We’re continuing this week with our series on emissions. This week we’re going to take a look at carbon monoxide (CO).
Carbon monoxide is a colorless, odorless, tasteless, and toxic air pollutant that is produced in the incomplete combustion of carbon-containing fuels, such as gasoline, natural gas, oil, coal, and wood. The largest global source of CO emissions is from vehicle exhaust. Let’s look at how CO is formed.
Let’s refer to the combustion equation for methane -
CH4 +2O2 + 7.5 N2 → CO2 + 2H2O + 7.5N2
While we would like to think it happens in a perfectly neat way like this, it doesn’t. There are many intermediate reactions that take place and you can never achieve complete combustion. One of the intermediate steps is the formation of CO. Some of this CO gets “frozen” and never gets fully oxidized to make CO2. The amount that gets frozen depends on a number of factors like:
- excess air
- environmental temperature around the reaction
- amount of time at an elevated temperature, and
The burner supplier has control over the excess air (as a design mandate) and the mixing. The mixing is controlled by the configuration of the gas jets relative to the incoming air stream. The burner has a throat region of reduced diameter that accelerates the air (via pressure drop) and fuel is introduced into that zone to enhance the mixing and shape the flame.
In typical heaters, the gases around the flame are inert and hot with normal temperatures between 1300 and 2200°F. The flame temperature is much hotter and that is where 99.99% (or higher) of the combustion reaction takes place. If the furnace gas temperature gets too low, even a good mixing burner can produce elevated levels of CO. So it takes the heater designer working with the burner supplier to ensure low CO levels are achieved.
A typical CO guarantee is in the range of 50 ppmv (or 0.04 lb/MMBtu). This is normally an acceptable level to meet environmental regulations. Sometimes regulations require lower numbers and burner suppliers will nervously make guarantees between 20 and 30 ppmv. If regulations require CO levels much lower than this, your only option is to use a CO catalyst and a split convection section. The catalyst used for this type of CO scrubbing is typically platinum, so it becomes a fairly expensive solution. The catalytic converter on your car performs the same function, helping to reduce CO emissions in your vehicle’s exhaust.
To get by with a burner only solution it’s mostly about the radiant box temperature. If the box temperature (called the bridgewall temperature) falls below 1300°F burner suppliers become reluctant to guarantee the CO. The auto-ignition temperature of CO is 1128°F, which means this is the minimum temperature that CO will oxidize in the presence of oxygen. Sometimes there are plant operating scenarios that require low heater turndown for extended periods of time. A low turndown on a heater means a low burner firing rate and a low box (flue gas) temperature in the radiant zone. There is very little a burner supplier can do in this situation to achieve low CO emissions and you may be back to the catalytic solution for these situations.
So the keys to minimize CO in a heater are:
- design the heater with a hot radiant box
- operate with excess air
- operate the heater at normal (or design) firing rates, and
- select a burner that does a good job of mixing the air and fuel
We’ll continue our series on emissions next week, so make sure to come back!