http://ago.helion.pl ISSN 1733-4381, Vol. 2 (2005), p-49-56
Investigations of Birch and Alder Bark Mixture Thermal
Utilization in the Heating Boiler Working in Unsteady Condition
Juszczak M., Ph.D., Eng.Poznan University of Technology, Institute of Environmental Engineering, Piotrowo Street 3a, 60- 965 Poznań
Phone: 061 6652 524, fax: 061 6652439 e-mail: marekjuszczak8@wp.pl
Abstract
The wooden waste thermal utilization study was performed. A mixture of birch and alder bark from plywood factory was utilized in the boiler of 25 kW heat power and 70 l water capacity. The process was performed in two steps: pyrolisis and combustion of wooden gas in the nozzle. The boiler was working without heat storage and therefore sometimes the boiler water temperature increased quickly above 80°C and the fan supplying the air for combustion stopped and started again when the temperature went down below 75°C. The measurements were carried out during the whole time of thermal utilization process including periods of low boiler water temperature and low flame temperature of in the nozzle. In this unsteady condition the value of the carbon monoxide concentration was too high and heat efficiency value was below 50%. The following parameters were measured: volume stream rate and temperature of boiler water, also heat quantity obtained by boiler water, heat power (using ultrasonic heat meter), flame temperature in the nozzle, temperature of flue gases, carbon monoxide and nitric oxide concentrations, air excess rate of the flue gases in the stack (using thermocouples and gas analyzer with electrochemical cells). All these parameters were registered manually every three minutes.
Streszczenie
Badania Termicznej Utylizacji Mieszaniny Kory Brzozy i Olchy w Kotle Grzewczym w Stanie Niestacjonarnym
Badano termiczną utylizację mieszaniny kory brzozy i olchy (uzyskanej z fabryki sklejki) w kotle o mocy 25 kW i pojemności wodnej 70 l. Proces przebiegał w dwóch stopniach: piroliza i spalanie gazu drzewnego w dyszy. Kocioł nie współpracował z zasobnikiem ciepła i niekiedy temperatura wody kotle przekraczała 80°C i wtedy wentylator powietrza do spalania wyłączał się, a włączał gdy temperatura była niższa niż 75°C. Pomiary prowadzono w czasie całego procesu termicznej utylizacji, włączając okresy niskiej temperatury wody kotłowej i płomienia w dyszy. W tym stanie niestacjonarnym stężenie
tlenku węgla było za wysokie, a sprawność cieplna kotła mniejsza niż 50%. Mierzono następujące parametry: strumień objętości i temperaturę wody kotłowej, również ciepło przejęte przez wodę kotłową i moc cieplną (przy pomocy ultradźwiękowego licznika ciepła), temperaturę płomienia w dyszy, temperaturę spalin, stężenia tlenku węgla i tlenku azotu, współczynnik nadmiaru powietrza w spalinach w kominie (stosując termoparę i analizator gazu z celami elektrochemicznymi). Wszystkie parametry rejestrowane były ręcznie co trzy minuty.
1. Wstęp
Wooden waste without chemical components, forest and agricultural residues are called biomass. Thermal utilization is frequently carried out in domestic heating boilers which are working without the help of water heat storage. Birch and alder bark mixture has low density, about 0.15 kg/m3 and therefore it is necessary to frequently put such fuel manually to the boiler. Sometimes in domestic conditions the fuel supply is too late and the temperature in the combustion chamber is too low and thus carbon monoxide concentration in flue gases is too high. There are rules in Poland [1] for permitted values of carbon monoxide, organic carbon and dust concentrations in flue gases from boilers firing solid fuels of heat power below 300 kW. These values depend on the heat efficiency value declared by the producer. Below, permitted values of pollutant concentrations for manually supplied biomass firing boiler of 25 kW heat power are presented (tab.1.1):
Tab.1.1. Permitted values of carbon monoxide, organic carbon and dust concentrations
(normalized to 10% oxygen, dry gas) in flue gases from the 25 kW boiler, biomass supplied manually
Tab.1.1. Wartości dopuszczalne stężeń tlenku węgla, węgla organicznego, pyłu (odniesione
do 10% tlenu, suchy gaz) w gazach odlotowych z kotła 25 kW, spalającego biomasę, zasilanie ręczne Heat efficiency % CO concentration mg/nm3 C org. concentration mg/nm3 Dust concentration mg/nm3 ≥ 75.4 (67 + 6 log25) 5000 150 150 ≥ 65.4 (57 + 6log25) 8000 300 180 ≥ 55.4 (47 + 6log25) 25000 2000 200
In addition, to obtain ecological certificate, the nitric oxides concentration (calculated to nitric dioxide) has to be below 400 mg/nm3 ( normalized to 10% oxygen, dry gas) [2].
