FEATURE
Controls for burning solid wastes -
-
-
-
-
-
-
- _ _--
Both incinerator and pyrolysis methods require considerable instrumentation and alarms for proper processing of wastes
Richard F. Tor0 Norman J. Weinstein Recon Systems, Inc., Princeton, N.J. 08540 Modern thermal processing systems are becoming much more complex, incorporating energy recovery, resource recovery, air and water pollution control, or other features that require instrumentation to a degree greater than that previously required just for proper combustion control. Instrumentation is needed to: control and monitor the basic incineration or pyrolysis process control and monitor associated subsystems such as steam generation, power generation, and resource recovery control and monitor environmental subsystems such as flyash collection, wastewater treatment, and visible plume control protect equipment against corrosion, heat destruction, mechanical destruction, and operator abuse collect data for calculating disposal costs and charges, making improvements, and designing additional facilities. The degree of automatic versus manual control depends not only on technical considerations, but on capital and operating budgets, and personnel policies regarding the experience and qualifications of plant operators. In some cases, television monitoring, computer control and digital data acquisition systems may be justified. Monitoring instruments measure, indicate, transmit and record important process conditions, including flow, temperature, pressure, weight, position, time, speed, voltage, and composition. Controls change these conditions, either manually or automatically, in response to a signal from a measuring instrument. Control systems are necessary because, as in any process, many input factors are variable, but the end result must be the same. That is, for example, an incinerator should process refuse to a substantially reduced volume of sterile residue containing no putrescible materials without damaging the environment, the equipment, or personnel, regardless of the composition and wetness of the refuse, atmospheric conditions, time of year, equipment condition, or other vagaries. incinerator process instrumentation Effective combustion in incinerator furnaces requires the use of manual and/or automatic controls. Combustion is controlled by residence time, furnace temperature, degree of 428
Environmental Science & Technology
~~~~
-v
-
-
-
~
~
-
-
Refuse incineration, its instrumentation and controls Wind meed. direction
Pressure
1
Tempekature Temperature Flame safeguard Source: Honeywell
mixing and use of auxiliary fuel. Residence time in turn is controlled by grate speed, assuming adequate raw feed (limited by crane capacity in conventional incinerators). Furnace temperatures are controlled by the ratio of total air flow to refuse and by overfire and underfire air rates. Mixing is primarily a function of grate design, but some control is possible by variations in underfire air and mechanical grate action. Most of these variables are interdependent and a change in one is likely to require a change in another. Underfire air control is the primary source of oxygen for the refuse combustion. Requirements are affected by the nature of the refuse, including percent combustibles, density, moisture content, and are determined only by measuring performance (for example, visible appearance of burning bed, reduction in refuse volume, burnout). Insufficient underfire air will result in incomplete combustion; large excesses may increase flyash generation. Since continuous sensing and measurement of refuse characteristics is impractical, the underfire air can be controlled at a preset level by sensing the flow in a duct and adjusting flow by adjusting damper or fan speed. Overflre air control is the secondary source of oxygen for combustion, especially for oxidizing refuse decomposition products contained in the gases, and the primary source of dilution air for cooling. Requirements are affected by the nature of the refuse, but can be dynamically controlled by temperature sensing. Temperatures that are too low result in unburned gases and odors; furnaces or steam tubes can be damaged (by heat destruction and/or corrosion) if temperatures are too high. The overfire air can be controlled at a preset level, as with the underfire air, or it can be controlled by adjusting dampers or fan speed in response to temperature measurement. Overfire and underfire air ratio control is sometimes used. Draft control. Draft refers to the pressure distribution required for maintenance of the proper flow in the desired direction. A prime requirement is to keep the furnace under a slight vacuum to prevent the escape of hot gases and odor. In a modern incinerator, draft is maintained by the use of an induced draft fan that compensates for variations in overfire and underfire air by drawing outside air into and through the furnace. Induced fan operation can be preset or its speed can be controlled in response to a pressure measurement.
