Effect of preheating on adiabatic flame temperature combustion efficiency and fuel saving
In this project, a furnace is used to burn Methane and Air Mixture. A recuperator is connected to this furnace to recover some energy from the flue gases and increase the temperature of the inflow air. Doing so will reduce the heat wasted through flue gases and is said to increase the efficiency of the combustion. The same is investigated in this project.
The reaction is:
`CH_4 + 2(O_2 + 3.76N_2) rarr CO_2 + 2H_2O + 7.52N_2`
Effect of preheating air intake on the adiabatic flame temperature:
To heat air seperately two instances of quantities class are created. One for air consisting of `O_2` and `N_2`molecules and other instance for fuel which is `CH_4` in this case. The temperature of the air is changed from 298k to 600k by 10k steps. Then, equilibrate function is used to determine the adiabatic flame temperature under constant enthalpy and pressure condition. The adiabtic flame temperature obtained is then compared against the preheated air temeperature and the plot obtained is attached below:
Effect of preheating on fuel savings:
To study the effect of preheating on the fuel savings, the effects of preheat temperature on combustion efficiency increase is studied first. For that, we assume that the furnace exit temperature is 1700K and the products of combustion as `CO_2`, `H_2O`, and `N_2`.
where the increase in combution efficiency is given by,
`eta = Q_(out)/(m_fxxLHV_f)`
we know that,
`m_a + m_f = m_p`
where,
`m_a = "Mass of air"`
`m_f = "Mass of fuel"`
`m_p = "Mass of products"`
since energy is released during combustion,
`m_a xx h_a + m_f xx h_f - m_p xx h_p ne 0`
`m_a xx h_a + m_f xx h_f - (m_a + m_f) xx h_p = Q_(out)`
`implies Q_(out) = m_axxh_a + m_fxxh_f - (m_a + m_f)xxh_p`
where,
`h_a = "Enthalpy of air"`
`h_f = "Enthalpy of fuel"`
`h_p = "Enthalpy of products"`
Air-Fuel ratio(AF) = `"Mass of air"/"Mass of fuel"` assuming equivalence ratio `phi` = 1.
`"Mass of air"/"Mass of fuel" = ("number of moles of air" xx "molar mass of air")/("number of moles of fuel" xx "molar mass of fuel"(CH_4))`
`"Mass of air"/"Mass of fuel" = (2xx(31.9988 + 3.76xx28.0134))/(1xx16.04246)`
`"Mass of air"/"Mass of fuel" = (274.658368g)/(16.04246g)`
`implies "Air-Fuel ratio" = (274.658368g)/(16.04246g)`
`implies eta =( m_axxh_a + m_fxxh_f - (m_a + m_f)xxh_p)/(m_fxxLHV_f)`
`implies eta =( (m_a/m_f)xxh_a + h_f - ((m_a/m_f) + 1)xxh_p)/(LHV_f)`
`implies eta =( AFxxh_a + h_f - (AF + 1)xxh_p)/(LHV_f)`
`LHV_f = 50(MJ)/(Kg)` for `CH_4`
To assume the furnace exit temperature as 1700K, another class instance is created. This class is defined to contain `CO_2`,`N_2` and `H_2O`as we are assuming that the products only contain `CO_2`,`N_2` and `H_2O`. After that, the inlet air temperature is varied from 298K to 600K by 10K steps and the corresponding efficiency increase is calculated, the preheated air temperature and the combustion efficiency increase is plotted. The graph of the same is attached below:
Now, the fuel saving is calculated. Higher the combustion efficiency higher is the heat that can be extracted from the fuel after combustion so lower is the amount of fuel required for combustion. So, the percent increase in combustion efficiency is calculated which directly relates to the fuel saved. For various preheat air temperature the fuel saved is calculated, the graph plotted comparing the preheat air temperature and percent fuel saved is given below:
Simulation wise, it is wrong to assume the exit furnace temperature because as the preheating temperature of air increases the adiabatic flame temperature increases thus it increases the exit furnace temperature resulting in reduced accuracy of combustion efficiency calculation which introduces a error.
Practically, it is right to fix the exit furnace temperature because the pipe material can only withstand up to a particular temperature and also because higher exit furnace temperature will result in higher air temperature. If the air temperature is high enough, the fuel + air mixture will start combusting before reaching the combustion chamber will is not a desirable circumstance.
The python program of the studies is attached via link below:
https://drive.google.com/file/d/1uU0AcvLuD4CgjCQppk23_3kAiLhutTjO/view?usp=sharing