R&D on Oil-Burning, Environment - Friendly, High-Efficiency Boiler

2001.M4.3.1 R&D on Oil-Burning, Environment - Friendly, High-Efficiency Boiler (Environment-Friendly, High-Efficiency Boiler Group) Takashi Murakawa, Hiroshi Kato, Hiroshi Matsumoto, Kentaro Sato, Yasuhiro Kotani, Atsuo Fujimune, Kazuhiro Kamijo, Koichi Tsujimoto 1. Contents of Empirical Research 1.1 R&D Background and Objectives In the cities of Japan, especially the large cities, the concentrations of NOx in air have not reached environmentally acceptable levels despite measures taken by the national government and local authorities. Reduction of NOx emissions, therefore, has become an urgent task. For this reason, environmental regulations on internal combustion engines and all other types of combustion equipment are being strengthened. For newly-established boilers, for instance, the Environment Agency has established a NOx emission guideline at 80 ppm (O 2 0% conversion), and the city of Tokyo s NOx emission guideline is at 60 ppm (O 2 0% conversion). At present, however, there are no oil-burning, small-scale boilers on the market that can meet these guidelines. What is more, it is difficult to meet these guidelines with current combustion technology; new R&D is necessary. From the standpoints of global warming prevention and resources protection, measures to improve the thermal efficiency of combustion equipment, to conserve energy and to reduce CO 2 emissions have also become vital issues. The purpose of the present R&D is to develop an environment-friendly, high-efficiency, small-scale boiler (once-through boiler or hot water boiler with combustion volume at 20 to 100 kg/hr and turndown ratio of 1 : 2) in which emissions are purified (NOx at 60 ppm or less, CO at 50 ppm or less) at high efficiency [thermal efficiency at 92% or more (boiler rated load), and air ratio at 1.2 or less], using kerosene and A heavy oil as fuel. 1.2 Contents of R&D In working to develop a burner of high-performance in terms of emissions purification, various burner types have been trial-produced and evaluated by applying combustion technologies, including exhaust recirculating combustion, flame cooling combustion, low pressure air atomizing combustion, and thick and thin fuel combustion, independently or in combination. Moreover, an exhaust flue condensation storage water heater has been designed and trial produced, based on the results of investigations of high-efficiency elemental technologies made up to the year 1999, and design materials have thus been obtained for the development of a high-efficiency storage water heater. 1

1.2.1 Development of High-Performance Burner With respect to the self-recirculating burner, which was trial-produced in FY 1999, the divided flame burner, the air atomizing burner, and the thick and thin fuel combustion burner, an investigation was made for achieving high performance (i.e., low NOx emissions, improved combustibility, compactness). Based on the findings thus obtained, various burner formats were trial-produced. Using the fuels given in Table 1.2-1, combustion tests were performed on the small-scale test combustion furnace shown in Figure 1.2-1 (hereinafter lateral test furnace ) and on the upright test furnace shown in Figure 1.2-2 (hereinafter upright test furnace). Gas flue Hot water outlet Cooling water inlet Mobile wall Smoke tube Peep Burner window Flame resistant sealing material Figure 1.2-1 Outline of Small-Scale Test Furnace Combustion emissions flow Measuring holes for emissions, etc. Mobile wall Gas flue Combustion emissions flow Hollow mobile wall Burner attachment Stop plug Stop plug Observation window Combustion chamber format A Smoke tube Ceramic fiber format B Figure 1.2-2 Outline of Upright Test Furnace Table 1.2-1 Properties of Fuel Used in Test Furnace Fuel Kerosene A heavy oil (1) A heavy oil (2) Density (15 C, g/cm 3 ) 0.791 0.867 0.877 Distillation properties ( C) First drop point 153 176 198 50% 193 289 295 95% 255 363 353 Final point 269 376 363 Aromatic content (vol%) 17 41* 44* Sulfur content (wt%) 0.004 0.06 0.15 Nitrogen content (ppm) <1 210 330 * Measured values respecting 95% distillate 2

