- Feb 10, 2009
- 410
- 23
Don't know if this will morph into the definitive solution to this age old debate, but there are some smart folks on this forum, so maybe it can be done.
What most Q-heads all seem to agree on is that white smoke is bad, blue smoke is good. But how do you get one while avoiding the other? I've done a bit of Internet research, along with some field testing on my patio, and here is what I've come up with. Feel free to join in to set me straight.
To start with, in the beginning.......there was wood. From the Q-head's standpoint, the smoke we are talking about comes from wood. Take a normal piece of wood, apply heat and things start happening. The catch all phrase for this is pyrolysis. A couple quick references to what happens when you heat wood:
Basic pyrolysis:
http://en.wikipedia.org/wiki/Pyrolysis
The process of making charcoal:
http://www.velvitoil.com/Charmake.htm
But if you read these, what you find is that as heat is applied to wood, solids and liquids trapped in the wood fibers, either by themselves or as components of other compounds) break free, turn into gases and start to evaporate off. One of the first is water. The guys making charcoal attribute the white smoke to being mostly water vapor. Mostly water, yes, but there are other things too. (This is the basic refining process used to make moonshine, ethanol and turning crude oil into the various petroleum products. All these compounds have different boiling points....apply heat....and keep catching the various compounds as they boil off).
Next comes blue smoke, which includes alcohols and other compounds evaporating, and lastly comes yellow smoke, which include tars. As this progresses, the residual wood chars (turns black). This is the basic charcoal making process. Apply enough heat to drive off the volatile compounds but not ignite the residual charred wood. How long is a function of how big are the chunks of wood you start with. How long does it take to heat the wood chunk to a certain degree to drive that stuff off? Small chips? Maybe minutes. Big chunks? Maybe hours or days even.
When all these volatile compounds are removed, what you are left with is the residual carbon. Shut down the process now (cut the heat and starve it for oxygen) and you get charcoal. Lump charcoal to be exact. Apply heat and turn the air up and oxygen in the air can now react with the carbon compounds to burn (orange glow....not a flame), which gives off heat and reduces the carbon compounds (charcoal) to it's mineral ash.
So that is the basic process, but there are subtle sub plots. Some of those gases that evaporate (like alcohol) will burn. For a long time now folks have been able to run internal combustion engines by putting what amounts to a charcoal kiln in the back of a vehicle and piping the smoke/gases that are being driven off to the engine. In those indirect methods, part of the heat to fuel the process comes from venting the gases and smoke driven off to the fire to help raise the temp to complete the process. It will also burn inside the smoker if given enough time and air (oxygen) to complete the process.
So when you put wood into your smoker, all this is going on. The speed at which it happens is also linked to how big your wood chunks are. The pyrolysis process starts at the surface, and works in. It's going to happen faster with small chunks (chips) than larger chunks. So if you want a lot of white smoke, toss on a large pile of wet chips. Starve it for air and those volatile compounds don't burn. They might escape through the vent, or left to mill around inside the smoker, might condense back to liquids when they come into contact with a cold piece of meat or the side of an uninsulated smoker. Creosote and the various tars are heavy and will be some of the first to condense....even on what appears to be a hot surface. Hot being relative to the burn temp of the compound. The longer it's in there, the more likely it is to condense. UDS comes to mind.
Taken to a different extreme, guys smoking with splits of hardwood alone are generally throwing larger chunks of DRY wood onto an already hot fire, the pyrolysis process works on the surface and combines the smoke and gases with heat already coming from previously burnt wood (charcoal). The offset fire box allows allows a mixing process and time for all this to happen, where the heat from the charcoal and burning gases combine to produce indirect heat with a little smoke thrown in. This is also why a lot of UDS guys use lump charcoal (briquettes have sawdust and other fillers that smoke) and just a few large chunks. Again, if you want a lot of white smoke in your UDS, put in a lot of wood chips or chunks, and starve it for air.
Lastly comes the wood itself. Wood density varies by species (hickory, pecan and oaks being some of the hardest), so you get more BTU's, but also the mix of volatile compounds vary. I have never seen a list of what those are, but we all know they exist. Cherry, apple and maple smell different than pecan or hickory. They also taste different. Walnut has compounds too, but they taste bitter. Softwoods like pine and fir have all kinds of stuff in them to generate heavy doses of creosote. Bad for smokers but also bad for wood stoves as the creosote condenses in the chimney and causes fire hazards.
