In chemical and industrial plants, several by-products are generated in solid, liquid, and gaseous forms that have to be safely destroyed to prevent potential environmental damage. These by-products come from petroleum refining and petrochemical, pharmaceutical, paper and pulp, and plastics production. Small quantities of by-products are stored in drums and placed in landfills, but the most effective method of rapidly destroying a high percentage of hydrocarbon contaminants is to oxidize the organic materials at elevated temperatures (1500-1800°C). For some vapor streams, effective destruction of contaminants can be achieved at lower temperatures. The carbon and hydrogen in the waste are converted to CO2 and H2O. If the gas stream contains sulfur or chlorine or similar substances they must be recovered or removed before venting the flue gases to the atmosphere according to local air quality regulations. Particulates are also generated that have to be removed.

The process of thermal oxidation of fumes, liquids, and gaseous wastes is often carried out in thermal oxidizers or incinerators. If the waste stream has a low heating value or low concentration, often natural gas or liquid fuels are fired alongside to improve the combustion process. In order to destroy most of the pollutants, incineration is carried out at temperatures ranging from 1500 to 1800°F with proper residence times, typically 1-2s. The exhaust gas stream contains a significant amount of energy and is recovered in the form of steam in waste heat boilers.

If the gas stream is greater than 100,000 lb/h and clean, then a water tube boiler with extended surfaces is the ideal choice. Fire tube boilers are also used in incineration plants if the gas is not likely to cause slagging. A superheater and economizer may also be used in fire tube boilers as shown iN Fig. 2.2. Because of the high gas temperature at the inlet to the boiler, 1500-2000°F, the superheater is often located downstream of the boiler as shown. The superheater steam temperature cannot be very high, obviously, with such an arrangement; it is typically 500-550°F depending on the steam pressure. The disadvantage of the fire tube design is that it is difficult to have two fire tube boilers with a superheater in between such as can be done with water tube designs. Hence we have to live with a steam temperature that is slightly lower than those feasible with water tube designs. Locating the superheater at the gas inlet can lead to corrosion due to the presence of corrosive gases in the gas stream.

Bare and finned tubes are used in the design of water tube boilers, depending upon the cleanliness of the gas, its fouling tendencies, and the gas temperature. Simple two-drum designs, such as those shown in Chapter 8 in Fig. 8.3, in which the steam drum and mud drum are connected by plain or finned tubes rolled into the steam and mud drums, are common. This design can have either a refractory-lined casing or a water-cooled casing. With the refractory-lined design, casing corrosion is a possibility if the gas stream contains corrosive acid vapors that can seep through the refractory. Access doors or lanes can be easily incorporated into this design. The water-cooled casing operates at the saturation temperature of steam and ensures that corrosion concerns are minimal. The two — drum crossflow design is suitable for small capacities, generally about 50,000­

75,0 lb/h of steam. When the amount of steam generated is much greater say above 100,000 lb/h, an elevated steam drum with external downcomers and risers may be justified. The steam drum should have the volume or holdup to handle a few minutes of residence time from normal level to empty. Some plants require this residence time to be 3-4min, and a few plants require 10-12 min. In large plants, multiple evaporators are connected to a common steam drum and circulation system.

Figure 2.3 Shows a water tube boiler consisting of a screen section, followed by a two-stage superheater with interstage attemperation, an evaporator, and an economizer that is used in large incineration projects handling clean effluents. The screen section is similar to the shield section used in a fired heater and protects the superheater from the hot gases and from external radiation from the incinerator flame. A minimum of four rows are required to absorb the external radiation from the cavity or flame, as discussed in Q8.09. The evaporator and the screen sections are in parallel and are connected to the same steam drum by external downcomers and risers. If the gas enters at a temperature in excess of 1500°F, the screen section is often designed with bare tubes. The superheater may or may not have fins, depending on the steam and tube wall temperatures. The evaporator has finned surfaces, which can vary from a low fin density section at the inlet (two fins per inch) to a high fin density section (four to five fins per inch) as the gas is cooled. This is done to minimize the heat flux inside the tubes and also to minimize fin tip temperatures.

The economizer uses a fin density of four to six fins per inch. The tubes of all the sections are generally vertical with horizontal gas flow, as in gas turbine HRSG plants. Superheaters are of T11, T22, or T91 material if the tube wall temperatures are close to 1000-1100°F.

In plants with large steam requirements, energy from the waste gas stream is augmented by firing natural gas or fuel oil. In these designs, a D-type boiler (Fig. 2.14) is an ideal choice. The burner is fitted at the front wall of the boiler and fires into a water-cooled furnace; the waste gas stream enters the convection bank, mixes with the furnace flue gases, then flows through the convection and economizer sections. A superheater can be located in the convection bank behind screen tubes. If the flue gases are clean, extended surfaces may be used in the cooler sections of the convection bank.

Various modes of operation have to be considered in these boilers, particularly if a superheater is used. If the waste gas stream supply is cut off, the steam generation is reduced. Hence the total steam flow is reduced which affects the steam temperature and the superheater tube wall temperatures. In some cases only the waste gas stream is used, and in some other modes only the burner is fired for generating steam. All these different cases generate different quantities of steam and flue gases at different temperatures that enter the convection section; hence the superheater performance has to be evaluated carefully in all these modes. The furnace pressure is maintained at nearly zero, and an induced draft fan handles the flow of the flue gases from the burner and the waste gas stream. The forced draft fan just handles the combustion air to the burner.

Figure 2.15 Is the schematic of a waste heat boiler for a dirty gas from a carbon black incineration system. A D-type boiler was also used for this application. The hot gas coming in at about 2100°F is cooled in the furnace and then enters the convection bank. A screen section with widely spaced bare tubes helps to minimize slagging concerns with ash particulates that have low melting temperatures. A retractable blower also helps to clean the front end. As the gas cools, the tube spacing can be closer.

Slagging is a serious concern when flue gases containing ash particulates with low melting point salts are used in heat recovery applications. The slag is a rocklike deposit that forms on cool surfaces such as tubes and solidifies as soon as it is formed. Retractable blowers can help minimize this problem but cannot eliminate it completely. The wide tube spacing ensures that tubes are not bridged by the molten mass of deposits, thus preventing the flow of gases. Ash particles, if any, are collected in hoppers located beneath the convection bank.