David Pawson

[David Pawson]

The reaction between steam and coal is endothermic so if you blow too much steam through the grate the fire will go out! In the complete absence of any experience or data my guess is that at best you can get no more than 20% of carbon into the gas phase by this process, which defines the maximum flow of steam possible. If anyone has any data I would love to see it. Even 20% will lead to a significant cooling of the fire. Porta as I understand it developed the technology to deal with very high ash coal. If the process keeps the fire below ash fusion temperature that will be a big boon, so you can see why it worked in this context. So if ‘bad’ means high ash, then yes, GPCS could work. And, as we have discussed, this should prolong the life of the grate. But Wardale’s general answer to the ‘bad coal’ question is ‘no’.

A cooler fire will also help reduce unburned coal loss it would seem. I think (having pored, glassy eyed for many hours at the Rugby boiler data) that the increase in unburned losses at very high specific combustion rates (above rates sufficient to allow very 700+lbs/sqft/hr evaporation), is related to the temperature of the fire surface, no hard data on this, certainly no experimentation- the production of char from lump coal is a black hole for data. This would fit with the idea that GPCS reduces unburned losses at high rates. A simpler solution is to increase the size of the grate!

What I would like to know is how a cooler fire impacts radiative heat transfer in the firebox, and whether this also leads to a reduction in superheat. Again, I don’t think there’s data, but if this did happen it would likely negate the effect of reduced coal loss.

I’m not at all sure about pulverized coal. If you pulverize coal in the screw feed of a mechanical stoker, the fines produced disappear up the chimney and boiler efficiency falls. Swindon showed that adding fines to coal also reduce boiler efficiency, because they too went straight up the chimney. So I think we are talking about a very fundamental redesign of the grate and firebox.

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