1.2.2 Development of High Efficiency Storage Water Heater (1) Design and Fabrication of Exhaust Flue Condensation Storage Water Heater In order to investigate the burner under development and its assembly to storage water heater, plus the thermal efficiency, combustion and emissions characteristics of this assembly, an exhaust flue condensation storage water heater was design and produced. An outline of the exhaust flue condensation storage water heater is presented in Figure 1.2-3. By installing a combustion emissions outlet to the storage water heater at the rear of the combustion chamber, a structure was established in which the combustion chamber space can be used adequately and in which a reduction in exhaust heat loss percentage, achieved through low NOx of the burner and through low air-ratio combustion, can be investigated. In addition, an exhaust heat recovery unit (water supply preheater) was installed at the storage water heater outlet so that improvements in recovery of heat from combustion emissions can be studied. Expansion tank Water chamber Connection adaptor Gas flue Measurement port Smoke tube Burner attachment Water chamber Combustion chamber Exhaust heat recovery unit Flow meter Thermometer Heat exchanger Cooling water outlet Water chamber Circulation pump Cooling water inlet Thermometer Figure 1.2-3 Outline of Exhaust Flue Condensation Storage Water Heater 3

2. Results of Empirical Research and Analysis Thereof 2.1 Development of High-Performance Burner 2.1.1 Improvement of Combustibility of Self-Recirculating Burner With the aim of improving the combustibility (CO, smoke density) of self-recirculating burner in an upright test furnace, combustion tests were performed in which the flow of combustion emissions was changed toward the rear of the combustion chamber [Figure 1.2-2: From emissions format (A) to upper combustion chamber (Figure 1.2-2: emissions format (B)]. The results are shown in Figure 2.1-1. From Figure 2.1-1 it can be seen that in the case of combustion emissions format (A) smoke is produced when the CO emissions value is increased sharply at an air ratio of 1.2 or below. However, if the flow of combustion emissions is changed from the rear to the top of the combustion chamber (emissions format B), we find that combustibility is improved and CO emissions value and smoke density are reduced. The reason appears to be as follows. In the case of combustion emissions format A, when the air ratio is lowered, the flame becomes long and during combustion it reaches smoke tube inlet at the bottom of the combustion chamber, where the flame is cooled, combustion becomes incomplete, and CO and smoke are produced. In emissions format B when the air ratio is lowered as in the case of format A, the flame becomes long but because the flow of combustion emissions is at the top of the combustion chamber, the flame does not reach the smoke tube inlet at the bottom of the combustion chamber and faces upward. Smoke density Air ratio Figure 2.1-1 Combustion Features with Each Format Test furnace: Upright test furnace, Fuel: Kerosene Burner: Self-recirculating burner : format (A), : format (B) 4

2.1.2 Improvement of Combustibility of Divided Flame Burner With the aim of improving the combustibility (CO, smoke density) of a divided flame burner, an investigation was made of blast tube diameter at the burner head with a lateral test furnace, using as fuel kerosene and A heavy oil having the properties given in Table 2.1-1. The results appear in Figure 2.1-2. These results were obtained when the blast tube diameter of divided flame burner shown in Figure 2.1-3 was 216 mm and 165 mm. It was found that combustibility at low air ratios can be improved by making the blast tube narrow in diameter. It appears that narrowing the blast tube promotes the mixing of fuel and secondary combustion air. Smoke density Air ratio Figure 2.1-2 Combustion Features with Each Blast Tube Diameter Test furnace: Lateral test furnace, Fuel: Kerosene, A heavy oil Burner: Divided flame burner Blast tube diameter ; 165 mmφ: Kerosene, A heavy oil (1), A heavy oil (2) 216 mmφ: (kerosene) 5