OK.....for starters, those are my thoughts. Yours?
What most Q-heads all seem to agree on is that white smoke is bad, blue smoke is good. But how do you get one while avoiding the other? I've done a bit of Internet research, along with some field testing on my patio, and here is what I've come up with. Feel free to join in to set me straight.
To start with, in the beginning.......there was wood. From the Q-head's standpoint, the smoke we are talking about comes from wood. Take a normal piece of wood, apply heat and things start happening. The catch all phrase for this is pyrolysis. A couple quick references to what happens when you heat wood:
Basic pyrolysis:
http://en.wikipedia.org/wiki/Pyrolysis
The process of making charcoal:
http://www.velvitoil.com/Charmake.htm
But if you read these, what you find is that as heat is applied to wood, solids and liquids trapped in the wood fibers, either by themselves or as components of other compounds) break free, turn into gases and start to evaporate off. One of the first is water. The guys making charcoal attribute the white smoke to being mostly water vapor. Mostly water, yes, but there are other things too. (This is the basic refining process used to make moonshine, ethanol and turning crude oil into the various petroleum products. All these compounds have different boiling points....apply heat....and keep catching the various compounds as they boil off).
Next comes blue smoke, which includes alcohols and other compounds evaporating, and lastly comes yellow smoke, which include tars. As this progresses, the residual wood chars (turns black). This is the basic charcoal making process. Apply enough heat to drive off the volatile compounds but not ignite the residual charred wood. How long is a function of how big are the chunks of wood you start with. How long does it take to heat the wood chunk to a certain degree to drive that stuff off? Small chips? Maybe minutes. Big chunks? Maybe hours or days even.
When all these volatile compounds are removed, what you are left with is the residual carbon. Shut down the process now (cut the heat and starve it for oxygen) and you get charcoal. Lump charcoal to be exact. Apply heat and turn the air up and oxygen in the air can now react with the carbon compounds to burn (orange glow....not a flame), which gives off heat and reduces the carbon compounds (charcoal) to it's mineral ash.
So that is the basic process, but there are subtle sub plots. Some of those gases that evaporate (like alcohol) will burn. For a long time now folks have been able to run internal combustion engines by putting what amounts to a charcoal kiln in the back of a vehicle and piping the smoke/gases that are being driven off to the engine. In those indirect methods, part of the heat to fuel the process comes from venting the gases and smoke driven off to the fire to help raise the temp to complete the process. It will also burn inside the smoker if given enough time and air (oxygen) to complete the process.
So when you put wood into your smoker, all this is going on. The speed at which it happens is also linked to how big your wood chunks are. The pyrolysis process starts at the surface, and works in. It's going to happen faster with small chunks (chips) than larger chunks. So if you want a lot of white smoke, toss on a large pile of wet chips. Starve it for air and those volatile compounds don't burn. They might escape through the vent, or left to mill around inside the smoker, might condense back to liquids when they come into contact with a cold piece of meat or the side of an uninsulated smoker. Creosote and the various tars are heavy and will be some of the first to condense....even on what appears to be a hot surface. Hot being relative to the burn temp of the compound. The longer it's in there, the more likely it is to condense. UDS comes to mind.
Taken to a different extreme, guys smoking with splits of hardwood alone are generally throwing larger chunks of DRY wood onto an already hot fire, the pyrolysis process works on the surface and combines the smoke and gases with heat already coming from previously burnt wood (charcoal). The offset fire box allows allows a mixing process and time for all this to happen, where the heat from the charcoal and burning gases combine to produce indirect heat with a little smoke thrown in. This is also why a lot of UDS guys use lump charcoal (briquettes have sawdust and other fillers that smoke) and just a few large chunks. Again, if you want a lot of white smoke in your UDS, put in a lot of wood chips or chunks, and starve it for air.
Lastly comes the wood itself. Wood density varies by species (hickory, pecan and oaks being some of the hardest), so you get more BTU's, but also the mix of volatile compounds vary. I have never seen a list of what those are, but we all know they exist. Cherry, apple and maple smell different than pecan or hickory. They also taste different. Walnut has compounds too, but they taste bitter. Softwoods like pine and fir have all kinds of stuff in them to generate heavy doses of creosote. Bad for smokers but also bad for wood stoves as the creosote condenses in the chimney and causes fire hazards.
OK.....for starters, those are my thoughts. Yours?
