Stainless Steel Usage

I think 321 would be sufficient.

There are more superior grades such as 347 but with the higher grades so are the costs.

You want to keep your costs down as much as possible and still maintain a suitable performance that won't result in failure later.

321 rates as a good corrosion resistant SS that is rated for high temp usage so I think it meets everything you've ever dreamed in a SS.

I don't know what the properties are in coal gas. What I do know is that 304 is sufficient for use with high chlorine concentrations. Chlorine is very corrosive to mild steel and rubber and plastic products not made to resist corrosion.

Is not easy to answer because the environments (coal gas flue) is complex and the effect of the temperature is different for different materials. As a general rule AISI 321SS can resist to the temperature you've indicated but it should be considered in the particular environment.

this paper can give you some good advise.

High Temperature Corrosion in Gasifiers

http://www.scielo.br/pdf/mr/v7n1/19330.pdf

Coal Ash Corrosion Resistant Materials Testing Program

http://www.osti.gov/bridge/servlets/purl/835656-QZppky/native/835656.PDF

S.

One of the flue gas constituents to be concerned with is the amount of hydrogen gas produced from combusting coal. I believe with pure SS, hydrogen gas can separate into hydrogen ions and enter the SS crystal structure and compromise the mechanical integrity of the vessel. Titanium or other metals are commonly "doped" into SS to improve overall strength and corrosion resistance. The company who the combustion technology is issued through should be able to answer this question with great certainty.