Crispin Pemberton-Pigott

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Recent posts by Crispin Pemberton-Pigott

Apologies if this posted twice.

Ted:

Unfortunately there is quite a bit of misdirection in the replies below and not wanting to offend anyone I will try to give you some guidance in principle and for your application.  Please everyone, tolerate my alternative interpretations as just that.

>I remember somebody saying that if you close up a chimney with a baffle that is just a metal plate that closes over it, that it would be friction and slow down the fire.

This is correct, such a device is called a chimney damper for the reason that you "dampen the fire" in the sense of slowing down the burn rate. A damper is quite permissible provided that it is not able to close off the chimney completely. There are often regulations about this where the area can be reduced by 2/3 leaving 1/3 open when fully closed.   This is essential for late fire emissions to escape.  Most stoves (including rocket mass heaters) have poor combustion in the late fire due to high excess air (far too much air for the fire's requirement) and poor control over it.  

Mostly, it is not a good way to control a fire's burn rate. In all cases possible, the system should be constructed so as to limit the ingress of air, not the egress of gases. When the entrance is limited, and the chimney is pulling, any leaks permit air to enter the stove, whereas in the opposite case the gases inside try to leak out.  A negative draft maintained on the top of a fire should be about 20 pascals if you can arrange it.  That is very little, but it means a warm chimney pulling and air entering the stove through the controller with the entrance as the restriction on the system.

CO leaking out of the stove into the room is not heavier than air. CO2 (44 mole mass) is and sinks, but not CO. CO is almost exactly the same density as air (28 mole mass) and disperses very effectively throughout the room causing problems. The mechanism is it displaces oxygen on the red blood cells so it lowers the oxygen carrying capacity of the blood.

>Does a hose nozzle work as a venturi or a restriction?  

Anything that reduces the flow up a chimney will reduce the exhaust gas flow rate.  The temperature of the chimney above and below the restriction will change if you change the amount of that restriction.  This will affect the total draft on the stove. Generally speaking, all stoves in the USA have exhausts that are too large in diameter to work properly, the chimneys are too large in cross-section, and the exit temperature is too low to be effective.  That is a pretty broad condemnation, but it is just how it is.   Most stoves require a barometric draft regulator but do no get it. If the chimney is hot enough to work properly it tends to generate thermal runaway: more heat leads to more firepower leads to more heat in the chimney leads to more firepower.  That is not how most people want their devices to operate.

Draft in a chimney is a source of "power".  You could use a fan, but a warm chimney is capable of "doing work".  So a chimney is really a natural fan.  The power available can be directed in various ways: You could turn the energy into a "suction" on the fire and the air entrance (the preferred option). You could dissipate the energy creating a high velocity flow gases past the restrictive plate you are considering, slowing down the total flow.  Given any constant temperature in the chimney, the available energy could pull a lot more air quickly, or more slowly but having a high flow rate past the restriction point.  You may have a reason for wanting that.  You could use a small diameter chimney so the energy was dissipated along the whole pipe length limiting the flow to a certain desirable rate.  Usually that is a good idea.  The main point is there is energy available and you can choose were it is applied.  It might be at the secondary air entrance adding velocity (one of my favourite design ideas.)

Now for your main motivation:

>...also at the same time be a restriction that would allow soot to burn up completely (actually in a restriction that speeds it up, soot particles should shrink just on that).  

What you describe is a fire that is not burning cleanly, followed by some attempt to fiddle with the flow rate of the gases in order to reduce the smoke generated.  It is already too late. If the fire is not burning cleanly in the first place, it is far better to burn better than to try to clean up what should not be there. Slowing down the gas flow rate in the chimney is very unlikely to correct a bad combustor design.  That is not how things work.  Poor combustion has several classic causes so addressing them is a better approach.

>...but really burn up those soot particles and shrinking them too

Smoke particles don't "shrink".  They grow, rapidly.  You are quite correct that they are combustible. Smoke is almost entirely made up of unburned fuel and water vapour. A small amount of a typical stove's emissions is ash, not the products of incomplete combustion. Creating good operating conditions for combusting biomass requires meeting certain conditions.

