[Stoves] Why not high mass stoves?

Sun Nov 2 03:57:33 CST 2014

Dear Michael

High mass stoves can have benefits for combustion. This is often overlooked when discussing high or low mass. What has to be taken into consideration is the burn cycle – the power, the duration, and of course if there is a dual purpose for the fire.

First, space heating. If the stove is used for space heating, meaning it will run for a long time, there is no concern at all about the thermal mass – it is basically irrelevant. High mass stoves are appreciated for keeping the local environment warm long after the fire has died down or gone out.

Second, combustion efficiency.  Once the region of the fire chamber is up to temperature, heat accumulates in the high mass behind the surface. How much depends on the material, the flame temperature and the conductivity. If heat can be put into the material, it can be recovered at some point.  If the material is a poor conductor, it will not be able to return heat to the combustion zone in an effective manner. This aspect is usually overlooked. The ‘gain’ from heat that is stored in a very hot combustion chamber wall that has a high conductivity is applied when the fire is dwindling or turned down.  The direct mechanism is to raise the temperature of the incoming air.

Towards the end of a fire, there is inevitably a reduction in combustion efficiency. CO2 starts to form a lower % of the total gases and CO starts to increase, normally stated as the CO/CO2 ratio.  If the combustion zone is hot, the conversion of CO to CO2 carries on longer than if the material had been either very light and insulative, or metal with little stored heat. The heat is added to the gas mix dropping the wall temperature. This added heat causes just that little bit more CO to burn properly to CO2, giving off 3 times more heat than the C to CO. That additional heat keeps the wall hot that little bit longer, providing more stored heat to give back again. Thus the efficiency of combustion is supported in a cooling chamber for much longer if the heat is )a stored there in the first place and b) is stored in a material with a sufficiently high conduction rate so as to be able to give it back at a rate high enough to support the combustion of CO. Heavy, hard and conductive combustion chamber consistently outperform light insulative ones if the burn cycle involves a low power phase or a burn down as part of the stove function.

Third, conduction v.s. insulation. The rate at which heat enters the combustion chamber materials is a natural property of the material. This can in effect be ‘altered’ by dividing the wall into layers with tiny gaps (not big ones that allow air circulation) or by grinding the material (like sand, for example).   Depending on the burn cycle, you could consider having a dense, conductive chamber shell in an insulating environment, having enough mass to give you the CO-burning boost you want towards the end.

Suffice to say, that if you have a long high power burn cycle and the insulative material used for the whole chamber is a lousy conductor, there will be little of the total stored heat in the stove body available to support the CO combustion because it can’t get there quickly enough to be useful. The chamber surface will cool rapidly and it will be as if it was made of a light material. If you don’t need the space heating from the exterior surface (which Lorena stoves are good at delivering) that stored energy is literally lost.

Fourth, recycling heat into the fire.  If you have a combustion chamber of any mass, there is the possibility of placing preheating channels behind it. This is the principle applied in a Lion Stove <http://www.newdawnengineering.com/website/library/Stoves/lionstove/>  which has primary air preheating using heat captured from a dense and conductive brick combustion chamber. The bricks are selected on the basis of high heat conduction ability because if they are not exposed to a strong side-to-side temperature differential, they will not crack.  Cheaper bricks are used for the main body and chambers within.  Several hundred of these stoves have been built for schools and orphan feeding sites. Because of the preheating, they can burn wetter wood than would otherwise be possible.  Burning it better means more heat to provide more preheating – it is a virtuous heat cycle like the CO combustion at the end. The benefit from the high mass and preheating is about 10% of value for the thermal efficiency. There is an ETHOS presentation by Peter Scott (about 2009) demonstrating this for the high mass chamber (alone) using a series of tests.

Where it is unwise to use such heat storage (except for preheating) is when the burn cycle is short and powerful, then ended until the stove is cold.

Regards

Crispin in clean and sunny Beijing

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