Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 33.4.8 Non-selective catalytic reduction of NO x and N 2 O in tail gasesDescriptionThe non-selective catalytic reduction of NO x enables the reaction of a reducing agent (fuel) withnitrogen oxides, to produce nitrogen and water. Although developed as a De-NO x system,NSCR also considerably reduces the emissions of N 2 O. This process is called non-selective,because the fuel first depletes all of the free oxygen present in the tail gas and then removes theNO X and N 2 O. The most used fuels are natural gas or methane (CH 4 ), hydrogen (H 2 ) orammonia plant purge gas (mainly H 2 ). An excess of reducing agent is required to reducenitrogen oxides and nitrous oxide to nitrogen. Catalysts for NSCR are usually based onplatinum, vanadium pentoxide, iron oxide or titanium; catalyst supports are typically made ofalumina pellets or a ceramic honeycomb substrate. The fuel requirement is the stoichiometricamount needed to reduce all the oxygen present (free and in nitrogen oxides) plus an excess(approximately 0.5 vol-% CH 4 ). As the catalyst ages, the amount of fuel is increased to maintainthe same NO x and N 2 O reduction values in the tail gas.The tail gas must be preheated before the reaction on the catalyst proceeds. The requiredtemperature depends on the fuel selected, varying from 200 – 300 °C (H 2 ) to 450 – 575 °C(natural gas). Due to the exothermic reactions in the NSCR facility, the tail gas temperature canbecome very high (>800 °C), exceeding the maximum for admission to the gas expander unit.To deal with these high temperatures, two methods of NSCR are developed: single-stage andtwo-stage reductionSingle-stage units can only be used when the oxygen content of the absorber tail gas is less than2.8 % (an oxygen content of 2.8 % will result in a tail gas temperature of ±800 °C after theNSCR facility). The effluent gas from these units must be cooled by a heat exchanger orquenched to meet the temperature limitation of the gas expander unitTwo-stage units with an internal quench section are used when the oxygen content is over 3 %.Two systems of two-stage reduction are used. One system uses two reactor stages with aninterstage heat removal. The other system involves preheating 70 % of the tail gas to ±480 °C,adding fuel, and then passing it over the first stage catalyst. The fuel addition to the first stage isadjusted to obtain the desired outlet temperature. The remaining 30 % of the tail gas, preheatedto only ±120 °C, is mixed with the first stage effluent. The two streams, plus fuel for thecomplete reduction, are passed over the second stage catalyst. After the second catalyst, the tailgas passes to the gas expander.Achieved environmental benefits• simultaneous abatement of N 2 O and NO x• reduction of N 2 O of at least 95 %, reducing emissions to well below 50 ppm N 2 O• reduction of NO x emission to 100 – 150 ppm (205 – 308 mg/m 3 ).Cross-media effects• when hydrocarbon fuels are used, emissions of carbon monoxide (CO), carbon dioxide(CO 2 ) and hydrocarbons (C x H y ) will take place. Normally, the carbon monoxide emissionwill be less than 1000 ppm (1250 mg/m 3 ), but emissions of hydrocarbons can be up to 4000ppm. Emissions of CO 2 can be over 6300 ppm (about 12 g/m 3 )• the tail gas needs a high preheat temperature, especially when hydrocarbon fuels are used.The tail gas needs heating from ±50 °C to ±250 – 300 °C (H 2 ) or to 450 – 550 °C (naturalgas). The energy to use this abatement technique can be obtained from the process, butreduces the possible amount of exportable steam.130 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers