THERMAL EQUILIBRIUM AND EQUIPMENT ENERGY CONSUMPTION ANALYSIS ON SCR DENITRIFICATION SYSTEM IN COAL-FIRED POWER PLANT

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1 THERMAL EQUILIBRIUM AND EQUIPMENT ENERGY CONSUMPTION ANALYSIS ON SCR DENITRIFICATION SYSTEM IN COAL-FIRED POWER PLANT Shihe Chen, Yuechao Wang, Fengping Pan, Lingling Shi, Zhiqiang Pang, Juanjuan Ren Electric Power Research Institute of Guangdong Grid Copany, Guangzhou 518, China ABSTRACT Selective Catalytic Reduction ( SCR) denitrification syste can effectively reove the nitrogen oxides in coal-fired power plants.however, the SCR syste will definitely have soe influence on the operation of boiler, such as inlet teperature distribution of furnace flue, boiler efficiency and equipent energy consuption. This paper studied the theral equilibriu of SCR syste to identify the flue gas teperature variation, and the heat loss of flue gas and boiler efficiency was also re-calculated in this paper. In addition, this paper build energy consuption odel for equipent energy consuption analysis of SCR syste. Based on the above discussion, 66MW unit was selected as the case study to validate the proposed theral equilibriu analysis of SCR syste. Fro the case calculation, the teperature variation of flue gas is less than 2 for SCR syste, and the variation of heat loss of flue gas and boiler efficiency caused by the SCR syste is not over. 5%, and the additional coal consuption caused by additional power consuption of the SCR syste is about.48 g/kwh. KEYWORDS: SCR, theral equilibriu analysis, equipent energy consuption analysis 1. Introduction In China,nitrogen oxides NO and NO 2 (collectively called NO X) produced fro burning coal is a ajor source of atospheric nitrogen oxide pollutants, and coal fired power stations produce a substantial aount of flue gas, where NO X accounts for ore than 4% of total NO X eissions [1].Currently, selective catalytic reduction (SCR) is regarded as one of the ost iportant technologies both in the doestic and overseas due to its high NO X reoval efficiency, low aonia slip rate and other advantages [2]. With the increasing awareness of environental protection, the power plant denitration syste attracts lots of researchers.sun et al. investigated catalyst arsenic poisoning, put forward ethods to reduce the ipact of arsenic poisoning [3] ; Lin et al. who studied the aonia evaporator within SCR in detail, gave out the design guidelines and precautions for the various coponents of the SCR syste [4] ; Yang Xuzhong gave soe suggestions on the catalyst layer for iproving denitration efficiency in view of the high catalyst price [5],which provided references for the installation of denitration syste. Currently studies have been reported in the literature on analysis of new types of catalysts and the overall perforance of SCR systes in specific theral power units.however, those analyses do not account forthe detailed quantitative analysis of the whole heat balance and energy consuption of the denitration syste. To fill this void, this paper focuses on quantitative analysis of heat balance and energy consuption of the coal-fired power plant SCR syste. The SCR unit is generally arranged in the flue pipe tail section between the

2 flue gas econoizer and air preheater. The introduction of the denitration syste will affect the teperature distribution of boiler heating surface, which includes that the cheical reaction heat generated in denitration syste will increase the flue gas teperature, and the use of dilution, the heat loss and the air leakage of denitration syste will all lead to the decrease of flue gas teperature. These factors will affect the boiler flue gas teperature and the exhaust volue, and ultiately affect the boiler flue gas heat loss. Thus, installation of denitration syste inevitably leads to additional energy consuption. By analyzing the energy consuption of SCR syste, energy saving optiization ethod can be found, which could further iprove the energy-saving effect of power plants SCR syste, and to increase plant the econoic and environental benefits. 2. The SCR syste and ajor energy-consuing equipent 2.1 SCR unit description aonia flow boiler aonia injection grid dilution fan aonia gas ixer air preheater FGD priary air fan ash collector induced draft fan aonia evaporator liquid aonia tank chiney aonia buffer tank Figure 1. The SCR syste technical process The SCR syste technical process is shown in Fig.1. Liquid aonia is stored in the liquid aonia tank and then enters into the SCR reactor after it evaporates in aonia evaporator. Aonia gas achieves stable pressure in the aonia buffer tank, and then enter the aonia gas ixer. The ixture is injected into the SCR reactor through the aonia injection grid and then aonia ixes with gas thoroughly. Aonia reacts with NO X in the flue gas to for N 2 and H 2O over catalysts, which effectively reove the NO X in flue gas. The ain reactions are as follows: 2.2 The ajor energy-consuing equipent 1 3 NO NH3 O2 N2 H2O (1) NO2 O2 2NH3 N2 3H2O (2) 2 2 The ajor energy-consuing equipent of SCR syste are: dilution fan, draught fan, aonia evaporator, aonia unloading copressor. Since the energy consuption of these devices is closely related to the deand for aonia, therefore accurate calculation of the required aount of aonia is essential to build the energy consuption odel. NO X quality eission concentration is defined as C NOx (g/n3), calculated by NO 2(standard state,dry flue gas,6%o2). The econoizer outlet excess air coefficient is, then the quality eission concentration of NO NO 2 are:

