[Stoves] Smoke-free biomass pellet fueled stove

Sat Nov 3 20:20:16 CDT 2012

Dear Ron and All

First congrats Dr Nurhuda on the stoves. The finish quality is very good. I was able to do one test with each version and will forward them to you privately. 

Re air controls, Ron perhaps you saw the recent message about finding an old Vesto in Yogyakarta. It has the same features you mention. We did only one test using it with locally sourced 8mm pellets. Air temp numbers are from earlier tests just to give you an idea. 

Fuel: wood pellets
Pot: large, thin aluminum with flat lid
Fuel load: 1120 g approx (detail not with me)
Water load: 4700 g
Initial temp: 31.2 C (-ish, have to look)
Local boiling point: 100.4 indicated
Time to boil from ignition: 12.5 minutes
Thermal efficiency (net): 35% ±1%
Primary air controller: rotating disk
Primary air preheat: 250 C
Secondary air controller: rotating sleeve
Secondary air preheat: 500 C
CO/CO2 ratio: steady at 0.3% ±0.1 but only after the fire was established. The fire was started with the secondary air controller closed. Once going it smoked until the secondary air was opened. (For this reason the feature was dropped in 2005. We did not repeat the test with this knowledge about the fuel and air. Another time...). I think the fuel will burn better using the modern combustion chamber with fewer holes. 

It was not operated in char-making mode so there was virtually nothing left from the fuel.

On a cautionary note to all the producers of char-making stoves conducting performance tests, please try to understand the following comment on the determination of thermal efficiency and a stove's fuel efficiency.  

There are several ways to report the thermal efficiency of a stove depending on the portion of the system being assessed. The heat transfer from the flame to the skin of the pot cannot be assessed directly through there are various ways to get close. The Indian and SeTAR tests are the common methods giving the nearest (though still incorrect) answers.

The stove 'efficiency' is usually calculated from the rise in water temperature which is actually not the heat transfer efficiency. It is the system efficiency of the portion of the stove from the flame onwards. 

If the whole stove is considered (which for fuel consumption and specific fuel consumption, it should be) one has to consider what fuel and unusable fuel remains. This has always been done with large boilers and industrial equipment assessments and will be done with stoves. 

During the recent testing methods conference this was highlighted as a major error in the common water boiling tests resulting in incorrectly reported efficiency and fuel consumption numbers. This has been a significant source of misdirection to policy makers.  

There has always been a general agreement in industry that the efficiency of the whole system is related to the fuel consumption numbers, not the other way round and not for just a portion of the system. In other words you calculate the efficiency from the fuel consumption, not the fuel consumption from the calculated efficiency. 

The implication is that the popular method of reporting the efficiency which treats the char remaining as 'unburned fuel' is not only incorrect, it is significantly misleading in the context of a char-producing stove. To get the correct answer requires a 'rational analysis'. 

For the purposes of calculating the heat transfer efficiency, important when designing a stove structure, the char should be deducted from 'fuel burned' or you do not get a reasonably accurate answer. However as mentioned above, the common WBT does not measure the heat transfer efficiency. It uses water temperature rise as a proxy and is a sub-system number only. 

The WBT 4.1.2 does not directly determine the fuel consumption either. When a whole stove is assessed for fuel efficiency it is not only the stove structure that is evaluated. It must be, of course, the whole system, its fuel consumption and its applied useful work (heat). 

Unburned char is a heat loss that falls into the same category as CO, H2 and other 'chemical losses'. Excess air is a 'mechanical loss'. See the recent discussion on the Siegert Formula for hints to follow on this if you are interested. 

The bottom line is that the popular WBT calculations of efficiency and specific fuel consumption return numbers that do not reflect the actual performance of the stove, particularly if it produces a significant quantity of char. The problem is not the char, it is the calculation method. 

The fuel consumed by a char-making stove has to be determined by the quantity of new fuel that is loaded into it each time it is ignited. This obvious point is broadly agreed. 

It is directly related to the amount of fuel that is drawn from the woodpile each day and is the basis for stating fuel consumption claims. Fire inefficiencies such as emitted CO, H2, carbonaceous PM, H2S, CxHy, the VOC's and so on are all system inefficiencies. So is char remaining. It is a combustion inefficiency because it was raw fuel placed into the stove that did not burn. 

An argument can be made that some stoves or even the same stove could burn it as fuel. In the latter case the proper cycle is to run the stove and produce the char. Then start the fire again with the char in it, or add it during the burn. In other words if the claim is that the char is fuel for the same stove, then burn it during the test so it does not show up as a combustion inefficiency/loss. 

The fuel consumed will be the total amount of new fuel added to the fire on each ignition, keeping an eye on the system as a whole. A fuel saving stove draws less wood from the pile each day. 

For program managers, choosing a stove to save fuel has to be done using a comparison of how much fuel is consumed from the pile, not a comparison of the heat transfer efficiency proxies recalculated to give a dry fuel mass equivalent. Fuel consumption relates to fuel consumed, not heat transfer efficiency only. 

When the thermal efficiency is properly calculated from the correctly determined fuel consumption, the results are informative and predict future performance during a similar burn cycle. 

The system efficiency during water heating or boiling is an engineering metric and can be reported in standard units. 

The performance of an arbitrary task such as boiling and simmering is not. That is why the IWA meeting discussed the removal of all references to 'simmering' which is not a scientifically defined term. Any task performed can have a fuel consumption number attached to it, providing the cautions above are applied. 

A task cannot be reported on an 'engineering basis' unless all portions of it are individually assessable in engineering terms. An example of an incorrect metric is 'specific fuel consumption when simmering'. 

First, simmering is not a defined term. Second, the fuel consumed is not correctly assessed on the basis described above. Third, the fuel consumption relates to the task and does not have a 'specific' component. Fourth, the fuel consumed during simmering is related to the pot diameter, not the mass of water in it so dividing by a number of litres, however determined, is meaningless. You might as well divide by the colour 'Red'. There is a 1991 paper from India on this subject. I think the Eindhoven Group also looked at the issue. 

As a result the 'specific fuel consumption' number for a stove as reported by the WBT 3.1 or 4.1.2 is quite far from the actual fuel consumed per initial litre of water in the pot. Even if the result was correct, it is by accident, not design. 

If you want to correct your methods you can start by deciding if you want engineering performance assessments or task-based metrics then make the appropriate calculations using appropriate units.

Best regards
Crispin kicking back on a Singapore Sunday
-----Original Message-----
From: rongretlarson at comcast.net
Sender: "Stoves" <stoves-bounces at lists.bioenergylists.org>
Date: Sat, 3 Nov 2012 16:47:13 
To: Discussion of biomass cooking stoves<stoves at lists.bioenergylists.org>; <mnurhuda at ub.ac.id>
Reply-To: Discussion of biomass cooking stoves
	<stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] Smoke-free biomass pellet fueled stove

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