If you just need a very simple model of the combustion of natural gas
Make sure you have oxygen, nitrogen, CO2 and water in your components list
Add CO to the component list for more accuracy
Create an air stream
Feed the air and the fuel streams into a Gibbs reactor
If there is no duty stream or if you set the duty to zero, you will get the adiabatic flame temperature
If you have a duty stream and you set an outlet temperature, then you get the heat you can recover
From here you can make things more complex, it all depends on your needs
If any of your compounds have other elements than carbon and hydrogen, you also need to include the combustion products of those other elements. For sulphur for example, the simple way out is SO2, but you might want to add SO3 and H2S and even the various forms of elemental sulphur.
Sometimes reaction kinetics come into play. For example, if the oxygen excess is low, it may be prudent not to specify too low an outlet temperature for the Gibbs reactor. The issue is the kinetics of the reaction between CO and CO2. The Gibbs reactor calculates chemical equilibrium, so if you were to cool the flue gas to ambient temperature, you'd get what the chemical equilibrium composition would be. In some cases, reaction kinetics will prevent the reactions from reaching equilibrium. For the CO/CO2 equilibrium, this will be quite noticeable, but not dramatic. A more dramatic example is nitrogen oxidation. If you were to add the various nitrogen oxides to the component list, you'd end up with a lot of nitrogen oxide in the model outlet, but in real life this doesn't happen because the reaction isn't fast enough.
All the above will work as long as you use HYSYS library components. With Aspen Properties library components things will usually work, but not always. The chemical formula and the heat of formation of the components needs to be known and that is not always the case for Aspen Properties components.
And then you get to the cases where your fuel is not so nicely defined. It doesn't have to be anything exotic, just imagine the case of a fuel oil and you'd typically no longer have a feed stream defined in terms of library components. You'd have hypo components and those do not have a chemical formula nor a heat of formation (or heart of combustion). Usually you would be able to obtain a lower heating value and an elemental composition for a fuel oil. Usually the elemental composition is given on a mass basis, you'll have to convert it to a mole basis to be able to create a chemical formula. You will have to manufacture a component with a high molecular weight to reasonably match your elemental composition. I on of the elements is only present in small amounts and you want to include it, the MW will have to be really high. Once you have settled on a formula, you'll have to use this information to create a combustion stoichiometry. Then you can us this, the heat of combustion and the heats of formation of all components except the fuel to calculate the required heat of formation of the fuel. Sounds tedious? It is ... But, if you need to do this on a regular basis, I can provide you with a small program that will do all the work for you. Just send an Email to wim@bpt.no.
This hasn't touched yet on anything that really requires combustion kinetics! This comes into play in the combustion of H2S for sulphur production for example. I can go into this in another post if lots of people ask for it.
Make sure you have oxygen, nitrogen, CO2 and water in your components list
Add CO to the component list for more accuracy
Create an air stream
Feed the air and the fuel streams into a Gibbs reactor
If there is no duty stream or if you set the duty to zero, you will get the adiabatic flame temperature
If you have a duty stream and you set an outlet temperature, then you get the heat you can recover
From here you can make things more complex, it all depends on your needs
If any of your compounds have other elements than carbon and hydrogen, you also need to include the combustion products of those other elements. For sulphur for example, the simple way out is SO2, but you might want to add SO3 and H2S and even the various forms of elemental sulphur.
All the above will work as long as you use HYSYS library components. With Aspen Properties library components things will usually work, but not always. The chemical formula and the heat of formation of the components needs to be known and that is not always the case for Aspen Properties components.
And then you get to the cases where your fuel is not so nicely defined. It doesn't have to be anything exotic, just imagine the case of a fuel oil and you'd typically no longer have a feed stream defined in terms of library components. You'd have hypo components and those do not have a chemical formula nor a heat of formation (or heart of combustion). Usually you would be able to obtain a lower heating value and an elemental composition for a fuel oil. Usually the elemental composition is given on a mass basis, you'll have to convert it to a mole basis to be able to create a chemical formula. You will have to manufacture a component with a high molecular weight to reasonably match your elemental composition. I on of the elements is only present in small amounts and you want to include it, the MW will have to be really high. Once you have settled on a formula, you'll have to use this information to create a combustion stoichiometry. Then you can us this, the heat of combustion and the heats of formation of all components except the fuel to calculate the required heat of formation of the fuel. Sounds tedious? It is ... But, if you need to do this on a regular basis, I can provide you with a small program that will do all the work for you. Just send an Email to wim@bpt.no.
This hasn't touched yet on anything that really requires combustion kinetics! This comes into play in the combustion of H2S for sulphur production for example. I can go into this in another post if lots of people ask for it.
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