Reducing Harmful Emissions Discharged into the Atmosphere from Operating Boilers by Applying a Combination of Low-Cost Technological Measures

Transcription

1 ISSN -65, Thermal Engineering,, Vol. 57, No., pp Pleiades Publishing, Inc.,. Original Russian Text P.V. Roslyakov, I.L. Ionkin, K.A. Pleshanov,, published in Teploenergetika. Reducing Harmful Emissions Discharged into the Atmosphere from Operating Boilers by Applying a Combination of Low-Cost Technological Measures P. V. Roslyakov, I. L. Ionkin, and K. A. Pleshanov Moscow Power Engineering Institute, ul. Krasnokazarmennaya, Moscow, 5 Russia Abstract The operational methods for suppressing nitrogen oxide emissions widely used in gas-and-oilfired boilers are described. Information is given on implementing integrated operational measures in BKZ-75-9GM and TGM-96B boilers. DOI:./S6598 Thermal power stations (TPSs) firing organic fuel are one of the main sources polluting the atmospheric air with toxic combustion products. The main part of equipment installed at TPSs had been commissioned before 985, and therefore, it does not comply with the modern requirements for environmental safety according to GOST (State Standard) R [] and for the efficiency of fuel combustion. It should be noted that this GOST is intended for being applied to newly commissioned boilers and the emissions from operating units must not exceed the maximum permissible levels established for them. However, the values of these limits are determined individually for each unit; therefore, in putting nature protection measures in operating boilers, attempts should be taken to comply with the requirements specified in []. As regards gas-and-oil-fired boilers, which make the main contribution in the production of heat and electricity in Russia, nitrogen oxides NO x are the most harmful component in the combustion products. By now, great experience has been accumulated with using different technologies for reducing NO x emissions. At TPSs that are already in operation, this is most frequently done by using so-called technological (or intrafurnace) measures, which differ from one another not only in the effectiveness of suppressing the generation of NO x, but also in money and time required for implementing them []. Application of technological measures makes it possible to reduce the generation of NO x by modifying the fuel combustion conditions. The rate with which nitrogen oxides are generated in boiler furnaces is determined by the characteristics of the active combustion zone (ACZ): air excess factors, temperatures, and time for which combustion products dwell in the zone of high temperatures []. It should be pointed out that the quantities in which nitrogen emissions are discharged from quite a large number of boilers (as a rule, these are hot-water boilers and boilers with a steam output of up to 67 t/h) are a factor of.5. higher than the levels specified by the relevant standards. Technological measures involving retrofitting are almost always economically unprofitable. Therefore, low-cost technological measures (that do not involve making any changes in the boiler design) are most promising for such boilers, in particular, organizing combustion with decreased air excess factors, nonstoichiometric, or simplified two-stage combustion. It frequently turns out that implementation of a single method does not make it possible to achieve the required reduction of NO x emissions. In such cases, a combination of these methods can be used. The table lists the low-cost mea- Low-cost technological measures Measure Implementation Reduction of NO x, % Drawbacks Use of two-stage combustion Use of nonstoichiometric combustion Reduction of air excess factor, combustion with moderate chemical underburning Disconnection of fuel supply from part of the burners Misalignment of the fuel-to-air ratio in different burners or in different tiers Reduction of the total air excess factor Difficult to implement in boilers with a small number of burners 5 55 Degraded efficiency at low loads if there is no automatic closed-loop control system Increased chemical underburning may occur, intricate control is required 5

