Multi-fuel burners: post-peak fuel efficiency

[DamianB]

During the summer I had a conversation with a very practical acquaintance of mine who told me that the reason Rayburns, and by implication multi-fuel burners in general, are inefficient (Rayburns in particular) is that they are designed with a compromise in mind. In this case, multi-fuel is taken to be wood or coal, not oak or beech .

He told me that wood burns most efficiently in its own hot ash and embers with the airflow coming in from the side whereas coal needs to sit on a grate and have the airflow come up from below. This makes sense to me; does anyone know it to be true?

If it is true, could I realistically have a piece of 20mm?, 10mm? steel plate cut to fit the grate in my Rayburn so that it left a 10mm gap around the sides, so that air could flow past and into the base of my fire?

I'd gladly cope with the more complicated ash removal if the very mechanism (riddling) that makes it easy, contributes to the vast amount of wood (3-4 cords) I use each winter.

[clv101]

He told me that wood burns most efficiently in its own hot ash and embers with the airflow coming in from the side whereas coal needs to sit on a grate and have the airflow come up from below. This makes sense to me; does anyone know it to be true?

That's certainly my understanding, coal fires need grates and wood doesn't. The problems occur when trying to burn coal without a grate than wood with a grate though!

[mikepepler]

I covered this on the biomass part of my MSc. Wood does need a grate, as you need to get rid of the ash, otherwise it reduces the airflow and combustion becomes inefficient. All commercial biomass furnaces have two air supplies: primary and secondary, or "underfire" and "overfire" as they are sometimes known. Underfire is air that comes through the grate, and overfire is air that feeds into the hot gases above the wood.

The wood undergoes pyrolysis in the heat of a fire, producing flammable gases and leaving charcoal behind. The underfire air is used in the combustion of the charcoal, while the overfire air is for combustion of the pyrolysis gases. Ideally a furnace will be able to control both of these air supplies, and maybe also limit the flow of hot gases out of the furnace.

We didn't cover coal specifically, but most coals will give off flammable gases when heated, so I imagine the same considerations will apply. However, coal is a very different fuel to wood, so I can imagine that it might be difficult to design a furnace that works well with both. Note - it's difficult, but not impossible. Having a different fuel-feeding system is probably the hardest bit, I imagine the only change inside the furnace itself is modifying the air supply.

But just to underline the point - wood does need a grate. Adding a grate is one of the first changes made when modifying wood-burning stoves in developing countries to improve their eficiency.

[biffvernon]

I covered this on the biomass part of my MSc. Wood does need a grate, as you need to get rid of the ash, otherwise it reduces the airflow and combustion becomes inefficient.

I haven't got an MSc in biofuels but I have used several different woodstoves over the last 30 years and now live in a house with 100% biofuel heating. Mike, you may have a point where industrial heating plant and automatic feed furnaces are concerned but for domestic woodstoves of the room heater sort you definately do not want a grate when burning wood. A grate is essential for coal but woodstoves work best with a good layer of woodash and no grate.

[mikepepler]

I covered this on the biomass part of my MSc. Wood does need a grate, as you need to get rid of the ash, otherwise it reduces the airflow and combustion becomes inefficient.

I haven't got an MSc in biofuels but I have used several different woodstoves over the last 30 years and now live in a house with 100% biofuel heating. Mike, you may have a point where industrial heating plant and automatic feed furnaces are concerned but for domestic woodstoves of the room heater sort you definately do not want a grate when burning wood.

This wasn't just commercial furnaces, it was also wood-fuelled cooking stoves for use in developing countries. Without fial, every single one I have seen uses a grate, and it is stated as neccessary to ensure complete combustion of the charcoal portion of the wood, and sufficient airflow. These stove have been designed by people who know what they're doing, and fine-tuned over the years in practical use around the world.

Of course, for best performance, you need to control the airflow as well as have a grate - too little air gives incomplete combustion, but too much air cools the fire down. Adding a grate allows sufficient air in, but some form of restriction on the air intake is required to control it. There some info on one of the best designs, the "Rocket stove" here:
http://www.efn.org/~apro/AT/atrocketpage.html
Here's a picture from it:

you'll see that this one actually has an air gap beneath the burning wood - which is taking the use of a grate to the extreme! You feed the sticks in slowly to control the heat output with wastefully burning too much wood.

A grate is essential for coal but woodstoves work best with a good layer of woodash and no grate.

Why do you think it's best not to have a grate, and what goes wrong if you have one? I had a glance at some woodstoves here: http://www.woodstoveshop.co.uk/prod01.htm and most of the ones I looked at used grates and primary/secondary air supplies. This one: http://www.hunterstoves.co.uk/main/stov ... r_hawk.htm said what you did about wood needing a bed of ash, but with no scientific or practical explanation of why this was best.

I'm not saying there isn't a good reason for not having a grate when burning wood, but given that everything I've learned about stove design says you should have a grate, I'd like to see a good explanation for not having one, other than just that it works best.

[MacG]

This little baby does over 70% efficiency without grating, and it's clean burning. I have lived with it and know what it's capable to. The people at J?tul are masters at what they do.

[Bozzio]

I have a Morso stove and it has a riddling grate. In their instructions on lighting a fire with wood they do recommend allowing the ash to build up in the bottom of the stove to act as insulation.

This page from the Hunter Stoves website describes the different set ups for solid fuel and for wood. With wood they recommend closing the grate such that it becomes a solid base and allows the ash to build up, as required for the correct combustion of wood.

I have to say that from my experience, my stove always performs better when the fire sits on top of a full ash tray.

[mikepepler]

This little baby does over 70% efficiency without grating, and it's clean burning. I have lived with it and know what it's capable to. The people at J?tul are masters at what they do.

