Heat Recovery Equipments

Whether heat recovery equipment should be applied to the exhaust of an industrial boiler depends on such factors as operational conditions, estimated maintenance, investments, payout time and fuel prices. The number of operating hours per year, boiler load, power consumption and fuel savings should be carefully balanced against costs. 

In general, low-loaded boilers do not justify the use of heat recovery equipment. In other applications, it may very well be worthwhile. The need for heat recovery equipment is sometimes difficult for a designer to determine, since he is often insufficiently familiar with the factors influencing an evaluation. In such instances, responsible selection of an economizer or air preheater is not possible. It should be remarked that adding heat recovery equipment means adding something that may influence availability. Fuel, excess air, flue gas temperature, air temperature or feed water temperature play a role here. 

If a study shows that heat recovery equipment does pay, the question remains whether an economizer or air heater should be used and what its 

With an economizer, the feed water temperature entering is of dominant importance for investment efficiency. This temperature should be high enough to avoid corrosion, especially when high-sulfur fuels are fired. A feed water temperature of about 280° F or higher can be safely handled with high-sulfur fuels. Part of the economizer can be made of cast iron, depending on the metal skin temperature and dew point. 

In the region where gas temperature levels out, an increase in surface is no longer economical. At lower feed water temperatures the flat gas-temperature curve shifts to the right and decreasing the gas temperature might pay. 

Most of the developments in the SCSA process were followed for the DCDA process with provision of WHRB and economisers along with steam superheaters to generate the maximum amount of steam. It was possible to produce power or to run the blower with the turbine running on the HP superheated steam. The exhaust steam was used for process heating—smelting of sulfur, evaporation of dilute alum/ phosphoric acid solutions/ for multiple effect evaporators, etc.  

Recent variants have attempted steam generation (as LP steam) or getting hot water from the hot acid circulating in the IPAT and FAT. Generation of LP steam needs very careful process control for the acid concentration to minimize the corrosion of equipment and piping. 

Hot water is obtained by cooling the acid by DM water in a closed circuit and the DM water is cooled by process water in another heat exchanger. The heated process water is then used as per need. 

A cooling tower can also be provided to cool the process water if there is no continuous need in the plant for the hot water. 

In another plant design, the air blower is arranged downstream of the drying tower to recover the heat of compression as pre-heated air to the sulfur burning furnace. This, however, required a very efQcient acid mist danister in the DT to prevent corrosion of the blower internals. 

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The gas leaving the heat recovery equipment contains soot and ash some ash is deposited in the bottom of the reactor for removal during periodic inspection shutdowns. The gas passes to a quench vessel containing multiple water-sprays which scmb most of the soot from the gas. Additional heat recovery can be accompHshed downstream of the quench vessel by heat exchange of the gas with cold feed water. Product gas contains less than 5 ppm soot. 

Heat Recovery Equipment. Factors that limit heat recovery appHcations are corrosion, fouling, safety, and cost of heat-exchange surface. Most heat interchange utilizes sheU and tube-type units because of the mgged constmction, ease of mechanical cleaning, and ease of fabrication in a variety of materials. However, there is a rich assortment of other heat exchangers. Examples found in chemical plants in special appHcations include the foUowing. 

Direct-flame afterburners are nearly 100% efficient when properly operated. They can be installed for approximately 350-700 per cubic meter of gas flow. Operating and maintenance costs are essentially those of the auxiliary gas fuel. On larger installations, the overall cost of the afterburner operation may be considerably reduced by using heat recovery equipment as shown in Fig. 29-16. In many industrial situations, boilers or kilns are used as entirely satisfactory afterburners for gases generated in other areas or processes. 

For question 7, if heat recovery equipment were installed to raise the incoming ctfluent to 425°C, how much natural gas would have to be burned per hour ... 

Purchase price of equipment turbine, compressor, auxiliary equipment, and heat recovery equipment, if desired. 

The cleanliness of the products of combustion is such that the use of heat-recovery equipment is possible without the risk of corrosion. This has led to the development of combined heat and power packages where the overall efficiency is high. 