Blast tube Secondary combustion air supply pipe Feed pipe Oil nozzle Figure 2.1-3 Structure of Divided Flame Burner 2.1.3 Low NOx of Air Atomizing Burner The air spray burner is a type of burner in which the energy of combustion air ejection is used to recirculate combustion emissions and curtail NOx generation. With the aim of expanding the emissions recirculating volume and lowering NOx, the structure of the emissions recirculating component shown in Figure 2.1-4 (burner trial-produced in 1999) was renovated into the recirculating component structure (burner trial-produced in 2000) shown in Figure 2.1-5 (insert diameter narrowed and large number of units used), and combustion experiments were conducted. Figure 2.1-6 shows the results. From the figure, a technological forecast was obtained in which the NOx emissions value is around 40 ppm with kerosene combustion, about 10 ppm lower than with the burner trial-produced last year, and 60 to 65 ppm with A heavy oil (N content: 210 ppm), thereby reaching the target value (60 ppm or less). Combustion Recirculating Component Oil atomizer Insert Spark plug Flame cone Fuel oil inlet Atomizing air inlet Sight hole Heat and flame resistant material Combustion air inlet Figure 2.1-4 Structure of Air Atomizing Burner Trial-Produced in 1999 6

Combustion air inlet Combustion Recirculating Component Atomizing air inlet Oil atomizer Spark plug Flame cone Fuel oil inlet Sight hole Insert Heat and flame resistant material Figure 2.1-5 Structure of Air Atomizing Burner Trial-Produced in 2000 Smoke density Burner trial-produced in 1999 (kerosene) NOx (0 %O2 conversion: ppm) Air ratio Figure 2.1-6 Combustion Features of Air Atomizing Burner of Different Recirculating Component Test furnace: Lateral test furnace, Burner: Air atomizing burner Fuel: Kerosene, A heavy oil (1) 7

2.1.4 Low NOx of Thick and Thin Fuel Burner (1) Investigation of Primary Air Volume The thick and thin fuel burner is one in which NOx generation is curtailed through the formation of flame in which there is little combustion air at the flame center as opposed to the fuel (thick combustion) and flame in which the flame outer portion has a surplus of air with respect to the fuel (thin combustion). In order to achieve low NOx in the thick and thin fuel burner, an investigation was made of the potential for reducing NOx by having little combustion air at the flame center, achieved by reducing the area of the primary air inlet shown in Figure 2.1-7. The results appear in Figure 2.1-8. From Figure 2.1-8 it can be observed that by lowering the primary air volume still further from 25% (1999 trial-produced burner), the NOx emissions value was reduced about 5 to 10 ppm, and that within the broad range of 1.1 to 1.5 for air ratio, combustibility (CO and smoke density) was favorable. With a drop in primary air volume, however, the formation (adhesion) of soot on the flame holder surface and on the inner surface of blast tube at the combustion outlet could also be observed. Blast tube Secondary air outlet Feed pipe Flame holder Primary air outlet Small hole (Primary air inlet) Oil nozzle Primary air control component Figure 2.1-7 Burner Head Structure of Thick and Thin Fuel Burner (Thick and Thin Fuel A) 8

Smoke density Air ratio Figure 2.1-8 Primary Air Volume of Thick and Thin Fuel Burner Test furnace: Lateral test furnace, Burner: Thick and thin fuel burner, Fuel: Kerosene Primary air volume: (4%), (8%), (15%), (25%) (2) Burner Head Structure and Secondary Air Nozzle Length In the previous section, it was noted that with a decline in primary air, soot forms (adheres) to the flame guard surface and to the inner surface of blast tube at the combustion outlet. Accordingly, with the aim of curtailing soot formation, an investigation was made to determine if the formation of NOx and soot can be curtailed by having a burner head structure (thick and thin fuel B) in which the secondary air nozzle remains but blast tubes around the nozzle are eliminated. Also investigated was the length of secondary air nozzle when there are no blast tubes around the nozzle. The results appear in Figure 2.1-10. From Figure 2.1-10 {comparison of mark (burner head structure: thick and thin fuel A (d=85 mm)) and mark (burner head structure: thick and thin fuel B (d=85 mm))}, it was observed that the NOx emissions value is somewhat lower (by several ppm) and that soot does not adhere to the flame holder surface when there are no blast tubes around the secondary air nozzle. This can be ascribed to the fact that combustion emissions can be circulated easily in the furnace since there are no blast tubes around the nozzle in the thick and thin fuel B burner head. On the other hand, in the range of 30 to 140 mm for secondary air nozzle length, the NOx emissions value hardly changed at all. 9

