[N.2.1.1] R&D on Oil-Burning, Environment-Friendly, High-Efficiency Boiler (Environment-Friendly, High-Efficiency Boiler Group) Takashi Murakawa, Yasuhiro Kotani, Kazuhiro Kamijo, Koichi Tsujimoto, Hiroshi Ikezawa, Nobuyuki Takahashi, Yoshihisa Kozawa, Shogo Suzuki, Akira Hattori, Hiroshi Kato, Hiroshi Matsumoto, Toshihiko Suetomi, Kentaro Sato 1. R&D Objectives In the cities of Japan, especially the large cities, it has been difficult to reach environmental levels for NOx concentrations in the atmosphere, despite countermeasures by the national government and local autonomies. Reduction of NOx emissions has thus become an urgent issue. For this reason, environmental regulations governing internal combustion engines and other types of combustion equipment are being made stricter. For example, the Environment Agency s NOx emissions guideline [80 ppm (O 2 = 0%)] and the Tokyo metropolitan NOx emissions standard [60 ppm (O 2 = 0%)] have been established for newly installed boilers. At present, however, small petroleum-burning boilers that can meet these regulations are almost totally absent in the market. What is more, with current technology, it is extremely difficult to reach these regulation levels; more R&D is required. In the interest of preventing global warming and protecting resources, other important issues include elevating the thermal efficiency of combustion equipment, conserving energy and working to reduce CO 2 emissions. Hence the objective of the present research is to develop an environment-friendly, high-efficiency, small size boiler (e.g. once-through boiler or hot water boiler with combustion volume at 20~100kg/hr and turndown ratio at 1: 2) that burns kerosene or A heavy oil at high efficiency [thermal efficiency at 92% or more (boiler rated load) and air ratio at 1.2 or below] and yields clean emissions (NOx at 60 ppm or below, CO at 50 ppm or below). 2. R&D Contents 2.1 Development of High-Performance Burner In the self re-circulating, divided-flame type or rich-lean combustion type burners trial-produced up to JFY2000, using kerosene or A heavy oil (Table 2.1-1) as fuel with lateral-type test furnace (Figure 2.1-1), NOx and CO targets (NOx: 60 ppm or less, CO: 50 ppm or less) have already been reached. Accordingly, an investigation was made of applications to commercial boilers (Figures 2.1-2 and 2.1-3) for the sake of early practical application of each developed burner. 1
Gas duct Hot water outlet Coolant inlet Mobile wall Flue Observation Burner window Flame resistance sealing material Figure 2.1-1: Diagram of Lateral-Type Test Furnace Steam Caster Fuel Burner Emission : Emissions flow Air/water separator Combustion chamber Water pipe Water supply Water chamber Blow Observation window Caster Figure 2.1-2: Diagram of Commercial Once-Through Boiler 2
: Emissions flow Fire bridge Heat exchanger Hot water Cold water Steam (100 C or below) Depressurizer Hot water (100 C or below) Burner Figure 2.1-3: Diagram of Commercial Hot-Water Generator Table 2.1-1: Fuel Properties Used in Combustion Tests Fuel Kerosene A heavy oil Density (15 C, g/cm 3 ) 0.791 0.865 Distillation properties ( C) Initial boiling point 154 194 50% 194 287 95% 254 348 Final point 270 359 Aroma content (vol%) 18 39 * Sulfur content (wt%) 0.004 0.07 Nitrogen content (ppm) < 1 210 * Values measured for 95% distillate 3
2.2 Development of High-Efficiency Boiler 2.2.1 Investigation of High-Efficiency Elemental Technology Using an emissions-duct, condensation-type canister (Figure 2.2-1) trial produced in 2000, an investigation was made of exhaust heat recovery and efficient combustion methods. Emissions Expansion tank Water chamber Connection adaptor Gas duct Measurement port Flue Burner attachment site Water chamber Combustion chamber Water chamber Thermometer Thermometer Exhaust heat recovery unit Flow meter Thermometer Circulation pump Thermometer Heat exchanger Thermometer Thermometer Coolant outlet Coolant inlet Figure 2.2-1: Diagram of Emissions Flue Condensation Type Canister In addition, a specially processed canister with heat transfer surface was designed and trial produced together with a water pipe heat exchanger. Exhaust heat recovery and efficient combustion methods were investigated with a high-efficiency boiler (Figure 2.2-2) in which the aforesaid canister and heat exchanger are combined. Damper Water pipe heat exchanger Emissions outlet Damper Combustion chamber Expansion tank Burner attachment site Observation windows Figure 2.2-2: Diagram of High-Efficiency Boiler 4
