Crispin, (Friends are not required to agree.)
Some of what you wrote is correct, but you go too far or leave out some important distinctions, thereby implying that some things are equal, that is, some are not really as worthy and distinctive as others (Ron and Paul) have stated. I give you four cases:
1. Pyrolysis progresses (migrates) through a piece or pile of biomass as heat is able to penetrate. True. The Adam retort (and other retorts) heat the biomass from many sides, but there is no flame (actual combustion) inside. It is a stretch of language to say that the pyrolysis in the retort
has an MPF [migrating pyrolytic front] proceeding in a roughly horizontal direction.
Sorry, that should say "has external heating proceeding inward from several directions from the sides toward the center." It is no more of a "front" than having the heat in a home reach to the center of the house from all of the furnace duct registers in each room. Furthermore, the processes in a retort are in sequential order during numerous minutes to 1) heat virtually all of the biomass to drive off the moisture, then 2) add more heat to virtually all of the biomass to drive off the low-temperature volatiles to create torrified material, and than 3) to finally get the temperature hot enough to have actual pryrolysis, first in the 400 deg C range and then at higher temperatures if the external heating is continued.
In sharp contrast, in a TLUD there is a distinctive zone of a few cm depth with actual small amounts of combustion (of some of the initial offgases). That zone migrates through the pile of biomass, heating mainly by radiation a small layer of biomass below the MPF, then with pyrolysis at a rather uniform temperature (generally from 450 to 700 C, depending on USER CONTROLED flow of primary air) to create the char as the gases move upward.
You may choose to try to equate the two descriptions above, but that would diminish your credibility as a precision-seeking scientist. And to equate a smouldering tree stump to MPF is a stretch beyond a stretch. Yes, there is fire there, and yes there is pyrolysis occuring. But such comparison is akin to saying that a ladybug and a moose are the same because both are living creatures; and in fact both are animals. Even a petunia and a whale are both living organisms. Scientific comparisons need precision and detail.
2. Ron correctly pointed out that in normal operation of TLUD stoves, there is zero O2 that gets past the MPF. That is vastly different from having so much primary air enter (as in Rocket stoves and other burners) that there is O2 moving upward above the fuel (where the gases were created). You are saying that is good as preheated secondary O2. Maybe, but not really. It is simply inadequate combustion, with the results of undesirable emissions. The excessive primary air (which you state should be secondary air) has had a cooling effect on the raw fuel that the combustion is trying to get hot enough to give off the pyrolytic gases. In other words, that secondary air that has passed over the fuel is part of the problem, not part of a solution because it has taken heat away from the fuel.
Your statements therefore are misleading, and a dis-service to any novice trying to understand the important differences about where and how primary and secondary air enters and performs.
3. Tertiary air: Defined as either 1) the needed secondary air that did not get into the combustion zone soon enough (meaning that there was poor stove design) or 2) excess air that will lower the temperature of the hot gases (meaning that this "tertiary air" was NOT needed nor part of the combustion process).
Either way, the concept of "tertiary air" is bogus in the context of cookstove combustion. The objective is to get the combustion to be complete with the proper amounts of primary and secondary air at the correct times and places.
4. You wrote:
This [MPF} process is … how the Terra Preta soils in the Amazon were created over 20,000 years of slash and burn agriculture.
The origins of Terra Preta are still being debated by the experts in that field, which is certainly not your claimed field of expertise.
The Amerindians cultivated land that was already productive, they did not create it de novo. Cecil confirms they farm patches of land that are already productive, not random areas. He observed this when he was doing PhD field research while at Harvard.
I have great respect for Cecil Cook. But somehow what you have attributed to him and his work has not reach my attention previously. And I do not accept your statements. What happened thousands of years ago was not witnessed by Cecil. He could only observe the current day activities on lands that somehow became fertile. You can take that topic to the Biochar Listserv, if you want to discuss it further.
Sorry, I reject much of what you wrote. I would not want you to be instructing people about TLUD stoves and char making and Terra Preta. IMHO, your comments about pyrolysis in different devices reflect poorly on your credibikity regarding your other strongly expressed positions, but that is a topic for others to discuss.
Paul
Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 12/9/2017 9:03 PM, Crispin Pemberton-Pigott wrote:
Dear Ron
I see no systematic differences between the uses of the terms. You are trying to create some special meaning for quite ordinary processes of combustion.
It is unfortunate you mix baseless assertions, projections of motive onto others and micro-aggressions into your erroneous analyses and combustion theory. It is wearying trying to separate your mistakes from your agendas.
For the rest of our readers, these definitions may be useful:
Primary air refers to that atmospheric oxygen used to convert a combustible fuel into a gaseous state through thermal decomposition. It is needed to combust gaseous fuels if the gas is not composed of molecules that can be oxidised directly (evaporated kerosene, for example).
