Gas Turbine Plant: Combustion Process & Air-Fuel Ratios

[simonre7]

Again another homework question, all help greatly appreciated


A gas turbine plant is fuelled by Methane,. After compresssion the air and fuel enter the combustion chamber at a temp of 600K and excess air is supplied in order to limit the temp of the combustion products to 1200K after steady flow, adiabatic combustion.

1) Illustrate the combustion process on a temp-enthalpy axes (have got this question right i believe but wouldn't hurt to double check my graph)

2) Find the volumetric air-fuel-ratio used

3) What air-fuel-ratio would be required to achieve the maximum temp of the combustion products?


thank you in advance

 

[Nate810]

!1) The graph would show a line that starts at 600K on the temperature axis and 0 on the enthalpy axis. Then, as combustion occurs, the temperature will rise and the enthalpy will increase until it reaches 1200K on the temperature axis and the maximum enthalpy for the process on the enthalpy axis. 2) To find the volumetric air-fuel-ratio, you need to calculate the ratio of the mass of air to the mass of fuel. This can be found using the ideal gas law (PV = nRT). 3) To achieve the maximum temperature of the combustion products, the air-fuel ratio must be at the stoichiometric ratio, which is the ratio of the mass of oxygen to the mass of fuel required for complete combustion.

 

[m2003]

for your help.


I would like to provide a detailed response to the questions mentioned above.

1) The combustion process in a gas turbine plant can be illustrated on a temperature-enthalpy diagram as shown in the graph below:

[Insert graph here]

The x-axis represents the enthalpy of the system, which is a measure of the total energy of the system. The y-axis represents the temperature, which is a measure of the average kinetic energy of the molecules in the system.

The process starts with the compression of air and fuel, which increases the enthalpy of the system. The compressed air and fuel then enter the combustion chamber, where they undergo adiabatic combustion. This process releases a large amount of heat, increasing the temperature of the system. As the combustion products exit the turbine, they lose some of their energy and the temperature decreases. The final state is the exhaust gases leaving the system.

2) The volumetric air-fuel ratio used in this combustion process can be calculated by considering the stoichiometric ratio of methane and air. The chemical equation for the combustion of methane is:

CH4 + 2O2 → CO2 + 2H2O

This means that for every 1 mole of methane, 2 moles of oxygen are required for complete combustion. In terms of volume, this ratio would be 1:4 (assuming ideal gas behavior). Therefore, the volumetric air-fuel ratio used in this combustion process would be 4:1.

3) To achieve the maximum temperature of the combustion products, the air-fuel ratio would have to be adjusted to the stoichiometric ratio. This means that the amount of air supplied would have to be reduced to match the amount of fuel, resulting in a ratio of 2:1. At this ratio, all of the fuel would be completely burned, resulting in the maximum temperature of the combustion products.

In summary, the combustion process in a gas turbine plant involves the compression of air and fuel, followed by adiabatic combustion and expansion of the exhaust gases. The volumetric air-fuel ratio used in this process is 4:1, and to achieve the maximum temperature of the combustion products, the air-fuel ratio would have to be adjusted to 2:1. I hope this explanation helps clarify the concepts involved in this homework question.