液相络合—铁还原—酸吸收回收法脱除烟气中NOx的研究
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摘要
燃料燃烧产生的NO_x是大气的主要污染物之一，它对人体健康和生态环境造成巨大的危害。目前，己商业化的烟气NO_x控制技术除低NO_x燃烧技术以外，主要还有选择性催化还原法和选择性非催化还原法。这二种方法都属于抛弃法，而且投资和运行费用较高。因此，研究和开发简单、廉价和适用的NO_x控制新技术，是近20多年来大气污染控制领域的一个热点课题。回收法脱除烟气中的NO_x，既可消除NO_x污染，又可回收其中的氮资源生产有价值的产品，具有重要的现实意义。
日本和美国从20世纪70年代开始就对液相络合法同时脱除烟气中SO_2和NO_x进行了大量研究。结果表明，对于亚铁氨羧螯合剂同时脱硫脱氮而言，处理过程中Fe~(2+)很容易被烟气中的O_2氧化为Fe~(3+)，而Fe~(3+)螯合剂与NO无亲和力，因此脱氮液的脱氮能力很快降低：此外，与螯合铁络合的NO能与溶液吸收SO_2而形成的SO_3~(2-)／HSO_3~-发生复杂的反应，形成一系列可溶于水的氮-硫化合物、S_2O_6~2-和N_2O二次气态污染物，这些液相产物在溶液中的积累，也会使脱氮液逐渐失去活性。因此，脱氮液难以再生和循环利用，这是阻碍该法进一步研究的根本原因。随后有人提出了用含-SH基的亚铁螯合剂进行烟气脱氮，该法研究中遇到的最大阻力也是其脱氮液的再生问题。
我们首次提出了“Fe~(2+)螯合剂络合吸收-铁还原-酸吸收”回收法脱除烟气中NO_x的新方法。首先用Fe~(2+)螯合剂将脱硫后烟气中的NO络合，使之进入液相，同时用铁屑(铁粉)将被络合的NO还原为氨，产生的氨随处理后的烟气带出，用磷酸或硫酸吸收，即可以制得磷酸铵或硫酸铵肥料；过程中所消耗的铁屑(铁粉)以铁沉淀物的形式从液相中分离，可用于生产氧化铁红颜料。本文主要研究内容和结果如下：
(1)研究了Fe~(2+)EDTA溶液络合吸收NO的影响因素和动力学。结果表明：纯水对NO几乎没有吸收能力，一定体积和浓度的Fe~(2+)EDTA溶液对NO的络合能力随时间延长而减弱，络合一定时间后即达到平衡；烟气中O_2含量从0％增加到4.2％，NO络合容量下降90％，O_2含量进一步增加，络合容量的下降趋势迅速减缓；在络合液起始pH=6，Fe~(2+)EDTA浓度为10mmol·L~(+1)，温度60℃时，NO的络合容量为NO在纯水中溶解量的1000倍，NO的络合容量随络合液中Fe~(2+)EDTA浓度的增加呈直线增长；NO的络合容量随温度的升高而降低，在络合液起始pH=6，Fe~(2+)EDTA浓度为15mmol·L~(-1)条件下，30℃时Fe~(2+)EDTA溶液对NO的络合容量为80℃时络合容量的5.3倍；当pH值较低时，NO的络合容量随络合液起始pH值的增加而增加，当pH值达到8左右时，络合容量达到最大值；继续增加pH值，络合容量逐渐降低；在25℃、45℃和65℃时，络合反应的平衡常数分别为2.15×10~6L·mol~(-1)、6.54×10~3L·mol~(-1)和2.67×10~5L·mol~(-1)；在实验条件下，Fe~(2+)EDTA络合NO的反应可以看成拟一级反应；络合反应的速率常数可通过下式求得：
k_2=E_i~2k_(L，NO)~2/(C_(B_o)D_A) (0-1)
(2)研究了铁还原亚铁亚硝酰络合物和Fe~(3+)的反应机理，并对酸吸收还原过程中产生氨的机理进行了讨论。结果表明：在Fe~(2+)螯合剂络合吸收-铁还原脱氮过程中，脱氮后气相中无N_2O生成，络合脱氮液中没有NO_2~-和NO_3~-存在，烟气中被脱除的NO全部转化成了氨；在无氧时，脱氮量与Fe~(x+)生成量的摩尔比约等于2／1，有氧时，脱氮量与Fe~(x+)生成量的摩尔比约等于1／2，在有氧条件下，脱氮时消耗的铁量为无氧时的4倍，其中1／4的铁粉消耗于还原NO，3／4的铁粉消耗于保持脱氮液的活性；无氧时，铁还原Fe~(2+)EDTA(NO)的反应式为：
2Fe~(2+)EDTA(NO)+Fe+8H~+→2Fe~(2+)EDTA+Fe(OH)_2+2NH_3 (0-2)Fe~(2+)EDTA络合-铁还原脱氮过程的总反应式为：
2NO+Fe+8H~+→Fe(OH)_2+2NH_3 (0-3)
NH_3+H~+←→NH_4~+ (0-4)有氧时，铁还原Fe~(2+)EDTA(NO)和Fe~(3+)的反应式为：
2Fe~(2+)EDTA(NO)+4Fe+10H~++2O_2+2H_2O→2Fe~(2+)EDTA+4Fe(OH)_2+2NH_3 (0-5)
2Fe~(3+)EDTA+Fe+2OH~-→2Fe~(2+)EDTA+Fe(OH)_2 (0-6)Fe~(2+)EDTA络合-铁还原脱氮过程的总反应式为：
2NO+4Fe+10H~++2O_2+2H_2O→4Fe(OH)_2+2NH_3 (0-7)
NH_3+H~+←→NH_4~+ (0-8)在实验条件下，络合脱氮液中氨的平衡浓度为1.159g·L~(-1)，用硫酸吸收氨的反应式为：
2NH_3+H_2SO_4→(NH_4)_2SO_4 (0-9)用磷酸吸收氨的反应式为：
NH_3+H_3PO_4→NH_4H_2PO_4(pH4.4～4.6) (0-10)
2NH_3+H_3PO_4→(NH_4)_2HPO_4(pH8.0～9.0) (0-11)
