(Tangshan Keyuan Environmental Protection Technology & Equipment Co.,Ltd Tangshan hebei 063300)
Abstract: Based on the structural characteristics of pyrolysis gasifier, this papar briefly describes coal nitrogen transformation and NH3 and HCN generation in gasifiication process; indicate that the main source of NH3 is in gasification process, then pyrolysis process and HCN in gas mainly comes from low temperature pyrolysis process in pyrolysis section. Also analyzes the principles and processes to the removal of NH3 and HCN in gas by using phenolic wastewater concentrated evaporation treatment and gas wet desulfurization process in pyrolysis clean gas station. Noting that in the process of phenolic wastewater concentrated evaporation treatment, some parts of NH3 and HCN in the gas can be oxidated into NO, and then reduced to N2; In the process of wet desulfurization process, the NH3 in gas will dissolve into desulfurization solutions and have desulfurization reaction with H2S in the gas, so some parts of NH3 in gas will be removed.
Keywords: pyrolysis gasifier; pyrolyzation; gasification; reduction; NH3; HCN; NOx; N2; phenolic wastewater concentrated evaporation treatment; wet desulfurization
1. Introduction
Coal gasification is one of the effective ways of coal transformation and reasonable utilization that conform to China's development of clean coal technology and energy diversification strategy. However, one of the most important nodes in the comprehensive coal utilization chain is how to combine the coal utilization together with strictly control of chain reaction problems of environmental pollution. NOx is one of the main harmful gases which generated during the process of coal transformation. Atmospheric fixed bed coal gasification is earlier clean coal technology in China which has always been the main gas supply technology of coal gasification in industrial fuel gas. For the removal of NOx precursor (NH3 and HCN) in the process of gas generating or purifying, it can effective control the generation of NOx during gas combustion.
2. Introduction of pyrolysis clean gas station
Please find the diagram of pyrolysis coal gasifier in picture 1. Gas generation process of pyrolysis coal gasifier is as follows: Air and steam are filled into gasifier from the bottom of gasifier as gasifying agent and have redox reaction with the carbocoal structure of coal in gasification section under the condition of high temperature, then forming coal gas which is mainly contain CO and H2. The coal gas upward runs into pyrolysis section and exchange heat with the moving down coal, the coal will be dried and distilled by the heat. Meanwhile, part of pyrolysis gas which is mainly high calorific value of alkane gas will be generated, that part of upward coal gas and pyrolysis gas will flow out from gas outlet.
Picture 1 Diagram of pyrolysis coal gasifier
The main reactions of coal in gasifier are pyrolyzation and gasification. There is no obvious boundary between dry layer, pyrolysis layer, reduction layer, oxidation layer and ash layer etc. in coal gasifer. The coal pyrolysis process is partially low temperature pyrolysis in pyrolysis section and the other part of pyrolysis is medium and high temperature pyrolysis in the upper section of reduction layer; the main gasification reaction（3）and （4）are reacted in reduction layer, and part of the gasification reaction such as（2）reacted in oxidation layer; nearly all the coal combustion reaction in (1) are reacted in oxidation layer.
C+O2=CO2；△H=-409 KJ/mol （1）
2C+O2=2CO；△H=-221.2 KJ/mol （2）
CO2+C=2CO；△H=162KJ/mol （3）
C+H2O=CO+H2；△H=119 KJ/mol （4）
The process of pyrolysis clean gas station is in picture 2. The outlet coal gas firstly flow into gas washing tower for primary cooling which can decrease the temperature of gas from 350-400℃ to 70-80℃, then it enters into the electrostatic tar collector for tar removing and enters into the indirect cooler for the secondary cooling that can decrease the temperature of gas to 35-45℃, then the cooling gas flow into the electrostatic light oil device for the removing of condensed water and light oil which are generated during the cooling process. Afterwards, the gas will be transported to wet desulfurization tower by gas booster for removal of H2S, finally, after the removal of droplet by spray catcher, the clean gas will be transported to the users.
Picture 2 Process of pyrolysis clean gas station
3. Coal nitrogen transformation and formation of nitride during the process of gas producing in pyrolysis coal gasifier
The flow chart of coal nitrogen transformation during the process of gas producing is in picture 3. Part of the coal nitrogen are transformed in gaseous tar, another part is transformed in coal gas in the form of NH3, HCN and N2 and the other part of nitrogen will be residued in ash.
