Carbon Monoxide Emission Control with Radiation Pyrometer

Conference paper

AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii Carbon Monoxide Emission Control with Radiation Pyrometer Donald L Corwin PE Therm-A-Cor Consulting Abstract Two stage medical waste incinerators are operated under a wide range of conditions with unknown waste feed characteristics. With only the weight of each waste charge into the incinerator, the total heat content of each change cannot be known until the material burns. With the very tight CO (carbon monoxide) emission rates for the medical waste incinerator, a method to rapidly respond to changes in the heat released of each charge needed to be implemented. Methods of improving the incinerator response to variations in the waste are critical to assure that the operational limitations of the unit are not exceeded. The current EPA (Environmental Protection Agency) regulations limit the CO emission rate to 11 ppmdv at 7% oxygen for medical waste incinerators. Prior to the latest regulatory changes, the regulatory CO emission rate was 40 ppmdv@7%. To maintain control of the medical waste incinerator emissions, air/oxygen must be added when the CO spikes to assure proper oxidization of the CO. A rapid temperature measurement system must be instituted to allow the control system time to react to the process changes that generated the CO spikes. The CO spikes are also indicative of spikes in other PICs. Radiation pyrometers provide the rapid temperature measurement capability. They are widely utilized on process with constant gas composition. A review of the installation and control results with the installation of a pyrometer on a two stage medical waste incineration is provided. Summary The regulatory requirements for medical waste incinerator facilities rely on the reliable and safe operation of the combustion process. The combustion process is the starting point for the destruction of the pathological and chemical medical waste materials. The combustion control system is needs to be able to respond to all types and characteristics of medical waste. Medical waste incinerators can be a rotary kiln or a two stage system. This evaluation focuses on the two stage incinerator handling continuous feeding of medical waste. The first stage serves to convert the medical waste to an organic 1 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii laden gas stream. The second stage is the main combustion section which fully oxidizes the organic materials generated in the first stage. All gases generated in the first stage must be fully combusted by the second stage to assure compliance with the emissions regulations set forth by EPA. For some waste feed batches, the first stage fumes will vary significantly in organic and water composition and flow rate over a short period of time. This rapid rate of change can produce large changes in the required response by the control device. With the reduction in the general firing rate, the medical waste incinerator response to changes in fume rate has been amplified. Sometimes, limitations in the control device response will place operation limitations on the process to assure the control device will function within it regulatory and operational limits. Operational tests were performed using radiation pyrometer of varying bandwidth response on an oxidizer system that is subject to large variations in its products of combustion. The tests demonstrated the improved response factor of the new radiation temperature measurement device. The tests also demonstrated the improved control that the oxidizer could achieve with the new temperature measurement device. Regulatory Limitations The medical waste incinerator is composed of two combustion chambers, and air pollution control equipment. The current regulatory requirements for the CO emissions from the incinerator are 11 ppmdv (parts per million, dry, by volume) corrected to 7% oxygen. This is a very low value. Currently the regulations require hazardous waste incinerators to have CO emission rate of approximately 17 ppmdv@ 7%. Initially good combustion was defined as operating with a CO emission level of 100 ppmdv@ 7%1. This was based on the incinerator studies of the 80's and 90's. The current regulatory limits are based on MACT (Maximum Achievable Control Technology). This method of defining the emission limits significantly reduce the allowable emissions from incinerators to the values listed above. The MACT system uses statistical evaluations to pick the lowest 12% of a specific compound from the sample group. This becomes the standard for all of the systems. Previously dispersion analyses were performed on the emissions to determine the emission limit that was deemed safe for the local population. This emission rate was based on a person staying at the same location for many years (60 or 70) and an associated cancer risk of a very low rate per million persons. Report on the Products of Incomplete Combustion Subcommittee of the Science Advisory Board, EPA-SAB-EC-90-004, 1989 1 2 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii MACT statistical evaluation