(3)研究了Fe~(2+)EDTA-铁屑脱氮。结果表明：在络合脱氮液中加有铁屑的情况下，Fe~(2+)EDTA浓度较低时，脱氮效率随着Fe~(2+)EDTA浓度的增加而迅速增长，而当络合脱氮液中Fe~(2+)EDTA浓度大于20mmol·L~(-1)以后，脱氮效率的增速变缓；在pH值2～6范围内，脱氮效率随pH值增加而缓慢增加，当起始pH值在6左右时，脱氮效率达到最高值，pH值继续升高时，脱氮效率较快降低；当铁屑用量较低时，脱氮效率随铁屑用量的增加而迅速增长，当100mL脱氮液中铁屑用量大于15g以后，脱氮效率的增幅变小；温度升高，脱氮效率增加，在65℃左右时，达到最大值，然后，脱氮效率随温度升高而降低；脱氮效率随烟气流量增加而降低；在实验的浓度范围内(120×10~(-6)～920×10~(-6))，进气NO浓度对脱氮效率几乎没有影响；脱氮效率随着烟气中O_2含量的增加而直线下降；在鼓泡反应器中，络合吸收-铁屑还原脱除烟气中NOx的最佳工艺参数为：吸收液中Fe~(2+)EDTA浓度20mmool·L~(-1)，起始pH值6.0，温度65℃，在此条件下，用两个鼓泡反应器串联，每个鼓泡反应器中吸收液体积100mL，铁屑用量15g，对O_2含量为10.5％的模拟烟气可取得90％以上的脱氮效率；过滤出脱氮液中的铁沉淀物即完成了脱氮液的再生，并恢复其脱氮性能；经过多次反复脱氮和再生以后，脱氮效果能够保持稳定；实验设计的用铁屑作填料的吸收塔可连续稳定地脱除烟气中的NO，在实验条件下可取得70％左右的脱氮效率，改变填料层高度和操作条件可进一步提高脱氮效率；用铁屑作填料的填料塔连续脱氮时，其脱氮效率的表达式为：
η=(C_1-C_2)/(C_1)×100≈(Y_1-Y_2)/(Y_1)×100=100{1-exp[-((k_2D_AC_(B_o))~(0.5)αZP)/Gm]} (0-12)由该模型所得的计算值能够和实验所得的实测值较好地相吻合，表明所建模型是合理的；用该模型可以很好地预测脱氮液流量、烟气流量和填料层高度等参数对脱氮效率的影响。
(4)研究了Fe~(2+)EDTA-铁粉脱氮。结果表明：当铁粉用量较低时，脱氮效率随铁粉用量的增加迅速增长，而当100mL脱氮液中的铁粉用量大于0.8g以后，脱氮效率的增长幅度变小，取得相同脱氮效率所需的铁粉用量约为铁屑用量的1／47；当铁粉目数较小时，脱氮效率随目数增加而迅速增加，而当铁粉目数大于200目以后，脱氮效率随粒径变化的增量减小；当反应器的搅拌速度小于900rpm时，脱氮效率随搅拌速度增加呈直线增长，而当搅拌速度达到900rpm以后，进一步加大搅拌速度，脱氮效率变化不大；液相络合-铁粉还原脱除烟气中NOx的最佳工艺参数为：脱氮液中Fe~(2+)EDTA浓度20mmol·L~(-1)，起始pH值6.0，温度65℃，铁粉用量0.8g，铁粉目数≥200目，搅拌速度900rpm，在此条件下，对O_2含量为10.5％的模拟烟气在搅拌反应器中可取得90％左右的脱氮效率；在优化实验条件下，铁粉在1台搅拌反应器中取得的脱氮效率和铁屑用2个鼓泡反应器串联所取得的效率相当；在实验条件下，铁粉还原Fe~(2+)EDTA(NO)为拟一级反应，在温度为25℃、45℃和65℃时，测得该反应的速率常数分别为2.64×10~(-2)min~(-1)、3.36×10~(-2)min~(-1)和4.28×10~(-2)min~(-1)，该反应的活化能为10096 J·mol~(-1)。
(5)研究了用脱氮过程中生成的铁沉淀物制备氧化铁红颜料。结果表明：X-射线衍射分析发现，脱氮过程所得的铁沉淀物风干后形成了Fe_2O_3·H_2O晶体，其三个主峰与Fe~(3+)OOH的标准谱线十分吻合，该风干沉淀物的外观为桔黄色，化学成分与铁黄相同。铁沉淀物经过滤、洗涤、干燥后，煅烧即成铁红颜料。正交实验结果表明，用脱氮过程生成的铁沉淀物制备铁红颜料的合适条件为：锻烧温度600-700℃，锻烧时间50min，沉淀物洗涤至水悬浮液的pH=7；在此条件下可生产出符合国家一级品标准的氧化铁红颜料。
(6)根据以上研究结果，结合实验过程的现象和操作，可将液相络合-铁屑(铁粉)还原-酸吸收回收法脱除烟气中NOx的过程分为如下三个阶段：
①亚铁螯合剂液相络合-铁屑(铁粉)还原阶段。脱氮液中的Fe~(2+)EDTA和溶解于其中的NO发生络合反应，形成亚硝酰络合物；同时，烟气中存在的O_2会将部分Fe~(2+)EDTA被氧化为Fe~(3+)EDTA：
NO(g)←→NO(aq) (0-13)
Fe~(2+)EDTA(aq)+NO(aq)←→Fe~(2+)EDTA(NO)(aq) (0-14)
4Fe~(2+)EDTA+O_2+4H~+→4Fe~(3+)EDTA+2H_2O (0-15)接着，在脱氮液中的铁屑(铁粉)将Fe~(2+)EDTA(NO)还原，生成NH_3和铁沉淀物，使Fe~(2+)EDTA再生；同时铁屑(铁粉)将Fe~(3+)EDTA还原为Fe~(2+)EDTA，维持了脱氮液的活性。
②酸吸收阶段。在液相络合-铁还原脱氮过程的初期，反应(0-7)生成的氨会在脱氮液中积累，随着脱氮过程的进行，液相中氨的浓度增加，氨会越来越多地从溶液中逸出。在一定的条件下，当脱氮进行一段时间后，脱氮所生成的氨和从液相中逸出的氨达到平衡，这时，从烟气中脱除NO的摩尔数将等于从液相中逸出的氨的摩尔数。用磷酸或硫酸吸收从脱氮液中逸出的氨，即可以制得磷酸铵或硫酸铵肥料。
③脱氮液的再生和铁沉淀物的综合利用阶段。已反应的铁屑(铁粉)以Fe~(x+)(OH)_x沉淀的形式存在于脱氮液中，过滤出脱氮液中的铁沉淀物，即完成了脱氮液的再生；铁沉淀物经洗涤、干燥后即被空气中的O_2氧化成了Fe_2O_3·H_2O晶体，再经过锻烧即可制得一级品氧化铁红颜料，过程的反应式如下：
