Combustion Air Control

Alternative configurations are possible which omit either of the fans. If both fans are omitted then recovery of heat from the flue gas is not possible using the type of air preheater shown. Control of air flow on such natural draught heaters is by manipulation of the stack damper, although only if this does not violate the need to keep firebox pressure negative. 

However, it is undesirable to opwate with excess air. Since air enters the heater at ambient conditions and leaves at flue gas temperature any unnecessary air increases the amount of fuel that is required to achieve the desired outlet temperature. Ideally we would like to maintain a stoichiometric air-to-fiiel mixture. In practice we need to provide a small amount of excess air to compensate for incomplete mixing and to ensure that fiill combustion has taken place before the products of combustion leave the firebox. 

The relationship between O2 and excess air can be developed by examining the chemical equations of combustion. If we first consider pure hydrogen then 

From Table 10.1 we see that air is 20.95 % O2. For simplicity let us assume the remainder 

On a dry basis, where the water remains as a vapour, the molar concentration of O2 in the flue gas is given by 

---

The settings of the combustion air control devices, flue gas dampers etc. during the test cycle have to be in accordance with the manufacturer s instructions. 

Therefore further experiments with a 250 kW Fan Conveyor Fed Furnace have been carried out with the aim of investigating the possibilities of NOx reduction by combustion air controlling. The investigations have been carried out with chipboard chips, chipboard sawdust, and spruce wood chips as fuel. 

Control Devices. Control devices have advanced from manual control to sophisticated computet-assisted operation. Radiation pyrometers in conjunction with thermocouples monitor furnace temperatures at several locations (see Temperature measurement). Batch tilting is usually automatically controlled. Combustion air and fuel are metered and controlled for optimum efficiency. For regeneration-type units, furnace reversal also operates on a timed program. Data acquisition and digital display of operating parameters are part of a supervisory control system. The grouping of display information at the control center is typical of modem furnaces. 

Control Systems. Control systems are used to regulate the addition of Hquid waste feed, auxiHary fuel, and combustion air flows to the incinerator furnace. In addition, scmbber operation is automated to help ensure meeting emission limits. Flows are measured using differential pressure... 

Ammonia vapor is mixed with air and converted into nitrogen oxide at an elevated temperature in the presence of a catalyst, which generally contains noble metals such as platinum and rhodium. The optimal gauge temperature is maintained by controlled ammonia and combustion air preheating. The reaction is highly exothermic ... 

Enclosed flares are composed of multiple gas burner heads placed at ground level in a staeklike enclosure that is usually refractory or ceramic lined. Many flares are equipped with automatic damper controls that regulate the supply of combustion air depending on temperature which is monitored upstream of the mixing, but inside the staek. This class of flare is becoming the standard in the industry due to its ability to more effectively eontrol emissions. Requirements on emissions includes carbon monoxide limits and minimal residence time and temperature. Exhaust gas temperatures may vary from 1,000 to 2,000 F. 

Combustion air blower located at the base of the flare stack, complete with the electric motor drive and proper control system. 

In large industrial installations, the latter is the most common arrangement. The burner has a profile plate that controls the rate of combustion air. The warm air delivery fan may be either centrifugal or axial. 

The air paths within these baffles are arranged to provide the correct amount of combustion air. Turndown ratios of up to 35 to 1 are obtainable, providing the correct gas flow for air temperature control. 

The advantages of thermal incineration are that it is simple in concept, has a wide application, and results in almost complete destruction of pollutants with no liquid or solid residue. Thermal incineration provides an opportunity for heat recovery and has low maintenance requirements and low capital cost. Thermal incineration units for small or moderate exhaust streams are generally compact and light. Such units can be installed on a roof when the plant area is limited. = The main disadvantage is the auxiliary fuel cost, which is partly offset with an efficient heat-recovery system. The formation of nitric oxides during the combustion processes must be reduced by control of excess air temperature, fuel supply, and combustion air distribution at the burner inlet, The formation of thermal NO increases dramatically above 980 Table 13.10)... 

A fireball is assumed to bum with a constant temperature in the isothermal fireball model of Lihou and Maund (1982). Combustion is controlled by the supply of air and ceases after a time which is correlated empirically with the mass of flammable gas in the initial vapor sphere. It is assumed that a fraction (1 — /c) of the fuel is used to form soot, and the remaining fractionbums stoichiometrically, producing an increase of /ij moles per mole of flammable gas. The stoichiometric molar ratio of air to flammable gas is p, and dVIdt is the volumetric rate of air entrainment. The rate of increase of volume can now be written as ... 

A boiler house enclosed eight large boilers attended by two men. Failure of the combustion air supply shut down one of the boilers. This boiler shutdown created conditions beyond the control of just two men and lack of proper combustion control equipment finally caused seven of the eight boilers to shut down. Amid the confusion caused by low instrument air... 

The discussion below will focus briefly on the design of the graphic displays in order to illustrate the methodology used. The aim of the furnace operation (see Figure 7.15) is to achieve a specified output temperature of the crude oil. This is done by means of a master temperature controller which regulates the pressures of the fuels used. An air/fuel ratio controller regulates the flow of the combustion air, receiving as input the flow rates of the fuels... 

Ash particles produced in coal combustion are controlled by passing the flue gases through electrostatic precipitators. Since most of the mass of particulate matter is removed by these devices, ash received relatively little attention as an air pollutant until it was shown that the concentrations of many toxic species in the ash particles increase as particle size decreases. Particle removal techniques become less efiective as particle size decreases to the 0.1-0.5 pm range, so that particles in this size range that escape contain disproportionately high concentrations of toxic substances. 

---