Ex10 VOC Thermal Oxidation 2024-2025 PDF

10 - VOC thermal oxidation Air Pollution and Control Engineering VOC THERMAL OXIDATION (BRING THE REPORT OF THE SOLUTION TO THE ORAL EXAM) (No excel files are admitted to the oral exam. The results should be discussed...

10 - VOC thermal oxidation Air Pollution and Control Engineering VOC THERMAL OXIDATION (BRING THE REPORT OF THE SOLUTION TO THE ORAL EXAM) (No excel files are admitted to the oral exam. The results should be discussed and synthesized by comments, tables and figures.) Introduction Thermal oxidation is the process of oxidizing combustible materials by raising the temperature of the material above its auto-ignition point in the presence of oxygen and maintaining it at high temperature for sufficient time to complete oxidation to carbon dioxide and water. Time, temperature, turbulence (for mixing), and the availability of oxygen all affect the rate and efficiency of the combustion process. These factors provide the basic design parameters for oxidation systems that are used to destroy mainly organic compounds classified as volatile organic compounds (VOCs), or generic hydrocarbons in the vapor phase in the flue gas, and carbon monoxide. Oxidation systems provide high-efficiency destruction of a wide variety of organic compounds, regardless of the inlet concentrations of the contaminants. The supplemental energy requirement needed to maintain the necessary operating temperature decreases as the contaminant concentration increases. Incomplete combustion of many organic compounds may create additional pollution problem because of incomplete combustion products such as aldehydes and organic acids. Furthermore, the oxidation products of organic compounds containing chlorine, fluorine, or sulfur include hydrochloric acid, hydrofluoric acid, sulfur dioxide etc. These unwanted pollutants may require additional removal units (e.g., scrubbers) prior to release into the atmosphere. For safety considerations, the maximum concentration of the VOCs in the waste gas must be substantially below the lower explosive limit (LEL) of the specific compound(s) being controlled. As a rule, a safety factor of four (i.e., 25% of the LEL) is used, although some direct-flame oxidizers can operate safely above this level. The waste gas may be diluted with ambient air, if necessary, to lower the concentration. The required level of VOC control that must be achieved in the time that the waste gas spends in the thermal combustion chamber dictates the reactor temperature. The shorter the residence time, the higher the reactor temperature must be. Most thermal oxidizers are designed to provide no more than one second of residence time to the waste gas with typical temperatures of 650°C to 1100°C. The required reaction temperature is a function of the particular gaseous species and the desired level of control. Even if some authors report lower average temperature ranges for normal operation (hydrocarbons: 510-760 °C, carbon monoxide: 680-790 °C, odor control 480-700 °C), to ensure 98% destruction of non-halogenated organics, thermal oxidizers generally should be run at 870°C with a nominal residence time of 0.75 seconds (guideline values). Destruction of halogenated organics may require an oxidation temperature closer to 1100°C and, most likely, a post-oxidation water or caustic scrubber to remove highly corrosive acid gases (e.g., HCl). The contaminated gas stream is almost always preheated prior to entering the oxidizer. For this reason, oxidation systems are practically independent of the temperature of the contaminated gas stream. Thermal oxidation systems are generally capable of treating contaminated gas streams Air Pollution and Control Engineering – A.Y. 2024-2025 1  10 - VOC thermal oxidation containing particulate matter. However, the particulate matter can deposit in the heat exchangers, which reduces the heat exchange efficiency and eventually plugs the system. Accumulated particulate material in heat exchangers must be removed on a routine basis to provide the necessary gas flow rates and to reduce the vulnerability of the system to bed fires caused by the ignition of these materials. There are three basic types of high temperature thermal oxidation systems: direct flame, recuperative, and regenerative. - Direct flame systems rely on