The recent flurry of posts regarding creosote with wood boilers piqued my interest in attempting to explain why, starting with the basics.
"Traditional" Outdoor Wood Boiler (OWB) and Gasification Wood Boiler (GWB)-- considering the simplest of these quickly shows the genesis of the problem with the OWB and the solution to that problem accomplished in the GWB.
The simple OWB is a steel firebox surrounded by a tank of water. Air (oxygen) is supplied by an open draft or draft fan. The wood burns in the firebox, essentially an open fire in a steel box, heat is transferred through the steel walls to the surrounding water, and flue gases go up the stack. You cannot heat water higher than 212F, so as the hot unburned gases collide with the low temperature steel walls, the gases condense on the walls of the firebox and droplets of condensation also are emitted with other flue gases up the stack, again condensing on contact with other surfaces. This condensation is creosote, water and other unburned material. Also, the rough flash point for wood is around 500F, and efficient combustion of wood starts to occur around 800-900F, with combustion efficiency increasing as the temperature continues to rise to a maximum of about 1800-2000F. Some of this lower (800F and higher) range efficient burning can occur in the hottest part of the fire, but it won't occur in the bulk of the firebox because of the "cold" steel walls encasing the combustion process. Therefore, lots of unburned gases will be exhausted up the stack and a portion will condense to creosote as the gases contact the low temperature in the steel walls, stack, and even in the cooling exhaust stream. Hence, often major creosote issues, low efficiency, and lots of smoke (unburned wood). Burn efficiency may range down to about 30%.
The simple GWB also is a steel firebox, quite small compared to an OWB, mostly surrounded by water but not on the bottom nor on the top. Condensation and creosote formation does occur on the walls in contact with water, and this is limited because the fire burns off the excess which accumulates and falls back into the burning wood. But the bottom of the firebox is a firebrick material with an opening (nozzle) to a lower chamber of ceramic/firebrick (tunnel). Air (oxygen) is supplied with a downdraft fan, with air divided between the firebox and the lower chamber. The downdraft forces the burning and unburned gases into the lower chamber, these are mixed with additional air to further the combustion, and the gases are momentarily trapped in the tunnel where further combustion occurs. This further combustion heats the tunnel to very high temperatures (up to about 1800-2000F) which almost completely burns the gases forced into the chamber from the downdraft. Burn efficiency in the tunnel can approach 98-99%. The extremely hot gases (almost all steam and carbon dioxide with excess oxygen and nitrogen at this point) in the tunnel are then forced by the draft through multiple firetubes surrounded by water in the back of the boiler, heat is transferred to the water, and the remaining steam and carbon dioxide and other gases are exhausted up the stack. Little or no condensation can occur in the firetubes because of the extreme heat, nor in the stack because exhaust temperatures are well above 212F. What may condense, if anything, is water and possibly a portion of the 1-2% of unburned gases, but again because of the heat and forced draft these normally are exhausted up the stack without condensing or formation of creosote. Smoke output up the stack is almost eliminated, and what may be seen as smoke is steam condensing as it contacts the cool atmosphere.
That's the basics. In the OWB arena design has moved in the direction of GWB design in an effort to increase efficiency and reduce smoke and creosote. In the GWB arena design has moved into computer controlled combustion with oxygen maximized to allow complete combustion, variable speed draft fans, and careful control of oxygen flow into both the upper firebox and lower tunnel, all to reduce emissions and maximize efficiency.
This explanation does not consider idling/slumbering in either boiler, and does not consider the impact of burning poorly seasoned wood or poor burning practices in either boiler, or the impact of often burning green wood in an OWB. The short answer is that any of this can greatly exacerbate the problem, or create a problem which otherwise may not exist, and those are additional discussions.
"Traditional" Outdoor Wood Boiler (OWB) and Gasification Wood Boiler (GWB)-- considering the simplest of these quickly shows the genesis of the problem with the OWB and the solution to that problem accomplished in the GWB.
The simple OWB is a steel firebox surrounded by a tank of water. Air (oxygen) is supplied by an open draft or draft fan. The wood burns in the firebox, essentially an open fire in a steel box, heat is transferred through the steel walls to the surrounding water, and flue gases go up the stack. You cannot heat water higher than 212F, so as the hot unburned gases collide with the low temperature steel walls, the gases condense on the walls of the firebox and droplets of condensation also are emitted with other flue gases up the stack, again condensing on contact with other surfaces. This condensation is creosote, water and other unburned material. Also, the rough flash point for wood is around 500F, and efficient combustion of wood starts to occur around 800-900F, with combustion efficiency increasing as the temperature continues to rise to a maximum of about 1800-2000F. Some of this lower (800F and higher) range efficient burning can occur in the hottest part of the fire, but it won't occur in the bulk of the firebox because of the "cold" steel walls encasing the combustion process. Therefore, lots of unburned gases will be exhausted up the stack and a portion will condense to creosote as the gases contact the low temperature in the steel walls, stack, and even in the cooling exhaust stream. Hence, often major creosote issues, low efficiency, and lots of smoke (unburned wood). Burn efficiency may range down to about 30%.
The simple GWB also is a steel firebox, quite small compared to an OWB, mostly surrounded by water but not on the bottom nor on the top. Condensation and creosote formation does occur on the walls in contact with water, and this is limited because the fire burns off the excess which accumulates and falls back into the burning wood. But the bottom of the firebox is a firebrick material with an opening (nozzle) to a lower chamber of ceramic/firebrick (tunnel). Air (oxygen) is supplied with a downdraft fan, with air divided between the firebox and the lower chamber. The downdraft forces the burning and unburned gases into the lower chamber, these are mixed with additional air to further the combustion, and the gases are momentarily trapped in the tunnel where further combustion occurs. This further combustion heats the tunnel to very high temperatures (up to about 1800-2000F) which almost completely burns the gases forced into the chamber from the downdraft. Burn efficiency in the tunnel can approach 98-99%. The extremely hot gases (almost all steam and carbon dioxide with excess oxygen and nitrogen at this point) in the tunnel are then forced by the draft through multiple firetubes surrounded by water in the back of the boiler, heat is transferred to the water, and the remaining steam and carbon dioxide and other gases are exhausted up the stack. Little or no condensation can occur in the firetubes because of the extreme heat, nor in the stack because exhaust temperatures are well above 212F. What may condense, if anything, is water and possibly a portion of the 1-2% of unburned gases, but again because of the heat and forced draft these normally are exhausted up the stack without condensing or formation of creosote. Smoke output up the stack is almost eliminated, and what may be seen as smoke is steam condensing as it contacts the cool atmosphere.
That's the basics. In the OWB arena design has moved in the direction of GWB design in an effort to increase efficiency and reduce smoke and creosote. In the GWB arena design has moved into computer controlled combustion with oxygen maximized to allow complete combustion, variable speed draft fans, and careful control of oxygen flow into both the upper firebox and lower tunnel, all to reduce emissions and maximize efficiency.
This explanation does not consider idling/slumbering in either boiler, and does not consider the impact of burning poorly seasoned wood or poor burning practices in either boiler, or the impact of often burning green wood in an OWB. The short answer is that any of this can greatly exacerbate the problem, or create a problem which otherwise may not exist, and those are additional discussions.