2.3 Matching High-Performance Burner with High-Efficiency Boiler Tests were conducted on matching high-efficiency boiler (Figure 2.2-2), to which high-efficiency technology was applied, with the self re-circulating type, divided flame type and rich-lean combustion type burners developed. 3. R&D Results 3.1 Development of High-Performance Burner 3.1.1 Investigation of Applications to Once-through Boiler of Developed Burner (1) Applications to Once-through Boiler of Self Re-circulating Burner Optimization of combustion cone guide, of secondary air nozzle diameter and of primary air volume was investigated for the purpose of matching to a commercial once-through boiler whose combustion chamber is of ω flow structure, using a self re-circulating burner trial produced for once-through boiler. Results appear in Figure 3.1-1.The figure reveals that a combustion cone guide is effective for improving combustibility and reducing NOx and that secondary air nozzle diameter and primary air volume are effective for even lower NOx. Under these conditions, therefore, combustion tests were performed with kerosene and A heavy oil as fuel at the rated load (capacity load: 1.42 million kcal/m 3 hr). Figure 3.1-2 shows the results. The figure indicates that the NOx emissions value of self re-circulating burner with kerosene fuel far surpasses commercial burner performance and reaches the target value. It also shows that combustibility (CO and smoke scale) is equivalent to that of commercial burner. With A heavy fuel, on the other hand, although combustibility (smoke scale) is somewhat inferior to commercial burner, it falls below the target upper limit. What is more, NOx emissions and CO emissions are equivalent to, or even superior to, commercial burner values. Given these findings, it was concluded that it is fully possible to use this burner with a once-through boiler of ω flow structure. 5
Smoke scale Cone guide effect Cone guide effect Optimization of primary air volume & secondary air nozzle diameter NOx (0% O2: ppm) Figure 3.1-1: Applications to Once-Through Boiler of Self Re-circulating Burner (Kerosene) Test conditions; capacity load: 1.17 million kcal/m 3 hr : Combustion cone guide absent, primary air 4%, secondary air nozzle diameter 20 mm : Combustion cone guide present, primary air 4%, secondary air nozzle diameter 20 mm : Combustion cone guide present, primary air 0%, secondary air nozzle diameter 19 mm Smoke scale Target upper limit value NOx (0% O2: ppm) NOx target upper limit value CO target upper limit value Figure 3.1-2: Self Re-circulating Burner Combustion Test (Rated Load), : Commercial burner ( : kerosene, : A heavy oil), : Self re-circulating burner ( : kerosene, : A heavy oil) 6
(2) Applications to Once-through Boiler of Divided-Flame Burner With respect to the divided-flame-type burner as well, an investigation was made of the number of secondary air nozzles and their arrangement, as well as optimization of combustion cone guide, for the purpose of matching with commercial once-through boiler of ω flow structure. Findings appear in Figure 3.1-3. Figure 3.1-3 presents the results of an investigation on secondary air nozzle count (6 nozzles -> 4 nozzles), on arrangements of secondary air outlet with 4 secondary air nozzles [cases (A) and (B)], and on optimization of combustion cone guide. Secondary air nozzle Once-through boiler Gas duct Emissions Emissions Gas duct Case (A) Case (B) The figure shows that by lowering the number of secondary air nozzles from 6 to 4, the NOx emissions value is reduced, but combustibility (CO and smoke scale) varies depending on the arrangement of secondary air nozzles. In other words, while combustibility (smoke scale) is improved in case (A), it declines in case (B). This is ascribed to the following. In case (B), in which the secondary air nozzles are arranged on the side of the combustion emissions port, because combustion occurs mainly at the emissions port side, non-combusted components can escape through the emissions port more easily than when there are 6 secondary air nozzles. In case (A), on the other hand, because combustion takes place on the side opposite the emissions port, although smoke scale dropped, CO combustibility was not improved so much over when there are 6 secondary air nozzles. Since it was found from an investigation of cone guide optimization that CO combustibility is improved, combustion tests were performed under these optimum conditions at the rated load (capacity load: 1.42 million kcal/m 3 hr) with kerosene and A heavy oil used as fuel. See Figure 3.1-4 for the results. 7
