Performance evaluation of atmospheric biomass integrated gasifier combined cycle systems under different strategies for the use of low calorific gases

Abstract

This work aims at a performance evaluation of atmospheric BIGCC (biomass integrated gasifier combined cycle) systems operating under different strategies for the use of low calorific fuel in gas turbines. The fuel is a synthetic gas derived from gasification of sugar cane residues. Two analysed strategies for surge control on gas turbines originally designed for operation on natural gas are de-rating and air extraction from the compressor. Another strategy for use of biomass derived gas is the retrofit of a gas turbine through modification of the expander geometry. Three generic gas turbines, whose main parameters have been taken from off the shelf machines, are proposed as representative of certain classes of gas turbines. Results are compared to the hypothetical situation in which it is possible to accept any increase in gas turbine pressure ratio. According to the results, de-rating is the worst surge control strategy, both from the point of view of power production and thermal efficiency. Conversely, results showed considerable gains that can be obtained on cycle power production when the gas turbine expander is modified. Additionally, an overview of issues concerning the suitability of sugar cane residues (bagasse and trash) for the production of gasified gas and its use in BIGCC plants is presented.

Introduction

Currently, most biomass electricity generation is based on steam cycles. These plants are usually small capacity (20–50 MW_e) 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 MW_e and 9 MW_th) built in Värmano (Sweden) [4], an 8 MW_e 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 MW_th processing refuse derived fuel (RDF), with the fuel gas being burned in a boiler [6]. A gasifier of 25 MW_th 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.