2Fe(OH)_2+0.5O_2→Fe_2O_3·H_2O+H_2O (0-16)
Fe_2O_3·H_2O(?)Fe_2O_3+H_2O (0-17)
尽管美国和日本对液相络合法脱除烟气中NO_x进行了大量研究，但是至今没有解决吸收液的再生问题，因而阻碍了该法的进一步研究和工业化进程。本文提出的“Fe~(2+)螯合剂吸收-铁屑(铁粉)还原-酸吸收”回收法脱除NO_x是一种新的烟气脱氮思路，其特色和创新之处在于：①按先脱硫后脱氮的烟气处理思路，提出了用本回收法单独脱除烟气中NO_x的新思路；②提出了Fe~(2+)EDTA络合-铁还原-酸吸收回收法脱氮过程的反应机理；③提出了用铁屑(铁粉)还原亚铁亚硝酰络合物中的NO为NH_3，并还原液相中Fe~(3+)为Fe~(2+)，维持液脱氮活性的新方法，使脱氮液实现了真正意义上的再生和循环利用；④提出了用酸吸收脱氮过程中形成的氨，制备农用肥料，以回收烟气中氮资源的新方法；⑤提出了利用脱氮过程中生成的铁沉淀物制备氧化铁红颜料的新方法。
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 HSO_3~-/SO_3~(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 SO_2 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 Fe~(2+)EDTA solution complex with NO are researched. Results show that the absorption capacity of pure water for NO is little. Complex capacity of Fe~(2+)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 O_2 content in the simulated flue gas increases from 0% to 4.2%. Downtread of the capacity slower rapidly when the O_2 content increases further. Under the conditions of initial pH=6, Fe~(2+)EDTA concentration lOmmol·L~(-1), and temperature 60℃, the complex capacity of Fe~(2+)EDTA aqueous solution increases by a factor of 1000 compared with that of pure water, and it increases linearly with increasing the Fe~(2+)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 2.15×10~6L·mol~(-1), 6.54×10~5L·mol~(-1), and 2.67×10~5L·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:
k_2=E_i~2 K_(L,NO)~2/(C_(Bo)D_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 N_2O formed in the exhaust and no NO_2 and NO_3 ion formed in the denitrification solution. All of the removed NO in the flue gas is converted to NH_3 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.
2Fe~(2+)EDTA(NO)+Fe+8H~+→2Fe~(2+)EDTA+Fe(OH)_2+2NH_3 (0-2)
2NO+Fe+8H~+→Fe(OH)_2+2NH_3 (0-3)
NH3_+H~+←→NH_4~+ (0-4) When where is oxygen, the reaction mechanism can be expressed as
2Fe~(2+)EDTA(NO)+4Fe+10H~++O_2+2H_2O→2Fe~(2+)EDTA+4Fe(OH)_2+2NH_3 (0-5)
2Fe~(3+)EDTA+Fe+2OH→2Fe~(2+)EDTA+Fe(OH)_2 (0-6)
2NO+4Fe+10H~++2O_2+2H_2O→4Fe(OH)_2+2NH_3 (0-7)
NH_3+H~+←→NH_4~+ (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 mechanism
2NH_3+H_2SO_4→(NH_4)_2SO_4 (0-9)
NH_3+H_3PO_4→NH_4H_2PO_4(pH4.4～4.6) (0-10)
2NH_3+H_3PO_4→(NH_4)_2HPO_4 (pH8.0～9.0) (0-11)