One is an oxygen level of about 8% to 10% in the exhaust.  This is air which is not required to burn the fire.  Biomass is not as easy to burn as gaseous fuels which can tolerate very low excess air levels (even 1% or less O2). If you have 18% O2 in the exhaust, that is 600% excess air and it FAR too much to burn cleanly.  

The calculation is:  Measured Oxygen %/(21% minus measured Oxygen %)  The answer is the excess air factor, so multiple by 100 to get %.  
Note that it is not the total air demand, which is Excess air + 100% also called "Lambda".

All that unwanted excess air cools the combustion.  Slowing down the exhaust does not guarantee the O2 level will drop. It might, it might not.  It depends on the architecture of the stove.  But one thing for sure, it should not be more than 10% when running on high power. If you have a technically sophisticated burner, you can get it down to 6% but that is not for the ordinary hacker. Instrumentation is required.

You are correct that if you burn the smoke it will yield more heat.  However burning the CO gives far more heat than burning the smoke. C to CO yields about 8 MJ per kg of fuel. CO to CO2 gives more than 24 MJ/kg.  So if you lose a lot of energy to CO (bad combustion), you can get it back by burning the CO instead of carefully venting it outside. Emitted CO is a called a "chemical loss".

To keep CO burning needs about 820 C above the combustion zone. This is NOT provided by blowing harder on the air supply, which will just force more cold air though the chamber, cooling the fire. Blowing on a charcoal or coke fire like a blacksmith usually create a lot more heat, but huge amounts of CO.  Such a fire is not a good example of clean combustion at all.  Hot, yes, but not clean.

To get a clean, hot burn, your fuel load should match the grate area and a lot of other things.  Most stoves have:

too large a grate
too large a fire chamber for the typical fuel load
too much primary air
too large a chimney diameter
too little insulation around the combustion zone to maintain a hot fire needed to light the CO produced
too low an operating temperature
too little draft or
Too much draft without a barometric draft regulator

Putting an air control at the air entrance is how to control the fire intensity.
Secondary air (if supplied) should be approximately twice the primary air supply and where possible, preheated. If you want to burn wet wood, the primary air has to be preheated (big emissions reduction).

Most stoves have far too much primary air so any secondary air is just a waste of energy as the air goes in, is not used or needed, and carries heat up the chimney.

Very clean burns can be obtained without any grate - see Masonry Heaters available from the Masonry Heater's Association website. If the floor of the chamber is made from firebrick, effective combustion can be maintained for a long time. Recent developments in combustion include placing a layer of small stoves (20-30mm) over the whole fuel bed (still no grate).  This may not suit people burning 24/7 however, try it and see the difference.  It reduces smoke generation a lot in some cases.

Very high performance stoves (called HELE for high efficiency low emissions) have a surprisingly low chimney gas velocity of 150 to 250mm per second in a chimney that is about 1 square inch (6.5 cm^2) per kW of firepower. When you compare that with what is usually provided or "required" you may be surprised.

General rules are: burn first, clean and hot.  Then transfer heat to some surface, then vent what is needed to operate the chimney properly. Do not use a chimney as a heat exchanger save for the first few feet.  Once heat enters the chimney, keep it there to operate the draft into the stove properly. You gain nothing from a warm chimney after it departs the heated envelope, i.e. above the ceiling.

Good luck with your investigations. Modern HELE stoves can be see (drawings and all) in the Library of www.newdawnengineering.com under Library / Stoves, then by country.  One you might check out is the KG2.5 under Kyrgyzstan.  Have a look at the function of the "flame tube" which burns smoke and CO as it enters the heat exchanger. It is so effective that the top of the heat exchanger serves as a third cooking position.  The Kyrgyzstan project is quite interesting. You can read about the development of the main stoves here:

https://openknowledge.worldbank.org/handle/10986/31282
(click the download button)

A precis is available at https://openknowledge.worldbank.org/bitstream/handle/10986/31774/Beyond-the-Last-Mile-Piloting-High-Efficiency-Low-Emissions-Heating-Technologies-in-Central-Asia.pdf