contact of the waste stream with a flame to achieve oxidation of the VOCs. These systems are the simplest thermal oxidizers and the least expensive to install but require the greatest amount of auxiliary fuel to maintain the oxidation temperature, thus entailing the highest operating cost. These systems are useful for destruction of intermittent streams. - Recuperative thermal oxidation systems use high efficiency metal, air-to-air tube or plate heat exchangers to preheat the effluent stream prior to oxidation in the combustion chamber. Thermal energy recovery efficiencies up to 80% are common in commercial use (typical 50%-80%). Supplemental fuel is usually required to maintain a high enough temperature for the desired destruction efficiency. Recuperative systems are more expensive to install than direct thermal oxidizers but have lower operating costs. - Regenerative thermal oxidation systems typically incorporate multiple ceramic heat exchanger beds to produce heat recovery efficiencies as high as 95%. An incoming gas stream passes through a hot bed of ceramic or other material, which simultaneously cools the bed and heats the stream to temperatures above the auto-ignition points of its organic constituents. Oxidation thus begins in the bed, and is completed in a central combustion chamber, after which the clean gas stream is cooled by passage through another ceramic heat exchanger. Periodically the flow through the beds is reversed, while continuous flow through the unit is maintained. Regenerative thermal oxidation systems are the most expensive thermal oxidizers to build, but the added capital expense is offset by savings in auxiliary fuel. Design principles Basic equations for the design of a high temperature thermal oxidation system are energy and mass conservation and the first order kinetics for the removal of the pollutants. Results of the design procedure are the geometrical dimensions of the system, the process operational parameters (temperature and residence time) and the required auxiliary fuel and combustion air flow rates. Following are the main steps in the design process: 1. Assume an acceptable process temperaturei in the combustion chamber Tp (°C) compatible with the pollutant remove (see introduction for typical ranges) and calculate the mass flow rates of combustion air m a , of auxiliary fuel m f and of flue gas produced during combustion m p by solving the following equation system: Energy balance for the combustion chamber Mass balance for the combustion chamber Combustion air/auxiliary fuel mass ratio 2. Assume a residence time of the gases in the chamber (typically 0.2-2 s, possibly no more than 1 s - see introduction) compatible with the pollutant to removal and verify that for the assumed i READ ME Process temperature is usually set 110-170 °C higher than the VOC autoignition temperature (for the concurrent removal of multiple contaminants should exceed the highest autoignition temperature involved) and should be in any case higher than 650°C for practical purposes. See introduction for typical ranges. Air Pollution and Control Engineering – A.Y. 2024-2025 2  10 - VOC thermal oxidation process temperature the residence time in the chamber is equal or higher than the reaction time determined based on the kinetic considerations and the required efficiency. 3. Estimate the volume and dimensions of the combustion chamber based on the residence time in the oxidation chamber, the flue gas actual flow rate and geometric considerations such as the chamber length-to-diameter ratio. Proper turbulence in the entrance of the chamber will be guaranteed by a proper inlet velocity. (see following sections for details). Figure 1. Energy and mass balance scheme for direct fired thermal oxidizer 1. Mass flow rate estimation Mass balance for the combustion chamber The mass balance for the combustion chamber (Figure 1) expresses the conservation of mass between mass flow rates at the inlet and the outlet of the chamber. Inlet flow rates are the auxiliary fuel m f , the combustion air m a and the input gas m  g , whereas outlet flow rate is given by the flue gas produced by the combustion process m p. Mass rate balance equation is as follows:  f +m m a +m g =m p (1)  g entering the combustion chamber is calculated based on gas The mass flow rate of the input gas m