Flue gases

Flue gas, as used here, is the byproduct of the combustion of fuels. Typically the fuels of concern here are natural gas, oil (and distillates from oil such as gasoline), coal, wood, etc. 

Combustion is a process involving oxygen. However, air is composed of only 21% oxygen, which is required for combustion, and 79% inerts, mostly nitrogen. Thus for every mole of oxygen consumed in the combustion of a paraffin hydrocarbon, more than 9.5 moles of air must be supplied. 

The combustion of a carbon-based fuel (coal, natural gas, or oil) produces a gaseous byproduct called flue gas. First consider the combustion of a paraffin hydrocarbon. 

So the combustion of a hydrocarbon releases carbon dioxide and water. In addition, the combustion of one mole of methane consumes 2 moles of oxygen. 

Fuel Moles of Oxygen Consumed Moles of Air Required (0% Excess) Moles of Air Required (15% Excess) Moles of co2 Produced 

In a fossil fuel power plant, the chemical energy stored in coal, fuel oil, natural gas or oil shale is converted successively into thermal energy, mechanical energy and, finally, electrical energy for continuous use and distribution. The complete combustion of fossil fuel using air as the oxygen source is summarized in the following chemical reaction  

---

In Fig. 6.27, the flue gas is cooled to pinch temperature before being released to the atmosphere. The heat releaised from the flue gas between pinch and ambient temperature is the stack loss. Thus, in Fig. 6.27, for a given grand composite curve and theoretical flcune temperature, the heat from fuel amd stack loss can be determined. 

In Figs. 6.27 and 6.28, the flue gas is capable of being cooled to pinch temperature before being released to the atmosphere. This is... 

Example 6.4 The process in Fig. 6.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C, and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume = 10°C for process-to-process heat transfer but = 30°C for flue-gas-to-process heat transfer. A high value for for flue-gas-to-process heat... 

Solution The first problem is that a different value of AT ,i is required for difi erent matches. The problem table algorithm is easily adapted to accommodate this. This is achieved by assigning AT ,i contributions to streams. If the process streams are assigned a contribution of 5 C and flue gas a contribution of 25°C, then a process-process match has a of 5 -H 5 = 10 C and a... 

Figure 6.30 shows the grand composite curve plotted from the problem table cascade in Fig. 6.186. The starting point for the flue gas is an actual temperature of 1800 C, which corresponds to a shifl ed temperature of (1800 — 25) = mS C on the grand composite curve. The flue gas profile is not restricted above the pinch and can be cooled to pinch temperature corresponding to a shifted temperature of 145 C before venting to the atmosphere. The actual stack temperature is thus 145 + 25= 170°C. This is just above the acid dew point of 160 C. Now calculate the fuel consumption ...

The fuel consumption is now calculated by taking the flue gas from theoretical flame temperature to ambient temperature ... 

The policy for waste heat recovery from the flue gas varies between incinerator operators. Incinerators located on the waste producer s site tend to be fitted with waste heat recovery systems, usually steam generation, which is fed into the site steam mains. Merchant incinerator operators, who incinerate other people s waste and... 

Desulfurize the flue gas. A whole range of processes have been developed to remove SO, from flue gases, such as injection of limestone into the furnace, absorption into wet limestone after the furnace, absorption into aqueous potassium sulfite after the furnace, and many others.However, the byproducts from many of these desulfurization processes cause major disposal problems. 

Flue gas recirculation. Recirculation of part of the flue gas as shown in Fig. 11.4 lowers the peak flame temperature, thus reducing formation. There is clearly a limit to how much flue gas can be recirculated without affecting the stability of the flame. 

NO reductions on the order of 40 percent are possible by flue gas recirculation. 

Chemical reduction. The injection of ammonia reduces NO emissions by the reduction of NO , to nitrogen and water. Although it can be used at higher temperatures without a catalyst, the most commonly used method injects the ammonia into the flue gas upstream of a catalyst bed (typically vanadium and/or tin on a silica support). 

Changes in thermal conductivity, e.g. carbon dioxide in flue gas. 

Flue gas handling Flue gas scrubber Flue-gas scrubbers Flue-gas scrubbing FlueUite Fluent... 

Regardless of method, desorption is never complete. Adsorbent capacity is always less following regeneration than it is on initial loading of adsorbent. Some adsorbable materials undergo chemisorption they chemically combine with the adsorbent. An example is the Reinluft process (52) for removing SO2 from flue gas on activated carbon. The SO2 is attached to the carbon as sulfuric acid. Desorption occurs only upon heating to 370°C a mixture of CO2, evolved from the chemically bound carbon, and SO2 are driven off. 

Limestone slurry scmbs flue gas. SO2 absorbed, reacted to CaSO. Further ak-oxidized to CaSO settled/removed as sludge. Lower cost and simpler than other processes. Disadvantages abrasive/corrosive, plugging and scaling, poor dewatering of... 

Pulverized lime or limestone injected into flue gas (often through burner). SO2 absorbed on soHd particles. High excess alkah required for fairly low SO2 absorption. Finer grindings lime preheat, flue gas humidification benefit removal. Particulate collected in baghouse. 

After flue gas pretreatment, SO2 absorbed into Na2S02 solution sohds and chloride purged, SO2 stripped, regenerating Na2S02, and SO2 processed to S. 

R. W. Coughlin, R. D. Siegel, and C. Rai, eds., AICbE Sjmp. Ser. 70, (137) (1974). Contains four papers on flue gas desulfurization, four papers on coal desulfurization, and three papers on petroleum desulfurization. 

AICbE Sjmp. Ser. 68, (126) (1972). Contains four papers on flue gas desulfurization and two on NO control. 

Sulfur Dioxide Processing, Repriuts of 1972—1974 Chem. Eng. Prog, articles, AIChE, New York (1975). Contaius thirteen papers on flue gas desulfurization, two on SO2 control iu pulp and paper, one on sulfuric acid tail gas, one on SO2 from ore roasting, and two on NO from nitric acid. 

---