2. Test facility, measuring equipment and study program
The thermal utilization of alder and birch bark mixture was carried out in the boiler of 25 kW heat power (fig.2.1), biomass manual supply.
Fig.2.1. Scheme of the boiler 25 kW; front side of the boiler with open combustion
chamber; boiler with thermocouple view
Rys.2.1. Schemat kotła 25 kW, przednia ściana kotła z otwartą komorą spalania; kocioł z
widokiem termopary
The process was performed in two steps: pyrolisis and biomass gas combustion in the nozzle located in the bottom of the pyrolisis chamber. The fan was supplying the air to the pyrlisis chamber and to the nozzle. Air stream value was regulated manually, according to flame color and temperature observations in the nozzle. Also the values of air excess rate and carbon monoxide concentration in flue gases in the stack (on display of gas analyzer with electrochemical cells) were taken into account. Observations of flame color for carbon monoxide and nitric oxides emission control have been used in the recent years in industry with very good results [3]. The flue gases went to the steel insulated stack, 7.5m high, 200mm inner diameter. Hot water from the boiler was cooled in the heat exchanger with fan, situated on the roof of the heat station. It was possible to obtain five angular fan velocities and three angular circulation pump velocities (fig.2.2).
Fig.2.2. Ultrasonic heat meter and the pump Rys 2.2. Ultradźwiękowy licznik ciepła oraz pompa
In this situation it was possible to adjust heat power of the heat exchanger to present thermal output of the boiler. The flame temperature in the nozzle was measured by radiation shielded thermocouple PtRhPt. The study was performed on 20.09.2004 and measurements of all parameters were registered manually every three minutes During the whole process which lasted 90 minutes 7.57 kg of wooden waste was utilized. Before the measurement, 1.35 kg of alder poles was burnt. The moisture of the bark mixture was about 20 % and calorific value about 14 kJ/kg. The bark contains about 0.2-0.4 % of nitrogen and about 48% of carbon. The following parameters were measured: volume stream rate and temperature of boiler water (supply and return), present heat power, quantity of heat obtained by boiler water - using an ultrasonic heat meter, temperature, air excess rate, carbon monoxide and nitric oxide concentrations in flue gases in the stack - using a gas analyzer with electrochemical cells.
3. Results
Measured parameters are presented in tab.3.1, fig.3.1 and fig.3.2. Two periods of thermal utilization can be distinguished – the first one when air excess rate value was below 4.0 and the second one when this value was above 4.0. In the first period, carbon monoxide concentration was below 3000 mg/nm3 (normalized to 10% oxygen, dry gas). In the second period, the temperature of the flame decreased, and carbon monoxide concentration increased. It is interesting to look at table.3.1 and analyze the relationship between flame temperature in the nozzle, air excess rate and carbon monoxide concentration. The measurements were carried out together with a student, during his master thesis [4]. A similar study was performed a few months earlier, utilizing oak poles [5].