3 C NO CNO 3 x C NOx (3) C NO2 Aonia consuption can be calculated by the following forula [6] : q.5cno 1.4.7C x NOx (4) H2 O H2 O qvg (1 ) CNO 17 qvg (1 ) CNO NH3 6 6 Aong which: NOx 1 C NO a 22.4 CNO (5) Where q NH3 is liquid aonia hours consuption (kg/h); q vg is the actual flue gas flow of SCR inlet (N 3 /h); HO 2 the reactor inlet flue gas ( g/n 3 ); SCR inlet aonia/no X ole rate; 2.3 The dilution fan is the volue fraction of vapor in actual flue gas (%); C NO2 NOx C NO is the NO ass concentration in is the NO and NO 2 concentration of inlet flue gas (g/n 3 ). M is is the SCR syste efficiency (%); a is aonia slip rate (PP). Dilution fan is selected according to the requireent of dilution air volue in the SCR syste when the concentration of NO x in flue gas is the highest. Generally, the aonia ass ratio of the diluted ixture is less than 5% and the dilution fan pressure is controlled at about 5.5 KPa [7]. Dilution air volue is estiated as follows: q V air 95 5 q VNH3 1 q (6).771 VNH3 NH3 Where V air is dilution air volue flow in standard state ( 3 /h), q VNH3 is NH 3 volue flow in standard state ( 3 /h), q NH3 is NH 3 ass flow (kg/h). The fan shaft power calculation forula is as follows: Vair P N (7) Where N is dilution fan shaft power (kw); Vair is dilution fan inlet flow ( 3 /h). P is dilution fan total pressure (Pa); η is dilution fan efficiency (%). 2.4 The draught fan The installation of denitration syste will produce pressure drop in flue pipe and inside the reactor [8]. And thus, it is necessary to increase the capacity of the draft fan. The draft fan power increase can be calculated as follows: (( t 273) / 273) pi q Pe 1236 Where T is the induced draft fan work teperature ( ); p i is the pressure drop caused by installation of SCR vg (8)