And the dramatic impact it produced on people here:

https://www.nature.com/articles/s41533-019-0144-8.epdf

Stay well.
Crispin
5 months ago
The stove is indeed a coal stove, though that is not the only model we used. Altogether 4 different stoves were developed, initially in Tajikistan, of which 2 were specifically for biomass in any form (dung is a very important fuel at high altitude).  Now there are 6.  There had never been a stove in Central Asia before which could burn dung without the addition of some sticks and wood.  The oft-assumed unburnability of dung was shown to be incorrect. It can be used to heat a home at -30C and cook properly, if operated in a new manner (back lighting) and if there is a "flame tube" incorporated into the heat exchanger.

It was decided to report the KG4 series in this document because it highlighted that if a concerted effort was made to apply modern engineering, assessment and testing, it was possible to produce a coal burning stove that was about as clean as propane. We won't quibble about numbers, it is just important to show that profoundly clean coal combustion is possible and that people dependent on it should not be denied access to modern engineered solution. In short, a pro-poor policy of doing what we can with what they have, given that nothing else will be done that helps them. We are also attaching the false claim that all solid fuels are "inherently dirty" - a position promoted by those advocating gas and liquid fuels. It's nonsense.

Touring the Kyrgyz uplands last year in February was shocking in some ways. People are so poor it is tragic, and they have terrible living conditions because of the poor stove designs and condition. Everyone over 40 is said by the Chief pulmonologist to have COPD. Everyone with one of these KG4.3 stoves was so-o-o happy. In most cases the stove saved more fuel expenses than they received per month from government in welfare payments.  The stove saves enough fuel to pay for itself after 4 months. So the issue is access, not affordability. All the designs and drawings are available at my website (in the public domain) www.newdawnengineering.com in the Library/Stoves/Kyrgyzstan then folder KG4.3 or 4.4 (there is a small difference in the overall length of those two versions).

In principle, the same combustion system (cross draft gasification) could be used to burn wood but we are not there yet.
1 year ago
This needs a new Forum category: Domestic space heating and cooking.

There is a new technical paper in the World Bank’s Open Knowledge Repository:

“World Bank Group. 2019. Advancing Heating Services Beyond the Last Mile : Central Asia Pilot Experience with High-Efficiency, Low-Emissions Heating Technologies. ESMAP Paper; World Bank, Washington, DC. © World Bank.

Please ask readers to download a copy from the website as it counts the number, and that ultimately influence pro-poor policies.  This paper on HELE stoves is downloadable from the following webpage:
https://openknowledge.worldbank.org/handle/10986/31282 or http://hdl.handle.net/10986/31282

Please share the link widely.

The direct link to the PDF is
https://openknowledge.worldbank.org/bitstream/handle/10986/31282/134620-WBAHSWEB.pdf?sequence=1&isAllowed=y

The document provides a detailed description of how one of the cooking and heating stove technologies used in the Kyrgyzstan Winter Heating Pilot was developed.  It is a comprehensive discussion of the social and technical factors that contributed to the success of this Pilot, producing significant impacts on health, income and comfort of people living “beyond the last mile”. A link is provided to the full set of drawings and some photos archived in the ESMAP Library.

The point of saying “beyond the last mile” is that whatever are the current plans to extend modern heating and cooking services, there are a large number of people who will not be reached. This group of 500m people “beyond” are, now and in the medium term, dependent on burning solid fuels (biomass, dung and coal).  The question addressed is: What can we do for this group of low income, deeply rural and hardly accessible families which suffer all the negative consequences of chronic cold, poor combustion, indoor leakage of smoke, inconvenient drudgery and a dire lack of access to modern energy?  How can we bring modern science and engineering to these spatially and culturally diverse groups?  

The article proposes a comprehensive approach and describes how this was done in highland, rural areas of Kyrgyzstan. It covers in some detail how the KG4.3 crossdraft stove was developed over 9 years and the scientific cooperation between groups in several countries that led to the development of this HELE technology.