volumetric flow rate Qg and on the gas density g, both expressed at the same reference conditions (actual or normal): m g = Q g  g (2) Energy balance for the combustion chamber (no heat recovery) The energy balance for the combustion chamber expresses the energy conservation between the amount of energy at the inlet and the outlet of the chamber (Figure 1). The energy (heat) rate balance (heat released by VOC oxidation is neglected) is as follows: flue gas + auxiliary fuel + combustion air + combustion heat release = combustion products + heat losses ( m g Cp g Tg − Tref ) ( + m f Cp f T f − Tref ) ( + m a Cp a Ta − Tref ) + m f LHV f  f −1 = (3) ( ) = m p Cp p T p − Tref +  m f LHV f  f −1 Energy at the inlet is the energy associated to flue gas, auxiliary fuel and air masses as well as the energy that becomes available in the chamber as a consequence of auxiliary fuel combustion. The Air Pollution and Control Engineering – A.Y. 2024-2025 3  10 - VOC thermal oxidation energy made available by the combustion of the auxiliary fuel is estimated based on its lower heating value LHVf (expressed as energy released by the combustion of a unit volume of gaseous fueiil) and on its density f. In case of the lack of detailed information specific heat capacities at constant pressure (Cp) may be assumed constant with temperature. Energy at the outlet section is given by the energy associated to the flue gas produced by the combustion process and by heat losses from the chamber, usually estimated as a fraction  (%) of the heat released by auxiliary fuel combustion. Energy balance for the combustion chamber (recuperative oxidizer) In case of a system with energy recovery by means of gas preheating (Figure 2), gas temperature Tg in eq. (3) should be substituted with the inlet gas temperature after pre-heating Tg,p. ( m g Cp g Tg , p − Tref ) ( + m f Cp f T f − Tref ) ( + m a Cp a Ta − Tref ) + m f LHV f  f −1 = (4) ( ) = m p Cp p T p − Tref +  m f LHV f  f −1 Outlet flue gas temperature after preheating of input gas (recuperative oxidizer) When the system includes an energy recovery section for inlet gas preheating up to temperature Tg,p (°C) and with the final temperature for the cooled gas Tout (°C) the energy balance around the heat exchanger is: ( ) m g Cp g Tg , p − Tg = m p Cp p T p − Tout ( ) (5) Maximum theoretical recovery is obtained for Tout = Tg,p. The efficiency of recovery for the exchanger can be calculated as follows:  hex = (Tg,p − Tg ) (T p − Tg ) Figure 2. Energy and mass balance scheme for a recuperative thermal oxidizer Combustion air/auxiliary fuel mass ratio The ratio between combustion air mass and auxiliary fuel mass can be derived by stoichiometric considerations for the reaction of combustion of the auxiliary fuel: ii If LHV is referred to the unit mass of fuel there is no need to use the density of fuel in equation 4. Air Pollution and Control Engineering – A.Y. 2024-2025 4  10 - VOC thermal oxidation  y  y y  y C x H y +  x +  O2 + 3.76  x +  N 2 → x CO2 + H 2 O + 3.76  x +  N 2 (6)  4  4 2  4 The ratio of the combustion air (including the excess air) mass flow rate and the fuel feed rate is calculated as follows  y −1  x +  MWO2   m a = (1 + ea )  4  0.21 mol O2 32 g O2 mol  (7) m f MW f  mol air 28.9 g air mol    where ea (%) is the excess air. The mass flow rates of combustion air, of auxiliary fuel and of the flue gas flow rate produced by combustion can be calculated by solving the system formed by equations (1), (3 or 4) and (7). The air and auxiliary fuel feeding systems are designed respectively based on air and fuel feed rates. The combustion chamber is designed based on the outlet flow rate of gas products. 2. Combustion chamber design Assuming that the high temperature thermal oxidation process follows first order kinetics the reaction time treax in order to obtain the required efficiency can be estimated from equation (8). treax is expressed as a function of the required removal efficiency η and the kinetic constant k with the following formulation (all in congruent units). ln(1 −  ) t reax = − (8) k The kinetic constant k depends on process temperature Tp (K) according to Arrhenius law, based on the values of the A factor, which measures the frequency of molecular collisions, and on the activation energy Ea of the various organic compounds (some