2 REDUCING HARMFUL EMISSIONS DISCHARGED INTO THE ATMOSPHERE 5 Afterburning zone α =.5 α >. Oxidation zone α =.5 Zone of afterburning and cooling Inlet of secondary air Burners Reduction zone Fig.. Scheme of implementing two-stage combustion. sures developed by specialists of the Moscow Power Engineering Institute (MEI) that have been studied well enough and put in use at many TPSs of different types, including the Bezymyansk cogeneration station, Kazan TETs- and TETs- cogeneration stations, Riga TETs- cogeneration station, and others. Simplified two-stage combustion is organized by disconnecting the supply of fuel from part of the burners while continuing the supply of air through them (Fig. ). The effectiveness with which the generation of NO x is suppressed in this case is determined by the possibility of organizing well-defined reduction and oxidation zones in the furnace chamber []. The following conditions must be satisfied for successfully implementing this method: A furnace must have a large number of burners, or these burners must be arranged in several tiers. The burners must have sufficient margin in their fuel throughput capacity. A sufficiently extended reduction zone must be set up in selecting the distance between the burner tiers. As it regards the drawbacks of this method, the main of them is that in some boilers it is not possible to disconnect part of their burners during operation at maximal loads. Nonstoichiometric combustion (Fig. ) is implemented by organizing a reduction (α r <.) and oxidation (α ox >..5) combustion zones in the furnace chamber volume (Fig. ) while keeping the air α <. α >. α <. (a) Reduction zone excess factors at the furnace outlet equal to their traditional levels [5]. Nonstoichiometric combustion of fuel in boilers can be implemented using different methods for upsetting the balance of the fuel-to-air ratio in burner devices. This imbalance can be obtained by redistributing air or fuel or both fuel and air among the burners. The main drawback of nonstoichiometric combustion is that the suppression of NO x becomes less efficient when the boiler load decreases. As a rule, this is connected with a change in the optimal fuel-to-air ratios in the burners. One way in which this drawback can be removed consists of tuning the system that controls the fuel-to-air ratio in the burners (if such a system is installed in the boiler). Firing with moderate incomplete combustion is one of the most widely used and easily implemented operational measures that consists in reducing the air excess factor in the furnace. This method can be widely used in operating boilers, which usually operate (b) Fig.. Schemes of implementing nonstoichiometric combustion. (a) By burner tiers and (b) opposite arrangement. NO, mg/m Yield of NO during usual combustion Yield of NO during nonstoichiometrical combustion.8 α red. α op. α ox Fig.. Yield of NO vs. the air excess factor. α THERMAL ENGINEERING Vol. 57 No.

3 5 NO x, mg/m ROSLYAKOV et al. CO, mg/m CO 5 No. No. No. 5 α cr..5 α op NO x 5 α max..5 α No. No. 5 No. 6 Media supplied to the burners: only air fuel and air (α > ) fuel and air (α < ) Fig.. Effect of air excess factor on the yield of NO x and CO. Fig. 5. Scheme of organizing a combined fuel combustion mode in the BKZ-75-.9GM boiler. with rather high air excess factors in their burners close to the values of α max (Fig. ). With the air excess factors reduced to the values α op = α cr +.., the emissions of nitrogen oxides usually drop by %. A still greater reduction of NO x emissions is observed as α is decreased further below the values of α op up to the occurrence of chemical underburning [6]. The best effect is achieved when the concentration of CO in flue gases is in the range 5 ppm [6]. We should point out that it is rather difficult to maintain such operating conditions in a boiler without using modern automatic control equipment; in addition, highly skilled personnel is required for running such modes. For this measure to be successfully implemented, the combustion process must be constantly monitored; therefore, we are dealing here with the firing of fuels with controlled moderate incomplete combustion, i.e., with adjusting and conducting operating modes using modern gas analyzers and automated control systems, which have been more and more frequently put in use in operating boilers in resent years. MEI specialists have gained great experience with putting different low-cost measures in operating boilers (BKZ-75-.9GM, TsKTI-75-.9RF, TPE-, TGM-8A, TGM-8B, TGM-96B, and others). However, the desired effect was not always achieved through using