I have been mulling over this discussion about heat in the reactor, and went back and reread the whole original thread on nozzles to reacquaint myself with what has already been discussed. It dawned on me that I do not really have a very good “big picture” idea of what it is that we are actually shooting for in the reactor. It seems to me that I need to answer the following:
1: The ideal temperature in the lobe - With the heavy steel nozzle, or Dave’s tungsten carbide version, we seem to have overcome the difficulty of catastrophic oxidation or outright melting, so what temperature are we looking for to create the best gas? It we are creating fused clinkers that suggests that without EGR the lobe must be reaching around 1300 C, right?
2: Where is the heat actually going inside the reactor? We know we are limited to around 50C on the outlet gas, or else it becomes too dilute to run the engine, and our plastic hoses start to melt. I looked up the heat capacity for charcoal (.24 BTU/(Lb * F)) which its similar to clay or stone, about twice the value of steel. But, its density is quite low, maybe 2lb/gallon? Okay, going back to standard units here… I have ~8gallons of charcoal = 16lb, and assuming 60 F of acceptable temperature rise (120F max minus 60 degree ambient) If I am doing my math right
BTUS = (lbs * F)/.24 or BTUS = 4000.
In another thread, @k_vanlooken you said
So what I am arriving at is that gasifying 1lb of fuel should liberate enough heat to bring my hopper temp up 60F (assuming no outside losses, no loss to the steel parts, etc). That actually seems like it might be about right, once you figure in those other heat sinks. I am getting to that temp before I manage to burn more than a gallon. (of course all these values are quite sketchy)
I drew a couple of reactor diagrams to try and illustrate what I think you are talking about (please correct me if I am wrong).
Koen, you say
By this I imagined you meant something like figure 2 - fins on the reactor are dumping heat and thereby pulling down the reaction temperature all the way to the lobe?
In figure 3 I imagined a layer of insulation keeping the heat from escaping as Dave suggested, which would presumably raise the temperatures throughout the hopper.
So what I am wondering is; if the goal is to keep the heat in, is the only way to increase runtime to add more mass of charcoal? Could heat be bled out of the system higher up in the hopper where it wouldnt interfere with the reaction? Also, how does the size of the charcoal factor into this? Is it simply increasing the density by being more tightly packed? For that matter, could I store my engine grade charcoal in the freezer?
Lots of questions, but that just means there must be lots of answers 
