[Stoves] Understanding TLUDs, MPF and more. (was Re: Bangladesh TLUD ) REVISED

[Crispin Pemberton-Pigott]

Dear Nikhil

As a back-slid chemist (as Cecil would term it) you find the parsing of fuels from stoves an interesting conversation.

>However, while I understand agree that "adding moisture to the fuel reduces the char yield and gives more gas," I am puzzled by the dogma that "fuel moisture ‘required more energy to remove it’."

The question was not, that I recall, discussed in the past 15 years on this list. Fuel contains moisture which can be expressed on a wet weight basis as "10% of this kg of fuel is water held within it."

Therefore only 90% of the mass is fuel. The energy needed to dry it is easily calculated. This energy for cooking stoves is regarded as useless. That is a convention. The HHV includes it as Norbert pointed out ‎for EPA efficiency calcs. The HVAC community in the US discounts a larger portion of the energy than the EPA or stovers.

>I interpret this to mean that higher-moisture fuel, for a given stove, scores low on the efficiency metric. Am I correct?

No. This is not a stove attribute, only applying to the fuel. The determination of the energy content of a fuel is important for all sorts of reasons including the efficiency of transport and determining system efficiencies however defined.

What has not been discussed is whether the energy invested in splitting water is higher than drying the fuel or how either changes the fuel energy rating, and how a stove efficiency rating might be affected by recombining the gases into water next to the pot.

Hydrolysis absorbs a great deal of energy but that might be an advantage: the energy could be released next to the pot instead of transporting it in hot gases from the oxidation point to the pot. There is no net gain in energy but it can be optimised as to where that energy is made available and how.

>Should it not be that one takes moisture level as a given - there goes the theology of oven-dried wood and computing efficiency of boiling water - and design stove functions around it, the actual fuel? `

That is the energy content (net) of the fuel.  The subtle point is that adding moisture to a stove with a distinct oxidation layer can split hydrogen from water. This fact is used in the gasifiers (or could be) to cool the reactor extending it's life and creating more combustible gas at a lower temperature - a straight trade with a beneficial outcome.

My gasification expert recommends adding upwards of 40% of fuel mass in the form of steam.

>That is what I interpreted from Frank's insistence on fixing the fuel.

It is a 'classic idea'. What I am describing changes the equations, perhaps a lot, with the 'same fuel' input. Suppose we have two stoves, one of which processes the fuel internally to make more gas at a lower temperature. The other doesn't. The service factor, durability, power control, emissions, cooking power, fuel requirements for the two may be quite different.

>Where possible, the moisture content can be adjusted - either drying in the sun and storing for rainy days or deliveries of pellet or charcoal.

Maybe in the 'right stove' that is a bad move. ‎Never assume anything.

>As far as I can tell, "wood gas" and "char making" are enough rationales for "more complete union" of carbon and oxygen. The obsession with efficiencies just distorts thinking, corrupts debates.

Efficiencies are just ratios. Hundreds are calculable. There is little point in objecting to them in principle. What is the efficiency of the water gas shift reaction in two pyrolysers? ‎If you are developing a new super stove that is an efficiency you want to maximize.

>Energy should be wasted. Economically. Then it is not waste.

Philosophically, yes.

>I don't think anybody would have an argument against the proposition that prudent public choice should favor the total cost of service from an appliance, whatever the fuel efficiency in thermodynamic terms.

That is what people do. Having 'the highest efficiency' is not the goal. Have adequate efficiency is all that is needed. There is reasonability behind a minimum efficiency requirement in a national standard - so the energy resources are not wasted.

For heating it is 70% everywhere, but calculated in different ways. It doesn't have to be 93%.

>Not all costs and benefits are knowable; consumer preferences vary, so that I might buy a CFL to replace one lamp in one room, an LED lamp in another, and leave incadescent lamp chandelier in the third. Millions of words have been written on the economics of appliance efficiency standard-setting and the behavioral economics of choice - the "nudge" vs. a command.

Killing trees and electrons in the process.

>But the general point remains - it is the total cost (including the capital cost of stove replacement) that matters to consumers. Some people may buy a Tesla with a solar batterybank; I won't make any argument that s/he derives enormous psychological benefit of self-righteousness, just that his/her economics are different.

The system efficiency sucks, big time. It is toys for big boys.

>Now look at this way: suppose a news stove costs 200% more while promising 20% higher fuel efficiency. It may still be a prudent choice depending on fuel prices and utilization rates.

Customers are surprisingly rational sometimes.

>...I know that with charcoal stoves, some CDM project developers demanded that the old stove be returned and destroyed, so the user can only use the new stove. If that new stove is not found as usable as the user wishes, rest assured s/he will find a version of the old store, or any other means of "stacking".

We had that in Ulaanbaatar. The stoves given over were parodies. Old, broken, tiny, ‎they were retrieved from the scrap heap to 'hand over'. It was a joke.

>You might respond - "So there should be higher efficiency stoves which are also found usable in practice, whether one stove costs 50% more and saves 30% fuel or one costs 200% more and saves 50% fuel."

Straight economics?

>...Which is why it makes perfect sense to waste, say 65% of energy in coal  in order to produce electricity. Or to use an electric kettle with electricity from that coal-fired plant to boil water to make tea.

Here in Bishkek the 65% "waste" heat is piped around the city to heat it with zero additional fuel needed. The system efficiency is high.

>...Then you'll stop wasting time on "international standards" with efficiency tiers.

‎There are good reasons to have minimum performance requirements in standards. There is not yet demonstrated any reason to include 'tiers' is such a document. Energy efficiency rating systems are not part of the Standards internationally, nor in the EU nor the USA. Tiers and rankings are properly the arena for consumer advocates and energy policy people.

Regulation of trade (permitted, not permitted) is not based on tiers of performance. It is a go/no-go permit to market system.

Regards
Crispin

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