(3) Secondary Air Blowout Method Based on results obtained up to the last section above, burner heads having the structures shown in Figures 2.1-11 and 2.1-12 were fabricated and secondary air blowout methods were investigated with the aim of further clarifying thick combustion at the flame center and thin combustion at the outer flame, and curtailing NOx generation. In addition, the total areas of the secondary air blowout ports in thick and thin fuel B, thick and thin fuel C and thick and thin fuel D structures were all the same. The results with kerosene and A heavy oil (N content: 210 ppm), together with thick and thin fuel B results, are given in Figures 2.1-13 and 2.1-14. Figure 2.1-13 reveals that NOx generation is curtailed as the blowout of secondary air becomes uneven with respect to fuel blowout component (oil nozzle). With thick and thin fuel D the NOx emissions value dropped to 10 to 15 ppm and combustibility was favorable with a CO emissions value of 50 ppm or less. With A heavy oil fuel, on the other hand, although some smoke formation was observed, the NOx emissions value (45 ppm or less) was far below the target value (60 ppm or less) regardless of the burner head structure of the burner employed. The reason for the reduction in NOx emissions can be ascribed to thick and thin fuel combustion and emissions recirculation, but plans call for clarification of this through measurements of flame temperature distribution and emissions compositions at combustion sites in the future. 2.2 Development of High-Efficiency Storage Water Heater 2.2.1 Combustion Test with Exhaust Flue Condensation Storage Water Heater The screw plate shown in Figure 2.2-1 was inserted into the smoke tube at the top of the exhaust flue condensation storage water heater (Figure 1.2-3), and the combustion features of thick and thin fuel burner and burner on the market were investigated together with the thermal efficiency of the storage water heater. Test results appear in Figures 2.2-2 and 2.2-3. Figure 2.2-2 shows the results of an investigation of combustion features in relation to air ratio in burner on the market and thick and thin fuel burner with the combustion chamber thermal load percentage held constant at 1.67 MW/m3. The figure shows that NOx emissions from thick and thin fuel burner (around 30 ppm) were drastically lower than those from burner on the market (80 to 100 ppm). In the thick and thin fuel burner, although CO emissions and smoke density are somewhat high at a high air ratio (1.25 or above), at a low air ratio (1.2 or below) combustibility is the same as in burner on the market. These findings indicate that the thick and thin fuel burner is an outstanding burner. Figure 2.2-3 gives the results of an investigation of storage water heater thermal efficiency and of total thermal efficiency when burner on the market and thick and thin fuel burner have been used. It was predicted that thermal efficiency of the thick and thin fuel burner would be low in comparison to burner on the market because NOx from the thick and thin fuel burner is made low through a reduction in flame temperature. The figure shows, however, that there is virtually no difference between the two burner types in terms of thermal efficiency. 10

Blast tube Secondary air nozzle Flame holder Feed pipe Primary air outlet Oil nozzle Primary air control component Secondary air blowout port Figure 2.1-9 Burner Head Structure of Thick and Thin Fuel Burner (Thick and Thin Fuel B) Smoke density Air ratio Figure 2.1-10 Burner Head Structure and Secondary Air Nozzle Length Test furnace: Lateral test furnace, Burner: Thick and thin fuel burner, Fuel: Kerosene Burner head structure: (Figure 2.1-7), (Figure 2.1-9) Secondary air nozzle length d (mm): (85), (0), (30), (85), (140) 11

Secondary air blowout port Oil nozzle Figure 2.1-11 Burner Head Structure of Thick and Thin Fuel Burner (Thick and Thin Fuel C) Secondary air blowout port Oil nozzle Figure 2.1-12 Burner Head Structure of Thick and Thin Fuel Burner (Thick and Thin Fuel D) Smoke density Figure 2.1-13 Secondary Air Blowout Method Test furnace: Lateral test furnace, Burner: Thick and thin fuel burner, Fuel: Kerosene Burner head structure: (Figure 2.1-9), (Figure 2.1-11), (Figure 2.1-12) 12