Yes, I've seen that there are some manufacturers making stoves without gratings, what I want is someone to explain in practical/scientific terms why this is a good idea. I'm assuming there can't be a "correct" answer to whether you have a grating or not, as there are obviously some made with them, and some without, all designed by people who should know what they're doing (though I've personally seen more designs with a grate). I know the reasons for having a grate, I'd just like to hear the reasons for not having one.

I've had a look around on the net, and found these so far:
http://www.hedon.info/goto.php/DesignPr ... ientStoves

Elevate the fuel and distribute airflow around the fuel surfaces. When burning sticks of wood, it is best to have several sticks close together, not touching, leaving air spaces between them. Particle fuels should be arranged on a grate.

So by a "particle fuel", this author probably means coal, charcoal or woodchip. When he's talking about burning sticks, he's thinking of the Rocket stove (see earlier post). However, the stoves we use here (I think) don't keep the fuel spaced out with air gaps between them. I wonder if these design differences are because we buy wood in bulk, while the people using a Rocket stove may have to walk miles to collect it by hand off the ground? I imagine this makes them much keener to only burn the wood they need (for cooking in this case), and no more.

A slightly more radical design of stove is here:
http://www.woodgas.com/Woodgas%20stove.pdf
I think it's advantages are more to do with cleanliness than anything else, as it's intended to be used indoors without a chimney.

This one, at last, has some explanation of the use of an ash bed:
http://www.plan.aau.dk/~ana/Education/Wood6.pdf

A special characteristic of ash is its heat conservation property. For wood stoves, the ash layer at the bottom of the stove forms a heating surface, transferring heat to the final burnout of the char. For heating systems using a grate, the ash content is important in order to protect
the grate against heat from the flames.

This makes some sense - the ash can absorb heat and re-radiate it to the char to keep it hot enough to burn. However, having done some practical lab experiments with this kind of thing, and given that most manufacturers (esp for developing countries) are using grates, it seems to me that it's a case of balancing several factors. For example, having a bed of ash might improve the insulation at the base of the fire and so assist char burning, but it will make the supply of sufficient air more difficult to achieve. There seems to be a tradeoff here, and different designs approach it in different ways, and could all be equally efficient, whether they use a grate or not.

Some of the key points I've seen from my course and the above links are:
- dry the wood before burning it. If your stove is designed so it can heat some wood, say using heat from the exhaust, that's a big advantage.
- make sure the stove design causes turbulence in the flames - this helps improve combustion of the pyrolysis gas products.
- make sure there is sufficient airflow, but provide a means of restricting it.
- design the stove so wood can be fed in gradually, so you only burn the fuel you need to for what you are doing.
- use the residual heat in the exhaust gasses for cooking (I've seen stove designs that give you 2 or 3 hot plates at differing temperatures, and they're built from clay and cow manure!)

I realise that the developing country stove designs are very different to the ones we have here, but they are often elegant in their simplicity, and have the advantage that you could build them in your back garden with just a few scavenged bits of metal.

[biffvernon]

Mike, I think I may be able to explain why we are both right. You are talking about cooking stoves and I'm talking about room heaters. For a cooking stove you want the maximum temperature for the least fuel for a short period of time. And you don't mind adding more logs while you're stirring the pot. This is where a grate gives good performance. For a room heater you want the wood to burn slowly, giving off its energy over a long period of time but the top temperature is less important than not having to keep refuelling. Room heaters can double up as places to keep the stew simmering but not so good for frying the sausages.

So, a grate allows the wood to burn quickly at a high temperature, good for cooking, while burning on a bed of ash allows the wood to burn slowly at a lower temperature, good for room heating.

[MacG]

So, a grate allows the wood to burn quickly at a high temperature, good for cooking, while burning on a bed of ash allows the wood to burn slowly at a lower temperature, good for room heating.

I think we are closing in! I have lived with iron stoves for cooking/baking also, and in addition to an always present grate, the combustion chamber is always much smaller than in the room heating stoves, and they are fed with MUCH smaller sticks of wood than the room heating units.

[mikepepler]

Mike, I think I may be able to explain why we are both right. You are talking about cooking stoves and I'm talking about room heaters. For a cooking stove you want the maximum temperature for the least fuel for a short period of time. And you don't mind adding more logs while you're stirring the pot. This is where a grate gives good performance. For a room heater you want the wood to burn slowly, giving off its energy over a long period of time but the top temperature is less important than not having to keep refuelling. Room heaters can double up as places to keep the stew simmering but not so good for frying the sausages.

So, a grate allows the wood to burn quickly at a high temperature, good for cooking, while burning on a bed of ash allows the wood to burn slowly at a lower temperature, good for room heating.

Yes, I think that's probably a good description of where it all ends up. It comes back to the issue that machines are usually designed for a specific purpose, and that's what they work best at. If you try to get two uses from the same machine, you have to compromise somewhere. I guess we have to have seperate stoves for heating and cooking, if we want to keep wood use to a minimum and flexibility to a maximum.

I'd be interested to get some input from Bozzio here, on the perspective of using a thermal store. In this case, I imagine it's OK to burn the wood fast and hot, so you get good efficiency at the heat exchanger in the stove. As the heat is going into the thermal store, it can then be released slowly later on for space heating. Does that make sense, or does it work differently?

The use of a grate on commercial furnaces also makes more sense then, as given that you have a automated fuel feeder anyway, you might as well go with the grate, high airflow, higher temperature, etc. So... does anyone know of a furnace/stove that has automated fuel feeding that is not electricity-dependant? I've heard of stoves that burn logs from an automated feeder - could you use a clockwork mechanism to trigger the dropping in of the logs??