4. Secondary combustion chamber 5 Natural gas burners 6. Undertake air supply 7. Overfire air supply 8 Reciprocating grate
13. 14 15 16 17
Subsiden'ce chamber Breeching Dust Collector Induced draft fan Stack
Auxiliary burner controls are sometimes used as a source of additional heat to sustain combustion during startup, transient, and low-Btu refuse periods. Generally on-off burners are sufficient, but standard burner train controls are required for safety; for example, flameguard and purging systems. Burners are normally required for systems generating steam to sustain output when refuse is not available. Grate controls. The grates must provide solids movement, agitation, and passage for underfire air. Grate speed controls residence time, and it can also be used to control temperature. As many as four grate sections with separate speed controls have been used. Residence time requirements are affected by the nature of the refuse, but, as previously noted, the nature of the refuse cannot be continuously measured. Therefore, sufficiency of residence time (and temperature and turbulence) is judged by visually observing combustion and periodically measuring burnout. Grate speed and, if possible, bed depth and turbulence are manually adjusted accordingly. Feed conveyors or cranes are required to bring the refuse from a storage area to the combustion zone. Cranes are manually operated simply to keep feed hoppers full. Feed conveyors, where used, require speed control, obviously to coincide with grate speeds. Flue gas cooling control. In practice, such cooling reduces gas volume and protects downstream equipment, such as induced draft fans, flyash collection equipment, and the stack from heat damage. Cooling has most often been accomplished with water sprays. Dilution with cold air can be used but the increased gas volume adds greatly to the expense of air pollution control. The newest systems incorporate boilers for heat recovery. If water is inexpensive, and if the pollution control device performance is not inordinately temperature sensitive (for example, scrubbers), simple water flow controllers or even manual valves to control water to the sprays may be sufficient, if accompanied by temperature indicators and alarms. In many cases, as for cooling prior to electrostatic precipitators, more sophisticated temperature control is required, such as flow control of water rate in response to temperature measurements. Flyash recovery controls are essential, but vary with the type of pollution control system(s). For example, a Venturi scrubber may operate with a fixed amount of water, and an Volume 9, Number 5,May 1975
429
Revelant categories from ES&T Directory 1974-75 1. 2. 3. 4. 5. 6.
7. 8. 9. 9a 10. 11. lla 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Alarm units &warning devices Analyzers & monitors Balances & scales Combustible gas monitors Controllers Flow measuring equipment Gauges Gates & regulators Meters Moisture determination equipment Monitoring systems complete pH controllers, indicators Pitot tubes Pressure transducers & indicators Pump control equipment Pyrometers Recorders Regulators Smoke measuring instruments & suppliers Stack emissions monitoring TV inspection equipment & supplies Thermometers Timers, switches & controls Valve operators & controls Valves Water quality monitoring instruments
Instrumentation for various control steps
air flow within the limits set by the maximum induced draft fan capacity and by the necessity for maintaining sufficiently high flow and pressure drop to ensure efficient particulate control. One method of controlling Venturi scrubber operation uses an air bleed damper to maintain scrubber pressure drop. In addition to gas temperature control, an electrostatic precipitator normally requires controls for voltage, rapping, hopper temperature, and ash discharge. The control systems are usually supplied by the manufacturer. Wastewater controls are normally required for at least pH and discharge and recycle flow rates, and in some cases, temperature. These may be necessary to meet pollution regulations, scrubber and cooling requirements, and to minimize corrosion. Other wastewater properties are usually monitored by laboratory tests. 430
Environmental Science & Technology
Both indicating and recording instruments are required for supervision of operation, troubleshooting, and record keeping (see box, p 428). In addition, a laboratory is useful for refuse, residue, flyash, water, and wastewater analyses, and in some cases for specification and quality control of materials purchased. An incinerator instrument list (see Table 1) shows a rather extensive array of devices that can be used to achieve proper control, even without steam generation or resource recovery. Steam generation and resource recovery. The addition of a steam boiler to the incinerator increases the instrumentation requirements. Controls for feedwater treatment and flow, steam pressure, steam temperature (if superheated), steam drum level, steam flow, condensers, and appropriate safety devices are required. Tube metal temperatures are particularly important because of potential corrosion problems. Steam controls are required for soot blowers, which are usually specified for tube cleaning, and for pump and fan drivers where steam is used. Feed preparation and resource recovery, such as magnetic metal separation, shredding, air density separation, glass separation, aluminum separation and other new developments require very specific operational controls as well as conveying systems controls, fugitive particle control, and other controls somewhat akin to the mining industry. The use of pretreated refuse as a supplementary fuel in pulverized coal suspension-fired boilers for power or steam generation, now under development, will require feed preparation controls as indicated above. However, it is expected that the boiler controls will remain essentially the same, since refuse is expected to be limited to 10-20% of the total fuel requirements. Pyrolysis process instrumentation Pyrolysis processes are in various stages of development.
Because of the complexity of these processes, which involve combustible fuel products, the instrumentation and control requirements are likely to be quite extensive. Since all the processes are, at this time, proprietary in nature, little information is available on instrumentation. It is likely that the process owners will supply a design package including necessary instrumentation and controls. Feed preparation and resource recovery practiced with pyrolysis will likely be similar to that practiced with incineration. Some idea as to the potential complexity of the pyrolysis step can be realized by referring to typical instrumentation for chemical reactors and pyrolysis furnaces.
