Combustion Basics and Pitfalls
Combustion is a subject that can be intimidating. The form of combustion that most people are familiar with is burning wood or paper. Believe it or not, most combustion processes are much simpler than those. We’ll get to that later.
With most combustion processes, a fuel reacts with an oxidant. Take your propane gas grill for example. The propane is the fuel and the oxygen in the air is the oxidant. Most fuels are made up of complex molecules containing carbon and hydrogen. When the fuel was made, energy was used to build those complex molecules and so when the molecule is broken down into simpler pieces, energy is released. Most of the time that energy is heat. An important aspect of combustion is the ratio of fuel to oxidant. For propane, an appropriate amount is 6% fuel and 94% air. These percentages are by volume which is just another way of describing the ratio of molecules. For the case of propane, it’s like saying there are 6 propane molecules to 94 air molecules. Since air is made up of 21% oxygen (O2), and oxygen is the oxidant in this case, .21 X 94 = about 20 molecules of O2. For methane (CH4), which is the primary gas in natural gas, the ratio is 10 CH4 to 90 air (19 O2 and 71 N2). The technical term for these ratios is stoichiometric. I like to call it the sweet spot! In the following equation, you can see the reactants (fuel, oxidant, and other gas) of a methane/air combustion process and the exhaust constituents.
1CH4 + 2O2 + 7.5N2 -> CO2 + 2H2O + 7.5N2
Notice that CO2 and water are produced. Basically, when the fuel was made, CO2, water, and energy were combined to make the methane and oxygen. The nitrogen just goes along for the ride but sometimes it can be changed too, which is not usually desired.
If the fuel amount drops too low, combustion cannot happen. This is called the Lower Explosive Limit (LEL). For propane, the LEL is about 2% and for methane, the LEL is about 5%. If the fuel amount gets too high, the Upper Explosive Limit (UEL) is reached. For propane, the UEL is about 10% and for methane, the UEL is about 17%. If the amount of fuel exceeds the UEL, again combustion cannot start.
The percentage of fuel in a combustion process can be tricky to control. Most things that utilize combustion in a process have what is called a burner. The burner handles the fuel and oxidant, mixes them, ignites them, and distributes the mixture in what is called a combustion chamber or to an open environment. Your gas water heater has a combustion chamber and the hot gasses from combustion pass through a tube or tubes that are surrounded by water that is heated by the gasses. Your gas grill has burners that discharge to the open area below the grates. The burner on the water heater is usually on or off. The burners on your gas grill can vary the amount of fuel and air to better control the temperature. This is where things often go wrong. The pressure in the propane tank is regulated down to an appropriate level that is about right for the burners but there is some variance. Changes in air pressure are so small they are insignificant. If the setup of the burners targets the sweet spot, the small variance in propane pressure doesn’t result in the fuel amount getting above the UEL or below the LEL. For large boilers and other industrial equipment fitted with burners, there are other factors involved. Environmental requirements or other factors may require some burners to operate away from the sweet spot. Remember that there is a lot of water in the exhaust. If the temperature of the exhaust drops below a certain point in the exhaust stack, the water can condense on the walls of the stack and run back down into the equipment. Not good. Recall as well that nitrogen was going along for the ride. In certain conditions, nitrogen can be split and bonded with oxygen atoms. These are called NOx (aka Nitrous Oxides) and they create smog. We can treat the exhaust using catalytic converters and other such devices but for a large industrial burner, that is sometimes cost-prohibitive. The solution to many of these problems is to set up the burners to operate with excess fuel or excess air which could make it much more probable that either the LEL or UEL is reached when relatively small variances in the facility are felt at the burner. An example is where a burner is set up to operate on the rich side of the sweet spot. One day it happens that just one boiler is running and all other natural gas users in the facility are off. This month, the natural gas has a different amount of water vapor in it and the result of these factors is a very fuel-rich mixture in the boiler. If the UEL is reached, the flame could go out and now there is a mixture of unburned fuel on its way to the stack.
Standards require very specific safety measures and/or devices on most equipment, which do a very good job of mitigating dangerous situations. However, I get calls all too often where the effectiveness of safety was defeated by a seemingly harmless decision and the result was a loss.
I told you that I’d discuss how burning wood or paper is more complex than most combustion processes. One thing that many do not know is that for a fuel to combust, it must be in vapor form. Methane and propane are already gasses but what about wood? At the microscopic level, the molecules of wood are changing into vapor at the surface. The reaction equations can look more like advanced theories of a rocket scientist than the simple one shown above. The same is true of candle wax. For the wood, radiant energy from the sun was captured by the tree and used to create a complex molecule out of CO2 and water while releasing oxygen. If the tree gets buried for millions of years, the molecules’ complexity increases as it becomes coal.
I very much enjoy working with combustion processes and equipment. A well-designed and properly maintained combustion system is truly beautiful. Burn baby burn!
About the Author
David S. Williams, P.E., CFEI is a consulting engineer with our Seattle-Tacoma Office. Mr. Williams provides consultation in the areas of mechanical component failure/fracture analysis, combustion systems, boilers, rotating power equipment, fluid flow control and waste gas handling systems, motor control, and sheet metal stamping equipment. You may contact David for your forensic engineering needs at firstname.lastname@example.org or (253) 345-5187.