How Can Primary Air Lift Unburnt Coal

[Greg Wetzel]

I was reading about GPCS, and it stated how primary air can lift unburnt coal, and how the unburnt coal would then possibly be discharged through an engine's chimney. I was wondering, would the primary air have to strong enough to send unburnt coal through the boiler tubes and out of the chimney, leading to the risk of lineside fire?

 

[John Webb]

What is GPCS, please? It's not a term that I can find in the BR "Handbook for Railway Steam Locomotive Enginemen" first published in 1957.

 

[Elecman]

GPCS - Gas Producer Combustion System,??

 

[John Webb]

GPCS - Gas Producer Combustion System,??

I wondered if it might be that. But from distant A-level chemistry I thought GPCS was more for fixed installations rather than railway locos, as it served better for less variable loads than a loco has to deal with.

 

[stuving]

GPCS - Gas Producer Combustion System,??

It must be - and I suspect the source for the question is this, about the 5AT project (q.v.). The key paragraph there says:

The advantage that the GPCS offers is that it allows the quantity of “primary” air passing through the grate to be significantly reduced perhaps by as much as 70%, thereby reducing the upward air velocity through the firebed that causes lifting of unburned coal particles off the fire surface and discharged through the chimney. The reduction in unburned fuel loss that can be achieved – and consequent increase in thermal efficiency – is significant especially at high steaming rates that approach the grate limit.

The producer gas is then burnt in secondary air entering above the grate. I have no feeling for what would be practical values for air volume or velocity through the grate, nor for the range of coal particle sizes actually used. But there must be relationship between the maximum size just lifted and the air velocity. Elsewhere there is mention of fluidised beds - which in my mind implies fuel crushed to quite small sizes to make sure it "dances".

 

[edwin_m]

I recall reading that one advantage of Stephenson's Rocket over earlier designs was that the multi-tube boiler improved efficiency, and that earlier locomotives such as Hackworth's had to be worked so hard that much of the fire went out of the chimney.

 

[John Webb]

It must be - and I suspect the source for the question is this, about the 5AT project (q.v.). The key paragraph there says:

The producer gas is then burnt in secondary air entering above the grate. I have no feeling for what would be practical values for air volume or velocity through the grate, nor for the range of coal particle sizes actually used. But there must be relationship between the maximum size just lifted and the air velocity. Elsewhere there is mention of fluidised beds - which in my mind implies fuel crushed to quite small sizes to make sure it "dances".

Many thanks. That seems to tie in with the OP's query.
Having read the article you linked to it seems to me that translating GPCS into a railway loco from industrial needs is an over-complex arrangements - for example the 'extra air tubes through the boiler into the firebox' introduces yet more potential problems. For example what happens passing through a narrow bore tunnel and these tubes no longer feed fresh air into the firebox? (At least in the UK's limited loading gauge.)
From reading various books and accounts about driving steam locos, it seems there were already well-proven techniques for balancing the thickness of the firebed, primary air through it and secondary air through the firebox door to maximise combustion and minimise fuel waste.