The Three T’s of Combustion
Most of us benefit from some sort of combustion every day. Whether for the release of heat or the expansion of gas to perform work, this special category of oxidation is probably the most widely-used chemical reaction in our daily lives. Like most chemical reactions, there are parameters that need to be present not only for the reaction to take place, but to be the most efficient.
For combustion, we follow the Three T’s…time, temperature, and turbulence.
The time of combustion refers to the rate of the reaction. The fuel (natural gas for example), is introduced to the furnace through injectors or pokers. These are usually pipes with a plate at the end with several holes (some call this a poker “shoe”). The volumetric rate, the size and the quantity of holes will result in a design gas velocity at each hole in the poker shoe. Remember also, most burners modulate, so as the firing rate changes so will the gas velocity. This range of gas velocities need to coincide with the rate of the combustion. Gas velocities too high result in the flame “leaving the zone” of combustion which can cause unburned fuel or flame outs. Gas velocities too low can result in “puffs” or a “punky” flame similar to when you shut the gas off to your grill (okay for barbecues, but not for your boiler).
The temperature of combustion is more intuitive. We all know we need to “ignite” the flame to start the reaction, but what does that mean? Simply, we are introducing heat with a spark or pilot flame at the point where air and fuel are mixed to start the reaction. Once the reaction is started, it provides the heat to maintain the temperature to keep the reaction going. Those familiar with gas pilots know adjusting the pilot gas pressure is key to making the pilot flame reliable. Make sure there is a separate pilot gas regulator to accomplish this.
The turbulence of combustion is the “mixing” of the ingredients…fuel and air. Those who have seen the heads of burners know the multitudes of designs to create turbulence at the combustion zone. This creates good mixing and efficient combustion. Poor mixing can allow unburned fuel to create unwanted compounds in the flue gas like carbon monoxide. Burner designers also consider the size and shape of the furnace. A turbulence pattern that makes the flame too wide or too long can result in flame impingement.
Burner technicians use a stack gas analyzer and a sight glass at the back of the boiler to make their final adjustments for the most efficient combustion. They also do this when the furnace is at normal operating temperature as a “hot” furnace has different combustion characteristics than a “cold” furnace.
So next time you’re in the boiler room, take a peek at the flame and look for the Three T’s.
– Robert Frohock, PE