Smoke scale Secondary air nozzle count + case (A) Secondary air nozzle count + case (B) Secondary air nozzle count + case (A) Secondary air nozzle count + case (B) Effect of cone guide NOx (0% O2: ppm) Figure 3.1-3: Applications to Once-Through Boiler of Divided-Flame Burner (Kerosene) Test conditions; capacity load: 1.17 million kcal/m 3 hr : 6 secondary air nozzles, cone guide absent, : 4 secondary air nozzles, combustion cone guide absent : 4 secondary air nozzles, combustion cone guide present The figure indicates that the NOx emissions value of divided-flame burner with kerosene fuel surpasses commercial burner performance and reaches the target value. It also shows that combustibility (CO and smoke scale) is equivalent to that of commercial burner. With A heavy fuel, on the other hand, although combustibility (smoke scale) is somewhat inferior to commercial burner, it falls below the target value. Moreover, NOx emissions value was superior to that of commercial burner and it fell far below the target value (60 ppm), so it was concluded that it is fully possible to use this burner with a once-through boiler of ω flow structure. 8
Smoke scale Target upper limit value NOx (0% O2: ppm) NOx target upper limit value CO target upper limit value Figure 3.1-4: Combustion Test of Divided-Flame Burner (Rated Load), : Commercial burner ( : kerosene, : A heavy oil), : Divided flame burner ( : kerosene, : A heavy oil) (3) Applications to Once-through Boiler of Rich-lean Combustion Burner The rich-lean combustion burner was matched with the commercial once-through boiler that was used with developed burner as described in Section 2 above. With respect to the rich-lean combustion burner, optimization of combustion cone guide (cone guide width L and burner head plus cone guide gap d) was studied as shown in Figure 3.1-5. Combustion air Fire-resistant material Burner head Wind box Cone guide Frame eye Figure 3.1-5: Structure of Rich-Lean Combustion Burner 9
The results appear in Figure 3.1-6. The figure shows that the cone guide is effective in improving combustibility. What is more, it was found that for even further reduction of NOx and CO, the position of combustion cone guide is effective. Under these conditions, therefore, combustion tests were performed with kerosene and A heavy oil as fuel at the rated load (capacity load: 1.42 million kcal/m 3 hr). Figure 3.1-7 shows the results. The figure indicates that the NOx emissions value of rich-lean combustion burner with kerosene fuel far surpasses commercial burner performance and reaches the target value. It also shows that combustibility (CO and smoke scale) is equivalent to that of commercial burner. With A heavy fuel, on the other hand, combustibility (smoke scale) was found to be somewhat inferior to commercial burner or other developed burners. This is ascribed to the following. Because the differential pressure (burner differential pressure = wind box pressure furnace internal pressure) of this burner, as compared to other developed burners, was low at around 50 ~ 60 mm H 2 O, combustion air flow speed became slow and the fuel/combustion air mixture became poor. Smoke scale Cone guide effect Cone guide effect Optimization of cone guide NOx (0% O2: ppm) Figure 3.1-6: Applications to Once-Through Boiler of Rich-Lean Combustion Burner (Kerosene) : Combustion cone guide absent : Combustion cone guide present (L = 50 mm) : Combustion cone guide present (L = 80 mm) 10
Smoke scale Target upper limit value NOx (0% O2: ppm) NOx target upper limit value CO target upper limit value Figure 3.1-7: Combustion Test of Rich-Lean Combustion Burner (Rated Load), : Commercial burner ( : kerosene, : A heavy oil), : Rich-lean combustion burner ( : kerosene, : A heavy oil) 3.1.2 Investigation of Applications to Hot-Water Generator of Developed Burner Based on the results obtained by matching once-through boiler with the burners developed, an investigation was done on applying self re-circulating burner, divided-flame burner and rich-lean combustion burner to commercial hot-water generator whose combustion chamber is of the reverse combustion type. With each burner, combustion characteristics were measured under optimum conditions with kerosene and A heavy oil used as fuel and at the rated load (capacity load: 0.82 million kcal/m 3 hr). The results appear in Figures 3.1-8 and 3.1-9. The figure indicates that the NOx emissions value of each developed burner with kerosene fuel is much less than that of commercial burner and is less than the target upper limit. It also shows that in the low air ratio domain (air ratio at 1.2 or below), combustibility (CO and smoke scale) is superior to that of commercial burner (Figure 3.1-8). 11