(3)The denitrification technology with Fe~(2+)EDTA-iron filings system is investigated. The removal efficiency increases rapidly with the increasing of Fe~(2+)EDTA concentration when the Fe~(2+)EDTA concentration is low. However, the augmentation of the NO_x removal efficiency lower after the Fe~(2+)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 O_2 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 Fe~(2+)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 O_2 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 as
η=C_1-C_2/C_1×100≈Y_1-Y_2/Y_1×100=100{1-exp「-(k_2D_AC_(Bo)~(0.5)αZP/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 Fe~(2+)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 Fe~(2+)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 O_2 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 Fe~(2+)EDTA(NO) by iron powder is pseudo-first-order. The rate constants of the reaction are 2.64×10~(-2)min~(-1), 3.36×10~(-2)min~(-1), and 4.28×10~(-2)min~(-1)respectively 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 Fe_2O_3·H_2O crystal after dried by air. Three characteristic peaks of the crystal is corresponding with the standard spectrum of the Fe~(3+)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 Fe~(2+)EDTA in the absorption solutions and the reduction of Fe~(2+)EDTA(NO) by iron powder/filings. In this stage, the dissolved NO binds with Fe~(2+)EDTA in the solutions to form ferrous nitrosyl complex
NO(g)←→NO(aq) (0-13)
Fe~(2+)EDTA(aq)+NO(aq)←→Fe~(2+)EDTA(NO)(aq) (0-14) and Fe~(2+)EDTA is oxidized partially to Fe~(3+)EDTA by the oxygen in the flue gas simultaneously
4Fe~(2+)EDTA+O_2+4H~+←→4Fe~(3+)EDTA+2H_2O (0-15) Subsequently, the NO coordinated with Fe~~(2+)EDTA is reduced into NH_3 by iron powder/filings. The Fe~(2+)EDTA can be regenerated in the denitrification process, and the Fe~(3+)EDTA is reduced into Fe~(2+)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 Fe_20O_3·H_2O 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 as
2Fe(OH)_2+0.5O_2→Fe_2O_3·H_2O+H_2O (0-16)
Fe_2O_3·H_2O(?)Fe_2O_3+H_2O (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.①A 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.②The 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.③A 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.④A 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.