of emissions from medical waste incinerators superseded that evaluation. Emission rates are now based on reported emissions during test burns without relative consequences or impact of the specific emission source. The CO emission limit for all large medical waste incinerators is 11 ppmdv@ 7% was imposed on all large medical waste incinerators. Additional information can be obtained on the EPA web site. Waste Description To comprehend the operational characteristics of the medical waste incinerator, the waste stream must be understood to the extent possible. Waste is placed into the primary in small batches ranging from 50 to 150 pounds per charge. Individual feeds of medical waste cannot be examined to determine its composition. The medical waste is received in sealed boxes, tubs, bags and other containers. The material in these containers cannot be examined because there may be pathological, infectious, or otherwise harmful materials in the container. The waste material must be incinerated to assure complete destruction of the potential hazards contained within each container. The medical waste materials will vary in composition relative to the energy in each container. Some of the containers will be contain garments and clothing that have residue of pathological or biological materials. Some of the containers will be benign materials such as saline solution, empty pill bottles, plastic containers, medical instruments that can be used only once, plastic hoses and anything that would be present in the hospital or doctor's office! Others may be the result of chemotherapy process. Many of the chemotherapy processes will contain alcohols of various concentrations or water. Thus the feed stream may be energetic or non-energet6ic depending on what was placed in the waste bag that could not be opened for review. System Description 3 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii The medical waste incinerator is typically composed of a two chamber combustion section. The incinerator is maintained under a negative pressure by an induced draft fan at the stack. The first combustion section is designated the primary chamber. This chamber operates under negative pressure and at a substoichiometric condition. The operation at a sub-stoichiometric condition is designed to limit the rate of organic gas production in the chamber. As such, the gases Figure 1 Typical Medical Waste Incinerator exiting the primary chamber will be fuel rich and oxygen deficient. This gas must be fully burned in the secondary, almost instantaneously, to assure total destruction of the organic content. Sufficient oxygen must be appropriately available to properly oxidize the organic gases exiting the lower chamber. Medical waste containers are fed into the primary chamber every 5 minutes on this unit. The waste charges are fed through a door that is open to the atmosphere. This process allows significant changes in the combustion process due to the sudden opening and closing of entry door. This door is called the fire door. The second chamber is typically mounted above the primary chamber. All of the gases generated in the primary pass through the secondary. As the gases enter the secondary, additional air and a stable ignition source are provided to assure proper ignition and combustion of the gases generated. The oxygen and temperature of the secondary chamber are controlled Figure 2 T/C Temperature Trends with the addition of secondary air. The primary temperature floats within a bound. The typical temperature trend for the primary and secondary are provided in Figure 2. Figure 2 Thermocouple Values 4 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii Since the primary chamber is maintained under a negative pressure, the opening and closing of the fire door significantly changes the pressure drops in the system. Opening the fire door will allow sudden increases in the air flow rate into the primary chamber. This increase in flow rate will radically change the primary chamber combustion characteristics. The door opening and subsequent closing takes approximately 5 seconds each. The temperature spikes seen in Figure 3 are mainly the result of change in system pressure and air flow rate as a result of opening and closing the fire door. Some of the spikes are the result of high BTU liquids Figure 3 Oxygen and Carbon Monoxide (alcohols) in the waste. No screening of the waste can be performed due to the health risks involved. Process Control The fumes generated by the primary chamber will contain significant quantities of organic gases. The actual composition of the gas cannot be determined since it changes with each charge. These gases when subjected to the oxygen in the air at the elevated temperatures will burn. But the correct amount of air in the secondary varies as the waste composition changes and as the fire door opens and closes. Large spikes in oxygen generally lead the corrected CO values. As is shown in the Figure 3, the CO spikes can be large. The spikes shown in this graph are well in excess of the 11 ppm rolling average as required by the regulations. Temperature Measurement The typical