As for NOx emissions of each developed burner with A heavy oil as fuel, on the other hand, the value is much smaller than that of commercial burner, the same as in the case of kerosene fuel, and it is lower than the target upper limit. Combustibility, however, especially CO emissions value, reaches the target value only in the self re-circulating burner. In the divided-flame burner and rich-lean combustion burner, at a high air ratio domain (1.2 or above), CO generation was found to be conspicuous in comparison to commercial burner. This is ascribed to the fact that in these two burner types, NOx is reduced somewhat at the expense of combustibility in comparison to self re-circulating burner, plus the fact that the hot-water generator used is of the reverse combustion type (combustion type in which there is a combustion emissions port near the burner. As the air ratio becomes higher, the flame approaches the burner vicinity, and non-combusted gas (CO) can easily escape through the combustion emissions outlet) (Figure 3.1-9). Smoke scale Target upper limit value NOx (0% O2: ppm) NOx target upper limit value CO target upper limit value Figure 3.1-8: Investigation of Applications to Hot-Water Generator (Kerosene) : Commercial burner : Self re-circulating burner : Divided flame burner : Rich-lean combustion burner 12
Smoke scale Target upper limit value NOx (0% O2: ppm) NOx target upper limit value CO target upper limit value Figure 3.1-9: Investigation of Applications to Hot-Water Generator (A Heavy Oil) : Commercial burner : Self re-circulating burner : Divided flame burner : Rich-lean combustion burner 3.2 Development of High-Efficiency Boiler 3.2.1 Investigation of High Efficiency through Emissions Duct Condensation Canister (1) Examination of Screw Plate Effect and Pitch In an effort to achieve high efficiency in the emissions flue condensation canister (Figure 2.2-1), an investigation was made of the impact of screw plate (Figure 3.2-1) effect and pitch when the screw plate has been inserted in the flue at the top of the canister. Since results from last year indicate that there is no difference in thermal efficiency between commercial burner and the developed burners, a commercial burner was used. Figure 3.2-2 shows the results obtained from a study done on the impact of screw plate pitch P on thermal efficiency when screw plate effect and screw plate total length L are kept uniform. The figure shows that thermal efficiency is elevated 2 ~ 3% in the range of air ratio evaluated, as compared to cases in which screw plate is absent. What is more, by shortening the screw plate pitch (75 mm), thermal efficiency is elevated about 1% in the range of air ratio evaluated, as compared to cases in which the pitch P is long (170 mm). This is explained as follows. By shortening the screw plate pitch P, swirling flow accelerated the flow speed at the flue wall surface, yielding an improvement in thermal transfer rate inside pipe. 13
Pitch (P) Figure 3.2-1: Total length (L) Screw Plate Structure Target value Thermal efficiency (%) Figure 3.2-2: Investigation of High Efficiency through Screw Plate (1) (2) Examination of Screw Plate Total Length Test conditions; Capacity load 1.44 million kcal/m 3 hr : Screw plate absent, : Screw plate (P170, L350) : Screw plate (P75, L350) Figure 3.2-3 depicts the results of an investigation of the impact of screw plate total length L (350 mm, 670 mm & 880 mm) on thermal efficiency when screw plate pitch P has been kept at the same value. It can be seen that, with air ratio in the range of 1.1 ~ 1.4, thermal efficiency is improved by about 1% when the screw plate total length has been elongated (880 mm) as compared to when it is short (350 mm). This is ascribed to the fact that heat transfer from combustion emissions was promoted more by having a long screw plate length rather than a short length. 14