日本和美国从20世纪70年代开始就对液相络合法同时脱除烟气中SO_2和NO_x进行了大量研究。结果表明，对于亚铁氨羧螯合剂同时脱硫脱氮而言，处理过程中Fe~(2+)很容易被烟气中的O_2氧化为Fe~(3+)，而Fe~(3+)螯合剂与NO无亲和力，因此脱氮液的脱氮能力很快降低：此外，与螯合铁络合的NO能与溶液吸收SO_2而形成的SO_3~(2-)／HSO_3~-发生复杂的反应，形成一系列可溶于水的氮-硫化合物、S_2O_6~2-和N_2O二次气态污染物，这些液相产物在溶液中的积累，也会使脱氮液逐渐失去活性。因此，脱氮液难以再生和循环利用，这是阻碍该法进一步研究的根本原因。随后有人提出了用含-SH基的亚铁螯合剂进行烟气脱氮，该法研究中遇到的最大阻力也是其脱氮液的再生问题。
我们首次提出了“Fe~(2+)螯合剂络合吸收-铁还原-酸吸收”回收法脱除烟气中NO_x的新方法。首先用Fe~(2+)螯合剂将脱硫后烟气中的NO络合，使之进入液相，同时用铁屑(铁粉)将被络合的NO还原为氨，产生的氨随处理后的烟气带出，用磷酸或硫酸吸收，即可以制得磷酸铵或硫酸铵肥料；过程中所消耗的铁屑(铁粉)以铁沉淀物的形式从液相中分离，可用于生产氧化铁红颜料。本文主要研究内容和结果如下：
(1)研究了Fe~(2+)EDTA溶液络合吸收NO的影响因素和动力学。结果表明：纯水对NO几乎没有吸收能力，一定体积和浓度的Fe~(2+)EDTA溶液对NO的络合能力随时间延长而减弱，络合一定时间后即达到平衡；烟气中O_2含量从0％增加到4.2％，NO络合容量下降90％，O_2含量进一步增加，络合容量的下降趋势迅速减缓；在络合液起始pH=6，Fe~(2+)EDTA浓度为10mmol·L~(+1)，温度60℃时，NO的络合容量为NO在纯水中溶解量的1000倍，NO的络合容量随络合液中Fe~(2+)EDTA浓度的增加呈直线增长；NO的络合容量随温度的升高而降低，在络合液起始pH=6，Fe~(2+)EDTA浓度为15mmol·L~(-1)条件下，30℃时Fe~(2+)EDTA溶液对NO的络合容量为80℃时络合容量的5.3倍；当pH值较低时，NO的络合容量随络合液起始pH值的增加而增加，当pH值达到8左右时，络合容量达到最大值；继续增加pH值，络合容量逐渐降低；在25℃、45℃和65℃时，络合反应的平衡常数分别为2.15×10~6L·mol~(-1)、6.54×10~3L·mol~(-1)和2.67×10~5L·mol~(-1)；在实验条件下，Fe~(2+)EDTA络合NO的反应可以看成拟一级反应；络合反应的速率常数可通过下式求得：
k_2=E_i~2k_(L，NO)~2/(C_(B_o)D_A) (0-1)
(2)研究了铁还原亚铁亚硝酰络合物和Fe~(3+)的反应机理，并对酸吸收还原过程中产生氨的机理进行了讨论。结果表明：在Fe~(2+)螯合剂络合吸收-铁还原脱氮过程中，脱氮后气相中无N_2O生成，络合脱氮液中没有NO_2~-和NO_3~-存在，烟气中被脱除的NO全部转化成了氨；在无氧时，脱氮量与Fe~(x+)生成量的摩尔比约等于2／1，有氧时，脱氮量与Fe~(x+)生成量的摩尔比约等于1／2，在有氧条件下，脱氮时消耗的铁量为无氧时的4倍，其中1／4的铁粉消耗于还原NO，3／4的铁粉消耗于保持脱氮液的活性；无氧时，铁还原Fe~(2+)EDTA(NO)的反应式为：
2Fe~(2+)EDTA(NO)+Fe+8H~+→2Fe~(2+)EDTA+Fe(OH)_2+2NH_3 (0-2)Fe~(2+)EDTA络合-铁还原脱氮过程的总反应式为：
2NO+Fe+8H~+→Fe(OH)_2+2NH_3 (0-3)
NH_3+H~+←→NH_4~+ (0-4)有氧时，铁还原Fe~(2+)EDTA(NO)和Fe~(3+)的反应式为：
2Fe~(2+)EDTA(NO)+4Fe+10H~++2O_2+2H_2O→2Fe~(2+)EDTA+4Fe(OH)_2+2NH_3 (0-5)
2Fe~(3+)EDTA+Fe+2OH~-→2Fe~(2+)EDTA+Fe(OH)_2 (0-6)Fe~(2+)EDTA络合-铁还原脱氮过程的总反应式为：
2NO+4Fe+10H~++2O_2+2H_2O→4Fe(OH)_2+2NH_3 (0-7)
NH_3+H~+←→NH_4~+ (0-8)在实验条件下，络合脱氮液中氨的平衡浓度为1.159g·L~(-1)，用硫酸吸收氨的反应式为：
2NH_3+H_2SO_4→(NH_4)_2SO_4 (0-9)用磷酸吸收氨的反应式为：
NH_3+H_3PO_4→NH_4H_2PO_4(pH4.4～4.6) (0-10)
2NH_3+H_3PO_4→(NH_4)_2HPO_4(pH8.0～9.0) (0-11)