This document can be used to inform policy in multiple ways. If we are to eventually bring modern energy services to everyone, which will take time, there are highly beneficial interventions which can be made a low cost per person, even in poorly served regions using carefully crafted technology transfers. Apart from all the social and health benefits, the project also makes positive impacts on everything from local employment, the social status of women, emissions of CO2, Black Carbon and PM2.5.

Two further papers reporting the health benefits of the project have been submitted by the Dutch International Primary Care Respiratory Group to the WHO Bulletin and npjPCRM (Primary Care Respiratory Medicine) which conducted a Fresh Air-sponsored study of the impacts of the stove and low pressure boiler replacements.

Please circulate the links given above to anyone who you think is interested in these topics.  Stove designers will find a good deal of technical information about construction and principles of combustor operation, as well as guidance on how to interact with the communities in which it is intended to provide such modern heating and cooking services.  This crossdraft technology has already been replicated in China, Kyrgyzstan, Mongolia, Russia, Poland, South Africa and Tajikistan. A few photos are provided.

Forthcoming: Complete details on construction of three versions of the technology will be included in the final report from the World Bank-funded UB-CAP Project in Ulaanbaatar, Contract C-5039-MN/KLTA-01.

Best regards
Crispin
1 year ago
There is a whole book printed privately by TATU, then called the ECATU, now a branch of the Eastern Cape Agriculture Department on building with gabions. It was dated about 1982 and has been used in hundreds of building in the Eastern Cape. There is a manual device from New Dawn Engineering that can make the gabions by hand called a Netwire Board. There is also a heavy duty version for making earth dam walls called TriNet based on the same principles. See www.newdawnengineering.com for ideas. /products /wire /fencing Something like that.

It is possible to contact Cecil Cook cec1863@gmail.com to ask how to get a copy of that Roneo publication. It used both the gabion style wire and soil cement bricks, in-filling between wooden posts. It was very cheap and relatively insulating. The TATU offices in Mthatha were made using that system.

Regards
Crispin
2 years ago
Brick greenhouses in China:

China Agricultural University has an experimental farm in Haidian North (Beijing) with several of the greenhouses described above. I am attaching several photo taken of these from inside and out.

This one is attached to the Biomass Stove Testing Laboratory which is run by the CAU College of Engineering at which I am a supervising 'visitor' (grad students, stove design and testing protocols).

The greenhouse is used as a storage facility though there are some biochar experiments going on in the photos. That is why it is not full of plants.  The fuels and stoves are stored prior to testing. All the stoves in the photos are low pressure boilers (hydronic heaters) made in Hebei Province. Testing is part of a clean air initiative for 18m farmhouses.

As described above the greenhouse top and south are curved, with a descending cover. The ropes and handles hanging from the ceiling are to control vents. There is a fan visible in the west wall. The building to which it is attached is occupied by not heated, it is just convenient to use the back was as part of another structure.

I can confirm that it is warm enough to grow things all year though I didn't pay much attention to what, or for how long. The structure was destroyed by the local government who declared that it had been built on 'farmland' illegally. No comment. The 'destroyed' picture shows the West wall, the fan and a bit of the structure - bricks.

The thermal mass of the walls is very large. I did not calculate the heat storage and return, but it is a substantial structure. There is every reason to think it is capable of keeping temperatures above some given value for a given period based on ordinary calculations. The length to width ratio would be important in that if it is really long, the influence of not having light enter on the east side is less important. I suspect the ratio is about 1:4 East:South. Although the West wall shelters the 'house from late sun, light still hits the inside of the East wall so the loss isn't all that much.

The metalwork is modern with modern conventional vents and levers. Nothing is automated except the fan thermostat.

Crispin in Muizenburg
3 years ago
Yes. Quotations come from Thabsile Shongwe. Sea Freight is possible. Air freight is sometimes cheaper. Other times it is better to make your own.

You can click on any of the email links to send an email to Thabsile (a lady).

There are masses of photos available of these technologies in use - just a taste on the web. Many have been on the market for 20 years. For home construction also see the hand operated rock crusher. There are 90 in Haiti.
3 years ago
The idea of using 'fencing' as a deterrent or structural element is good in that you can make by hand precisely what you need. For example if you are making Square Mesh on site, you can make a single sheet of 'fence' as wide as you want - no joints for example over an entire roof.

