[Stoves] Increasing ND-TLUD riser height accelerates gasification and increases bed temperature

[Crispin Pemberton-Pigott]

Dear Julien

 

I agree with much of what you say. No need to repeat it.

 

I do take issue with your comments about a systematic investigation into the effects of preheating and so on. It is a standard part of the stove design training course given in Mongolia and South Africa. I have never been under the impression that this was novel.  

 

>What I used for this research is (1) a scale to measure fuel and char, (2) an overhead scale to measure mass loss during gasification, (3) a four channel thermocouple data logger, and (4) an oven I can hold to 105°C to measure fuel moisture content.  For unfunded, recreational research, that is not bad. 

 

I have not disputed that at all – I think you are doing fine. The questions you want to investigate further require additional equipment.

 

>What would have been the biggest improvement for my experiment would be to set-up a more sophisticated method of feeding primary air, and measuring the primary air superficial velocity (SV).  

 

There are some low cost instruments that can be helpful for that. There is a good example at the Alexander Batsulin’s site shown measuring intake velocity with a propeller type velocity meter. That would give you the raw data you need.

 

>As for trying to explain my observations, just about all explanations for ND-TLUDs are speculative, because ND-TLUD SCIENCE DOES NOT EXIST.  There are at least 50 peer-reviewed papers on 'FD-TLUDs' dating back to 1947.  No all of them are appropriate to cookstoves, but a lot of them are.  For ND-TLUDs there is nothing.   

 

A TLUD is an upward burning downdraft stove and there is a great deal in common with them. There are investigations and patents going back to the 17th century. FD-downdraft stoves are rare though known. There is a good reason why Dr Tom’s stove was called an upside down downdraft combustor. It was a downdraft stove turned over, burning the same fuel in the same manner internally, with the fire on top. 

 

>For now what we need are some very basic empirical studies on ND-TLUD performance, so that builders of stoves can make more deliberate decisions, so that rural extension workers can have confidence that their advise is well-grounded, and so that both can respond effectively to unexpected situations.

 

That would be great. I wouldn’t second-guess what people are doing or have done, and not published it in a way you could find it. There is a lot in Chinese which is not very accessible. 

 

>We need to start building ND-TLUD science at a very basic empirical level.  Nobody has actually measured primary and secondary air flow, so nobody really knows the their optimum proportions for low emissions and how that changes with gasification temperature, types of burner and fuel.  

 

I believe that has been done for BLDD gasifiers long ago. Such gasifiers were used for making town gas. It is (as I understand it) not wise to estimate that the gases produced somehow set the emissions of the fire. That is a bit like saying ethanol is a ‘clean fuel’.  I can suggest that you try two sets of experiments: use a fixed burned design and vary the gas production parameters, then see how the burner handles the gaseous fuel.

 

Then leaving the gas production in one of several starts, vary the burner to get the best burn for the gas it happens to produce. What you will find is that there are dozens of combinations that produce very low emissions. The combination of a gas generation method (creating a significantly different gas composition) will burn best in a particular architecture of burner, mixing and secondary air pre-treatment regime. 

 

>Nobody has actually proven that preheating secondary air is effective; it is just assumed to work, even though we are taking heat from the reactor to create more viscous and less dense air.  

 

I have personally conducted many tests involving this and so has Roger Samson who has been manufacturing rice hull gasifiers for years. I have been making downdraft stoves more than 10 years and they all have (now) preheated secondary air because the benefit is so great.  The effect of preheating is strong, reproducible, and is best demonstrated when using a gas analyser which tracks the CO/CO2 ratio. Completeness of combustion is a good metric. Taking heat out the combustion process generates CO sooner than PM.

 

ND-TLUD design has been investigated by me and Prof Lodoysamba many times in Mongolia, first over a period of 18 months, and later for several shorter periods.  We have many hours of tests showing the clear advantage of preheating the secondary air and how to introduce it, and more hours of showing the poorer results of not doing so. Two stoves that failed to make the cut this year were failed precisely because they did not incorporate secondary air preheating, combined with poorly conceived operating instructions.

 

>If preheating makes no difference, then it becomes easier to design the gas burner and the gasification reactor as separate modules, that are mixed and matched according to need.  

 

Yeah, but it is a big damper on combustion efficiency (pun intended).  Cold air entering the gas stream simply cannot compete with hot air entering the same gas stream. Cold air injection produces a combustion efficiency that will meet US and EU CO targets, but the approved stoves in Ulaanbaatar demonstrate a far lower sustained CO level – often 0.03% or lower. This is only achieved in those TLUD’s that have preheated secondary air and have it injected in the right amount in the right manner.

 

>Nobody has shown that TLUD biochar has safe levels of polycyclic aromatic hydrocarbon and dioxins.  

 

No idea. I have not researched what is in biochar though I think the Japanese have been looking at that for some time. 

 

>All this, and more, needs clarification in replicated, controlled experiments ... and open access publication.

 

There is a significant problem with that in that as the money pool spent on stoves drives the creation of larger private participants, they have proven to be less likely to share as openly as in the past (unless it is in the patents they file). This group is far more likely to participate in a sharing exercise, but even here, times they are a-changing. 

 

I appreciate your willingness to share your work and thoughts about your investigations. 

 

Regards

Crispin

 

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