In "Flame Temperature: What Becomes of It?", I looked at the factors affecting burner flame temperatures. One big player is the fuel-air ratio. At stoichiometric, or correct, ratio, flame temperatures and combustion efficiencies are highest. But just how is the correct ratio figured, and is it important for our process?
Ask someone the correct combustion ratio for air and natural gas, and they might reply, "Ten to one" -- that is, ten volumes of air for every one volume of natural gas. If you're used to figuring these things on a weight basis, the weight ratio is about 16.5 parts air to one part gas because air is denser than natural gas. Ten to one is a nice number -- it allows you to do air/gas flow calculations in your head -- but just how accurate is it?
Not bad, actually, and for most purposes, good enough. Let's take a closer look just to see how the number is arrived at.
Natural gas is a mixture of gases, and its composition will vary with the well it's drawn from and any processing steps that alter its makeup. Components like propane and helium will be removed because of their commercial value. Hydrogen sulfide is a toxic, corrosive, foul-smelling contaminant in some gases, so it's taken out before the gas is sent into the pipeline. What's left is usually at least 90 percent methane with small amounts of ethane, propane, carbon dioxide and nitrogen. Some butane, pentane and even hexane also might be present.
Pure methane has a stoichiometric air-gas ratio of 9.53 to 1 on a volume basis (the weight ratio is 17.2 to 1). The other combustible hydrocarbons have these stoichiometric ratios (table 1).
If natural gas were 100 percent methane, stoichiometric ratio would be 9.53 to 1, but heavier hydrocarbons in the mix raise the ratio in proportion to the amount present. On the flip side, inert (noncombustible) gases like nitrogen and CO2 tend to lower the ratio of the mix. If, for example, a gas contained 50 percent methane and 50 percent ethane, its stoichiometric air requirement would be 13.11, the average of 9.53 and 16.68. A gas containing 50 percent methane and 50 percent nitrogen would need only 4.77 ft3 of air for every cubic foot of gas. If the gas contains both heavier hydrocarbons and inerts, the two groups could nearly cancel each other out, leaving you with a gas very similar to straight methane in its combustion air requirements.
The Gas Engineers Handbook contains a table of natural gas compositions delivered to 48 major U.S. cities in 1962. Their heating values ranged from 945 to 1,121 BTU/ft3, with an average of 1,049. Stoichiometric air-gas ratios ranged from 8.85 to 10.49, with an average of 9.61.
Delivered gas compositions will change over time as old wells go off line and new ones are connected to the supply system, so these 40-year old numbers are surely out of date. In fact, where pipelines draw from more than one gas field, it's not unusual to see small, day-to-day fluctuations. Most likely, though, overall averages haven't changed much.
The thought of daily fluctuations makes some people uneasy. Will burners flame out? Will the process be starved for heat? Will product quality be affected? The answers to these questions are "No," "No," and "Only in a few specific cases."
Industrial burners are designed for stable operation over a range of air-gas ratios -- they have to be because their manufacturers can't count on customers having exactly the same gas composition as they do. Besides, most industrial operations draw their gas from the same mains as homes and small commercial establishments. The distribution utilities can't allow gas properties to vary enough to cause home furnaces and water heaters to misbehave, so industrial operations are insulated from any drastic swings in gas makeup.
Heat input to the industrial process shouldn't be a concern, either. We're talking about a BTU value fluctuation of 1 or 2 percent, at most, and industrial process temperature control systems usually can compensate by increasing or decreasing the burners' firing rate to maintain the desired temperature.
Most products will be oblivious to small fluctuations in gas composition. There are a few areas, however, where the process or product might be affected. Processes depending on very accurate and repeatable flame dimensions -- glass fire polishing or some types of flame sealing, for example -- may suffer if the composition change affects flame length. Some types of liquid or powder coatings cured in direct-fired ovens may show subtle color shifts if the gas composition fluctuates enough, and the composition of synthetic furnace and oven atmospheres generated from natural gas will vary as the gas does.
There are two things to keep in mind. First, gas composition changes are usually very small over the short term, and their effects, if noticeable, are likely to be equally small. Second, most low temperature ovens and dryers are designed to operate with large amounts of excess air, so the operating burner ratio may be in the 50 or 60 to 1 range. With all that excess air diluting the combustion products, the impact of changes in the gas usually will be negligible.