4 syste (Pa). η is induced draft fan efficiency (%). 2.5 Theaonia evaporator In general, SCR syste ainly adopts the spiral tube aonia evaporator, and inside the tube is liquid aonia, outside the tube is war water. liquid aonia is heated by the low grade stea extraction sprayed directly into war water and the evaporation of liquid aonia begins when the teperature up to 4 or so. Saturated stea pressure required for this process is about.9mpa, and teperature is about 17 [9]. The consuption of stea flow coputation forula is: q stea qnh ( h 3 2 h1) ( h h ) Where: q stea is the stea ass flow consuption (kg/h); q NH3 is NH 3 evaporation ass flow (kg/h); h 1 is the liquid aonia enthalpy (kj/kg); h 1 is the gaseous aonia enthalpy (kj/kg); h 3 is the vapor enthalpy (kj/kg); 3 4 h 4 is waste water enthalpy (kj/kg); η is the heat change efficiency (%). 2.6 The aonia unloading copressor The aonia unloading copressor is generally opened twice a onth, each tie about one hour, whose power is calculated according to the design value. The annual electricity consuption of the copressor can be calculated by the following forula: W Pe Pe (1) Where: W is the copressor annual electricity consuption (kwh); Pe is the copressor power (kw). 3. Therodynaic calculation of the SCR syste theral equilibriu 3.1 Flue gas teperature rise caused by exotheric cheical reaction Within the denitration reactor, the two ain reactions of reductant (NH 3) and NO X are shown in (1) and (2), where Q 1 and Q 2 are the reaction heat of reaction (1) and (2), respectively, which are calculated through the standard foration enthalpy. Q kj/ol, Q kj/ol. The reaction heat of reaction of (1): CNO 3 Qfy 1 1 qvg Q1 (11) 3 The reaction heat of reaction of (2): CNO 2 3 Qfy2 1 qvg Q2 (12) 46 The total cheical reaction heat: Q Q Q (13) fy fy1 fy2 (9) Where: Q fy1 is the reaction heat of reaction (1) (kj/h); Q fy2 is the reaction heat of reaction (2) (kj/h); C NO is the concentration of NO in flue gas (g/n3); NO2 qvg is the flue gas volue flow rate(n3 /h); is denitration efficiency(%). Based on the total reaction heat, the heat balance equation is listed: t 1 C C is the concentration of NO 2 in flue gas (g/n 3 ); Q fy Q p, y y, Where : t 1 is the increased teperature of reaction ( ); Q fy is the total reaction heat of reaction (kj/h); Q y, is the flue gas ass flow rate (kg/h); py, C is the flue gasspecific heat at constant pressure (kj/(kg K)). 3.2 Flue gas teperature drop caused by heat absorption of dilution air

5 Dilution air inlet teperature is defined as gas teperature is equation is as follows: t t 11, flue gas teperature of SCR syste is t 2, and the ixed flue. The absorbed heat of dilution air is equal to the released heat of flue gas. The heat balance C Q ( t t ) C Q ( t t) (14) p, k air 11 p, y y, 2 Where : C pk, is the air specific heat at constant pressure (kj/(kg K)); Q air is the ass flow rate of dilution air (kg/h); C py, is the flue gas specific heat at constant pressure (kj/(kg K)); Q y, is the ass flow rate of flue gas (kg/h). According to the balance equation, stability teperature of the reactor is calculated as follows: The flue gas teperature drop is calculated as C Q t C Q t t C Q C Q p, k air 11 p, y y, 2 p, k air p, y y, t t t Flue gas teperature drop caused by heat dissipation of the reactor and flue pipe A typical arrangeent of tail flue pipe fro inside to outside can be divided into: refractory layers, theral barrier, theral insulation layer, protective plate. Due to the influence of insulation on energy consuption is not large, single-layer insulation way is adopted in the theral design of syste [1]. Supposing flue denitration syste is coposed of N layers aterial, ignoring the contact theral resistance between the layers, using one-diension conduct equation to calculate the heat flux, and then the heat loss of the denitration syste can be obtained. Since the SCR reactor is filled with catalyst hierarchically, it is assued that the flue pipe inner wall teperature is the sae with flue gas teperature, without considering the ipact of the heat transfer of the flue pipe inner wall.. (15) The flue inner wall teperature is set as t 1, the outer wall of the flue teperature is t n, the coefficient of theral conductivity of each layer is 1, 3,, n 2, 3,,, respectively. Then the heat loss can be siplified as follows: t1 tn q n n, and the thickness of each insulation layer is 1, Where: q is the heat loss per square eter of the wall (W / 2 ). The total area of the denitration reactor and flue pipe is referred to as A, then the heat balance equation is: qa36 Cp, y Qy, t3 (17) 1 qa36 t3 1C Q p, y y, Where: t 3 is the teperature drop caused by heat loss (K); A is the total area (2 ). 3.4 Flue gas teperature drop caused by air leakage of the reactor and flue pipe Teperature of flue pipe air leakage is given as t 12 n, the flue gas teperature of SCR syste is 2, (16) (18) t 2, the ixture flue gas teperature is t.. And the absorbed heat of leaks is equal to the released heat of flue gas. The heat balance equation is: C Q ( t t ) C Q ( t t ) (19) ' ' p, k lf 12 p, y y, 2

