Combustion with excess air

The only available small-scale system is a packaged two chamber incinerator with waste heat recovery. This technique is practical at the 25 to 100 tons per day (TPD) scale. In these units, partial oxidation occurs in the first section of the unit and causes a portion of the waste material to degrade and give off combustible gases. These gases, as well as products of combustion and particulate from the first chamber, flow to a second chamber where they are combusted with excess air and a natural gas or oil pilot flame. The combustion products then flow through appropriate heat transfer equipment to produce steam, hot water, or hot air. Today, four small cities and more than sixty industrial plants use the technique with heat recovery equipment. 

In the ATR, feed is partially combusted with excess air to supply the heat needed to reform the remaining hydrocarbon feed. The hot autothermal reformer effluent is fed to the shell side of the KRES reforming exchanger, where it combines with the reformed gas exiting the catalyst-packed tubes. The combined stream flows across the shell side of the reforming exchanger where it efficiently supplies heat to the reforming reaction inside the tubes. 

Sharp Fire. Combustion with excess air and short, hot flame. 

The analysis of the start-up procedure, as described in Section 5.1.3, was extended to the heat-exchanger and the shift reactor [152]. The reformer reactor was pre-heated by catalytic combustion with excess air, as described in Section 5.1.3, and then switched to the autothermal mode. As shown in Figure 5.58, the reformer itself could be heated to the operating temperature within 60 s, but the pre-heating of the two devices downstream of the reformer took about 6 min. An intermediate shift of the position of the maximum temperature within the reformer occurred, which was related to the introduction of cold steam into the device. 

To simplify calculations, but also by convention, the amount of excess reactant in a reaction is defined on the basis of the reaction going to completion for the limiting reactant. In the case of methane (CH4) burned with excess air, the volume of air needed to combust the methane is calculated as though there is complete combustion of the methane, converting it entirely to carbon dioxide and water. 

In the conventional gas turbine plant, a hydrocarbon fuel (e.g. methane CH4) is burnt, usually with excess air, i.e. more air than is required for stoichiometric combustion. 

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

Once a carbonaceous solid or liquid material is converted to a gaseous state, undesirable substances such as sulfur compounds and ash may be removed from the gas. In contrast to combustion processes, which work with excess air, gasification processes operate at substoichiometric conditions with the oxygen supply controlled (generally 35 percent of the... 

In contrast to combustion process, which works with excess air, the gasification process works on partial combustion of coal with the oxygen supply controlled (generally 20 to 70% of the amount of 02 theoretically required for complete combustion) such that both heat and a new gaseous fuel are produced as the coal is consumed. In simplest terms, the stoichiometric reactions are as follows ... 

Methane, also referred to as marsh gas, is a gas composed of carbon and hydrogen with a chemical formula of CH4. It is the first member of the paraffin or alkane series of hydrocarbons. It is lighter than air, colorless, odorless, tasteless and is flammable. It occurs in natural gas and as a by-product of petroleum refining. In atmospheric burning no smoke production normally occurs. In air methane bums with a pale, faintly luminous flame. With excess air carbon dioxide and water vapor is formed during combustion, with an air deficiency carbon monoxide and water is formed. It forms an explosive mixture with air over a moderate range. Its primary uses are as a fuel and raw feedstock for petrochemical products. 

A gaseous paraffinic hydrocarbon, CHj.CHj that is colorless and odorless and normally found in natural gas, usually in small proportions. It is slightly heavier than air and practically insoluble in water. When ignited in atmospheric burning it produces a pale faintly luminous flame with little or no smoke production. With excess air during combustion it produces carbon dioxide and water, with limited air supplies the combustion process will produce carbon monoxide and water. It forms an explosive mixture with air over a moderate range. 

Desulfurized feed is reacted with steam in the primary reformer (1) with exit temperature at about 700°C. Primary reformer effluent is reacted with excess air in the secondary reformer (2) with exit at about 900°C. The air compressor is normally a gas-driven turbine (3). Turbine exhaust is fed to the primary reformer and used as preheated combustion air. An alternative to the above described conventional reforming is to use KBR s reforming exchanger system (KRES), as described in KBR s KAAP plus. 

However, the main problem with excess air is that it introduces oxygen into the flue gas. Typical flue gas concentrations for natural gas and coal combustion in power production are given in Table 6.3. 

Step 15. In the indirect contact furnace, natural gas containing 92.5 mole% methane, 4.8 mole % ethane, and the remainder propane is burned completely with excess air (combustion air). The dryer air feed stream (the combination of makeup dryer air and humid air recycled from the dryer outlet) passes through metal channels within the furnace combustion zone and then out of the furnace to the rotary dryer air inlet. The hot gases in... 

There is concern about the levels of carbon dioxide emissions from cars, as well as the three main pollutants from combustion HC, CO, and NO. Lower CO2 emissions result from improved fuel economy that can be obtained in several ways. An important, more fuel efficient approach is through the use of so-called lean-bum engines that operate with excess air rather than with stoichiometric air-fuel mixtures. 

This problem can be circumvented in a fuel-rich approach to catalytic combustion for gas turbines recently proposed. In this method fuel is mixed with air to form a fuel-rich mixture that is reacted over the catalyst to produce both partial and total oxidation products. The reaction products are then mixed with excess air and burned in a homogenous flame. Because the gases exiting the catalyst are fuel-rich, they cannot sustain combustion in the event of a homogenous flame backup. The promise of this method needs to be confirmed in full-scale turbine tests. 

Needless to say, the focal point of an entire carbon black plant is the reactor. The basic conversion process is universal in that all reactors utilize the concept of burning a fuel with excess air to completion, atomizing oil feedstock into the hot products of combustion for thermal decomposition, and quenching the reaction with direct water sprays. 

The CO and inert gas mixture leaves the vessel approximately at the bath temperature. The CO mixture exiting the vessel is mixed with excess air in order to fully bum the CO to CO2 very early in the exhaust duct. This is done to prevent the presence of combustible or e q)losive mixtures persisting downstream into the duct to the filter equipment. 

When systems operate with excess air that results in stack O2 above 7% d.v., the CF is less than 1. If the system operates with stack O2 less than 7% d.v., the CF is greater than 1. This impacts systems that dilute the combustion gases with excess air. This CF is used when air (21% O2 by volume) is used as the combustion air gas. 

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