Bit and Biomass: Possible in Same Boiler

I am just starting my research in building my own boiler to heat my house. I love having options and the ability to heat my house with wood and coal has its appeal, but I am not sold on firewood. I got plenty of trees, its the process of cutting and splitting that really gets me. I love the thought of burning chips, but have been told it takes a special boiler to get enough fire to do so.

It seems that burning bituminous coal kind of needs the same thing with secondary, preheated air.

The only boilers I have been in that could burn both coal and biomass, have been liquid bottom boilers and I am not really sure I could replicate that in a residential type setting. Don't get me wrong, I would love to have one, but I am thinking a chain stoker would be a better arrangement.

I was really looking at something like this, scaled down of course. Obviously the feed intake would have to be adjusted, but do you think the same general design would tolerate bituminous coal and wood chips?

http://www.hurstboiler.com/boilers/solid_fuel_fired/firebox_low_pressure_chain_grate

I see a lot of moving parts that will have to be made. And of course motors to move all those moving parts.

I think you would be far better off to get a underfed stoker unit.

Are you looking for somthing like this boiler with an underfed stoker, instead of the tri grate.
http://alternateheatingsystems.com/MultiBoilers.aspx

Maybe, it was hard to tell from the pictures how it was designed.

I was told I really will need to get a lot of air movement in to get wood chips to really burn, which is why you need a chain stoker, and once burned (like all wood) secondary air is important as well.

I really like the idea of the cyclone scrubber because in all the boilers I ever worked in, the heavier particles were dumped back into the boiler for reburn again and again until all their fines were burnt and fine enough in size to go up the stack, but these were liquid bottom boilers too and not chain stoked. If it could be incorporated however, that would not only be more efficient, it could act as a pre-heater for the incoming secondary air. Simply channel the incoming secondary air around the cyclone and glean some heat off that before it gets dumped into the firebox. I don't see why that would not work.

The real challenge I see is not in making the boiler burn efficiency a full throttle, but in getting it to maintain a burn at idle when the water is already heated. Anyone can toss a 55 gallon drum on a bonfire and get water hot, it is feeding that bonfire constantly that becomes a chore. Getting the unit to idle down, then comes back to life would be the challenge I think, but would also drastically reduce fuel consumption, whether you were burning coal or woodchips.

Carlherrnstein is right though, there is a fair amount of moving parts, and the only real area I can see minimizing this, is in the ash pan area. That could be taken out by hand with ease.

An underfeed stoker burning bituminous is about as good as it gets for bit coal - and you can burn anthracite in most bit underfeed stokers with some tweaking of the air/fuel. An underfeed can also feed wood pellets, but, in my experience, it works best with short "off" times and high feed/firing rates. An underfeed stoker w/ bit coal burns it smokeless w/ or w/out overfire air - heated overfire air really only is used on batch-feed hand-fired stoves and such, not on stokers. An underfeed stoker w/ bit can achieve high 80's combustion efficiency if tuned right with a quality coal - there's not much need to look further than this.

Wood chips (usually fired green) are a very low btu product and are a serious pain handling-wise. Any trucking over any distance makes their cost per btu increase even more substantially.

Any particulate or fly ash emission from a small boiler can easily be dealt with via a settling chamber - no cyclone necessary (basically a steel box where the gasses enter one side, loose momentum and drop what they're carrying before exiting the other side).

Wow, I guess I just never knew that an underfed boiler was so versatile. I will have to look into them more I guess.

My father has one; a brand new boiler that he bought but has had nothing but problems with it. I am not sure what is broke on it now, but he is burning oil instead of pellets. This new boiler is broken down far more then it runs unfortunately. On the last point...that is the truth. I tried to talk him into burning coal and going with that sort of boiler, but he is stubborn sometimes, and when he gets something in his head, he cannot get it out. He works part-time at a hardware store and they sell pellets but not coal, and so was convinced that is the way to go thinking coal will always be hard to get, but me...I greatly dislike pellets.

Now wood chips, biomass, whatever you wish to call it; I am really interested in that. As I type this I can hear a wood chipper on my neighbors land churning trees into chips. Any logger of any size has one around here and I have the capacity to make them myself if needed. Someone told me it requires a lot of air coming up through the grates from beneath and that a chain stoker is required to get them to burn. I figured that would be a lot like bituminous coal with some preheated secondary air. Our local school has one, having switched over from oil a few years ago. It is a huge school, holding about 1200 kids from K-12 and uses about one load of chips per week in the winter, BUT it requires clean chips. Still it has enough capacity so that their sidewalks are all radiantly heated and even added a few more this year because the boiler has excess heating capacity. One of the maintenance guys is a good friend of ours and goes to our church so I will have to ask him about the design of it. Maybe it can be scaled down.

Thanks for your input. I know you have built quite a few boilers and stoves and so I know you realize that it can be done. After watching my father pay 5K for his junk boiler, I figure I can do the same thing for a whole lot less. heck I might even be able to make something that works for a whole lot less!

I have some experience co-firing and handling wood chips on a traveling grate spreader stoker boiler. It cannot be scaled down well. When chips have been fired in very small btu plants they use a modified underfeed stoker either fed with a screw (like coal) or pusher block/ram; as the plant gets a little bigger, you can start using chain grate, inclined grate, and many other furnace designs. As you have discovered, they have to be CLEAN otherwise you will have problems feeding them. Typically they're fired green to prevent an excess of airborne ash and to keep the fuelbed together. When there's enough material burning, the green chips burn well - as I mentioned, it doesn't scale down well because of the heat needed to ignite and efficiently burn the incoming fuel. It is not at all like burning bit coal. Their is a LOT more flyash when burning wood chips than when burning stoker coal in these plants, so much so, on plants not designed for it, even firing 10% will overwhelm the flue gas particulate removal systems.

That is too bad.

I do not think I have ever been inside a truly dedicated biomass boiler. There was a boiler out in Rumford (Actually 8 of them) that could burn biomass I guess, but were always burning coal when I was there. They were all liquid bottomed boilers and that was when I first heard of liquid bottom boilers and how efficient they were. I learned a lot about the scrubbers while I was there, but as you say, were pretty intensive set-ups. I did work on the scrubber cyclones once, but that was not in Rumford, but actually in your neck of the woods, at Ticonderoga NY. Not sure what type the boiler design was though. Certainly not a very big boiler by any means, but the view from the top looking out across Lake Champlain was impressive. The only chain stoked boiler I have been in burned municipal garbage which made my one visit there my last one; too smelly and dirty for me.

I have looked at the bituminous coal stove you drew up a long time ago on here, and liked what you had, but was curious if you think it could be modified as a boiler with a water compartment? I am not sure I have been clear on this, but in referring to a boiler I meant a non-pressurized one, so I would only need to heat the water to a maximum temperature of 150 degrees.

(Because of what you said, I am not looking for it to burn chips however).