Is the "water effect" from combustion of propane significant in roasting?

Propane, C3H8:
Density: 582 g/L (liquid) (1.013 bar at boiling point)
Molar mass: 44.10 g/mol

Water, H20:
Density: 1000 g/L
Molar Mass: 18.016 g/mol

Chemical reaction: C3H8 + 5O2 --> 3C02 + 4H20 + heat
One mole of Propane produces 4 moles of water.


How many moles of propane from 1 liter?

1L * 582 g/L * 1/44.1 mol/g = 13.2 moles of propane

How many grams of water does combustion of 1 L propane produce?

13.2 mol * 4 * 18 g/mol = 950 grams of water

1 Liter of propane = 24000 BTU


Assumption:

1 kg gas roaster, 10.000 BTU max
average 70% on a 15 min roast
10000/4 *0.70 = 1750 BTU

24000 BTU / 1750 BTU = 13.7

950 grams of water / 13.7 = 69 grams of water.

During a "normal" roast the combustion of propane gives off 69 grams of water.

Is this a significant amount compared to the water from the ambient air and the beans?


1000 g green beans, 15 % weight loss
150 grams of water vapor from beans.

Normal airflow through a roaster:
60 m^3/h
15 min = 15 m^3 of air.

50 % relative humidity, normal indoor humidity.
50 % rel. at 25 C = 10 grams of water per kg of air.

Density air at 25 C ,1 atm, 1.1839 kg/m^3

10 g/kg * 1.1829 kg/m^3 = 11.8 g/m^3

15 m^3 of air gives 177 grams of water

Conclusion:

Combustion of 1750 BTU of propane gives 69 grams of water.
Roasting 1000 grams of green beans produces 150 grams of water, 15% weight reduction
Humidity of the ambient air, indoor and a airflow of 60 m^3/h equals 177 grams of water.

The total water "content" is 20% higher in a gas roaster compared to an electrical roaster is this case.

But you have to remember that most of the beans have given up their free water by the end of the drying phase. It's not like theres an constant, even quantity of water vapor present from the beans throughout the roast. And that obviously goes for both gas and electric roasters. And the ambient conditions (humidity) will be the same as well. I can see why you're wanting to calculate some generalized numbers, but the only one thats relevant is the water vapor produced by combustion. It's the only delta that is different.

Granted, this doesn't even touch on various air flow designs and how they affect the roasting process.

Given that the first crack is a direct result of the water vapor expanding inside the beans its reasonable to assume that the beans are losing water content all the way up to and through first crack. This is however easy to find out by dumping the beans at different times and measuring the weight.

TomC wrote:but the only one that's relevant is the water vapor produced by combustion. It's the only delta that is different

But how can you say anything about the size/effect of the water content from the combustion without comparing it to something?



A simple experiment would be to add the equal amount of water to an electrical roaster and analyse the results.

"Given that the first crack is a direct result of the water vapor expanding inside the beans its reasonable to assume that the beans are losing water content all the way up to and through first crack. This is however easy to find out by dumping the beans at different times and measuring the weight".

I know what first crack is. My point was the water vapor from the beans will essentially be non-existant towards the end of the roast, when the beans are at their most vulnerable. This is more prominent for people who want to roast past Full City.

"But how can you say anything about the size/effect of the water content from the combustion without comparing it to something?"

easily, because it's the only topic we're discussing. water vapor is produced from burning propane, not the energy from an electric roaster. So you're comparing a measurable quantity to "0". You might as well compare the propane roaster to a big yellow ball. One is relevant, the other isn't. A big yellow ball doesn't produce water vapor either.

"A simple experiment would be to add the equal amount of water to an electrical roaster and analyse the results."

not simple at all. It's not even feasible in an experiment, it wouldn't be the same phenomena at all.

TomC wrote:"A simple experiment would be to add the equal amount of water to an electrical roaster and analyse the results."

not simple at all. It's not even feasible in an experiment, it wouldn't be the same phenomena at all.

I think It is doable. I've done it, but was not interested in numbers. I have a 1/2HP water AC unit, it emits cool and humid air, and my vacuum cleaner motor pulls air from it into my electric fluid bed roaster. Was planning to compare results with and without, but got busy with propane conversion all together.

I would love to see it done, and be done precisely, consistently and relevant. You would need to be able to create the EXACT amount of water vapor as produced by burning propane, and be able to get it into the roaster without altering the flow mechanisms and characteristics of the native roaster at all, in order to do draw any meaningful conclusions. You couldn't just add in humidified air and call the experiment a success. You'd be changing too many variables.

I've even pondered adapting an ultrasonic nebulizer, since I use them at work and they're disposable, to create a micro fine mist of water vapor and somehow route it into the roaster. The Quest lends itself to an adaption like this, due to it's odd flow designs ( it could just be setup in a small box that sits underneath the intake fan). But there's no way to titrate the quantity of vapor produced. It's on or off. And I wouldn't be surprised if it was too much.

sversimo wrote:Is the "water effect" from combustion of propane significant in roasting?

Conclusion:

Combustion of 1750 BTU of propane gives 69 grams of water.
Roasting 1000 grams of green beans produces 150 grams of water, 15% weight reduction
Humidity of the ambient air, indoor and a airflow of 60 m^3/h equals 177 grams of water.

The total water "content" is 20% higher in a gas roaster compared to an electrical roaster is this case.

No.

There's a difference between closed and opened systems. The burner in a gas roaster is part of an open system, It has to be, because that's where it gets the oxygen for combustion and voids the carbon dioxide (CO2, water and energy are the major products of propane combustion). The earth's atmosphere is a pretty big place, while a roaster is a fairly small thing. The atmosphere manages to suck up the water from the roaster's burner cracking propane molecules without much of a hiccup -- not even a local hiccup.

Furthermore, the roasting chamber itself is also open to the atmosphere. The significant parts of the roasting process occur at temperatures well above 100C and at normal atmospheric pressure. 100C is the boiling point of water; it's also hot enough to expand the air in the roaster and create a continuous exhaust. Unless you find some way to hold the steam in, it ain't gonna stick around for long in the roaster.

Better Conclusion:
It's extremely unlikely that you'll find find measurable humidity in an ordinary gas-fired drum roaster which (a) is not extremely evanescent; and (b) derived from any source other than the beans themselves. If you do, you're in a universe with a different set of physical laws -- Florida or Texas maybe.

BDL

There have been experiments roasting in pure nitrogen, pure steam, pure CO2, etc. The net result is that it makes no difference at all, since the beans are outgassing throughout the roast. This means their surrounding atmosphere can't get in.

Steam roasts can finish in under three minutes, of course, since the convection rates are much higher. This would make the the roasts worse due to the defective profile, not the steam.

I do not know if the higher moisture and CO2 content of combustion air makes for a higher convective heat transfer. If it does, it would knock a few seconds off the roast time

I agree. The only thing you can push into the coffee is heat. Otherwise, it's a just little pressure cooker with relatively small surface area.

You and Jim have a point -- as to the bean.

However this thread was split off from another thread to address sversimo's hypothesis that a propane fired drum roaster puts more humidity into the drum than an electric roaster. As to that, sversimo's premise, chemical analysis, and "proof," that it would do so -- because water was a byproduct of cracking propane -- was poorly taken. The premise fails to account for the effect of the global environment, so the analysis is not useful.

First things first.
BDL