11 replies to this topic

[LittleRain]

Hi all.

I was assigned to design (chemical design) the furnace for the combustion of H2S to SO2 according to chemical equation as below:

H2S + 3/2 O2 -----> SO2 + H2O

Inlet stream contains 96.5 mol% water, 1.8 mol% CO,1mol% CO2, 0.6 mol% H2S, 0.05 mol% H2 and 0.005 mol% COS. Combustion of H2S with oxygen in the air occurs in furnace at 1.5 bar and 540 deg Celsius. Molar flow rate of fuel stream is 9040 kmol/hr.

I would like know:
1. Should air be in excess in this combustion?
2. The total reaction rate of the equation above is not known. So to obtain the reaction rate, I'd written down the reaction mechanism (as per attached) which I found in the book of Combustion, 4th edition. (First 4 reactions are not given by the book..)The book suggested that at high temperature, three steps of reactions will occur. I wonder, is 500++deg Celsius high enough to be consider as high? I am aware that the furnace of Claus process operates at temperature above 1000 deg Celsius? Am I getting the correct reaction mechanism or did I miss any important reaction steps?
3. Will sulfur be produced at this temperature?
4. How does the real WSA furnace look like? Does WSA furnace burn up H2S completely in any existing plant?
5. To simplify the design process, can I assume that there is no presence of CO, CO2 and COS?

Any tips and tricks in this furnace design are welcome. Thank you in advance..

Attached Files

•  IMG_0936.JPG   2.45MB   39 downloads

[kkala]

Some points are noted as opinions (hoping that someone having expertise will help).
1. WSA seem to mean "wet sulfuric acid", see http://books.google....furnace&f=false; but it is also the name of a German Institute dealing with furnaces, see http://www.wsa.rwth-...oCO2Process.pdf. The furnace described there burns coal with pure O2, so it does not seem to have some relation with a furnace burning H2S+CO2.
2. The gas to be burnt contains 97.5% H2O+CO2 (inerts) and only 2.5% combustibles. Probably a gas should have a minimum heating value (kcal/Nm3) to be burnt, as I can recall (from API 537? - I have looked into Coulson & Richardson Vol 6 to no avial). At any case this gas should be forced to combustion in an incinerator, with excess of O2.
3. Emission limits of H2S, H2, COS to atmosphere should be posed. Since this gas contains so much water vapor, an alternative would be to cool it and remove most of water vapor as liquid. This would facilitate burning of the remaining gas. A simulation could reveal quality of liquid condensed, that is H2S, CO2, COS content; hence way of liquid treatment.
4. Formation of elemental sulfur is not expected as long as excess of O2 is used.
5. Beware of acidic dew point (dew to SO2 or H2S), even though combustion temperature would be much higher.

Edited by kkala, 26 February 2011 - 03:15 PM.

[LittleRain]

Thank you Mr.kkala. I appreciate your help. Your reply somehow gave me some new thoughts.

Now, I have a question that might seem very ignorant. But I'd like to know, whether atomizer can be used for gas fuel too? What kind of atomizer is more suitable for gaseous fuel? I suppose it would be different from the one for liquid.

Thanks in advance.

[kkala]

... I'd like to know, whether atomizer can be used for gas fuel too? What kind of atomizer is more suitable for gaseous fuel? I suppose it would be different from the one for liquid.

Contrary to fuel oil burners or dual firing, no atomizing media seem to be used for gaseous fuels, since gas can be considered already atomized. There are premixed burners (like the bunsen) and nozzle mix burners. See Perry, 7th edition, p. 27-34 to 27-39 for explanations on several burner types (Heat generation).

I think there is no atomizer for gaseous fuels, only a ΔP in some cases for effective mixing with air.

Injected fluid can have other purposes, e.g. water is injected into gas turbines combustion chambers to reduce nitrogen oxide formation.

[LittleRain]

Thanks for the reply.

Now that you know I have a very lean-fuel stream (gaseous form) with less than 5% (molar) combustible components with a lot of water vapour. If I want to burn that gaseous stream with excess air, should I preheat that stream or add in some auxiliary fuel?

I did energy balance, and found out that if I were to preheat the stream, I will have to preheat it to the temperature same as outlet stream. So what is the risk of using this method? Should I preheat the auxiliary fuel stream and air stream separately or together? If I were to mix them together before heating them out, I should be aware of the autoignite temperature and things like that, am I correct?

This link:http://www.freepatentsonline.com/4632043.html, provided a method of treating low quality gas. It doesn't mix the auxiliary fuel with my lean-fuel stream. So basically it is preheating the lean-fuel gas, am I correct?

If I want to mix the auxiliary fuel with my existing fuel gas, what is the disadvantages? And I suppose the auxiliary fuel is in gaseous form..(low boiling point I supposed)?

