Performance evaluation of atmospheric biomass integrated gasifier combined cycle systems under different strategies for the use of low calorific gases
Introduction
Currently, most biomass electricity generation is based on steam cycles. These plants are usually small capacity (20–50 MWe) and present relatively low electric efficiency (≈20%) [1]. Conversely, the biomass integrated gasifier gas turbine/combined cycle (BIGGT/CC) technology, which is still in development, is expected to achieve efficiencies around 40% for atmospheric gasification of wood at modest scales [2], [3], but further improvements are necessary for it to become competitive. Cost reduction seems especially possible by improving the conversion efficiency [2], which could be achieved using existing larger natural gas combined cycles with efficiencies over 50–55% electric.
Following the pioneer demonstration cogeneration plant (6 MWe and 9 MWth) built in Värmano (Sweden) [4], an 8 MWe atmospheric BIGCC unit was built in UK (the ARBRE – ARable Biomass Renewable Energy – plant). In 2002, the gas turbine was fed with biomass derived gas (BDG) during a short period of time. The project was ended in the same year, apparently due to a contractual dispute and not due to technological problems [5]. The atmospheric biomass gasifier that is proposed for this work has been previously demonstrated in Italy in two units of 15 MWth processing refuse derived fuel (RDF), with the fuel gas being burned in a boiler [6]. A gasifier of 25 MWth based on the same technology was used in the ARBRE project.
Despite the efforts towards engineering development and cost reduction, technological drawbacks of the BIGGT/CC technology still have to be overcome. One of the main technological issues relates to adaptation of gas turbines to low heating value fuels. This involves hardware adaptation, the demonstration of a reliable cleaning process and the off design operation of gas turbines that have been designed for natural gas.
The use of BDG may lead to unacceptable pressure ratios of gas turbine operation as a result of a larger mass flow led through the expander if design heat input and combustion temperature are maintained. The low heating value of BDG can be 10 times lower than the natural gas heating value, so that a much larger fuel flow is necessary for an equivalent heat input. Accepting an increase in pressure ratio might lead to substantial reduction of compressor surge margin. Surge is a risky transient condition involving reverse flow through the compressor and can occur as the pressure ratio increases beyond the maximum safe value [7]. In those cases, other operational parameters should be changed to keep the compressor operation far enough from the surge limit and under torque limits, as well as to avoid over speed for the case of twin shaft engines [8].
The use of low calorific gas requires that “commercial off the shelf” gas turbines run in off design mode. Off design operation of gas turbines, in order to accommodate the larger mass flow, can be accomplished through de-rating by lowering the temperature, bleeding air from the compressor or inlet guide vane (IGV) control, or even by accepting the increase on pressure ratio. The use of the IGV for control was not considered within this work due to the lack of reliable information; IGV use is specific for individual gas turbines. Another strategy is to consider a gas turbine that has been designed for use of low calorific gas, operating on design conditions. The redesigned gas turbine is considered to be able to accommodate a larger mass flow and operate on the design condition because of its expander with a modified geometry (i.e. increasing the blade height or nozzle discharge angle).
This paper assesses the impact of different strategies for BDG use on the performance of BIGCC power cycles. This work is part of a broader research that aims at investigating co-firing (burning a mix of natural gas and biomass derived gas) as a mechanism to improve short term BIGCC cost-effectiveness [9], [10].
The biomass considered within this study is sugar cane bagasse and sugar cane trash (leaves and points of the plant). Bagasse is a residue of the sugar cane crushing process and is relatively cheap. Trash is becoming increasingly available in Brazil due to the increasing implementation of green cane harvesting.
Section snippets
Issues on suitability of sugar cane residues for BIGG technology
Physical and chemical properties of the biomass affect the suitability of BDG for BIGCC technology. Physical properties, such as size and density, affect the handling and feeding systems. Chemical properties, such as composition, heating value and ash content, have a strong impact on gasification and cleanup systems. There are limitations on experimental data for sugar cane residues and, more importantly, the cleaning of BDG has not been demonstrated commercially so far. Hence, the assessment
Required modifications on gas turbines
Natural gas and light oil distillates are the preferred fuels for gas turbines, but many other fuels have been used successfully, including low calorific value (LCV) gases such as gases from iron and steel industries with heat content as low as 3 MJ/m3 [3]. Some modifications should be necessary for the use of LCV fuels in a gas turbine originally designed to use natural gas as fuel. These modifications comprise gas turbine layout for the changed mass flow, fuel delivery/injection system, fuel
Power system description
The power plant considered here is based on biomass gasification that occurs in an atmospheric air blown gasifier similar to those developed by TPS. This type of gasifier is technically proven, and some units have operated for some time. It is considered that before the gasifier, the biomass moisture content should be reduced to 15% by using flue gas from the HRSG and that the drying design conditions require a minimum flue gas temperature (200 °C). Gasification occurs within a circulating
Simulation results
Calculations were performed for the cycles for each of the three gas turbine control strategies considered. Overall thermal efficiencies are presented on the lower heating value basis and include the gasification conversion efficiency. The effects of each strategy on the cycle performance are investigated below.
Conclusions
This work aimed at a performance evaluation of atmospheric BIGCC cycles of different capacities fuelled by sugar cane derived gases. Regarding the suitability of sugar cane derived gas for use in gas turbines, most of the issues related to the physical and chemical properties of sugar cane bagasse and trash seem to be possible to overcome. Gas cleanup systems should be effective to fulfil the strict requirements of gas turbines regarding particulate, alkali and tars.
From the point of view of
Acknowledgements
Monica Rodrigues Souza is grateful to the Brazilian agencies CNPq and CAPES for the financial support received during her work at the University of Campinas, Brazil, and Utrecht University, The Netherlands. This paper also reports results of research developed with the financial support of Fundação do Amparo a Pesquisa do Estado de São Paulo – FAPESP (project 2001/14302-1). The authors would like to thank Eileen and Michael Zimmerman for their help on language editing.
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