Automatic Combustion Control

Seven to ten percent excess air seems a realistic figure for the average industrial boiler. 

Air pollution legislation gradually prevents the flaring of process waste gases and liquids. Increasing requests are put forward to industrial boiler makers to destroy burnable waste by firing it in the furnaces simultaneously with the main fuel — either in dual fuel guns or in a separate burner. 

Waste quantities able to provide 50% of the load have been encountered. In examples like these, the combustion control system has to be adapted to insure safe, automatic operation. This boils down to adding a cutback system which automatically cuts back the waste flow if steam demand falls 

Combustion control on some packaged boilers is on a fuel and air flow differential and not on a fuel pressure and windbox-to-fumace differential, because the burner associated with these boilers operates with a variable damper position. With simultaneous dual fuel firing, flow-type combustion control is mandatory for both packaged and field-erected boilers. With some field-erected boilers, both flow and pressure/pressure differential systems are used for single string firing systems. 

In the flow case, the air register dampers should be automatically actuated for open/shut positions, because on losing one flame out of several, local air deficiency can occur. This cannot happen with the p/Ap control system. In the latter case an extra amount of air excess will result, which is not unsafe. In the p/Ap system, it is necessary to keep air registers in one position while firing, because Ap is used as an airflow indication. 

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This paper deals with real time energy measurement from the industrial gas users point of view. It illustrates how real time energy measurement by the user allows him to optimize his combustion process utilizing automatic combustion control and high speed measurement devices for feed forward and feedback systems. 

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. 

Automatic firing controls arranged to control the supply of fuel and air to the combustion appliance. A shutdown will occur in the event of one or more of the following occurring ... 

For gas heaters, intelligent firing systems are the way ahead, which means improved combustion control due to automatic monitoring of the combustion process and closed-loop control at all times. 

Flares ideally bum waste gas completely and smokelessly. Two types of flares are normally employed. The first is called the open flare, the second is called the enclosed flare. The major components of a flare consist of the burner, stack, water seal, controls, pilot burner, and ignition system. Flares required to process variable air volumes and concentrations are equipped with automatic pilot ignition systems, temperature sensors, and air and combustion controls. 

NFPA 85, Boiler and Combustion Systems Hazards Code, provides guidance for steam boilers and similar high reliability automatic combustion systems. In general, NFPA 85 combustion system control and safety instrumentation systems requirements exceed those defined in NFPA 86 and in API RP 556. 

It is not possible to operate on automatic temperature control on the wrong side of the point of absolute combustion. 

If you try to operate a furnace, fired heater, or boiler with too little combustion air to starve the burners of oxygen to smother or bog down the firebox, then you will likely cause afterburn or secondary combustion in the stack, you will not be able to operate on automatic temperature control, and may even destroy the equipment altogether. 

If we have the furnace on automatic temperature control while we are not using enough combustion air, and if the control valve on the fuel gas then opens to allow more fuel to the burners in order to either maintain or perhaps increase the furnace outlet temperature, the extra fuel will not burn efficiently. In fact, the extra fuel is likely to reduce the heater or furnace outlet temperature rather than increase or maintain it, as there is already a shortage of air and it cannot bum properly and tends to cool the firebox. The automatic temperature controller... 

Good J. (1994) Combustion control for automatic wood firings. In Advances in Thermochemieal Biomass Conversion. (Ed. by A. V. Bridgwater), Vol. 1, pp. 590-604. Blackie Academic Professional,... 

In most practical combustion installations there are two separate parts of the equipment system (1) the burner itself and (2) all of the peripheral equipment necessary to control the burner operation efficiently and safely. The control equipment includes fuel and oxidant pressure and flow controls automatic shutoff controls flame supervision equipment furnace purge equipment and other related devices. The safety issues of the burner are significantly different in character as compared with the safety issues of the control equipment. In many ways, these two safety issues are diametrically opposed. Within the burner, fire is a desired condition, whereas within the control system and surrounding environment, fire is to be avoided entirely. 

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. 

With internaJ-combustion engines, automatic transmissions are frequently used these are easily justified when vehicles must make many moves during the day. Smooth as is the control afforded by automatic transmissions, it is neveriheless inferior to that provided by electric trucks, especially those with solid-state controls. Gasoline and diesel power are also used, but mostly for outdoor equipment and very-heavy-duty units. 

The starting system can be manual, semiautomatic, or automatic, but in all cases should provide controlled acceleration to minimum governor speed and then, although not called for in the standards, to full speed. Units that do not have controlled acceleration to full speed have burned out first- and second-stage nozzles when combustion occurred in those areas instead of in the combustor. Purging the system of the fuel after a failed start is mandatory, even in the manual operation mode. Sufficient time for the purging of the system should be provided so that the volume of the entire exhaust system has been displaced at least five times. 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

Prevention of arson Control access at all times Screen employees and casual labour Lock away flammable substances and keep combustibles away from doors, windows, fences Provide regular fire safety patrols, even where automatic systems are provided Secure particularly storage and unmanned areas... 

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