6 Where : C pk, is the air specific heat at constant pressure (kj/(kg K)); Q lf is the ass flow rate of air leakage (kg/h); flue gas (kg/h). C py, is the flue gas specific heat at constant pressure (kj/(kg K)); Q y, According to the equilibriu equation, the stable teperature of ixtyre can be calculated: ' t C Q t C Q t C Q C Q p, k lf 12 p, y y, 2 p, k lf p, y y, is the ass flow rate of (2) And the teperature drop of flue gas is t t t ' Influence of SCR syste on the boiler exhaust heat loss A large nuber of literature [11,12,13] shows that copared with the inlet teperature, outlet teperature of the denitration syste changed little. So this article ainly considered the exhaust loss caused by the increase of leakage rate which lead to increase of soke volue. Exhaust gas heat loss is equal to the enthalpy difference between the cold air entering the boiler and the exhaust [14]. The calculating forula ay be expressed in the following for: Where: preheater; h lk h py q h h h h 1 1 (1 )% (21) Q2 py py lk 1 q4 py py lk 2 q4 Qr Qr 1 Qr is exhaust gas enthalpy (kj/kg); py is the outlet excess air coefficient of flue gas side air is the theory enthalpy of cold air (kj/kg); q 4 is the echanical incoplete cobustion heat loss, which can be given an appropriate value depending on different types of coal when only take into account changes in boiler efficiency due to air leakage and it is considered the other heat loss is unchanged. 4. Calculation and analysis of case unit This article selected a doestic 6MW unit as a case, conducted an in-depth study and quantitative analysis of the syste. The coal ain paraeters of case sets are suarized in table 1, the ain design paraeters of SCR syste are suarized in table 2. Table 1. The coal ain paraeters of case unit Ite (as received coal) units Coal type C ar % H ar % 3.36 O ar % 7.6 N ar %.79 S ar %.75 M ar % 9.61 Table 2. Main design paraeters of SCR syste Ite units value reductant - Liquid aonia Gas flow(hygroetric state, practical oxygen) 3 /h denitration efficiency % 8 Inlet NO X(calculated by NO2,6%O2) g/ 3 35 NH 3/NO X %.88

7 Aonia escape rate pp Calculation results analysis of the ain energy-consuing equipent Table 3 shows the energy consuption calculation results of the 6MW unit denitration syste. Copared with the original unit, coal consuption for power generation of new unit with denitration syste increases by g/kwh, aong which the additional coal consuption value of induced draft fan is up to.438 g/kwh, taking up to 93.79% of the largest proportion of energy consuption. It proves that the optiization of induced draft fan is advantageous to the SCR syste energy conservation operation. Table 3. Energy consuption calculation results of the 6MW unit denitration syste Ite Use nuerical Additional coal consuption Proportion value (g/kwh) (%) liquid aonia kg/h - - Extraction stea kg/h induced draft fan 817 kw dilution fan 24 kw Unloading copressor 18.5 kw.18.4 Total additional coal consuption Figure 2 shows the additional coal consuption and induced draft fan capacity caused by the variable load. It can be seen fro the graph, induced draft fan capacity increases with the increasing of load because of that induced draft fan power will inevitably increase when flue gas flow becoes larger under high load. Instead, the additional coal consuption caused by denitration syste power consuption decreases with the increasing of the load, which eans that the higher the load, the better the perforance of the syste, relatively, the lower the coal consuption of the syste itself. Although draft fan capacity is larger under high load, but the additional coal consuption of the unit is relatively low. Figure 2. The additional coal consuption and induced draft fan capacity caused by the variable load Figure 3 shows the change of induced draft fan additional coal consuption with the variation of pressure loss. As can be seen fro the figure, when the pressure drop is the sae, additional coal consuption reduces as the load increases; while at the sae load, the greater pressure drop, the greater the additional coal consuption. It iplies that when the pressure drop becoes greater, draught fan needs to provide uch higher pressure head to overcoe it in the syste, with greater power consued, higher additional coal consuption.