Early SM boilers were manufactured with between two and four corrugated furnace tubes in wet-back and dry-back versions and generally incorporated heat recovery equipment such as economizers and air heaters. Some designs also provided for superheaters and for coal, oil, or gas fuel firing. Many of the best features are incorporated in the SM boilers commonly available today. 

CMCs are used in the manufacture of preheaters and recuperators in heat recovery equipment. They are used for indirect heating and energy-intensive industrial internal processes such as glass melters, steel reheaters and aluminium remelters. 

An alternative method of cooling metallurgical offgas is to pass it through sprays of water. Spray cooling avoids investment in waste heat recovery equipment but wastes the heat of the gas. It also generates acidic waste liquid that must be neutralized and treated for solids removal/recycle. 

Many considerations are necessary when deciding if heat recovery equipment should be included in an incineration facility. Unless a practical use for the recovered heat exists, it is usually not advisable to include heat recovery equipment at a facility since the equipment is expensive. If an incineration facility operates at a large capacity, the generation of power from heat recovery equipment is generally economical. Municipal solid waste incineration facilities are widely used to produce steam for electric power generation. 

The only available small-scale system is a packaged two chamber incinerator with waste heat recovery. This technique is practical at the 25 to 100 tons per day (TPD) scale. In these units, partial oxidation occurs in the first section of the unit and causes a portion of the waste material to degrade and give off combustible gases. These gases, as well as products of combustion and particulate from the first chamber, flow to a second chamber where they are combusted with excess air and a natural gas or oil pilot flame. The combustion products then flow through appropriate heat transfer equipment to produce steam, hot water, or hot air. Today, four small cities and more than sixty industrial plants use the technique with heat recovery equipment. 

The design of any downstream heat recovery equipment. Any SO3 in the flue gas will react with the NH3 added in the SCR to form ammonium sulfate salts (ammonium sulfate and ammonium bisulfate). These salts form a solid at high temperatures and... 

The reviews by Spivey [3] and by Jennings et al. [156] are excellent sources for further details on catalytic incineration of volatile organics emissions. Spivey [3] describes two types of techniques for removal of VOC from off-gases, namely one without preheater and one with a direct flame preheater. From an economically point of view it is more beneficial to carry out the catalytic oxidation at lower temperatures. In a catalytic incinerator, sometimes called an afterburner, VOCs are oxidized into carbon dioxide and water. The efficiency is about 70-90%. The incinerator has a preheat burner, a mixing chamber, a catalyst bed, and a heat recovery equipment. Temperatures of about 590 K are sirfficient for the destruction of VOCs. Various catalyst geometries have been used metal ribbons, spherical pellets, ceramic rods, ceramic honeycombs, and metal honeycombs. Precious metals such as platinum and palladium are often used in catalytic incinerators. 

Thermal incinerators depend upon contact between the contaminant and the high-temperature combustion flame to oxidize the pollutants. The incinerator, generally consists of refractory-lined chamber, one or more burners, a temperature-control system, and heat-recovery equipment. 

The lower capital cost figures for various conservation measures (relative to the investment needed for new energy supplies) provide only a rough indication that such measures will yield attractive economic returns to the investor. Detailed payback calculations, similar to those discussed later for waste heat recovery equipment, are needed to establish economic feasibility in each specific case. 

Thermal oxidizers are usually designed as adiabatic chambers. Heat recovery equipment is common but is almost always located downstream of the thermal oxidizer chamber. Therefore, quench fluids are used to control the temperature of the flue gases in the thermal oxidizer chamber by direct cooling. The most common quench fluids are liquid water, steam, or air. 

Figure 1-27. Fuel savings at MCR vs. investment increase for heat recovery equipment (1968 cost basis).

An ell (90-degree turn) is recommended in a flue line to prevent straight-line furnace radiation out the flue, wasting fuel, and chilling part of the load. This is particularly important if there is cleanup or heat recovery equipment beyond the flue because of possible radiation damage to that equipment. 

Sankey diagrams (visual heat balances) assist overseeing the Btu checkbook, that is, to analyze where heat is being wasted and how to divert wasted heat to optimum use. Figures 5.11 and 5.12 are Sankey diagrams before and after addition of heat recovery equipment to a furnace. 

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