Types of instruments & controls Each parameter that is measured or detected can be indicated and/or recorded locally at the point of detection; transmitted to a remote indicator or recorder; or the measurement can be used to actuate local or remote control or alarm circuits. Numerous choices are available for each instrument function depending on the service required. For example, the choice of a temperature sensor depends on applicable temperature ranges, accuracy, physical size, stability, repeatability, response time, sensitivity, interchangeability, maximum distance to readout, and suitability for the control or alarm devices to be used. The application of various types of sensing and measuring devices to thermal processing equipment is outlined in Table 2. A few of the other instrumentation choices to be made are related to pneumatic vs. electronic controls; type, size, and speed of indicating and recording instruments; modes and mechanisms for control; remote vs. local instrumentation;
control room layout; and selection of mechanisms for the control action, including control valves, dampers, motor speed controls and the like. Pneumatic controls are much more widely used than electronic controls, but the increasing availability of electronic controls, and their adaptability to computers without transducers to convert pneumatic signals, may increasingly justify their use. Instrumentationdesign is the function of instrumentation specialists working for the design engineering organization, guided by the needs of the client and by the recommendations of the major equipment manufacturers.
Operational problems An instrumentation system that is carefully thought out and well designed with cost-effectiveness choices should result in a successful operating facility. However, proper installation, routine calibration, and maintenance are essential for accurate, safe, and reliable operation. Contract maintenance service should be considered if staff personnel are not appropriately qualified, but operators should be traihed for every day problems such as chart changing, inking, and part replacement. Enclosed or protected instruments will have less problems from dust, dirt, water, and abuse. Sensing devices will require protection in certain services such as in the combustion chamber and flue gas ducts. Consideration may be given to duplication of critical sensors and/or instruments. Pneumatic instruments require dry, clean air. Especially good drying is essential where air lines are exposed to ambient conditions in cold climates. Virtually all instruments and controls are directly or indirectly dependent on electrical power. The system Volume 9, Number 5, May 1975
431
NMCIITA1 MONl
should be "fail-safe" in the event of a power outage or other instrument failure. Of course, an inventory of ink, charts, and spare parts should be maintained and used. From an operator standpoint, simplicity and ruggedness are desired. Dampers, for example, should be capable of withstanding the abusive gases, have fine adjustment, lock in place, and have remote bearings, if possible. TV monitoring of unmanned and critical areas can reduce manpower needs and minimize down time, damage, and unsatisfactory performance. It has been used to monitor conveyors for jams, fire boxes and good combustion, automatic weigh scales, and stacks for smoke. Difficulties may be encountered in keeping camera lenses clean in dirty services; similar problems may be encountered with smoke meters. Air purging can help overcome this problem.
Future needs With the advent of complex sophisticated thermal processing systems-including steam generation, pyrolysis, resource recovery, and pollution controls-instrumentation and control systems needs become correspondingly more extensive. As in other process systems, TV monitoring and computer control should become useful tools. The most important single undeveloped area appears to be the control of parameters dependent on the nature of the refuse. For example, feed rate and underfire air cannot be automatically controlled because of a lack of a sensor for refuse moisture, Btu content and burning rate. Additional reading Liptak. B. G. Instrumentation in the Processing industries. New York, Chiiton Book Company, 1973,950 pp. Corey, R. C. Principles and Practices of Incineration. New York. Wiley-Interscience, 1969. pp 190-191. Niessen, W. R.. et al., Systems Study of Air Pollution From Municipal Incineration. Volume II. A. D. Little. Inc. Cambridge. National Technical Information Service No. PB 192-379, March 1970, p H-28-30. Proceedings, 1968 National Incinerator Conference, New York. American Society of Mechanical Engineers, May 5-8, 1968, pp 287-294 303-308. Proceedings, 1970 National Incinerator Conference. Cincinnati. American Society of Mechanical Engineers, May 17-20, 1970, pp 93-106 128-140: 141-148: 157-166. Special Studies for Incinerators for the Government of the District of Columbia. Day 8 Zimmerman, Philadelphia Public Health Service No. 1748. U.S. Department of Health. Education, and Welfare, 1968, pp ~~~
~~
.~~~
~~~~
29-37.
This article is based on work done by Recon Systems, Inc. under contract to the U.S. Environmental Protection Agency Office of Solid Waste Mariagement Programs Systems Management Division (Cantract No. 6; 8-03-0293). Richard k. ioro IS vice-presioent of Recon Systems, Inc., an environmental engineering consulting and testing firm. A chemical engineer by training, Mr. Tor0 is an industrial and governmental consultant on technical, economic and legal aspects of pollution control problems.
Norman J. Weinstein is oresident of Recon Systems, Inc. He has been a consultant to industry and government for nine years in chemical, petroleum, gasification, and combustion processes. Prior to consulting, Dr. Weinstein spent more than 10 years with Esso Research and Engineerlng Co. and played a key role in the development of the NOR fluidlzed solids process for direct reduction of iron ore. CIRCLE 2 ON READER SERVICE CARD
132
EnvironmentalScience &Technology
. .