Target value Thermal efficiency (%) Figure 3.2-3: Investigation of High Efficiency through Screw Plate (2) Test conditions; Capacity load 1.44 million kcal/m 3 hr : Screw plate (P170, L350), : Screw plate (P75, L670) : Screw plate (P75, L880), : Screw plate absent (3) Examination of Operating Conditions An attempt was made to optimize both screw plate pitch and total length, but even when the air ratio was 1.2 or less, thermal efficiency of the canister main body could not reach the target value. Nevertheless, because the average capacity load of this class in the market is around 1 million kcal/m 3 hr, an investigation was made to see if the target value could be reached by lowering the capacity load to 1 million kcal/m 3 hr. Figure 3.2-4 gives the results. Figure 3.2-4 also shows the thermal efficiency with supply water preheater included (hereinafter, general thermal efficiency ). From the figure, it was confirmed that the thermal efficiency target (92% or higher) in the range of air ratio evaluated could be reached by lowering the capacity load from 1.33 million kcal/m 3 hr to 1 million kcal/m 3 hr. In addition, general thermal efficiency was elevated to 95 ~ 96%. 15
Thermal efficiency (%) Target value Figure 3.2-4: Investigation of High Efficiency through Screw Plate (3) Screw plate size: P170, L880 Capacity load 1 million kcal/m 3 hr: canister thermal efficiency, general thermal efficiency Capacity load 1.33 million kcal/m 3 hr: canister thermal efficiency, general thermal efficiency 3.2.2 Test of Combustion by Canister with Specially Processed, Heat Transfer Surface In an effort to improve the thermal efficiency of the convective heat transfer component (water pipe component at rear of combustion chamber) of specially processed canister with heat transfer surface, the volume of coolant in the canister combustion chamber was investigated. The results are shown in Figure 3.2-5. The figure represents the coolant volume (150 ~ 300 L/min.) of the water pipe component when the circulating water at the canister shell side is held constant (100 L/min.). By means of the heat loss method (JIS B 8418), canister thermal efficiency was determined from the temperature of emissions at the canister outlet (inlet of water pipe heat exchanger), and general thermal efficiency (1) was determined from the temperature of emissions at the duct (water pipe heat exchanger outlet). For general thermal efficiency (2), water (latent heat) condensed on the water pipe heat exchanger was taken into consideration together with general thermal efficiency (1). The figure indicates that canister thermal efficiency is improved by increasing the volume of coolant in water pipe. In the range of 200 ~ 300 L/min., there are no major differences in canister thermal efficiency. It was also confirmed that in terms of general thermal efficiency (1), there are virtually no differences with the volume of coolant in water pipe in the range of 150 ~ 300 L/min. With the volume of coolant in water pipe at 250 L/min., condensed water in the water pipe heat exchanger was measured (22 ~ 24L/hr), and general thermal efficiency (2) was determined at 102 ~ 103% (low calorific standard). 16
General thermal efficiency (2) Thermal efficiency (%) General thermal efficiency (1) Canister thermal efficiency Figure 3.2-5: Test of Combustion by Canister with Specially processed, Heat Transfer Surface (Thermal Efficiency) Burner: Commercial burner, Fuel: Kerosene, Capacity load: 1.2 million kcal/m 3 hr Coolant volume (L/min.): 150, 200, 250, 300 3.3 Matching of Self Re-circulating Burner with High-Efficiency Boiler As a result of matching developed burners with once-through boiler and hot-water generator, matching with the high-efficiency boiler shown in Figure 2.2-2 was investigated, using a self re-circulating burner that, among the developed burners, exhibits especially outstanding combustion characteristic. The results appear in Figure 3.3-1. The figure also presents the results of an examination of combustion characteristic as opposed to air ratio in commercial burner and in self re-circulating burner, with capacity load held constant (kerosene: 1.2 million kcal/m 3 hr, A heavy oil: 1.28 million kcal/m 3 hr). The figure shows that the NOx emissions values of the self re-circulating burner (kerosene: 23 ~ 25 ppm, A heavy oil: 42 ~ 44 ppm) were drastically low in comparison to the NOx emission values of commercial burner (kerosene: 68 ~ 82 ppm, A heavy oil: 81 ~ 93 ppm). As for combustibility of the self re-circulating burner, in relation to A heavy oil, although smoke scale was somewhat high in the domain of high air ratio (air ratio: 1.25 or above), values are below the target upper limit. In the domain of low air ratio (1.25 or below), the combustibility shown was greater than that of commercial burner. It was thus confirmed that the self re-circulating burner is an outstanding burner. 17
