Natural gas fired combined cycles are fired with a large excess of air for cooling purposes. The exhaust gases have a high temperature and contain still a fair amount of oxygen (vitiated air). Use of this air for subsequent coal combustion should lead to a mixed natural gas/coal fired cycle with increased efficiency of the electrical power generation and a decrease of CO2 emissions.
The main objective of this project is to improve coal combustion with vitiated air (turbine exhaust gas) in combined cycle applications by improving the combustion and catalytic oxidation of unburnt fuel.
In the extensive coal combustion experiments that were carried out, pyrolysis occurred in three independent steps, where each step could be described by a complex n-th rate equation. Variations in the oxygen content and the heating rate had a significant influence on the combustion and burn-out of the coal. From these results, a combustion model could be derived with parameters that were applicable to high heating rates. Catalysts were successfully identified that could oxidise carbon monoxide and propane in unburnt flue gases completely, but no catalyst could oxidise methane or solid carbon. The results were applied in computer modelling of the combined cycle with vitiated air to identify optimum operational cycles. The feasibility and profitability study concluded that the use of vitiated air in coal combustion was feasible, identified opportunities to optimise the cycle's efficiency and estimated the possible CO2 reductions.
Modern natural gas fired combined cycle plants reach overall net efficiencies of up to 55% for electrical power generation. These plants use modern gas turbines with inlet temperatures up to 1300 C which result in low O2 content in the turbine exhaust gas (<17%).
This project investigates the possibility to use the turbine exhaust gas as vitiated air for the coal combustion.
The low O2 content of the exhaust gas from modern gas turbines leads to problems in ignition, combustion, stability and burnout of pulverized coal flames. To overcome these difficulties, the following work will be done: 1. Experimental investigation of coal combustion under the conditions of vitiated air in a differential thermal analysis apparatus and a fluidized bed (laboratory scale).
2. Development and experimental investigation of catalyst materials for the combustion of the remaining unburnt components in the flue gas (laboratory-scale test facility).
3. Numerical calculations to design a pulverised coal-burner for combustion with vitiated air.
4. Feasibility study and efficiency optimization of combined cycles with pulverized coal combustion and catalytic oxidation and assessment of the possible carbon dioxide reduction.
Funding SchemeCSC - Cost-sharing contracts