficient for power (and heat) production than those for medium-value syngas. Parallel-powered hybrid schemes did not show thermodynamical benefits in the studied options. Levelized cost of electricity (LCoE) for integrated options was found to be in the range of, at most, 10-20 EUR/MWh lower than non-integrated options. The best cost performance was found for options with the highest efficiencies (gasification to medium value syngas, hybrid options at some conditions) or for those with simple design at conditions of high DH prices and/or low biomass fuel costs. Drying of biomass with CCGT low temperature exhaust gases was found to be cost-efficient. Obtaining modest efficiency increase by employing more advanced technology, such as some configurations of hybrid cycles, did not show significant cost benefits. The choice of most cost effective technology was found to mainly depend on district heating prices, whereas fuel costs and discount rates had less but still important influence. The overall conclusion of this work is that biomass integration with existing combined cycle natural gas-fired plants has the potential to lower costs and increase the competitiveness for biomass power compared with other low carbon alternatives (fossil power with carbon capture), although attributed with some risks and technical difficulties. Biomass integration in existing natural gas power plants can, therefore, be part of a pathway to increase employment and development of biomass thermal conversion technologies.