Picture 3 Coal nitrogen transformation during the process of gas producing
Chang Liping【1】 and Zhao Wei etc.【2】 found in the experiment that when the heating temperature is 500-600 ℃, they can test the existing of NH3 and HCN. The higher the temperature is, the more NH3 and HCN will be generated by pyrolyzation. Moreover, the slow heating is benefit to generating of NH3 and HCN. The pyrolysis section in pyrolysis coal gasifier is 6 meters height and 4 meters thick. The pyrolytic thermal medium in pyrolysis section is all the coal gas generated in gasifying section. the thermal medium temperature is about 500-600℃ and the coal in pyrolysis section have slowly low temperature pyrolysis process under this temperature, that can remove volatile and form semicoke which contain less than 10% (or =10%) of volatile. Then the semicoke enters into reduction layer for further medium pyrolysis in the upper section of reduction layer at 700-800℃ so that the volatile can be removed completely. During the volatile removal process, NH3 and HCN will be generated at the same time, the higher the pyrolysis temperature is, the more generating of NH3 and HCN will have.
NH3 and HCN are not only produced during the pyrolysis process of coal but also exist during the process of coal gasification, their generation are related to the reaction atmosphere and temperature. Zhao Wei and other professional personnels【3】 found that, the nitrogen in coal can transform into NH3 and HCN under the atmosphere of CO2 and steam, also their production will increase with the rise of temperature. It was showed in the testing result of Chang Liping【1】: with the increase of reaction temperature , auxo-action of CO2 to the generating of NH3 and HCN is obvious; the main factor that influence the generating of NH3 and HCN in O2 gasification process is reaction temperature, the addition of O2 makes the coal’s low temperature gasification reactivity increased obviously and the generating temperature of NH3 and HCN obviously decreased. In the bottom of reduction layer of prolysis coal gasifier, the coal reacts with CO2 and steam that mentioned in the above（3）and（4）reaction, in the upper section of oxidation layer, coal have reduction reaction under the atmosphere of O2 and steam that showed in the above （2） reaction. Coke nitrogen is the main source of NH3 and HCN during the gasification process, the formation rate and production are related to the reaction atmosphere and reaction temperature.
Liu Meirong【4】 found from the experiment that there is only NH3 generated during the process of coke gasification while there are NH3 and HCN in the coal gasification process, so it can be considered that the generation of HCN during the coal gasification process is come from the volatile gasification. Therefore, the HCN in pyrolysis coal gas is mainly from the low temperature pyrolysis process in pyrolysis section, and the other part is from the medium pyrolysis process and volatiles gasification in the upper reduction section. As the production of NH3 and HCN are obviously larger than pyrolysis conditions【1,3】 under steam atmosphere, so we can know that during the process of gas generating of pyrolysis coal gasifier, gasification process is the main source of NH3 and HCN, while the pyrolysis process takes the second place. Gasifing the coal from a coal mine with double-stage coal gasifier in Datong, the content of NH3 in lower section of coal gas is 597mg/Nm3 and 619mg/Nm3 in upper section. It can be seen that each Nm3 of NH3 in upper section of coal gas only increased 22mg during the low temperature pyrolysis process in pyrolysis section that implies the production of NH3 during the gasification process is much higher than pyrolysis process.
There is only part of coke participate in reduction reaction in coal gasifier, the coke which didn’t join the reduction reaction as well as other part of coke enter into the combustion area of oxide layer and partially react with O2 to form NO， the transformation rate from nitrogen to NO is related to reaction temperature. CO, semicoke as well as volatile and NH3 dissolved out from coal all have reducing action to NO. CO will start to reduce NO to N2 over 600℃, its temperature range of reduction reaction is around 600-1000℃, the temperature for semicoke to reduce NO to N2 is around 680℃, When the temperature is in the range of 850-1100℃, NH3 can achieve a better reduction【5,6,7】 effect on NO. The NO and other gases that generated in combustion area of oxide layer go upward into the reduction layer. CO, semicoke in reduction layer as well as volatile dissolved out from coal and NH3 etc. all have reducing action to NO, moreover, the temperature of reduction layer is precisely conform to the required temperature range of NO reduction, so NO in reduction layer will be reduced into N2 and flow into coal gas.
