Week 4.2 - Combustion Efficiency Calculation after Preheating

                                                  Combustion Efficiency calculation after Preheating

Objective

A furnace is used to burn Methane and Air Mixture. A recuperator is connected to this furnace to recover some energy. 

To find out What is the effect of preheating on the adiabatic flame temperature by Changing pre-heating from 298 to 600K

To find out whether  preheating helps with fuel saving.If so, how does combustion efficiency go up as a function of pre-heating temperature.

To find out Is it right or wrong to assume a fixed furnace exit temperature.

Theory

A recuperator is a special purpose counter flow energyrecovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat. Generally, they are used to extract heat from the exhaust and use it to preheat air entering the combustion system. In this way they use waste energy to heat the air, offsetting some of the fuel, and thereby improves the energy efficiency of the system as a whole.

In many types of processes, combustion is used to generate heat, and the recuperator serves to recuperate, or reclaim this heat, in order to reuse or recycle it. The term recuperator refers as well to liquid-liquid counterflow heat exchangers used for heat recovery in the chemical and refinery industries and in closed processes such as ammonia-water or LiBr-water absorption refrigeration cycle.

Recuperators are often used in association with the burner portion of a heat engine, to increase the overall efficiency. For example, in a gas turbine engine, air is compressed, mixed with fuel, which is then burned and used to drive a turbine. The recuperator transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the fuel burner stage. Since the gases have been pre-heated, less fuel is needed to heat the gases up to the turbine inlet temperature. By recovering some of the energy usually lost as waste heat, the recuperator can make a heat engine or gas turbine significantly more efficient.

Coding:

In order to  find out What is the effect of preheating on the adiabatic flame temperature by Changing pre-heating from 298 to 600K

Cantera -Python code is given below

results:

Observations:

•  Adiabatic flame temperature varies linearly as preheating temperature increases.
• Possible theory behind this will be turbulance and preheated air fuel mixture has more enthalpy or energy making it easier to combust,since The AFT depends on the heating value of the fuel, the temperature and amount of oxidizer, and final combustion product composition.It make sense.

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To find out whether preheating help with fuel saving.

Combustion efficiency, defined as the ratio of heat released by the fuel to the heat input by the fuel.

  Ncombustion=(Q/(mfuel*LHV)

where Q=(mair*hair)+(mfuel*hfuel)-(mproduct*hproduct)

m=mass,h=enthalpy

Code:  #note"I used equivalance ratio as a function to find efficiency for both rich and lean mixtures"

Below i have used equivalance ratio as 1| but can be changed as user wanted

Results:

 Observations:

• from the plot we can note that the amount of heat energy generated increases when the inlet temperature of air is increased.
• efficiency of the system incresed linearly corresponding to preheating temperature
• in other word preheating increased the efficiency.So a lees amount of fuel is only needed to generated desired work output.

the efficiency is  measured by assuming constant exit temperature

But the enthalpy of combustion at exit depends upon the amount of heat that converted to work by the system and quantity of fuel before combustion.Here we assumed that the useful heat lost or work is not constant and the excess heat that generated as a result of preheating gets converted to more work.This creates the exit enthalpy or temperature not to change so we can say it is right to assume fixed furnace temperature if the work is not constant.

 To access the codes: https://drive.google.com/drive/u/0/folders/1iKZro4WpOK4Bj-LxSWTtp5eeIQlGeV4t