Cycle Gas Turbine Power Research Articles

Reduction of Combustion Irreversibility in a Gas Turbine Power Plant Through Off-Gas Recycling

Combustion in conventional fossil-fueled power plants is highly irreversible, resulting in poor overall energy conversion efficiency values (less than 40 percent in many cases). The objective of this paper is to discuss means by which this combustion irreversibility might be reduced in gas turbine power cycles, and the conversion efficiency thus improved upon. One such means is thermochemical recuperation of exhaust heat. The proposed cycle recycles part of the exhaust gases, then mixes them with fuel prior to injection into a reformer. The heat required for the endothermic reforming reactions is provided by the hot turbine exhaust gases. Assuming state-of-the-art technology, and making a number of simplifying assumptions, an overall efficiency of 65.4 percent was attained for the cycle, based on the lower heating value (LHV) of the methane fuel. The proposed cycle is compared to a Humid Air Turbine (HAT) cycle with similar features that achieves an overall efficiency of 64.0 percent. The gain in cycle efficiency that can be attributed to the improved fuel oxidation process is 1.4 percentage points. Compared to current high-efficiency gas turbine cycles, the high efficiency of both cycles studied therefore results mainly from the use of staged compression and expansion with intermediate cooling and reheating, respectively.

A Methane-Steam Reformer for a Basic Chemically Recuperated Gas Turbine

Several illustrative designs are presented for a methane-steam reformer (MSR) that is used as a chemical recuperator in a Basic Chemically Recuperated Gas Turbine power cycle (a “Basic” CRGT is defined as one without intercooling or reheat). In this cycle, an MSR, heated by the turbine exhaust flow, converts a methane-steam mixture into a hydrogen-rich fuel that powers the gas turbine. A computer code was developed to calculate the size and performance characteristics of counterflow reformers. The code consists of a one-dimensional marching scheme that integrates the chemical, thermodynamic, and geometric variables along the heat exchanger/reformer tubes. The calculated designs were selected to give near-minimum catalyst volumes. These designs show that maintaining a high reformer gas temperature, using combustion-side heat transfer augmentation techniques, and using a catalyst of high reactivity are critical to obtaining a compact reformer design.

A Decomposition Strategy for Thermoeconomic Optimization

An optimal thermal design of a considered system configuration is conveniently decided when the system is modeled as made up of one thermodynamic subsystem and of the essential number of design subsystems. The thermodynamic subsystem decides the performance of the components and the design subsystems decide their best matching geometry and costs. An optimizer directs all decisions to an extremum of a given objective function. This decomposition strategy is illustrated by investigating the optimal values of seven decision design variables for a regenerative gas turbine power cycle when a cost-objective function is minimized. The results seen from the point of view of second law analysis and costing are discussed.

The Effects of LBG Composition and Combustor Characteristics on Fuel NOx Formation

The low Btu gas (LBG) combined gas and steam turbine power cycle is a potentially attractive alternative to the direct coal fired steam cycle because of the potential for low sulfur emissions and high overall cycle efficiency. However, LBG may contain ammonia (NH3) which could be converted to nitrogen oxides (NOx) under typical combustion conditions. This paper examines the effects of LBG composition and combustor design on NOx emissions. Low Btu gases of varying compositions were synthesized from bottled gases and fired in three atmospheric pressure flame reactors: diffusion flame reactor, flat flame reactor and catalytic reactor. Nitrogen oxide emissions were found to be most sensitive to the concentrations of NH3 and hydrocarbon fuel gas in the synthetic LBG. Lowest NOx emissions were produced by the diffusion flame reactor operating at near stoichiometric conditions and the catalytic reactor operating fuel rich.

Performance of N2O4 Gas Cycles for Solar Power Applications

The use of N2O4 as the working fluid in gas turbine power cycles is recognized as a potential instrument for improving cycle efficiency at moderate top temperatures while maintaining the technical advantages connected with the waste heat rejection at a comparatively high average temperature. Solar central receiver power stations, whose economic effectiveness is very sensitive to cycle efficiency and which must often reject their waste heat into the atmosphere, could usefully adopt this fluid. The thermodynamic reasons which explain the peculiar behaviour of N2O4 as the Brayton cycle working fluid are discussed. With respect to inert gas cycles, N2O4 permits, for a given efficiency, a reduction in turbine inlet temperature by 200-250°C. At a given turbine inlet temperature, the dissociating character of N2O4 allows overall efficiencies similar to those of steam cycles (at least for moderate plant capacities and provided N2O4 and steam cycles reject their waste heat at comparable temperatures). The relatively long relaxation time of the second step of the N2O4 dissociation can represent a problem mainly for the regenerator. A cycle is presented where regeneration at a pressure higher than the compressor inlet pressure can alleviate this problem.

Effects of Steam Injection on the Performance of Gas Turbine Power Cycles

The effect of injecting steam generated by exhaust gas waste heat into a gas turbine with 3060°R turbine inlet temperature has been analyzed. Two alternate steam injection cycles are compared with a combined cycle using a conventional steam bottoming cycle. A range of compression ratios (8, 12, 16, and 20) and water mass injection ratios (0 to 0.4) were analyzed to determine effect on net turbine power output per pound of air and cycle thermodynamic efficiency. A water/fuel cost tradeoff analysis is also provided. The results indicate promising performance and economic advantages of steam injected cycles relative to more conventional utility power cycles. Application to coal-fired configuration is briefly discussed.

The Energy Exchanger, a New Concept for High-Efficiency Gas Turbine Cycles

A method of circumventing the turbine inlet temperature limitation of present-day gas turbines is presented. This method is based on a direct fluid-to-fluid energy exchanger whereby the available energy of expansion of the hot combustion gas in a gas turbine cycle is transferred directly to a colder gas. The aerodynamic wave processes in several possible modes of operation are examined to determine the inherent limitations in efficiency of direct fluid-to-fluid energy exchange processes. In particular, it is demonstrated that, by using a system of isentropic compression waves to avoid shock losses and by carefully choosing the molecular weights of the fluids utilized in the energy exchanger, perfect energy exchange is possible in principle. When allowances are made for losses due to mixing, leakage, and viscous effects, an energy exchanger utilizing heated combustion air at 3240 deg F to drive steam at 1500 deg F with a potential energy exchange efficiency of 85 percent is feasible. Applications of the air-steam energy exchanger operating in gas turbine cycles utilizing a conservative choice of component efficiencies indicate that thermal efficiencies of gas turbine power cycles of 50–60 percent may be possible.