examples may be found in Table 1):  E  k = A exp − a  (9)  R Tp    For the application of eq. (9) the activation energy Ea and the gas constant R must be in coherent units, so that their ratio is expressed in Kelvin. The flue gas residence time tres in the chamber should not be less than the reaction time for the least oxidizable compound (tres ≥ treax). As previously mentioned, it practically takes values in the range 0.2-2 s. (reasonably 0.5-0.8 s, see introduction). Once tres is set to a value in this range, the volume V (m3) of the post combustion chamber is calculated based on the residence time of the gases in the chamber:  Dchamber 2 V = Q p t res = Lchamber (10) 4 Assuming a circular section for the combustion chamber, the chamber diameter Dchamber and length Lchamber (m) can be obtained from the geometric considerations knowing the volume of the chamber V (i.e., calculated based on the residence time) and the L/D ratio (chamber length/diameter) which is usually equal to 2 - 3 (typically 2.5). Average throughput velocity of the gases in the chamber will be as follows: Air Pollution and Control Engineering – A.Y. 2024-2025 5  10 - VOC thermal oxidation Lchamber v chamber = (11) t res Combustion chamber velocities vchamber in the range of 3.0 - 4.5 m/s are enough in the combustion chamber. The diameter of the entrance to the combustion chamber (i.e., throat) Dthroat (m) is calculated based on the inlet velocity to the combustion chamber v (m s-1). Gas velocities v in the range of 4.5–9.0 m/s in the throat regions of the thermal oxidation unit will suffice to promote the desired degree of turbulence to mix the combustion products and pollutant gasses. Qp 4 Dthroat = (12) π v Table 1. Values of the frequency factor A (s-1) and of the activation energy Ea (kcal mole-1) for the combustion reaction of some organic compounds (1 kcal = 4.186 kJ) Air Pollution and Control Engineering – A.Y. 2024-2025 6  10 - VOC thermal oxidation PROBLEMS –VOC THERMAL OXIDATION UNITS DESIGN AND VERIFICATION (READ ME: if necessary, assume or calculate any quantity that is not provided in the text based on the notions of past precepts) 1. Design a thermal combustion facility to remove toluene from a gas stream with an efficiency of 99%. Since flue gas contains also benzene verify whether the facility (with the geometry designed in the first step) allows complying with the emission limit with a required removal efficiency of 99.5%. What should be the operational conditions (process temperature, combustion air and auxiliary fuel requirements) to comply with both benzene and toluene emission limits? (flue gas flow rate at NTP: 2.5 m3 s-1; flue gas temperature: 20 °C; flue gas specific heat: 1 kJ (kg °C)-1; combustion air temperature: 20 °C; fuel (methane) temperature: 15 °C; fuel LHV at NTP: 35600 kJ m-3; air excess: 15%; combustion gas specific heat: 1.15 kJ (kg °C)-1; fuel specific heat: 2.2 kJ (kg °C)-1; thermal dispersion 10%; toluene auto-ignition temperature: 535 °C; benzene auto-ignition temperature: 560 °C). 2. The exhaust from an industrial process contains 3 g m-3 of 1,2-dichloroethane. Design the recuperative oxidizer to treat the exhaust stream (8490 m3 h-1 at NTP) with a removal efficiency of 99%. (flue gas temperature: 32 °C; flue gas specific heat: 1 kJ (kg °C)-1; combustion air temperature: 20 °C; fuel (methane) temperature: 15 °C; fuel LHV at NTP: 35600 kJ m-3; air excess: 10%; combustion gas specific heat: 1.15 kJ (kg °C)-1; fuel specific heat: 2.2 kJ (kg °C)-1; thermal dispersion 10%; auto-ignition temperature: 413 °C; thermal recovery efficiency 60%). 3. Discuss the variation of process and operational parameters (e.g., air and fuel supply, reaction time, required residence time) for different process temperatures in a thermal oxidizer removing vinyl chloride with an efficiency of 95% if the oxidizer had the combustion chamber volume to provide a residence time of 1 s at 1090°C. (flue gas flow rate at NTP: 0.8 m3 s-1; flue gas temperature: 15 °C; flue gas specific heat: 1 kJ (kg °C)-1; combustion air temperature: 20 °C; fuel (methane) temperature: 15 °C; fuel LHV at NTP: 35600 kJ m-3; air excess: 10%; combustion gas specific heat: 1.15 kJ (kg °C)-1; thermal dispersion 10%; vinyl chloride auto-ignition temperature: 472 °C). Air Pollution and Control Engineering – A.Y. 2024-2025 7