Stoichiometric combustion is the hypothesized point at which the fuel-to-air ratio is optimum, resulting in complete and efficient combustion. Although stoichiometric combustion is not achievable, all combustion processes seek towards it in order to optimize profits.
Fuels
Coal, Oils (#2, #4, and #6), Diesel Oil, Gasoline, Natural Gas, Propane, Coke Oven Gas, and Wood are among the most often utilized fuels in combustion processes around the world. Each fuel has its own chemical properties, such as a distinct C/H2 ratio and calorific value, among others. The amount of combustion air needed to thoroughly burn a particular fuel is determined by those parameters, particularly the C/H2ratio. To ensure complete combustion, the higher the carbon content of the fuel, the more air is required. It is critical to know the fuel being burned while monitoring the efficiency of a combustion process, as this information will help not only establish the boiler’s best working conditions, but also maximize the boiler’s efficiency.
Coal
Anthracite, bituminous, sub-bituminous, and lignite are the most common types of coal used in combustion processes around the world. Due to the extremely high quantities of carbon in coal, a significant amount of carbon dioxide is produced when it is burned; because carbon requires more oxygen to burn, coal requires more combustion air than other fossil fuels.
Other pollutants produced by coal combustion include NOx, sulfur dioxide (SO2), sulfur trioxide (SO3), and particle emissions, in addition to carbon dioxide. Sulfur dioxide reacts chemically with air vapor to make a weak form of sulfuric acid, which is one of the main sources of acid rain.
Oil
Oil fuels are mostly made up of a blend of highly heavy hydrocarbons with higher hydrogen content than coal. At the same time, because oil contains less carbon than coal, it takes less combustion air to complete the combustion process. As a result, while burning oil emits less carbon dioxide than coal, it emits more carbon dioxide than natural gas. The majority of pollutants produced by coal combustion are likewise created by oil combustion.
Natural Gas
Because of its low carbon content and high hydrogen content, natural gas uses substantially less air in combustion. Natural gas combustion is cleaner than oil and coal combustion. When gas is burned with insufficient combustion air, volatile hydrocarbons might form, posing a safety risk; caution should be exercised to avoid harmful situations.
Natural gas combustion creates fewer greenhouse gases, which are thought to be one of the primary causes of global warming. Burning natural gas creates 30 percent less carbon dioxide than burning oil and 45 percent less carbon dioxide than burning coal in equal proportions.
Gas combustion produces NOx emissions in addition to carbon dioxide, but sulfur dioxide (SO2) and particle emissions are insignificant.
Other fuels, such as wood, diesel, gasoline, propane, butane, and biofuels like ethanol, all have different combustion qualities that affect the process’ efficiency and emissions.
How much air is necessary for natural gas combustion?
In any combustion process, there is a balance between squandering energy by utilizing too much air and wasting energy by running too richly. The optimal combustion efficiency occurs when the air-to-fuel ratio is optimized, and managing this ratio ensures maximum efficiency. In most cases, a liquid or gas fuel burner achieves this desirable balance by running at 105 to 120 percent of the theoretical air. The stoichiometric air required for natural gas-fired burners is 9.4-11 ft.3 / 1.0 ft.3 of natural gas, or a gas-to-air ratio of around 10:1. In this instance, there is a 2% surplus oxygen level.
Excess air is difficult to measure in the combustion zone. Oxygen analyzers, on the other hand, can easily measure it in the stack. It would correlate to a 1% to 3% oxygen reading in the stack when running with 5 percent -20 percent surplus air.
At changing operating loads, the optimal air-to-fuel ratio will change. Tuning is the process of determining the ideal air-to-fuel ratio under a variety of operating situations. When evaluating parameters in the stack, such as temperature, oxygen content, carbon monoxide, and NOx emissions, this can be performed.
In part three of this five-part series, we’ll look at how to enhance the combustion efficiency of industrial boilers, steam generators, furnaces, ovens, smelters, and process heaters by monitoring flue gas oxygen and combustibles, as well as altering air and fuel pre-combustion flows.