only one of the methods considered above. Essentially more efficient suppression of nitrogen oxides can be achieved by a combined use of lowcost technologies. In this paper, we describe the experience gained at MEI in using combined measures. The results obtained from putting environment protection measures in use in operating BKZ-75-.9GM and TGM-96B boilers, which are presented below, can serve as an example. The BKZ-75-.9GM boiler (station No. ) installed at the cogeneration station of OAO ChMZ is equipped with six swirl burners arranged in two tiers (Fig. 5). The tests were carried out with firing natural gas and fuel oil. The results from these tests have shown that the best effect in terms of suppressing nitrogen oxides is obtained by using the combined mode of nonstoichiometric and two-stage combustion of natural gas, which is implemented by disconnecting the supply of fuel from the middle burner in the second tier (with retaining the supply of air to it) and reducing the supply of air to the burners in the lower tier. Still better reduction of NO x emissions during the combined firing mode can be achieved by additionally implementing controlled chemical underburning of fuel. With the combined firing of natural gas organized according to such a scheme, the nitrogen oxide emissions dropped from 7 to 5 mg/m (i.e., below the levels specified in the relevant standards) in the load range D = 5 75 t/h. However, the use of this scheme during operation at lower loads did not allow the standardized values of NO x emissions to be achieved because of essentially higher values of the air excess factor in the furnace. Therefore, at D = 5 5 t/h burner No. 5 was also disconnected from the fuel supply with retaining the supply of air to it. With such a scheme, the quantity of nitrogen oxide emissions discharged at the minimal load equal to 5 t/h remained higher than their standardized level and were equal to 5 mg/m, which is also attributed to a high air excess factor in the furnace required for maintaining the necessary steam superheating temperature. The content of O in flue gases downstream of the steam superheater at D = 5 t/h was equal to %. It should be noted that these boilers operate in the load range 5 5 t/h only in the startup and shutdown modes. During operation at the maximal load D = 8.5 t/h, the NO x emissions were equal to 6 mg/m. The level of these emissions was reduced to mg/m by organizing combined firing with controlled chemical underburning. The concentration of CO down- THERMAL ENGINEERING Vol. 57 No.

4 REDUCING HARMFUL EMISSIONS DISCHARGED INTO THE ATMOSPHERE 55 NO x, mg/m.5.5 NO x, mg/m (a) (b) stream of the exhaust fan in this case did not exceed mg/m (Fig. 6). During operation in low-toxic fuel oil firing modes, the use of the scheme with combined firing (see Fig. 5) at maximal loads was not possible due to limitations on the fuel oil pressure upstream of the burners. Therefore, during operation with D = 7 75 t/h, the scheme of nonstoichiometric combustion was organized by supplying the larger portion of air to the upper tier of burners (burner No. operated in the same mode as burners Nos. and ). In this case, the NO x emissions dropped from to 5 mg/m, or by 57 58% (see Fig. 6a). The standardized level equal to 5 mg/m can be achieved in this range of loads only by organizing nonstoichiometric combustion with controlled underburning. According to the existing operating chart, fuel supply is disconnected from burner No. 5, and part of air is supplied for cooling during operation at loads below 65 t/h. During the tests, a combined arrangement for reducing NO x emissions was studied, which consisted of nonstoichiometric combustion at the edges and a simplified scheme of two-staged combustion in the central part of the furnace. This scheme is implemented by disconnecting fuel supply from burner No. with retaining the supply of air through it. In this case, the NO x emissions become lower than their standardized level (5 mg/m ) both when fuel oil is fired without underburning and in the modes with controlled chemical underburning (see Fig. 6a). A tendency is Fig. 6. Content of NO x vs. the load during traditional and low-toxic combustion of natural gas (a) and fuel oil (b). () Traditional combustion, () low-toxic combustion, () low-toxic combustion with underburning, and () maximum permissible value according to GOST R α ox > No. No. α red < No. No. Fig. 7. Scheme of organizing nonstoichiometric combustion in the TGM-96B boiler. observed in which the concentration of NO x increases as the load is decreased from 65 to 55 t/h, which is due to an increase in the total air excess factor in the zone of active combustion. Thus, the use of nonstoichiometric combustion in the upper range of loads and two-stage and nonstoichiometric combustion in the lower range of loads in combination with controlled chemical underburning makes it possible to reduce the emissions of nitrogen oxides to their standardized values []. Tests aimed at developing low-toxic combustion modes were carried out on the TGM-96B boiler (station No. ) installed at the Riga TETs- cogeneration station. The standardized levels of maximum permissible emissions of NO x that are in force in Latvia differ from those in Russia and are equal to mg/m when recalculated for dry gases and air excess factor α =.5 (% O ). The boiler is equipped with four powerful swirl gasand-oil burners designed by the Kharkov Branch of TsKB-VTI, which are installed on the furnace front wall in two tiers with the burner axes inclined upward by (Fig. 7). The boiler is furnished with the TELEPERM XP process control system of Siemens, which monitors all parameters of its operation. Preliminary tests showed that nitrogen oxide emissions exceeded their standard level ( mg/m ) in almost the entire range of studied loads. The average values of NO x concentrations were found to be 5 5 mg/m at low and medium loads and 5 6 mg/m at loads close to the nominal level. So high concentration of NO x is attributed to the large capacity of the installed burners. A combined scheme of nonstoichiometric combustion was used in the boiler. The imbalance between the tiers was achieved by redistributing the flowrate of gas among the burners: approximately % of gas was supplied to the upper-tier burners and approximately 6%, to the lower-tier burners (the /6 ratio). In order to achieve the maximal effect, the total quantity of air was also reduced, and the air flowrate was redistributed among the burners (a larger portion of air was forwarded to the upper tier). The flowrates were THERMAL ENGINEERING Vol. 57 No.

5 56 ROSLYAKOV et al. NO x, CO, mg/m 7 O, % 7 Efficiency, % selected so as to have the concentration of CO in flue gases equal to 5 mg/m. The results obtained from adjustment of low-toxic modes are shown in Fig. 8. Attempts to reduce NO x emissions below mg/m during operation at the maximum loads were not met with success. This is explained by the fact that the limitation imposed on the maximal flowrate of natural gas to the burner ( m /h) does not make it possible to distribute the fuel among the burner tiers in the /6 ratio. Nonetheless, by distributing gas supply among the tiers in the 5/55 ratio and organizing controlled chemical underburning, the NO x emissions were reduced to approximately 5 mg/m against 59 mg/m that were generated during the usual combustion (i.e., by around %). The use of nonstoichiometric combustion during boiler operation with a steam output of t/h and with nominal steam parameters made it possible to reduce the emissions of nitrogen oxides to below the standardized level equal to mg/m. During operation in this load range, the ratio of gas flowrates supplied to the tiers was close to its optimal value equal to /6. The NO x emissions generated in these modes were equal to 6 68 mg/m against 5 mg/m during the usual combustion. A considerable scatter of NO x concentrations generated during usual combustion can be attributed to a change in the concentration of oxygen due to insufficient accuracy of the operation of the TELEPERM XP process control system Fig. 8. Concentrations of NO x, CO, and O vs. the load of the TGM-96B boiler during usual and low-toxic combustion of natural gas. (, ) NO x generated during traditional combustion, (, ) NO x generated during nonstoichiometric combustion, (, ) NO x generated during nonstoichiometric combustion with controlled underburning, (, ) O during usual combustion, (5, ) O during nonstoichiometric combustion, (6, ) O during nonstoichiometric combustion with controlled underburning, (7, ) CO generated during nonstoichiometric combustion with controlled underburning, and (8, ) CO generated during nonstoichiometric combustion Fig. 9. Efficiency of the TGM-96B boiler vs. the load during usual () and low-toxic () combustion of natural gas. The level of NO x emissions will be considerably lower if the fuel is fired with chemical underburning (Fig. 8). Thus, these modes can be subdivided into two categories: without chemical underburning (curve ) and with controlled chemical underburning (curve ). It is also well noticeable that the level of NO x emissions is reduced much more efficiently during combined use of nonstoiciometric combustion and underburning than during simple nonstoichiometric firing. The modes of low-toxic firing of natural gas with low parameters of steam were adjusted at the loads equal to and 87 t/h. Nonstoichiometric combustion could be implemented in these modes of operation only by redistributing air flowrates among the burner tiers. Fuel could not be redistributed due to the limitation on the minimal gas pressure upstream of the burners. During operation at a load of t/h, it became possible to reduce the emissions of nitrogen oxides to 68 mg/m. Such an insignificant reduction is explained by the fact that the furnace operated with rather high air excess factors and considerable misalignments between the furnace sides. During operation at a load of t/h and below, attempts to decrease NO x emissions by using nonstoichiometric combustion were not met with success, because the conditions of furnace operation did not allow reduction and oxidation zones to be set up. In view of this circumstance, the method of simplified two-stage combustion was tried out: the entire gas flowrate was supplied through the peripheral channels of the lower burners, and air was supplied through all the burners. This made it possible to reduce NO x emissions to mg/m, or by almost a factor of. The influence of low-cost methods on the efficiency of boiler operation was estimated by comparing the efficiency during operation in the usual and lowtoxic combustion modes. The calculated efficiencies of the TGM-96B boiler during its operation on natural gas are shown in Figs. 9 and. The efficiency of the boiler does not decrease during low-toxic firing of natural gas; moreover, it can even be stated that its efficiency slightly increases as compared with traditional THERMAL ENGINEERING Vol. 57 No.

6 REDUCING HARMFUL EMISSIONS DISCHARGED INTO THE ATMOSPHERE 57 ϑ f.g, C combustion (see Fig. 9). This is due to the fact that despite some increase in the flue gas temperature, a decrease in air excess factors occurs at the same time (see Fig. ). CONCLUSIONS 5 O, % () Combined use of low-cost technologies makes it possible to reduce nitrogen oxide emissions by a factor of.5. without any essential decrease in the boiler efficiency. 6 Fig.. Flue gas temperature and oxygen concentration in the operating section of the TGM-96B boiler vs. the load during usual and low-toxic combustion of natural gas. (, ) Temperature of flue gases during usual and low-toxic combustion, and (, ) concentration of oxygen during usual and low-toxic combustion. 5 () The costs for implementing these methods boil down mainly to the cost of operational-adjustment tests, development of operational charts, and tuning the automatic control system. () A combination of low-cost technological measures can be put in use in a large number of operating hot-water and steam boilers with an output of up to 5 t/h. REFERENCES. GOST (State Standard) R 58-95: Boiler Units. Heat- Generating and Mechanical Equipment. General Technical Requirements (Izd. Standartov, Moscow, 996) [in Russian].. P. V. Roslyakov, L. E. Egorova, and I. L. Ionkin, Technological Measures for Reducing Harmful Emissions from Thermal Power Stations into the Atmosphere (MEI, Moscow, ) [in Russian].. L. E. Egorova, Development Methods for Calculating the Generation of Nitrogen and Sulfur Oxides in Steam and Hot-Water Boilers, Candidate s Dissertation in Technical Sciences (Moscow, 995).. I. L. Ionkin, Ways of Making the Two-Stages Combustion of Natural Gas and Fuel Oil in Steam and Hot-Water Boilers More Efficient, Candidate s Dissertation in Technical Sciences (Moscow, ). 5. P. V. Roslyakov and I. A. Zakirov, Nonstoichiometrical Combustion of Natural Gas and Fuel Oil at Thermal Power Stations (MEI, Moscow, ) [in Russian]. 6. P. V. Roslyakov, I. L. Ionkin, and K. A. Pleshanov, Efficient Combustion of Fuels with Controlled Chemical Underburning, Teploenergetika, No., (9) [Therm. Eng., No. (9)]. THERMAL ENGINEERING Vol. 57 No.