Removal Of NOx From Flue Gas With The Recovery Process Of Absorption With Acid Following Complex In Aqueous Solution And Reduction With Iron

The NO_X (nitrogen oxides) from combustion of fuels is one of the major air pollutants.It is very harmful to human health and to the environment. In addition to low-NOx combustion technologies, the principal processes for NO_X control from flue gas that have been commercialized are selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) now. Both SCR and SNCR belong to discarded process, and their first cost and operation cost are high. Therefore, it is a urgent task for air pollution control to research and develop new NO_X abatement processes that will be uncomplicated, low-cost, and adaptable widely in recent more than 20 years. It has very important implication to employ recovery process for removal of NO_X from flue gas, because the pollution of NO_X can be abated and the nitrogen can be recovered to obtain a valuable production in the recovery process.The complex process has been investigated considerably in United States and Japan since the late 1970s. The results show that it is difficult for the absorption solution to be regenerated and reused circularly, and the further research of the process have been impede d because of the following problems. First, ferrous ion can be easily oxidized to ferric ion by the oxygen in the flue gases, and the ferric chelates can't complex with NO, therefore, the amount of removal NO in the process decreases rapidly. Second, the complicated reaction between NO coordinated to ferrous chelates with HSO₃^-/SO₃^2- form a series of nitrogen-sulfur compounds, dithionate ion, and nitrous oxide that is a secondary gaseous pollutant. The reaction activity of the absorption solution disappears gradually because of the accumulation of the above byproducts formed in the solution in the process. It was proposed subsequently to use ferrous chelates of SH-containing amino acids and peptides for the removal of NO_x and SO₂ from flue gas, but the main problem hampering the research further of the process is still the regeneration of the absorption solution.A novel recovery process for removal of NO_X from flue gas with the sequence of absorption with acid following complex in aqueous solutions of ferrous chelates and reduction with iron filings/powder is proposed. Firstly, NO in the flue gas is complexed with ferrous chelate, and form ferrous nitrosyl complex in the absorption solution. The bound NO is converted to ammonia by iron filings/powder at the same time. The ammonia formed is entrained in the treated flue gas, and is absorbed by phosphoric acid or sulfuric acid to produce the fertilizers of ammonium phosphate or ammonium sulfate. Iron filings/powder consumed in the denitrification process is converted to iron precipitate. It can be separated from the solution and used for produce iron oxide red pigments. The issues and results that are investigated in this dissertation are as follows.(l)The influence factor and the kinetics for reaction of Fe²⁺EDTA solution complex with NO are researched. Results show that the absorption capacity of pure water for NO is little. Complex capacity of Fe²⁺EDTA solution for NO weakens with time. The complex reaction reaches the equilibrium after a period of time. Complex capacity for NO decreases by 90% when the O₂ content in the simulated flue gas increases from 0% to 4.2%. Downtread of the capacity slower rapidly when the O₂ content increases further. Under the conditions of initial pH=6, Fe²⁺EDTA concentration lOmmol·L^-1, and temperature 60℃, the complex capacity of Fe²⁺EDTA aqueous solution increases by a factor of 1000 compared with that of pure water, and it increases linearly with increasing the Fe²⁺EDTA concentration in absorption solutions. The capacity decreases with raising in temperature, and it is 5.3 times at 30℃as much as that at 80℃. The capacity increases with increasing initial pH when the pH of absorption solution is below 8. It reaches the maximum at pH 8 and decreases with increasing initial pH after the pH is above 8. The equilibrium constants of the complex reaction are 2.15×10⁶L·mol^-1, 6.54×10⁵L·mol^-1, and 2.67×10⁵L·mol^-1 respectively at temperature 25℃, 45℃and 65℃. The complex reaction can be regarded as pseudo-first-order. Rate constant of the reaction can obtain with following equation 0-1:k₂=E_i² K_L,NO²/(C_BoD_A) (0-1)(2)The reaction mechanism of reduction of nitrosyl complex and ferric ion by iron is investigated, and the reaction mechanism of absorption ammonia formed in the denitrification process by acids is discussed also. The results show that there is no N₂O formed in the exhaust and no NO₂ and NO₃ ion formed in the denitrification solution. All of the removed NO in the flue gas is converted to NH₃ in the denitrification process. The molar ratio of denitrification amount to the consumption