Smoke scale Target upper limit value NOx (0% O2: ppm) NOx target upper limit value CO target upper limit value Figure 3.3-1: Matching of Self Re-circulating Burner with High-Efficiency Boiler Testing condition; capacity load: 1.22 million kcal/m 3 hr, : Commercial burner ( : kerosene, : A heavy oil), : Rich-lean combustion burner ( : kerosene, : A heavy oil) 4. Synopsis 4.1 Results of R&D 4.1.1 Development of High-Performance Burner (1) Investigation of Applications to Commercial Boiler 1) With the aim of early practical application for developed burners, an investigation was made of self re-circulating burner, divided-flame burner and rich-lean combustion burner in terms of application to commercial once-through boiler in which the flow of combustion emissions is of the ω flow type. It was found that with kerosene and A heavy oil fuels, the target values (NOx: 60 ppm or less, CO: 50 ppm or less) were cleared by all three burner types and that all three types could be applied to once-through boiler. 2) Also investigated was application of the aforesaid three burner types to commercial hot-water generator in which the flow of combustion emissions is of the reverse combustion type. With kerosene fuel, target values were cleared by all the burners. With A heavy oil, target values were reached only by the self re-circulating burner; it was found that this type burner could be applied to hot-water generator. 18
4.1.2 Development of High-Efficiency Boiler (1) Investigation of High-Efficiency Elementary Technology 1) To achieve high efficiency in the boiler s convective heat transfer component, an investigation was made of the structure (pitch and total length) of the screw plate inserted in the flue component of duct-condensation canister. By shortening the pitch and expanding the total length, thermal efficiency of the canister main body reached 90 ~ 91%. 2) Moreover, by lowering capacity load from 1.33 million kcal/m 3 hr to 1 million kcal/m 3 hr, thermal efficiency of the canister main body reached 92% or more, and general thermal efficiency, with supply water preheater included, reached 95% or more. 3) In an effort to improve thermal efficiency even further, an investigation was done on achieving high efficiency by using commercial burner and high-efficiency boiler in which a supply water preheater (water pipe heat exchanger) was combined with a specially processed canister with heat transfer surface in which the heat transfer surface of the new trial-produced combustion chamber was rendered into a corrugated structure. General thermal efficiency reached 100% or above (low calorific standard). 4.1.3 Matching of High-Performance Burner with High-Efficiency Boiler The self re-circulating burner was matched with a high-efficiency boiler and the optimum conditions for eliciting the combustion performance of the self re-circulating burner were identified. It was confirmed that under these conditions, the self re-circulating burner is outstanding, with its NOx emissions at 23 ~ 25 ppm (kerosene) or 42 ~ 44 ppm (A heavy oil), falling far below the target value (60 ppm or below). 4.2 Summary The present R&D was completed in JFY2001. The development target a small, environment-friendly and high-efficiency boiler was reached. To further promote practical application of this burner, however, it was found that a latent-heat-recovery canister must be developed, in consideration of economy, so as to facilitate low NOx at high-load combustion and energy conservation (CO 2 reduction) in the future. Practical equipment was neither developed nor commercialized. Accordingly, because advances must be made in the development and practical application of new technologies, based on the results of the present R&D, plans call for R&D aimed at commercialization from JFY 2002 under the new theme: Development of clean, high-level combustion technology and fuel technology for petroleum combustion equipment. Copyright 2002 Petroleum Energy Center. All rights reserved. 19