Tail gas temperature

The advantages of the SCR system are97 ammonia is readily available in a nitric acid plant a low NOx content can be achieved the increase in tail gas temperature is negligible and no oxygen is consumed. 

The disadvantages of the SCR system are97 the tail gas temperature after the expander must be kept high enough to avoid any ammonium salt deposits a... 

Figure 9.10 shows how N2O destruction varies with tail gas temperature and with the fuel that is used221. 

Figure 9.10. Nitrous oxide destruction at different tail gas temperatures. (Reproduced by permission of Energy Research Centre of the Netherlands)...

The selective catalyst reduction of both NOx and N2O can take place when a second metal-exchanged, zeolite catalyst is combined with the catalyst that removes N20. Propane is used as the reductant. Destruction efficiency of both NOx and N20 is higher than 80% at a tail gas temperature above 280°C, a pressure of 4 bara and a space velocity of 15,000 h 1. At atmospheric pressure, conversions of NOx and N2O are somewhat lower. A space velocity of 15,000 h 1 and a temperature of 350°C results in 80% destruction of both NOx and N20221. 

Direct Decomposition Upstream of Expander Tail Gas Temperature = 450 °C Space Velocity = 45,000 h-1 N2O conversion = 70% Retrofit Factor = 50%... 

End-of-Pipe SCR of N2O Using 500 ppmv Propane Tail Gas Temperature = 100 °C ... 

Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

Seleetion of a tail gas expander depends on the temperature and pressure on the inlet eonditions of the nitrous gas. The pressure equals that of the eompressor outlet minus the pressure losses in the eyele. Depending on proeess and plant size, these losses amount to 0.3-2.0 bar. The inlet temperature may vary widely from plant to plant. Figure 4-19 shows an expander in the 10,000 kW power output eategory. 

Low-temperature activity promotion is an issue in mobile (diesel) applications, but may not be a critical issue in several stationary applications, apart from those where the temperature of the emissions to be treated is below 200°C (for example, when a retrofitting SCR process must be located downstream from secondary exchangers, or in the tail gas of expanders in a nitric acid plant). In the latter cases, a plasmacatalytic process [91] could be interesting. In the other cases, the use of NTP together with the SCR catalyst is not economically viable. However, the synergetic combination of plasma and catalysts has been shown to significantly promote the conversion of hazardous chemicals such as dioxins [92], Although this field has not yet been explored, it may be considered as a new plasmacatalytic SCR process for the combined elimination of NO, CO and dioxins in the emissions from incinerators. 

In the first step the chlorine from the tail gas and chlorine feed reacts with the caustic in the jet-loop reactor. The advantage of the jet-loop reactor is that it also acts as a suction device for the gas stream. The residence time of the liquid in step one is dependent on the capacity of the hypochlorite production and liquid level in the tank and varies between 1 and 4 h. A heat exchanger in the loop controls the temperatures in steps one and two. The amount of caustic in the feed-tank of step two is the back-up for failure of chlorine liquefaction. 

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

Extended Claus processes maintain the 2 1 ratio of HS to SO2 in the tail gas. The reaction is extended at low temperatures, below the dewpoint of sulfur. Some processes of this type are the Sulfreen, IFP Clauspol, and CBA processes (9). 

Furnace temperatures have also been shown to be important in controlling the formation of COS. While COS has little effect on the downstream Claus catalyst efficiency its presence in the gas stream leads to higher loading of the reductive tail gas clean-up processes (e.g. SCOT, BSR, see environment) or to higher SO2 emissions in the stack gas. The recent developments regarding the control of its formation in the front end furnace are thus a significant contribution to the improvement of environmental quality control. 

Elsewhere in this review we have commented on the problem of COS production as a result of high temperature reactions occurring in the front end furnace. Sulfur in this form is not subject to conversion to elemental sulfur in the catalytic redox Claus reaction and thus appears as COS in the tail gas where it is incinerated to SO2 thus adding to losses to the environment. The COS and any CS2 can be hydrolyzed to H2S which can then be converted by the redox Claus reaction. 

In this new process the H2S/SO2 reaction is carried out in liquid sulfur at pressures in excess of five atmospheres. Typical Claus catalysts are still employed but temperatures are lower (below the dewpoint of sulfur) and thus the redox reaction occurs in the liquid sulfur phase at the surface of the catalyst. Vapor losses due to sulfur mist entrainment are reduced and interstage condensers in the tradition Claus train are not required thus avoiding wasteful heat transfer problems. The authors claim that overall sulfur recoveries in excess of 99% are possible without the use of tail gas clean up units. 

---