of iron powder is 2/1 without oxygen in the simulated flue gas, and that is 1/2 when the oxygen content is 5%. The amount of iron consumed to reduce NO in the process is a quarter, and th tee quarteres of iron powder is consumed to keep the activity of denitrification solution. The ferrous nitrosyl complex can be reduced by iron according to equation 0-2 and 0-3 without oxygen in the simulated flue gas.2Fe²⁺EDTA(NO)+Fe+8H⁺→2Fe²⁺EDTA+Fe(OH)₂+2NH₃ (0-2)2NO+Fe+8H⁺→Fe(OH)₂+2NH₃ (0-3)NH3₊H⁺←→NH₄⁺ (0-4) When where is oxygen, the reaction mechanism can be expressed as2Fe²⁺EDTA(NO)+4Fe+10H⁺+O₂+2H₂O→2Fe²⁺EDTA+4Fe(OH)₂+2NH₃ (0-5)2Fe³⁺EDTA+Fe+2OH→2Fe²⁺EDTA+Fe(OH)₂ (0-6)2NO+4Fe+10H⁺+2O₂+2H₂O→4Fe(OH)₂+2NH₃ (0-7)NH₃+H⁺←→NH₄⁺ (0-8) Under the experimental conditions, the equilibrium concentration of ammonia in the denitrification soultion is 1.159g·L^-1. Ammonia entrained in the treated flue gas is absorbed by sulfuric acid or phosphoric acid according to the mechanism2NH₃+H₂SO₄→(NH₄)₂SO₄ (0-9)NH₃+H₃PO₄→NH₄H₂PO₄(pH4.4～4.6) (0-10)2NH₃+H₃PO₄→(NH₄)₂HPO₄ (pH8.0～9.0) (0-11)(3)The denitrification technology with Fe²⁺EDTA-iron filings system is investigated. The removal efficiency increases rapidly with the increasing of Fe²⁺EDTA concentration when the Fe²⁺EDTA concentration is low. However, the augmentation of the NO_x removal efficiency lower after the Fe²⁺EDTA concentration is above 20mmol·L^-1. The more the NO_x removal is, the higher the pH is, when the pH is in the range of 2-6, and the NO_x removal efficiency is largest at pH6. The efficiency decreases rapidly with increasing the initial pH when the pH is above 6. The efficiency almost increases linearly with increasing the amount of iron filings, when the amount of iron filings in the absorption solution is not enough. After the amount of iron filings is more than 15g in 100mL absorption solution, the increment of the efficiency lower with increasing further of the iron filings amount. When the temperature is below 65℃, the efficiency increases with raising in temperature. After the temperature is above 65℃, the lower the NO_x removal efficiency is, the higher the temperature is. The inlet NO concentration has every little influence on the NOx removal efficiency under the experimental conditions. The efficiency decreases with increasing the O₂ concentration in flue gas. The optimal conditions for removal NOx from flue gas with iron filings reduction following liquid phase complex in a bobble column is Fe²⁺EDTA concentration 20mmol·L^-1, pH6.0, and 65℃. Under the circumstances, and iron filings 15g in 100mL absorption solution, the NOx removal efficiency in two series absorbers for flue gas containing O₂ 10.5% exceeds 90%. The ability of absorption solution for NOx removal can be recovered completely after filtration, and the efficiency keeps up steady after regeneration many times. NOx can be removed continuously and steady in a laboratorial scrubber that is packed with iron filings. The NOx removal efficiency is about 70% under the experimental conditions. For the scrubber packed with iron filings, the NOx removal efficiency can be expressed asη=C₁-C₂/C₁×100≈Y₁-Y₂/Y₁×100=100{1-exp「-(k₂D_AC_Bo^0.5αZP/Gm」}0-12) The calculation number is accordant with the measure number. It shows that the model is useable. The influence of the denitrification liquid rate, flue gas rate, and fillings height can be predetermine with the model.(4)The denitrification technology and kinetics with Fe²⁺EDTA-iron powder system is investigated. The NO_x removal efficiency increases rapidly by an increase in the amount of iron powder when the amount of iron powder in the absorption solution is low. After the amount of iron powder is more than 0.8g in 100mL absorption solution, the increment of the efficiency lower with increasing further of the iron powder. The amount of iron filings in 100mL absorption solution is 47 times as high as that of iron powder at the same NO_x removal efficiency. The NO_x removal efficiency increases rapidly with decreasing the size of iron powder particulate when the particulate is large, and the increment of the efficiency lower after the size is less than 200 mesh. The efficiency increases linearly with raising the stirring speed of the stirrer, and the augmentation of the efficiency is slow when the stirring speed excesses 900rmp. The optimal circumstances for removal NOx from flue gas with iron powder reduction following liquid phase complex in a stirred reactor is Fe²⁺EDTA concentration 20mmol·L^-1, pH6.0, 65℃, 0.8g iron powder in 100mL absorption solution, and the size of the iron powder less than 200 mesh. Under the conditions, the NOx removal efficiency in one reactor for flue gas containing O₂ 10.5% can reach about 90%. The efficiency with iron powder in a reactor can be comparable with iron filings in two series bubble column. Under the experimental conditions, the reduction reaction of Fe²⁺EDTA(NO) by iron powder is pseudo-first-order. The rate constants of the reaction are 2.64×10^-2min^-1, 3.36×10^-2min^-1, and 4.28×10^-2min^-1respectively at 25℃, 45℃, and 65℃. The activation energy of the reaction is found to be 10096J·moo^-1.