(3)研究了Fe~(2+)EDTA-铁屑脱氮。结果表明：在络合脱氮液中加有铁屑的情况下，Fe~(2+)EDTA浓度较低时，脱氮效率随着Fe~(2+)EDTA浓度的增加而迅速增长，而当络合脱氮液中Fe~(2+)EDTA浓度大于20mmol·L~(-1)以后，脱氮效率的增速变缓；在pH值2～6范围内，脱氮效率随pH值增加而缓慢增加，当起始pH值在6左右时，脱氮效率达到最高值，pH值继续升高时，脱氮效率较快降低；当铁屑用量较低时，脱氮效率随铁屑用量的增加而迅速增长，当100mL脱氮液中铁屑用量大于15g以后，脱氮效率的增幅变小；温度升高，脱氮效率增加，在65℃左右时，达到最大值，然后，脱氮效率随温度升高而降低；脱氮效率随烟气流量增加而降低；在实验的浓度范围内(120×10~(-6)～920×10~(-6))，进气NO浓度对脱氮效率几乎没有影响；脱氮效率随着烟气中O_2含量的增加而直线下降；在鼓泡反应器中，络合吸收-铁屑还原脱除烟气中NOx的最佳工艺参数为：吸收液中Fe~(2+)EDTA浓度20mmool·L~(-1)，起始pH值6.0，温度65℃，在此条件下，用两个鼓泡反应器串联，每个鼓泡反应器中吸收液体积100mL，铁屑用量15g，对O_2含量为10.5％的模拟烟气可取得90％以上的脱氮效率；过滤出脱氮液中的铁沉淀物即完成了脱氮液的再生，并恢复其脱氮性能；经过多次反复脱氮和再生以后，脱氮效果能够保持稳定；实验设计的用铁屑作填料的吸收塔可连续稳定地脱除烟气中的NO，在实验条件下可取得70％左右的脱氮效率，改变填料层高度和操作条件可进一步提高脱氮效率；用铁屑作填料的填料塔连续脱氮时，其脱氮效率的表达式为：
η=(C_1-C_2)/(C_1)×100≈(Y_1-Y_2)/(Y_1)×100=100{1-exp[-((k_2D_AC_(B_o))~(0.5)αZP)/Gm]} (0-12)由该模型所得的计算值能够和实验所得的实测值较好地相吻合，表明所建模型是合理的；用该模型可以很好地预测脱氮液流量、烟气流量和填料层高度等参数对脱氮效率的影响。
(4)研究了Fe~(2+)EDTA-铁粉脱氮。结果表明：当铁粉用量较低时，脱氮效率随铁粉用量的增加迅速增长，而当100mL脱氮液中的铁粉用量大于0.8g以后，脱氮效率的增长幅度变小，取得相同脱氮效率所需的铁粉用量约为铁屑用量的1／47；当铁粉目数较小时，脱氮效率随目数增加而迅速增加，而当铁粉目数大于200目以后，脱氮效率随粒径变化的增量减小；当反应器的搅拌速度小于900rpm时，脱氮效率随搅拌速度增加呈直线增长，而当搅拌速度达到900rpm以后，进一步加大搅拌速度，脱氮效率变化不大；液相络合-铁粉还原脱除烟气中NOx的最佳工艺参数为：脱氮液中Fe~(2+)EDTA浓度20mmol·L~(-1)，起始pH值6.0，温度65℃，铁粉用量0.8g，铁粉目数≥200目，搅拌速度900rpm，在此条件下，对O_2含量为10.5％的模拟烟气在搅拌反应器中可取得90％左右的脱氮效率；在优化实验条件下，铁粉在1台搅拌反应器中取得的脱氮效率和铁屑用2个鼓泡反应器串联所取得的效率相当；在实验条件下，铁粉还原Fe~(2+)EDTA(NO)为拟一级反应，在温度为25℃、45℃和65℃时，测得该反应的速率常数分别为2.64×10~(-2)min~(-1)、3.36×10~(-2)min~(-1)和4.28×10~(-2)min~(-1)，该反应的活化能为10096 J·mol~(-1)。
(5)研究了用脱氮过程中生成的铁沉淀物制备氧化铁红颜料。结果表明：X-射线衍射分析发现，脱氮过程所得的铁沉淀物风干后形成了Fe_2O_3·H_2O晶体，其三个主峰与Fe~(3+)OOH的标准谱线十分吻合，该风干沉淀物的外观为桔黄色，化学成分与铁黄相同。铁沉淀物经过滤、洗涤、干燥后，煅烧即成铁红颜料。正交实验结果表明，用脱氮过程生成的铁沉淀物制备铁红颜料的合适条件为：锻烧温度600-700℃，锻烧时间50min，沉淀物洗涤至水悬浮液的pH=7；在此条件下可生产出符合国家一级品标准的氧化铁红颜料。
(6)根据以上研究结果，结合实验过程的现象和操作，可将液相络合-铁屑(铁粉)还原-酸吸收回收法脱除烟气中NOx的过程分为如下三个阶段：
①亚铁螯合剂液相络合-铁屑(铁粉)还原阶段。脱氮液中的Fe~(2+)EDTA和溶解于其中的NO发生络合反应，形成亚硝酰络合物；同时，烟气中存在的O_2会将部分Fe~(2+)EDTA被氧化为Fe~(3+)EDTA：
NO(g)←→NO(aq) (0-13)
Fe~(2+)EDTA(aq)+NO(aq)←→Fe~(2+)EDTA(NO)(aq) (0-14)
4Fe~(2+)EDTA+O_2+4H~+→4Fe~(3+)EDTA+2H_2O (0-15)接着，在脱氮液中的铁屑(铁粉)将Fe~(2+)EDTA(NO)还原，生成NH_3和铁沉淀物，使Fe~(2+)EDTA再生；同时铁屑(铁粉)将Fe~(3+)EDTA还原为Fe~(2+)EDTA，维持了脱氮液的活性。
