Dear All,
I am writing to seek inputs concerning zero-dimensional energy balance around a gasifier; say, for instance, this gasifier is fed with coal at certain rate along with steam and air. The product stream(s) includes producer gas and unconverted coal/char. Considering no heat losses, overall energy balance can simply be expressed as:
total enthalpy in (includes all reactant streams, i.e., coal and steam/air) = Total enthalpy out (includes all product streams, i.e., unreacted coal/char and producer gas).........(1)
Now, the question is how to write down enthalpies of these reactant and product streams; in absolute format or enthalpy change format?....what I meant is....say, steam is supplied at 1 bara 500 K...then, absolute enthalpy is expressed as: (heat of formation of water at 298 K) + (Sensible heat of liquid water until 398 K at 1 bara) + (Latent heat at 398 K) + (Sensible heat of water vapor until 500 K)...the same approach is extended to all the reactant and product gas streams and if the same species appears both in reactant and product streams, only then, heat of formation term cancels out in the overall energy balance shown in Eq. (1). Otherwise, it remains and could significantly influence the energy balance calculations (e.g., producer gas comprises of CO, H2, and CH4 which do not appear in the reactants.).
Also, what about enthalpy of solid fuel being fed; could that be set equal to HHV or LHV?
Any inputs are greatly appreciated.
Thanks and Regards,
Kareem
In an energy balance around a gasifier, it is generally best to express the enthalpies of the reactant and product streams in terms of enthalpy changes rather than absolute enthalpies. This is because the enthalpy changes of the reactants and products can be directly measured or calculated from thermodynamic data, whereas the absolute enthalpies of the reactants and products depend on the reference state that is chosen.
For example, when calculating the enthalpy of a gas stream, it is common to use the enthalpy of the gas at a reference temperature and pressure (such as 25°C and 1 atm), rather than the absolute enthalpy at the temperature and pressure of the gas stream. This is because the enthalpy of a gas is largely determined by its temperature and pressure, and the reference state (25°C and 1 atm) is chosen as a convenient reference point that allows the enthalpy changes of different gas streams to be compared on a common basis.
In the case of solid fuels, it is generally best to use the lower heating value (LHV) rather than the higher heating value (HHV) when calculating energy balances. This is because the LHV is based on the heat of combustion of the fuel minus the heat of vaporization of the water produced during combustion, whereas the HHV includes the heat of vaporization of the water. In a gasifier, the water produced during combustion is typically used to produce steam, which is then used as a reactant in the gasification process. Therefore, the LHV is more appropriate for calculating the energy balance around a gasifier, as it takes into account the energy used to produce the steam.
I hope this helps! Let me know if you have any additional questions.
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