(5)It is investigated that the iron precipitate formed in the denitrification process is used for production iron oxide red pigments. It is showed from the X-ray diffraction analysis that the iron precipitate converts to Fe₂O₃·H₂O crystal after dried by air. Three characteristic peaks of the crystal is corresponding with the standard spectrum of the Fe³⁺OOH. The precipitate dried by air appears saffron, its composition is the same with iron oxide yellow pigments. The precipitate converts to iron oxide red pigments after filtrated, washed, dried, and calcined. Orthogonal experimental results show that the optimal conditions for manufacturing iron oxide red pigments from the precipitate is calcination temperature 600～700℃, calcination time 50min, and pH number 7 of the precipitate suspension. Under the conditions, iron oxide red pigments of grade A can be manufactured from the precipitate.(6) Removal of NO_x from flue gas with the recovery process of absorption with acid following complex in aqueous solution and reduction with iron powder/filings can be divided into following three stages according to the above investigation results, the experimental phenomenon and operations.The first is the complex of NO with Fe²⁺EDTA in the absorption solutions and the reduction of Fe²⁺EDTA(NO) by iron powder/filings. In this stage, the dissolved NO binds with Fe²⁺EDTA in the solutions to form ferrous nitrosyl complexNO(g)←→NO(aq) (0-13)Fe²⁺EDTA(aq)+NO(aq)←→Fe²⁺EDTA(NO)(aq) (0-14) and Fe²⁺EDTA is oxidized partially to Fe³⁺EDTA by the oxygen in the flue gas simultaneously4Fe²⁺EDTA+O₂+4H⁺←→4Fe³⁺EDTA+2H₂O (0-15) Subsequently, the NO coordinated with Fe^<sup>2+EDTA is reduced into NH₃ by iron powder/filings. The Fe²⁺EDTA can be regenerated in the denitrification process, and the Fe³⁺EDTA is reduced into Fe²⁺EDTA by iron powder/filings simultaneously. The activity of the absorption solution can be maintained.The second is the absorption of the ammonia by acid. The ammonia is formed according to the equation 0-4 in the denitrification process, and it accumulates in the solution in initial period. The concentrations of the ammonia in the denitrification solution and in the treated flue gas increase with the removal of NOx from flue gas. The amount of ammonia formed from removal of NOx in the flue gas is equal to that desorbed from the denitrification solution after a period of denitrification. Fertilizers of ammonium sulfate/phosphate can be produced from the absorption of ammonia desorbed with sulfuric/phosphoric acid.The third is production of iron oxide red pigments with iron precipitate. Iron precipitate, Fe^x+(OH)_x, is formed in the denitrification precess according to equation 0-15. The precipitate can be seperated from the denitrification solution by filtration or sedimentation, and it oxidized into the Fe₂0O₃·H₂O crystal by the oxygen in air dried process. Iron oxide red pigments can be manufactured from the calcination of the precipitate. The reactions can be expressed as2Fe(OH)₂+0.5O₂→Fe₂O₃·H₂O+H₂O (0-16)Fe₂O₃·H₂O(?)Fe₂O₃+H₂O (0-17)Though removal of NO_x in flue gas with the complex process has been investigated considerably in United States and Japan, the further research and commercialization of the process have been impeded because it is difficult for the absorption solution to be regenerated and reused circularly. A novel recovery process for removal of NO_x from flue gas with the sequence of absorption with acid following complex in aqueous solutions of ferrous chelates and reduction with iron filings/powder is proposed. The innovation and characteristic of the process are as follows.â‘ A new recovery process for removal of NO_x with the sequence of absorption with acid following complex in aqueous solutions of ferrous chelates and reduction with iron filings/powder is proposed according to the concept of desulfurization first and denitrification then.â‘¡The mechanism of removal of NOx from flue gas with the recovery process of absorption with acid following complex in aqueous solution and reduction with iron powder is proposed.â‘¢A new method for reduction of ferrous nitrosyl complex and ferric ion by iron filings/powder is proposed. The activity of the denitrification solution can be maintained, and the solution can be regenerated and reused circularly.â‘£A new method for absorption of ammonia formed in the denitrification process with sulfuric/phosphoric acid to manufacture agricultural fertilizers of ammonium sulfate/ phosphate is proposed.⑤A new recovery method for production of iron oxide red pigments with iron precipitate formed in the denitrification process is proposed.