②酸吸收阶段。在液相络合-铁还原脱氮过程的初期，反应(0-7)生成的氨会在脱氮液中积累，随着脱氮过程的进行，液相中氨的浓度增加，氨会越来越多地从溶液中逸出。在一定的条件下，当脱氮进行一段时间后，脱氮所生成的氨和从液相中逸出的氨达到平衡，这时，从烟气中脱除NO的摩尔数将等于从液相中逸出的氨的摩尔数。用磷酸或硫酸吸收从脱氮液中逸出的氨，即可以制得磷酸铵或硫酸铵肥料。
③脱氮液的再生和铁沉淀物的综合利用阶段。已反应的铁屑(铁粉)以Fe~(x+)(OH)_x沉淀的形式存在于脱氮液中，过滤出脱氮液中的铁沉淀物，即完成了脱氮液的再生；铁沉淀物经洗涤、干燥后即被空气中的O_2氧化成了Fe_2O_3·H_2O晶体，再经过锻烧即可制得一级品氧化铁红颜料，过程的反应式如下：
2Fe(OH)_2+0.5O_2→Fe_2O_3·H_2O+H_2O (0-16)
Fe_2O_3·H_2O(?)Fe_2O_3+H_2O (0-17)
尽管美国和日本对液相络合法脱除烟气中NO_x进行了大量研究，但是至今没有解决吸收液的再生问题，因而阻碍了该法的进一步研究和工业化进程。本文提出的“Fe~(2+)螯合剂吸收-铁屑(铁粉)还原-酸吸收”回收法脱除NO_x是一种新的烟气脱氮思路，其特色和创新之处在于：①按先脱硫后脱氮的烟气处理思路，提出了用本回收法单独脱除烟气中NO_x的新思路；②提出了Fe~(2+)EDTA络合-铁还原-酸吸收回收法脱氮过程的反应机理；③提出了用铁屑(铁粉)还原亚铁亚硝酰络合物中的NO为NH_3，并还原液相中Fe~(3+)为Fe~(2+)，维持液脱氮活性的新方法，使脱氮液实现了真正意义上的再生和循环利用；④提出了用酸吸收脱氮过程中形成的氨，制备农用肥料，以回收烟气中氮资源的新方法；⑤提出了利用脱氮过程中生成的铁沉淀物制备氧化铁红颜料的新方法。
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 HSO_3~-/SO_3~(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 SO_2 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 Fe~(2+)EDTA solution complex with NO are researched. Results show that the absorption capacity of pure water for NO is little. Complex capacity of Fe~(2+)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 O_2 content in the simulated flue gas increases from 0% to 4.2%. Downtread of the capacity slower rapidly when the O_2 content increases further. Under the conditions of initial pH=6, Fe~(2+)EDTA concentration lOmmol·L~(-1), and temperature 60℃, the complex capacity of Fe~(2+)EDTA aqueous solution increases by a factor of 1000 compared with that of pure water, and it increases linearly with increasing the Fe~(2+)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 2.15×10~6L·mol~(-1), 6.54×10~5L·mol~(-1), and 2.67×10~5L·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:
k_2=E_i~2 K_(L,NO)~2/(C_(Bo)D_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 N_2O formed in the exhaust and no NO_2 and NO_3 ion formed in the denitrification solution. All of the removed NO in the flue gas is converted to NH_3 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.
