Need help on how to find the Equivalence Ratio of combustion for plastic waste

Hello
Actually, I'd like to find the effect of different Equivalence Ratios on my gasification experiment using plastic waste as feedstock fuel but I'm confused as to how I can find the Equivalence Ratio because I don't know the exact chemical formula of plastic waste I'm using. So I'm curious how can I calculate the stoichiometric Air-Fuel ratio of combustion? Can I use the ultimate analysis to calculate the stoichiometric Air-Fuel ratio?

This is the analysis of my feedstock (%w/w),
Moisture =10%
Ash =0.09%
VM =89.7%
FC =0.21%
C =83.1%
H =11.77%
O =4.83%
N =0.14%
S = 0.16%

I specify waste mass flowrate = 20 kg/h,
air mass flowrate = 10 kg/h

rhmourwa said:

Can I use the ultimate analysis to calculate the stoichiometric Air-Fuel ratio?

Yes, you can, but the total is not 100%.
What is the VM =89.7%, is that volatiles such as water ?
Does that mean the remainder is the dry fuel = 10.3% ?

If you know the carbon and the hydrogen, then you can make up a hypothetical hydrocarbon and evaluate the oxygen required for its combustion.
Some of the O =4.83% may be available.
Is the ash a silicate?

Baluncore said:

Yes, you can, but the total is not 100%.
What is the VM =89.7%, is that volatiles such as water ?
Does that mean the remainder is the dry fuel = 10.3% ?

If you know the carbon and the hydrogen, then you can make up a hypothetical hydrocarbon and evaluate the oxygen required for its combustion.
Some of the O =4.83% may be available.
Is the ash a silicate?

Thank you for answering. Yes, the VM means volatile matter and VM=89.7% is the weight percent of nonwater gas that is released when this plastic waste is heated in the absence of air.

So according to this reference website (this), Is this the same combustion reaction you mean? and to make up a hypothetical hydrocarbon, I change %mass basis in analysis to moles basis, and the combustion reaction will be:
C_0.069H_0.117 + 0.0465(O₂ + 3.76N₂) → 0.069CO₂ + 0.0585H₂O + 0.1748N₂

From my understanding, this means oxygen required theoretically for combustion is 0.0465 mole but there is also 4.83%weight oxygen (=0.003 mole) in fuel. So, the amount of oxygen required is actually 0.0465-0.003 = 0.0435 mole and the stoichiometric air-fuel ratio will be = 0.0435+(0.0465*3.76) = 0.2183 mole. Am I doing it correctly? if you can correct me step by step, I'm very thankful.

and I have another question. If I want to change my actual air mass flow rate to achieve ER=1 for complete combustion. Can I determine it by using this equation:
air mass flow rate actual= Equivalence ratio * mass flowrate of fuel used * stoichiometric air-fuel ratio

rhmourwa said:

Am I doing it correctly? if you can correct me step by step, I'm very thankful.

I need a diagram that shows the mass flow through the analysis process. That way, I can see where the double counting is being done. For example, are the VM gasses being burnt? What do they include?

Following on from that diagram, I would use a spreadsheet to account for the many burning processes, to produce the mixed exhaust gas stream. The oxygen demand needed for that process can be computed in the spreadsheet.

There is a simple rule of thumb that says, for each one pound of hydrocarbon fuel burnt, about 14.5 pounds of air are required. That will give you a sanity check of your spreadsheet.

rhmourwa said:

... and I have another question. If I want to change my actual air mass flow rate to achieve ER=1 for complete combustion. Can I determine it by using this equation:
air mass flow rate actual= Equivalence ratio * mass flowrate of fuel used * stoichiometric air-fuel ratio

When it comes to burning a waste stream in a real furnace, there will be oxygen sensors in the exhaust, adjusting the air supply and secondary burning, or catalytic conversion of flue gasses. The precise computation of the stoichiometry will become moot, to be replaced by analysis of the bag house filtering to remove pollutants. The spreadsheet can help estimate the quantities and size of the exhaust gas processing plant.