A cleaner wood fired boiler

[drkillroy]

Hi:

I am looking for info on how to design a better boiler. I don't know which is the best forum to post my questions. The system is built by Switzer. It is company a short distance from my home builds wood fired boilers for residential heating. I believe I can make some improvements over what he does and would like to find the correct forum to post these questions.
The boiler is located outside in its own building with all the circulation pumps, expansion tanks, and dampening controls. His theory (a sensible one) is that the wood burns more completely, efficiently, when burning hot for a short period of time rather than an over the night smolder. The booiler is well insulated and holds anywhere from 200 to 1000 gallons of water. This way a fire is only needed every 2 to 4 days depending on heat storage capacity and demand. There is far less smoke than the other style wood burning boilers. I was really sold on the Switzer design since I saw it in action on a farmers home. He started the fire with a small amount of kindling, some paper and saw dust. Within moments, the fire was roaring with large pieces of wood being added to the fire. Two hours later he added cut up pieces of auto tires. He burned about 4 car tires in the next few hours. I saw very little smoke coming out of the chimney.

Please let me know where to post my questions related to the boiler.
Thanks drkillroy

 

[stanlsimon]

I'm not sure what you mean by by improvements. Automatic stoking, better fuel efficiency, longevity of fire tubes, are some things that come to my mind.

Safety is a big concern with solid fuel fired boilers. You can't just cut the power to a solenoid valve like a gas boiler. There are people making boilers fired on corn, wood pellets, or whatever, they stop the feed auger in an over temperature situation. If you've taken a boiler operators class you know how many people were killed by boilers blowing up back in the good old days.

A friend of mine wrecked his by turning on the circulating pump when there was a big steam bubble inside. The steam bubble collapsed when the cold water came in. The whole thing got crunched down like a pop can.

 

[drkillroy]

Hi:

Thanks for responding! I'm looking for better more complete combustion. Secondary combustion air in the Switzer boiler is not preheated and no seperate chamber. I was planning on lining the fire box with fire brick leaving a gap between the brick and the water jacket. Secondary combustion air will be heated as it flows through the gap on its way to the secondary combustion chamber. There the air will either breathe onto or vent from a 'catalytic converter' (I don't know what else to call it) to improve secondary combustion. I wasn't planning on actual heat transfir until after the secondary combuation chamber. Do you believe I'm on the right track? What should my converter be made out of?
Thanks for your time.
Drkillroy

 

[stanlsimon]

Many of the wood fire heaters are required to have catalytic "converters" to help with the pollution aspects.
Maybe you can buy one somewhere. Usually they go in the flue where you don't get much of the extra heat out of it.
I've never actually seen one of them up close. I would suspect that they have a matrix of ceramic with platinum or palladium for the catalyst. If this is an experimental thing maybe you could take apart an automotive converter and try to modify it. The Ford converters used a solid matrix that tends to get plugged up. Chevy used round beads that vibrated around and tend to be self cleaning.

There is a company called HS Tarm that sells a cast iron boiler with some of the features you mentioned. They are not cheap. Also they require wood that is dried for a year or two.

There was an article in HPAC magazine back in the 80's which claimed that the key to clean combustion is to not oversize the firebox.
If the firebox is oversized the you get a lot of extra wood gasifying at too low of a temperature and not enough heat and air to burn completely. The small firebox radiates more heat back onto the fuel.

I have wondered if the combustion air preheater should consist of a pipe running back down the flue into the fire box. This would give you counterflow heat exchange. You would need a forced draft fan to make it work. It would have to be made of a material that could take the heat.

Bigger commercial boilers have modulating combustion air control along with modulating fuel feed rate. This leads to cleaner combustion because you always get the right mix of air and fuel.

Are these Switzers related to the Switzers that have a christian commune in Minnesota, something like Mennonites?

Hope this helps.

 

[drkillroy]

Hi;
Thanks for your reply

I haven't seen any wood burners actually have a catylic converter. I also never thought of incorperating an automotive catylic converter for that use in a wood fired boiler. I was planning on the superheated secondary supply air to "breathe" on the converter and have the majority of heat exchange take place after the converter. The stack will have a fan pulling the exaust gasses out and is controlled by stack temperature and heating demand. A bypass switch is set to run the fan while starting a fire.

That is good to know about the effect the size of fire box and efficiency has with burning. My friend that has the Switzer boiler says he can add some green wood after a proper bed of coals are burning.

I don't know if he is related to the people in Minnesota, but I'll ask.

Thanks, Drkillroy.