Smoke density Air ratio Figure 2.1-14 Secondary Air Blowout Method Test furnace: Lateral test furnace, Burner: Thick and thin fuel burner, Fuel: A heavy oil (1) Burner head structure: (Figure 2.1-9), (Figure 2.1-11), (Figure 2.1-12) Figure 2.2-1 Screw Plate Structure 13

Smoke density Air ratio Figure 2.2-2 Combustion Test by Exhaust Flue Condensation Storage Water Heater (Kerosene) : Thick and thin fuel burner, : Burner on the market Thermal efficiency (%) Air ratio Figure 2.2-3 Combustion Test by Exhaust Flue Condensation Storage Water Heater (Kerosene) Storage water heater efficiency: Thick and thin fuel burner, Burner on the market total thermal efficiency: Thick and thin fuel burner, Burner on the market 14

3 Results of Empirical Research 3.1 Development of High-Performance Burner (1) Improvement of Self-Recirculating Burner Combustibility In order to improve the combustibility (CO, smoke density) of self-recirculating burner in upright test furnace, combustion emissions formats were investigated. As a result, combustibility was improved by changing the outflow of combustion emissions from the combustion chamber rear to the combustion chamber top. (2) Improvement of Divided Flame Burner Combustibility With the aim of improving the combustibility of divided flame burner, an investigation was made of blast tube diameter at the burner head. By making the blast tube narrow in diameter, CO emissions were 10 ppm or below at low air ratios (1.2 or below) and no smoke formation was noted. In addition, NOx emissions was 30 ppm or less in kerosene combustion, dropping 5 to 10 ppm from the previous year, and in A heavy oil combustion (N content: 210 ppm), 50 ppm or less was reached. (3) Low NOx of Air Atomizing Burner With the aim of lowering NOx from air atomizing burner, the structure of the emissions recirculating component was investigated. It was found that in kerosene combustion the NOx emissions value is around 40 ppm, about 10 ppm lower than in the previous year, and in A heavy oil (N content: 210 ppm) combustion, a technological forecast of 60 to 65 ppm, reaching the target value (60 ppm or less), was obtained. (4) Low NOx of Thick and Thin Fuel Burner In order to achieve low NOx in the thick and thin fuel burner, an investigation was made of primary air volume, burner head structure, secondary air nozzle length and air blowout method. From an examination of primary air volume and secondary air blowout method, it was found that in kerosene combustion NOx emissions reached 10 to 15 ppm, down about 15 to 20 ppm from last year. In A heavy oil (N content: 210 ppm) combustion, on the other hand, although some smoke formation was noted, NOx emissions was 45 ppm or less, far below the target value (60 ppm or less). Patent application is in preparation based on these results. 3.2 Development of High Efficiency Storage Water Heater (1) Combustion Test with Exhaust Flue Condensation Storage Water Heater A comparative investigation was made of thermal efficiency and combustion characteristics in kerosene fuel when thick and thin fuel burner and burner on the market have been combined with an exhaust flue condensation storage water heater. It was confirmed that NOx emissions from the thick and thin fuel burner (around 30 ppm) was drastically lower than that from burner on the market (80 to 100 ppm), while the thermal efficiency of thick and thin fuel burner was roughly the same as that of burner on the market. 15

4. Synopsis In a test furnace, NOx emissions from the burner were reduced while maintaining a fairly high level of combustibility even with A heavy oil fuel (N content: 210 ppm). In the future, issues pertaining to practical application, in combination with practical boiler (once-through boiler, hot water boiler and hot water generator) will have to be abstracted and investigated, and findings will have to be incorporated into burner at the practical level. In addition, trial-produced storage water heater and developed burner must be combined and the relationships between storage water heater structure, efficiency and NOx emissions must be investigated. The optimum boiler storage water heater for a small-scale, oil-burning boiler must then be developed. Copyright 2001 Petroleum Energy Center all rights reserved. 16