[kkala]

As said, a good option is to cool the fuel stream in order to remove most of water vapor as condensate, then try to burn remaining gas. Cost is not the most important factor in environmental projects.

Also note that the mentioned patent concerns min H2S about 30%, plus 2% combustible gases, and only 3% water vapor. 95%+ water vapor will not help burning, good knowledge on incineration (by a connoisseur) is needed to clarify your questions.

Anyhow, gas stream temperature should be high enough to prevent acidic corrosion. This is particularly true for the gas going into burner (acidic corrosion due H2O vapor + H2S), with a margin of (say) 20 oC over estimated dew point value. This also depends on the gas max operating pressure.

Premixing should take autoignition temperature (under the specific conditions) into consideration; two different streams can be more practical (and safer), majority of modern burners are not of the premixing type (Perry 7th ed, 27-34 and on). But these are subjects for the incinerator expert or supplier.
I understand design of such furnace falls into proprietary knowledge; a way out for a student would be to "design" a simple incinerator (even not realistic) based on data found and engineering knowledge, trying to justify assumptions made.

Edited by Art Montemayor, 07 March 2011 - 01:18 PM.

[LittleRain]

kkala:

What is the best way to eliminate water vapour from the inlet stream? Cooling the inlet fuel stream to condense out the water vapour? I suppose it has to be very low before the water vapour starts to condense. And after that I will have to heat it up. It is a little not 'efficient'.

Second way I thought of is using H2SO4 (that I will be producing in the end) to absorb the water vapour. I do not know if it could work. Do you have any information regarding this?

Third way is that, instead of eliminating water vapour, why dont I extract/adsorp/absorb the H2S from the stream? Any information on this too?

[kkala]

From given data, gas composition is in kmol/h: H2Og=8723.6, CO2=90.4, CO=162.7, H2S=54.2, H2=4.5, COS=0.5, with some 4 kmol/h unknown (specify). Total 9040 kmol/h ~202000 Nm3/h.

This is actually water vapor with a small content of other substances, some of then noxious; abatement of the latter is understood to be the main purpose of the task (clarify). Condensing the water vapor to some extent seems to be a solution, since the stream is almost saturated. There may be other solutions, too, as you mention, so one of them should be chosen and justified / supported. Compression along with cooling is another option.

I think a student may not chose the best possible solution, but one that is understood and supported by his/her calculations and judgment. Time and effort is not enough for the former in most cases.

Please specify inlet gas temperature and pressure, along with requested clarifications (as a minimum). Also emission limits applicable for the noxious substances. Having this information, somebody with relevant experience could hopefully help.

Edited by kkala, 07 March 2011 - 01:41 PM.

[LittleRain]

I attached an excel file which contains the basic information.

Sorry, I'm totally clueless what is the next step I should do.

Attached Files

•  H2S furnace.xlsx   26.48KB   57 downloads

Edited by LittleRain, 08 March 2011 - 08:13 AM.

[kkala]

Data of excel file indicates a lot of work; hopefully someone with expertise in WSA process can offer real help.
My comments on the speadsheet can be found attached (bold notes have some significance, non bold notes are just observations), but they are trivial and do not promote the "project". It seems to be a case not met in a refinery Claus reactor, or in its tail gas treatment.
This is a tail gas treatment after shift conversion, and after first treatment of gases for CO2 and H2S removal, as understood. In my opinion such a small percentage of combustibles in inert gas (H2O) will need an incinerator to burn, using more excess air. But catalyst is unknown and much may depend on it. And probably you have been instructed to look into tai gas burning.
Anyway, a close tail gas treatment from Claus furnaces is the Dyna Wave process, e.g. http://wikibin.org/a...e-scrubber.html. The gases are treated with soda solution for Na2SO4.

Attached Files

•  WSA.xls   114KB   49 downloads

[LittleRain]

Data of excel file indicates a lot of work; hopefully someone with expertise in WSA process can offer real help.
My comments on the speadsheet can be found attached (bold notes have some significance, non bold notes are just observations), but they are trivial and do not promote the "project". It seems to be a case not met in a refinery Claus reactor, or in its tail gas treatment.
This is a tail gas treatment after shift conversion, and after first treatment of gases for CO2 and H2S removal, as understood. In my opinion such a small percentage of combustibles in inert gas (H2O) will need an incinerator to burn, using more excess air. But catalyst is unknown and much may depend on it. And probably you have been instructed to look into tai gas burning.
Anyway, a close tail gas treatment from Claus furnaces is the Dyna Wave process, e.g. http://wikibin.org/a...e-scrubber.html. The gases are treated with soda solution for Na2SO4.

Thanks for your reply and opinions. I appreciate your help very much.

[LittleRain]

I hope someone can tell me what are the consequences if the furnace size is too big? And what about if it is too small?

What I can think of is just, if the furnace is too small, incomplete combustion occurs. If it is too big, we spent extra money.