Secondary air is that atmospheric oxygen which is used to combust those gases at some location generally distinct from the solid fuel.
Tertiary air is that atmospheric oxygen separately added to the gas combustion phase in order to complete the combustion and or to moderate the gas temperature to fall within some desired range (for example a low-NOx burner or a fish dryer).
While a TLUD’s migrating pyrolytic front (MPF) can produce char from biomass, there are other methods of doing so. Two that have a higher theoretical char yield are the charcoal kilns designed by Chris Adams (the Adam Retort) and the World Stove. Their working principles are not the same as the TLUD. Both produce a combustible gas that is applied to a process outside the charring of biomass.
In the char producing section of the Adam Retort there is no primary air supplied at all but it still has an MPF proceeding in a roughly horizontal direction. The World Stove supplies some primary air in the beginning but only secondary air later. Still later, primary air is again supplied if the operator wishes to burn the char produced.
All three of these approaches have proven effective. Of the three, the Adam Retort has the highest char production fraction per kg of input material, the widest range of usable materials and the least possible preparation required. The seven drum kiln designed by Dr AD Karve operates on the same principles and is specialised to char leaves. A small percentage of the fuel ends up in the ground as biochar.
A higher fraction of the char ends up in the ground when it is produced using the earth pit charcoal production process. I have not seen this discussed on the Stoves list. An investigation by Cecil Cook in Mozambique into the ultimate destination of char conducted while working for ProBEC/GTZ showed that a considerable fraction of char produced ended up under the pot and the rest ended up on or in the ground, which you have termed variously as ‘sequestration’ and some form of fertilization.
There is nothing magical or new about an MPF and it does not need a special vocabulary to describe it. In Southern California the devastating forest fires are leaving behind millions of smouldering tree stumps each of which has an MPF moving along its stump and major underground roots. This process is as old as fire. It is how the Terra Preta soils in the Amazon were created over 20,000 years of slash and burn agriculture. The Amerindians cultivated land that was already productive, they did not create it de novo. Cecil confirms they farm patches of land that are already productive, not random areas. He observed this when he was doing PhD field research while at Harvard.
I hope others can get contribute to especially the technical aspects of the discussion. I will close with a reminder that the highest char production fraction I have encountered (45%) is the Hawaiian pressurised kiln which uses a controlled primary air supply and an MPF to char a five foot long log one foot in diameter in 45 minutes. "45 in 45".
Amazing.
Regards
Crispin
Ron Larson wrote:
Todd:
Thanks for the comment. I urge readers to visit https://www.silverfire.us/. Some nice looking designs.
I can’t quite accept your message below (and one from Crispin that we received 1:23 hours later). You are both using the terms primary and secondary in a different (not wrong – just different) sense from those of us interested in char-making. We char-makers (or at least the TLUD design – Nat Mulcahy and World Stove have a still different meaning to this nomenclature) use primary air (meaning oxygen) as something that is totally depleted at/near the pyrolysis front. No oxygen in the exiting pyrolysis gases, where they are met by secondary air (oxygen).
I am pretty sure you and Crispin (who is also discussing char-using stoves) do not mean the above. Rather “primary” means plenty of air (controllable to a minimum degree) to both create and consume char. Your and most Rockets (regardless of pre-heating) find char to be a defect; you would have preferred to consume it, I believe. (I’d love to hear that you welcome the char.) So your “secondary” is to be sure that the “small” amount of uncombusted gases leaving the fuel bed are combusted. I am NOT disputing your phrase below (“Secondary combustion is possible with a rocket stove”). I am only saying that secondary air is mandatory with TLUDs, and has a totally different function than “primary” air. Most of your primary air consumes char.
The dialog between Paul and myself below on secondary air is different than your own explanation below on its preheating.
Do we agree that the terms “primary” and “secondary” have very different meanings in these two different parts of the stove world?
Ron
On Dec 9, 2017, at 12:32 AM, Todd Albi <todd.r.albi@gmail.com> wrote:
Ron:
Not quite correct. Secondary combustion is possible with a rocket stove. Our Survivor Rocket stove launched 5 years ago introduced preheated ventilation ductwork at launch. In fact we only offer a primary & secondary combustion design rocket stove. Thats why our stove incorporates 360 degree ventilation through base of stove, tied to ventilation ductwork behind combustion chamber walls. The double insulated walls allow preheated air to travel behind firebox and mix at chimney base, before exiting cooktop. Others have also now added preheated channels and secondary gasification to rocket stoves. It is not an inclusive to TLUD designs and can be adapted to any stove firebox.
—
Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com