Postcombustion

NO Emission Control It is preferable to minimize NO formation through control of the mixing, combustion, and heat-transfer processes rather than through postcombustion techniques such as selective catalytic reduction. Four techniques for doing so, illustrated in Fig. 27-15, are air staging, fuel staging, flue-gas recirculation, and lean premixing. 

Reduction of exhaust emissions is being tackled in two ways by engineers, including precombustion and postcombustion technology. One of the most effective methods now being researched and adopted includes use of synthetic fuel made from natural gas. This fuel is crystal clear, and just like water, it has no aromatics, contains no sulfur or heavy metals, and when used with a postcombustion device such as a catalytic converter any remaining NO, or other emissions can be drastically reduced. Estimates currently place the cost of this fuel at 1.50 per gallon, with availability in 2004 to meet the next round of stiff EPA exhaust emission standards. 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

Combustion modifications and postcombustion processes are the two major compliance options for NO., emissions available to utilities using coal-fircd boilers. Combustion modifications include low-NO burners (LNBs), overfire air (OFA), reburning, flue gas recirculation (FGR), and operational modifications. Postcombustion processes include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). The CCT program has demonstrated innovative technologies in both of these major categories. Combustion modifications offer a less-expensive appiroach. 

Postcombustion processes are designed to capture NO, after it has been produced. In a selective catalytic reduction (SCR) system, ammonia is mixed with flue gas in the presence of a catalyst to transform the NO, into molecular nitrogen and water. In a selective noncatalytic reduction (SNCR) system, a reducing agent, such as ammonia or urea, is injected into the furnace above the combustion zone where it reacts with the NO, to form nitrogen gas and water vapor. Existing postcombustion processes are costly and each has drawbacks. SCR relies on expensive catalysts and experiences problems with ammonia adsorption on the fly ash. SNCR systems have not been proven for boilers larger than 300 MW. 

The term cold end commonly refers to the back-end convection area of the boiler system where an economizer, air-heater, or ID fan may be found, together with the stack and (with high dust-burden flue gases) possibly a cyclone scrubber or electrostatic precipitator. Any fuel treatment applied in this area may be considered to act as a postcombustion additive. 

The technologies currently available for post-combustion capture are classified into five main groups absorption, adsorption, cryogenics, membranes and biological separation. The most mature and closest to market technology and so, the representative of first generation of postcombustion options, is capture absorption from amines. 

In order to provide further insight into the post-combustion ratio and the heat transfer efficiency, the factors that affect the PCR and HTE will be delineated. The factors that affect the PCR and HTE will be discussed separately with the understanding that a complex relationship may exist between the two parameters. The factors that affect the PCR are shown in Table 4, and Fig. 3 demonstrates the primary conditions for postcombustion. The PCR should be kept relatively high, since the fuel consumption decreases with an increase in the PCR at the same HTE (Aukrust, 1993). However, as mentioned, high PCR may lead to problems due to increases in ... 

Reducing the amount of C02 in the atmosphere could involve prescrubbing to take the carbon out of fuels before combustion, leaving only hydrogen to be burned. Another approach is postcombustion scrubbing which removes C02 from the emissions stream after burning. 

Calculated lifetimes of N20 in combustion products indicate that for temperatures above 1500K, the lifetime of N20 typically is less than 10ms, suggesting that except for low-temperature combustion, as found in fluidized bed combustors and in some postcombustion NO removal systems, N20 emissions should not be significant, a conclusion that is in agreement with the most recent measurements of N20 emissions from combustion devices. 

PCDD/F and other chlorinated hydrocarbons observed as micropollutants in incineration plants are products of incomplete combustion like other products such as carbon monoxide, polycyclic aromatic hydrocarbons (PAH), and soot. The thermodynamically stable oxidation products of any organic material formed by more than 99% are carbon dioxide, water, and HCl. Traces of PCDD/F are formed in the combustion of any organic material in the presence of small amounts of inorganic and organic chlorine present in the fuel municipal waste contains about 0.8% of chlorine. PCDD/F formation has been called the inherent property of fire. Many investigations have shown that PCDD/Fs are not formed in the hot zones of flames of incinerators at about 1000°C, but in the postcombustion zone in a temperature range between 300 and 400°C. Fly ash particles play an important role in that they act as catalysts for the heterogeneous formation of PCDD/Fs on the surface of this matrix. Two different theories have been deduced from laboratory experiments for the formation pathways of PCCD/F ... 

As a consequence of the detection of catalytic pathways for formation of PCDD/F, special inhibition methods have been developed for PCDD/F. By this approach the catalytic reactions are blocked by adding special inhibitors as poisoning compounds for copper and other metal species in the fly ash. Special aliphatic amines (triethylamine) and alkanolamines (triethanolamine) have been found to be very efficient as inhibitors for PCDD/F, and have been used in pilot plants. The effect can be seen in Figure 8.6. The inhibitors have been introduced into the incinerator by spraying them into the postcombustion zone of the incinerator at about... 

Desulfurization of fossil fuels was the subject of an authoritative review by J. B. Hyne (Alberta Sulphur Research Institute). This is a topic of increasing importance as Canada relies more and more on sulfur-containing fuels such as tar sands and heavy oils. Hyne reviewed the present state of the chemistry and technology for both precombustion desulfurization of natural gas and crude oils and postcombustion tailgas clean up of coals and cokes. He clearly identified areas of possible future research such as the high temperature-high pressure chemistry pertaining to in-situ desulfurization processes. 

The selective Noncatalytic reduction (SNCR) process is a postcombustion NO reduction technology. NO is reduced through the controlled injection of a reagent, either ammonia or urea, into the combustion products of boiler, heater, or FCC regenerator. This process is typically applied on partial burn applications with a CO boiler (COB). 

A number of vendors offer SNCR technology based on either ammonia or urea. Exxon Mobil Thermal DeNO (TDN) technology is a common SNCR process applied to FCC units. The technology is licensed exclusively to Hamon Research-Cottrell Inc., and has been utilized to achieve postcombustion NO reduction in CO furnaces, thermal oxidizers, overhead regenerators, and power boilers. Thermal... 

The postcombustion systems used at power generating plants and factories are somewhat different. These systems remove nitrogen oxides from the waste gases jlue gases) processes using classified as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR). Oxides of nitrogen are also removed by some systems... 

Most postcombustion cleaning systems make use of scrubbers to remove sulfur dioxide. Scrubbers are devices tbat contain some chemical that will react with sulfur dioxide in flue gases. Two kinds of scrubbers are used, wet and dry. As their names suggest, the two... 

Natural gas combined cycle power plants with highest efficiencies (NCCC) and pre-and postcombustion capture of COj. 

Flue gas desulphurization (FGD) is postcombustion technology that cleans gases after they leave the boiler or combustion zone. The basic approach to cleaning flue gas is to react alkaline minerals with the gaseous S02. Calcium is the principal alkaline sorbent used, although Mg or K are also used. Slurry made from limestone and water, or lime and water,... 

Figure 9.1 Basic schematic diagram of (a) postcombustion capture, (b) precombustion decarbonization, and (c) oxy-fuel processes.

---