You may have seen the Diamond Stucco Mesh on the same website. This was specifically designed to make a wire product that could hold cob or plaster without and backing - sort of like a chicken wire that is sized to hold plaster. It has a thickness of about 5mm meaning if you push it against a mould, the cement still gets behind half the wires and onto the other half.

The purpose is to be able to add a very strong reinforcement to plastered things like hay bales without investing a lot of money.

The Diamond Stucco Mesh machine is also used to make stainless steel mesh for bird cages. Unlike chicken wire (rigid hexagonal holes) it is stretchy so it can form over complex shapes like barrels. The machine is not easy to learn to use it - it takes a few days. Experts make about 25 m^2 per day (14mm holes). There are no machines in North America.

The Square Mesh has been made by hand down to 30x30mm. It is used to make gabions with internal dividers. The usual size is 76mm (3") holes.

The savings over buying fence may not be much where you live - check the cost of plain wire in bulk per kg and fence per kg first. The difference is the available savings. Most places it is 40% but watch out. Same warning for barbed wire.

If there is any interest in making water tanks from cement and mesh I can forward drawings for making a mould based on a galvanised corrugated iron water tank. We made dozens of such moulds for thousands of tanks in Southern Africa. The finished tank is 40mm thick (1-1/2"). Sizes were 1000 and 2000 Imperial gallons (4500/9000 litres)
3 years ago
Gabion making by hand.

There is a set of hand operated equipment (two actually) for producing gabions from plain galv wire.

The idea is to make one of three types of mesh, then fasten it to a wire frame that is held on a large spindly jig. Then fold it up.

The three types of mesh are Diamond mesh (chain link) and Trinet which has triangular holes and Square Mesh which looks like diamond mesh turned 45 degrees. All three can be hand made under a tree.

The website is www.newdawnengineering.com under fence products.

The Trinet jig product was designed for Dam construction and bad be made with quite thick wire. The others are normally 11 gauge or thinner, 12 gauge being most common (2.5mm).
3 years ago
On the thermodynamic side I can make two small contributions.

The angled riser AKA chimney: a chimney's draft is related to its average temperature (of that section) and its vertical height, even if it is on an angle. Your system consists of several sections with different drafts, at least one of which is negative (being a Rocket Mass Heater). There is a draft calculator in the library at bioenergylists.org (search for draft calculator nigel). You can use it to determine the sum of five or eight sections, can't recall which. At least five.


Second, the 'corrugated' flexible pipe sections which you can use for elbows: the flow 'restrictions' which these represent is not really because of the bumpiness. It is that they have a more effective heat transfer rate (higher surface area) than a straight smooth surface. In short, they drop the temperature more, reducing the heat available at the exit thus reducing the net draft on the system because it will be cooler at the end.

You have two main sources of draft: the initial internal chimney (use bricks) and the final exhaust. The sum of these is your total draft, pretty much. You should measure the temperatures involved at the end to prove you have 'some draft' at the late fire stage when things are the most dangerous. The heat stored in the bricks of the internal riser help greatly at that time.

Generally speaking feeding into the leg of a Tee is a bad idea, rather use a branch Y + 45 degree elbow on one side then a 90 degree elbow for the other direction, which doesn't have to go directly opposite because you can rotate it on its axis. The combination has less flow resistance that a tee heading off in two directions.

In that same draft calculator there is a section on the left which you can use to 'burn' your fuel and it will tell you the gas flow rate if you know the excess air level (separate discussion). The flow rate can be used to calculate the total flow resistance of the proposed pipe network using conventional HVAC methods. The bottom line is that below about 10 feet per second 'resistance' to flow is not significant, and is far outweighed by changes in total draft caused by small changes in temperature (as can quickly be seen using the draft calculator) and chimney height.  

There are municipal rules for the exit height of the pipe above the roofline for very good safety reasons (to do with wind).

It is common here in Central Asia to wrap the outside portion of the chimney in insulation to improve/conserve the draft at the end.

Good luck
Crispin in Bishkek
3 years ago