Why is it necessary to avoid a high amount of surplus air in a gas calorimeter?
Many industrial furnace operators are wasting a lot of energy because there is too much air in the furnace, which causes heat loss through flue gases. Excess air produces oxygen that isn’t burned during combustion, which absorbs otherwise useable heat and transports it out of the stack. The chemically optimal amount of air that enters a furnace is just enough to consume all of the oxygen in the air. However, because fuel and air do not entirely mix, a certain quantity of extra air will always be required for complete combustion. This ideal (known as the stoichiometric air-to-fuel ratio) is difficult to achieve. In reality, insufficient extra air leads to inefficient fuel combustion, soot formation, and wasteful greenhouse gas emissions.
Extra air levels will vary depending on the furnace and application, but in general, 10-15% excess air is an achievable, optimum aim while keeping the existing input temperature or production output level, whichever is required. If your furnace has more than 10-15% surplus air, you have a clear chance to reduce your energy expenditures by limiting air input at the burner and sealing any furnace leaks. When the air/fuel ratio is tuned, the resulting energy savings typically vary from 5% to >25%.
Analyzing the amount of oxygen in the flue gas might reveal how much surplus air is present in the system. As an example, consider natural gas combustion. The oxygen content of flue gas can be assessed in two ways: dry reading A percent or wet reading B percent, assuming the CO level in flue gas is very low and incomplete combustion can be ignored. The following formulas can be used to compute extra air using those measurements: If the oxygen dry reading in the flue gas is 2.5 percent, the excess-air calculation is 0.895 x 0.025 / (0.21-0.025) = 12.1 percent excess air. Excess air causes the flame temperature to drop. As a result, less heat enters the system. Furthermore, excess air must be heated to flue gas temperature, which requires additional energy. Using an excess air chart (Figure A), you may figure out how much more available heat you can get by reducing excess air to the smallest amount that still allows complete combustion in your furnace. The quantity of heat remaining in the furnace (i.e., not lost by flue gases or leakage) as a fraction of the heat input is commonly given as a percentage.
You can then calculate how much money you’ll save by reducing superfluous air and increasing accessible heat. Excess air, combustion air temperature, flue gas temperature, fuel cost, and other factors influence total energy savings (Figure B).
What happens when there’s too much air in a combustion?
With more excess air delivered to the combustion chamber, the velocity of air increases, resulting in a decrease in moles of CO 2 due to decreased particle residence time in the combustion chamber at higher excess air factors. With increasing particle size, char combustion decreases.
Is natural gas more or less dense than air?
Natural gas is lighter than air, therefore when it is discharged, it quickly dissipates into the atmosphere. When natural gas is burned, it produces a high-temperature blue flame and complete combustion, which results in just water vapor and carbon dioxide. Its heating value per cubic foot is around 1000 BTUs.
What does “extra air” imply?
The amount of air over the stoichiometric requirement for full combustion is referred to as the percentage of extra air. The amount of oxygen in the incoming air that is not consumed during combustion is known as excess oxygen, and it is proportional to the percentage of extra air. When using natural gas, for example, 15 percent surplus air equals 3 percent oxygen.
What does “extra air” imply? What is the relevance of having too much air?
Excess air is air given in excess of the theoretical amount required for full combustion of all fuel or combustible waste material present.
What exactly is the extra air factor?
An incineration process’ surplus air coefficient is defined as the ratio of the amount of supplied air Mair (oxygen) to the stoichiometric amount of air MStoich (oxygen) required to ensure complete oxidation of the waste components: (16.11)
Why is there so much air delivered to the burners of all gas-burning appliances?
Why do all gas-burning appliance burners get too much air? In the mixing tube chamber, air and gas never completely combine. Extra air is pumped in to ensure that the methane burns fully.
What happens if the furnace receives too much extra air during the coal combustion process?
What happens if the furnace receives too much extra air during the coal combustion process? Explanation: In most furnaces, 50 to 100 percent more air is given than is required. There is a loss of heat in the furnace if the surplus air is provided above this quantity.