8 Figure 3. Change of induced draft fan additional coal consuption with the variation of pressure loss. Therefore, in order to prevent the pressure drop increases sharply caused by ash blocking, it is necessary to to pay attention to the change of the pressure drop at any tie in the operation, ensure that the pressure drop changes in a reasonable scope. When the pressure drop increases, blow the soot of catalyst in tie to reduce the syste power consuption. If necessary, also need to stop air preheater and use high pressure water washing it to reduce the blocking of aoniu hydrogen sulfate and lower the pressure drop. 4.2 Energy balance of denitration syste Figure 4 shows the change of SCR reactor flue gas teperature in case unit with various load. As it can be seen fro the figure, when the unit load is certain, air leakage teperature drop is greater than the reaction teperature rise, and the dilution air teperature drop is iniu; when under different load, the three teperature decreases with the decreasing of the load. Fig.4 also iplies that the flue gas teperature drop is less than 2 after it flow through the SCR reactor, and the teperature drop becoes saller with the decreasing of load. Heat loss can be neglected in the calculation as a result of sall heat flux density of reactor and flue pipe. Figure 4. Change of flue gas teperature in SCR syste of theral power unit Figure 5 shows change the change of exhaust heat loss in case unit with various load. As shown in Fig.5, the use of denitrification syste has little effect on the boiler exhaust heat loss which decrease with load reduction, and the average values of heat loss change is less than.5%. Exhaust heat loss and the change of it are the highest in BMCR condition. In the point view of the calculation forula of exhaust heat loss, when the load is high, the boiler exhaust soke teperature is high, but the excess air coefficient is relatively sall, which leads to a higher exhaust heat loss.

9 5. Conclusion Figure 5. Change of exhaust heat loss in theral power unit This article ainly analyzed the heat balance of SCR syste reactor and the flue pipe fro four aspects: flue gas teperature rise caused by exotheric cheical reaction, teperature drop caused by heat absorption of dilution air, heat dissipation of the reactor and flue pipe and air leakage of the reactor and flue pipe.meanwhile, this work conducted a detailed therodynaic analysis and calculation of the teperature distribution of SCR reactor and flue pipe, as well as the effect of SCR syste on boiler flue gas heat loss. At the sae tie, SCR syste energy consuption odel was established, and the coparison and optiization of ain equipent energy consuption was copleted. Finally, design data of a typical 6MW unit was analyzed, and it was found that: (1) It can be known through calculation that the flue gas teperature drop is less than 2 after it flow through the SCR reactor, which is coincide with the engineering experience data in pertinent literature [15,16,17]. The use of denitrification syste has little effect on the boiler exhaust heat loss, and the change values of heat loss is less than.5% in BMCR condition. It is inevitable that the heat loss increases due to installation of denitration syste and boiler air leakage. Thus, in order to reduce the energy loss of the syste, the reactor heat preservation easures should be paid attention to. (2) Denitration syste ain energy-consuing equipent is: the dilution air fan, induced draft fan, liquid aonia evaporator, unloading copressor. Copared with the original unit, coal consuption for power generation of new unit with denitration syste increases by g/kwh, aong which the additional coal consuption value of induced draft fan is up to.438 g/kwh, taking up to 93.79% of the largest proportion of energy consuption. By analyzing induced draft fan energy consuption in varying load and different pressure drop, it is concluded that induced draft fan should try to keep running under high load, in which condition the aount of coal consuption is relatively low. At the sae tie, in order to prevent pressure drop increase sharply caused by ash blocking, it is necessary to pay attention to the change of the pressure drop at any tie in the operation, ensure that the pressure drop changes in a reasonable scope. References [1] HE Z, REN G, QI L. Study on the Disadvantageous Influence of SCR Denitrification Syste [J]. Power Syste Engineering, 29, 3: 22. [2] Busca G, Lietti L, Rais G, et al. Cheical and echanistic aspects of the selective catalytic reduction of NO x, by aonia over oxide catalysts: A review [J]. Applied Catalysis B Environental, 1998, 18(18):1-36. [3] Ke-qin S U N, Ai-hua Z Q Y U. Study on Arsenic Poisoning of SCR Catalyst[J]. China Environental Protection Industry, 28, 1: 18.

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