A Method to Evaluate the Emissions of Gas Turbine for Power Generation

Abstract

This paper focuses on emission prediction for plants which use gas turbines for electrical power generation. A Techno-Economic, Environmental and Risk Analysis (TERA) framework, developed at Cranfield University, is used as the modelling philosophy. Thermodynamic performance simulation is at the core of the study whilst the risk, lifing, economics and environmental modules are built around the performance. Recently, the public agenda has emphasised environmental issues and new restrictive legislation on emissions can be expected. It means electrical power generation companies will have to look for ways to reduce their emissions. The replacement of out-dated and/or obsolete machinery having lower overall energy efficiency is one way. However, selection of new machinery will not only require economic and technical risk assessment but also its environmental impact. In-house software (Turbomatch) is used to calculate thermodynamic performance and an adaptable aviation emissions model, to fit industrial applications is presented here with the emissions model focusing on NOx, CO2, H2O, CO and unburned hydrocarbons. Then, the environmental module has been fed by the levels of NOx, CO2 and H2O to estimate the damage the engine will cause to the environment over several years with respect to global warming. Based on both field and public domain data two hypothetical engine configurations are investigated. One of them, a 165MW single shaft industrial machine, is used as the baseline to compare against the second one which is a 30MW aeroderivative single shaft machine. The results predict that the 165MW single shaft engine model is more sensitive to an increase in ambient temperature than the 30MW aeroderivative single shaft engine model. The larger engine thermal efficiency reduces by 4%, for 30°C increase in ambient temperature above design point. That of the smaller engine model decreases by 2 1/2%. The loss in shaft power is also sharper for the 165MW engine model; however the significantly greater stability shown for the 30MW engine with respect to ambient temperature variation comes at a price. This engine emits higher levels of all pollutants, especially NOx, compared to the 165MW engine due to relatively higher firing temperature in relation to the engine size. This paper helps to establish the basis of a methodology to analyse GT emissions for power generation. The other aim is to integrate these findings into a system which will act as optimiser for TERA analysis. The effectiveness of the system is that it will allow designers and users to compare between alternative possibilities for turbomachinery selection and replacement. There are other models being developed at Cranfield University which combine systems to give an overall evaluation in terms of technical, economic, environmental and risk perspectives for power generation [1].