2Fe~(2+)EDTA(NO)+Fe+8H~+→2Fe~(2+)EDTA+Fe(OH)_2+2NH_3 (0-2)
2NO+Fe+8H~+→Fe(OH)_2+2NH_3 (0-3)
NH3_+H~+←→NH_4~+ (0-4) When where is oxygen, the reaction mechanism can be expressed as
2Fe~(2+)EDTA(NO)+4Fe+10H~++O_2+2H_2O→2Fe~(2+)EDTA+4Fe(OH)_2+2NH_3 (0-5)
2Fe~(3+)EDTA+Fe+2OH→2Fe~(2+)EDTA+Fe(OH)_2 (0-6)
2NO+4Fe+10H~++2O_2+2H_2O→4Fe(OH)_2+2NH_3 (0-7)
NH_3+H~+←→NH_4~+ (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 mechanism
2NH_3+H_2SO_4→(NH_4)_2SO_4 (0-9)
NH_3+H_3PO_4→NH_4H_2PO_4(pH4.4～4.6) (0-10)
2NH_3+H_3PO_4→(NH_4)_2HPO_4 (pH8.0～9.0) (0-11)
(3)The denitrification technology with Fe~(2+)EDTA-iron filings system is investigated. The removal efficiency increases rapidly with the increasing of Fe~(2+)EDTA concentration when the Fe~(2+)EDTA concentration is low. However, the augmentation of the NO_x removal efficiency lower after the Fe~(2+)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 O_2 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 Fe~(2+)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 O_2 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 as
η=C_1-C_2/C_1×100≈Y_1-Y_2/Y_1×100=100{1-exp「-(k_2D_AC_(Bo)~(0.5)αZP/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 Fe~(2+)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 Fe~(2+)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 O_2 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 Fe~(2+)EDTA(NO) by iron powder is pseudo-first-order. The rate constants of the reaction are 2.64×10~(-2)min~(-1), 3.36×10~(-2)min~(-1), and 4.28×10~(-2)min~(-1)respectively 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 Fe_2O_3·H_2O crystal after dried by air. Three characteristic peaks of the crystal is corresponding with the standard spectrum of the Fe~(3+)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 Fe~(2+)EDTA in the absorption solutions and the reduction of Fe~(2+)EDTA(NO) by iron powder/filings. In this stage, the dissolved NO binds with Fe~(2+)EDTA in the solutions to form ferrous nitrosyl complex
NO(g)←→NO(aq) (0-13)
Fe~(2+)EDTA(aq)+NO(aq)←→Fe~(2+)EDTA(NO)(aq) (0-14) and Fe~(2+)EDTA is oxidized partially to Fe~(3+)EDTA by the oxygen in the flue gas simultaneously
4Fe~(2+)EDTA+O_2+4H~+←→4Fe~(3+)EDTA+2H_2O (0-15) Subsequently, the NO coordinated with Fe~~(2+)EDTA is reduced into NH_3 by iron powder/filings. The Fe~(2+)EDTA can be regenerated in the denitrification process, and the Fe~(3+)EDTA is reduced into Fe~(2+)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 Fe_20O_3·H_2O 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 as
2Fe(OH)_2+0.5O_2→Fe_2O_3·H_2O+H_2O (0-16)
Fe_2O_3·H_2O(?)Fe_2O_3+H_2O (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.①A 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.②The 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.③A 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.④A 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.
引文
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