4. Nitrogen removal of pyrolysis clean gas station
4.1 NH3 and HCN removal during the process of phenol water treatment
The phenol water of pyrolysis clean gas station comes from indirect cooler, electrostatic light oil remover device , gas booster and condensate water from factory’s gas pipeline. The literature【8】 introduced the process of phenol water treatment with " concentrated evaporation treatment ", it centralized collected each node condensation of phenol water and preprocess with filtering, quenched and tempered etc., then the pre-treated phenol water will be pumped into water cooling tank of coal gasifier and be gasified by part of gas sensible heat, meanwhile, part of benzene and phenolics with low boiling point in phenolic wastewater will be gasified into steam and bubbled into the bottom of coal gasifier as gasifying agent. Benzene and phenol substance are oxidative cracked into water and carbon dioxide in high temperature of oxide layer. The solubility of NH3 which was generated during the pyrolysis and gasification process in water is 1:700, HCN can miscible with ammonia, benzene and water etc.. In the gas cooling process, part of NH3 and HCN soluble in condensed phenol water, NH3 and HCN are easy to volatile after heating, so during the process of “concentrated evaporation treatment”, with the heating evaporation of phenol water, NH3 and HCN volatilizes quickly and mixes with phenol water vapor. After NH3 and HCN flow into the gasifier together with phenol water vapor, they through the ash layer and enter into the high temperature of oxide layer, NH3 and HCN will be oxidized into NO in this layer. Along with the gas generated in oxide layer flow into reduction layer, NH3, semicoke, CO and volatile dissolved out from coal in reduction layer all have strong reducing action【3,4,5】 to NO, most of NO can be reduced to N2 here.
4.2 NH3 removal of wet desulfurization process
During the spray process of desulfurization solution to coal gas in wet desulfurization tower, the residual NH3 and HCN in coal gas dissolve in desulfurization solution, while the NH3 dissolved in desulfurization solution can totally or partially replace Na2CO3 that can have desulfurization reaction with H2S in coal so that to decrease the addition amount of Na2CO3 in desulfurization solution. Zhong Jinming【9】 and Guo Jinhua etc.【10】separately described liquid phase catalytic oxidation method of wet desulfurization process with HPF and Cobalt sulfonated phthalein cyanide as catalyst, NH3.H2O as absorbent, the source of NH3.H2O is all from the NH3 in coal gas. Moreover, although the proportion between ammonia and sulfur in coal gas is only 0.71, when circulating 4-5g of desulfurization solution each hour, the desulfurization efficiency can also reach 99%. There is a double-stage coal gasifier which has 600-700 mg/Nm3 of H2S in coal gas in a Shanxi enterprise, it only uses NH3 in coal gas as absorbent and doesn’t need to add with sodium carbonate, the desulfurization degree of droplet is around 5.7 and its desulphurization efficiency can reach 99%. It can be seen that, during the process of wet desulfurization process, the NH3 from coal gas can replace NaCO3 as desulfurization absorber, it can not only remove the NH3 in coal gas but also saved the cost of gas desulfurization。
5. Conclusion
(1) The gasifying process of Pyrolysis coal gasifier is the main source of NH3 and HCN, while pyrolysis process takes the second place; HCN in coal gas is mainly come from the low temperature carbonization of pyrolysis in pyrolysis section and the other part is from medium temperature pyrolysis in upper section of reduction layer and gasification of volatiles
(2)During the gas cooling process, part of NH3 and HCN in coal gas dissolved into condensed phenol water and in the phenol water treatment process with “concentrated evaporation treatment”， NH3 , HCN and phenol water vapor enter into high temperature of oxide layer and be oxidized into NO, then NO will be reduced into N2 by semicoke, CO and dissolved volatile and NH3 in reduction layer, so that to remove part of NH3 and HCN in coal gas.
(3) During the wet desulfurization process, the residual NH3 and HCN in coal gas dissolved into desulfurization solution, and the dissolved NH3 in desulfurization solution have desulfurization reaction with H2S from coal gas, it means part of NH3 in coal gas can be removed during the wet desulfurization process.