thermocouple controlled oxidizer response to the changes in fume composition on a shorter time period than is seen in the CO spikes. With the delay associated with a controller, the large CO spikes were generated by the system. A pyrometer was installed to improve the temperature response and Figure 4 Radiation Wavelength 5 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii therefore the air addition of the unit at the appropriate time. In a previous paper2, a comparison was made between a thermocouple, thermopile pyrometer and a narrow band pyrometer. A new narrow band pyrometer was installed on the medical waste incinerator to provide rapid temperature response. The carbon dioxide and water vapor were monitored by the narrow band pyrometer. As is shown in Figure 3, the wavelengths between 2 and 3 microns have overlap to allow for reasonable measurement of the gas temperatures simultaneously. This proved to be a better temperature measurement unit. The response of the narrow band pyrometer experienced some degradation with time and gas composition. The main cause for the degradation is water condensation and dust accumulation on the lens. Subsequently a wide band pyrometer, with a filter allowing wave lengths similar to the thermopile was tested to improve the operational performance. Gas Composition Figure 5 Carbon Dioxide The absorbance and emittance of these two gases have peaks in the 2.0 to 3.0 µm range as is shown in Figure 5. Therefore variations in the water and carbon dioxide concentrations will the temperature response of the pyrometer. Operational data from incinerator system had a wide variation in the water concentration. The waste fed into the unit on each individual charge could be alcohol or water or a combination some form. The carbon dioxide concentration can vary from 3 % up to 9% over a one hour period. During this period, the water concentration varies from 30 to almost 75%. These wide variations in the hot gas concentration will radically change the emissive properties of the gas. But by concentrating on the 2 to 3 micron wavelengths, the total temperature variation was acceptable. This allows for the controller to be tuned to the 1 second response time of the pyrometer. Temperature Control Corwin, D.L., "Rapid Process Control with Radiation Thermocouple", Incineration and Thermal Treatment Conference, Phoenix, AZ, May 2004. 2 6 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii The oxidizer used in this study is controlled to always be above a specific operational temperature. The temperature spikes are controlled by adding secondary air. The rapid temperature changes as monitored by the pyrometer require a very fast response by the high gain temperature controller on the secondary air supply. The pyrometer recorded larger changes in temperature and generally led the thermocouple. As is seen in Figure 6, the magnitude of the temperature spikes is more evident in the pyrometer temperature Figure 6 Pyrometer and T/C measurement even after the additional quenching from additional air be added quicker. The thermocouple had a generally higher temperature to the radiation component from the flame located in the front of the secondary chamber. System Control The system control was modified and several secondary changes were made to control components. Figure 7 provides the initial CO trend with the rolling average. After inclusion of the pyrometer and tuning, the CO rolling average stayed below 4 ppm as is provided in Figure 8. Conclusions The operation of incinerators and oxidizers generates a wide variation in the products of combustion gases. The carbon monoxide concentrations Figure 7 Initial CO Trend will be directly impacted by the variation in oxygen and temperature of the unit. The installation of a radiation pyrometer provides almost instantaneous temperature readings. The instantaneous temperature change in a well-mixed system is an indication of rapid change in organic content. This must be compensated by a rapid change in the oxygen to properly combust the organic material. This needed response time is significantly faster than that of ceramic or metal sheathed thermocouples. This quicker response time can allow for more accurate process control and thereby prevent extreme process excursions. However, a potential problem may occur 7 Corwin AFRC General Meeting 12 & 13 September 2016 Kauai, Hawaii when the water vapor and carbon dioxide concentrations vary widely. The increase in one component will reduce the concentration of the other component. The radiation bandwidth viewed by the radiation pyrometer must be sufficiently wide to allow for all variations in the gas composition. Tests indicate that a bandwidth on the order of 2.0 to 3.0 µm provides a excellent response when the gas Figure 8 Pyrometer CO Trend composition varies widely. The degradation observed in a narrow pyrometer has not been observed to date in the wide bandwidth unit. Thus care should be exercised when incorporating the radiation pyrometer into the control system to ensure that it is appropriately matched with the anticipated range measured gas composition. 8 Corwin