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markmai86
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Join Date: Jun 2009
Posts: 43

Preheating Biogas (Methane)

03/30/2010 1:26 PM

Hello,

I am curretnly looking to preheat biogas prior to inject into the air intake. The biogas must be above 100°C to make sure there is no condensation (and no corrosion problem).

Therefore, I want to make sure the biogas is hot enough but I lack some thermodynamics knowledge.

The exhaust temperature is about 250°C (as I measured it) and the flow of exhaust gas is about (23m³/hr).

The biogas (we can consider it's 100% methane) going in the exhaust gas (in a stainless steel tube) is at about 20°C before entering the exhaust (with a volumetric flow of 1.5m³/hr).

I want to determine the length of the stainless steel tube to make sure the biogas reaches a temperature of 100°C. (Cp of stainless steel = 1.5kJ/kg.K)

Could anyone give me some information on how to solve this problem?

Thank you

(PS and also how can I make paragraphes, when I preview the thread is shows as if I had never pressed the enter key! maybe it's chrome fault...)
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markmai86
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Join Date: Jun 2009
Posts: 43
#1

Re: Pre-heat Gas (Methane)

03/30/2010 3:09 PM

In fact we can just say that the exhaust temperature is constant at 250°C (because of the big flow rate) and now I just need to find the amount of time (or dimension of stainless steel tube) to preheat the methane above 100°C.
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dkwarner
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Location: Placerville, CA (38° 45N, 120° 47'W)
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#2

Re: Preheating Biogas (Methane)

03/30/2010 11:33 PM

The air intake of what? A furnace? An engine?...

I assume you are feeding the biogas through a smaller tube that is inside a larger exhaust tube. If length is a problem, add fins. The smaller tube could also be wrapped in a spiral around the exhaust, with insulation outside the spiral, thereby avoiding the possible corrosion problems of the exhaust gasses.

Experiment!

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markmai86
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Join Date: Jun 2009
Posts: 43
#6
In reply to #2

Re: Preheating Biogas (Methane)

03/31/2010 8:35 AM

The air intake of an engine. Thank you for the information, but you wouldn't have any rule of the thumb concerning the length required to heat the gas above 100°C?
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Anonymous Poster
#3

Re: Preheating Biogas (Methane)

03/30/2010 11:43 PM

Chrome and CR4 have issues. The formatting does not function the same way it does with Explorer for example.
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Anonymous Poster
#4

Re: Preheating Biogas (Methane)

03/30/2010 11:45 PM

I think there is a simulation software called Comsol that would calculate this easily..I don't have access to it unfortunately.
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Anonymous Poster
#5
In reply to #4

Re: Preheating Biogas (Methane)

03/30/2010 11:48 PM

This book has some helpful info on mass/heat transfer as well:

http://www.amazon.com/gp/product/013101367X/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=0205059392&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0F29RQRFFHDS2QJ92S2C
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Carlos J. Valdez
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Join Date: Jun 2008
Location: San Antonio, Texas
Posts: 47
Good Answers: 1
#7

Re: Preheating Biogas (Methane)

03/31/2010 10:10 AM

i feel I should tell you that you will need more initiative to complete your project. I am no engineer but am sure you can find these things out on your own. Try this: Take temp. readings of the entering & exiting gas using at least three (incremental) lengths of exhaust pipe. You will need to control the rate of fuel flow and be able to observe the state of your exiting gas. Try using a Bunsen burner set up at a comfortable distance from your heat source. I think you will find it (the length/time of fuel line heat) of minor import since you will need only a minimum length/time - since you will want to run a variety of fuels through your system. PS I have seen videos on-line of just such a device. Look for them. Good luck. Carlos
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markmai86
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Join Date: Jun 2009
Posts: 43
#8
In reply to #7

Re: Preheating Biogas (Methane)

03/31/2010 10:34 AM

Thank ou for your input. Machining takes some time and can be costly therefore, I wanted to have some kind of order of magnitude. I guess I have to head back to the library and some thermodynamics books.
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simonsd
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Join Date: Nov 2008
Location: Port Glasgow, Free Republic of Scotland
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Good Answers: 30
#9

Re: Preheating Biogas (Methane)

04/01/2010 9:16 AM

There are a few bits of information that would be useful - the most significant of which is

what is the pressure of the biogas as it has a major impact?

I have assumed around 1barg but you will have to rework the calcs for another condition.

Also the flue temperature seems a bit low but if you have measured we will go with this.

First you have to calculate the duty

This why the pressure is key as pressure directly impacts density and you have given us a volumetric flow

Duty = mass flow (m) * Specific Heat Capacity (Cp) * Teperature Difference (dT)

At 1 barg density = 1.33 kg/m3 => m= 20kg/hr = 0.00554 kg/s

Cp = 2.243 kJ/kg.K (note this also varies with pressure but not massively over a small pressure range)

dT = 100 - 20 = 80 K

Duty = 0.99kJ/s (ie kW) which is pretty small

= 990 W

To size a heat exchanger

Duty = U A dT

U = Overall heat Transfer Coefficient 15 W/m2K (estimate from http://www.engineeringtoolbox.com/heat-transfer-coefficients-exchangers-d_450.html )

A = Log Mean Temp Difference = 187 K (I concur with the assumption above that we will assume that the exhaust gas does not change in temperature to simpify the analysis in practice it will drop about 5°C)

Rearranging Area = Duty / (U * DT)

= 0.35 m2

As you know the outside area of the tubing you have then this will give you a length

Finned tubes are common if the exhaust gas is clean (otherwise the fins get clogged up) and increase the effective area ie you need a shorter length of finned tube than bare tube

Other points to watch include

Although I dont think its an issue do an enthlapy calc (duty = m CP dt) for the exhaust gas to ensure you dont condense anything here as this will be far more aggressive. Exhaust gases always include CO2, often SOx and so when water condeses from exhaust gas you get acids - weak H2CO3 and H2SO3/H2SO4

velocity in the tube should be no more than 10m/s

you need to allow for fouling factors as well although these shouldnt be too bad for your case

Chem Eng Handbook by Perry

Process Heat Transfer by D Kern

would be the books I would start with

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sceptic
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Good Answers: 65
#10

Re: Preheating Biogas (Methane)

04/06/2010 5:45 AM

Presumably your biogas has been cleaned and the CO2 scrubbed out.

If not you will probably have around 25% by volume of CO2 present.

If no pretreatment, there will be a percentage of H2S and tiny amounts of stinky nasties (mercaptans spring to mind but they are not the only ones). Although they burn, they can be quite aggressive from a corrosion viewpoint.
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