[Gasification] why adding water isn't likely to get you the results you want

[AJH]

On Wed, 07 Feb 2007 16:11:42 -0800, drew wrote:

>    A related technology that is interesting was the coal gas plants 
>that produced "blue water gas" from coal,   these plants would use a 
>large coal bed with a sizable air blast to heat a large quantity of coal 
>to incandescence,

And because this  incandescent coal had mass and specific heat a large
amount of energy was stored in the ~1100C coal/coke


> the resultant hot gases would be routed through a 
>large brickwork heat absorber,

I didn't realise it was brickwork, still this would have removed some
of the sensible heat from the CO+N2 mix.

These hot gases also had the chemical energy of the CO still in them,
this was burned I think this was burned to superheat the steam.

We had a coal gas (town gas) plant in this town till 1967, almost
certainly a Lurgi design but I never found out what system it used.
 
>heat absorbed from the coal.   The gas would exit through another heat 
>exchanger as a very high heat value H2 CO often with a substantial 
>amount of tar, this gas (with no nitrogen) was compressed and sent out 
>via pipes and resulted in gas being available for many uses in early 
>industrialization.    It turns out though that this is not a very clean 
>way to burn coal (massive bed temp temperature changes and intermittent 
>high volume air blasts) and the resultant bi products in the tar in many 
>cases are still in need of cleaning up. 

I suspect most of the contamination was from the earlier pyrolysis
stage unless they combined pyrolysis and alternating producer gas and
blue water gas in one reactor.


>   The relevant issue is 
>efficiency, these plants were not at all efficient in terms of coal in 
>to gas out, 

Actually I recall reading a book, "elements of combustion technology"
in the 70s that had a schematic, this suggested an overall thermal
efficiency of 85% for a town gas plant. You can easily see the losses
as only relating to sensible heat from the apparatus and there is a
lot of heat recycling inside the plant, like the brickwork regenerator
and superheated steam raising from burning the producer gas.

Also because of the way switching between processes was done there was
a bit of mixing, such that there was 10% nitrogen in the resulting
town gas.


>and they were very dirty causing lots of pollution (think 
>1800's London smog).    

I don't think they were particularly dirty but the sites did get badly
contaminated meaning a lot of remedial work was necessary when the
site here was redeveloped, how much of this contamination was from the
earliest use of the site (1900s) compared with when it closed I don't
know. I suspect it was from the gas clean up processes.

The London smog was cause by poor combustion of coal in open fires, if
you go to London you will see town hoses with rows of chimneys which
would have been pouring sulphurous smoke into the atmosphere.

AJH