Tab.3.1. Selected results of birch and alder mixture thermal utilization in the boiler of 25 kW (wooden gas is combusted in the nozzle)
Tab.3.1. Wybrane wyniki termicznej utylizacji mieszaniny kory brzozy i olchy w kotle 25 kW (gaz drzewny spalano w dyszy)
τ min N kW Tz K Tp K Ts K Tb K ˘λ - CO 10%O2 mg/nm3 NO 10%O2 mg/nm3 NO→NO2 10%O2 mg/nm3 18 5.9 316 310 471 633 2.93 2038 225 345 21 7.1 320 313 478 653 1.77 501 231 354 24 8.6 326 314 502 658 1.35 905 168 258 27 10.5 333 323 522 633 1.30 4236 118 181 30 16.0 340 325 526 685 1.27 3930 116 178 33 23.1 346 333 520 693 1.31 3810 135 207 36 20.7 348 335 501 743 1.29 1539 132 202 39 18.0 347 336 476 783 1.36 2059 164 251 42 22.0 346 336 463 703 1.54 2137 167 256 45 21.3 345 334 450 653 2.03 3230 148 227 48 17.5 344 333 440 523 2.64 3852 125 192 51 19.6 343 332 430 583 3.30 3214 125 192 54 17.1 341 330 420 533 3.95 3012 135 207 57 16.4 339 329 423 673 4.84 6461 120 184 60 15.9 339 326 414 633 5.53 4705 140 215 63 15.0 339 326 405 533 5.07 5278 157 241 66 14.1 337 324 398 513 4.91 5512 181 277 69 13.4 335 323 391 483 6.29 12095 196 302 72 12.4 333 322 384 453 8.30 12887 190 291 75 11.7 331 320 378 423 10.73 11719 198 304 78 11.3 329 319 373 403 13.67 10510 198 304 81 10.4 327 318 369 393 16.98 9374 197 303 84 10.0 325 316 364 383 21.20 8459 196 300 87 9.5 324 315 361 373 25.83 7879 198 304 90 9.0 322 314 357 373 29.81 7790 198 304 94 8.5 320 313 354 363 34.52 8438 205 314 96 8.0 319 312 351 363 39.98 8861 216 331 99 7.6 318 311 349 353 45.17 8720 214 328 102 7.1 317 310 346 353 49.89 8773 216 331 105 6.7 315 309 344 353 56.35 9066 225 345 A 13.1 332 323 421 531 13.50 11014 176 260
Symbol explanation for table 3.1:
τ – time from the beginning of the measurement period N – heat power
Tz – boiler water temperature (supply)
Ts – flue gas temperature in the stack
Tb – temperature of the flame in the nozzle
λ - air excess in flue gases in the stack
CO (10%O2) – carbon monoxide concentration in flue gases (normalized to 10% oxygen) ,
dry gas
NO (10%O2) – nitric oxide concentration in flue gases (normalized to 10% oxygen), dry gas
NO→NO2 (10%O2) – nitric oxide concentration, calculated to nitric dioxide (normalized to
10% oxygen), dry gas A - medium values 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 1 4 :4 2 1 4 :5 1 1 4 :5 7 1 5 :0 3 1 5 :0 9 1 5 :1 5 1 5 :2 1 1 5 :2 7 1 5 :3 3 1 5 :3 9 1 5 :4 5 1 5 :5 1 1 5 :5 7 1 6 :0 3 1 6 :0 9 1 6 :1 5 1 6 :2 1 1 6 :2 7 1 6 :3 3 1 6 :3 9 1 6 :4 5 1 6 :5 1
Measurement time [h:min]
te m p e ra tu re [ K ] 0 2 4 6 8 10 12 14 16 18 20 22 24 v o lu m e s tr e a m r a te [ m 3/h ] h e a t p o w e r [k W ] a ir e x c e s s r a te [ L a m b d a ]
Tz Tp air excess rate heat power volume stream rate Lambda +273,13
Fig.3 .1. Variation of thermal utilization parameters Rys.3.1. Przebieg parametrów termicznej utylizacji
0 4 0 8 0 12 0 16 0 20 0 24 0 28 0 32 0 36 0 40 0 44 0 48 0 52 0 56 0 60 0 1 4 :4 2 1 4 :4 8 1 4 :5 1 1 4 :5 4 1 4 :5 7 1 5 :0 0 1 5 :0 3 1 5 :0 6 1 5 :0 9 1 5 :1 2 1 5 :1 5 1 5 :1 8 1 5 :2 1 1 5 :2 4 1 5 :2 7 1 5 :3 0 1 5 :3 3 1 5 :3 6 1 5 :3 9 1 5 :4 2 1 5 :4 5 1 5 :4 8 1 5 :5 1 1 5 :5 4 1 5 :5 7 1 6 :0 0 1 6 :0 3 1 6 :0 6 1 6 :0 9 1 6 :1 2 1 6 :1 5 1 6 :1 8 1 6 :2 1 1 6 :2 4 1 6 :2 7 1 6 :3 0 1 6 :3 3 1 6 :3 6 1 6 :3 9 1 6 :4 2 1 6 :4 5 1 6 :4 8 1 6 :5 1 M ea su rem e n t tim e [h :m in ] T e m p e ra tu re [ K ] 0 5 0 1 00 1 50 2 00 2 50 3 00 3 50 4 00 4 50 5 00 5 50 6 00 6 50 7 00 7 50 8 00 P o ll u ta n ts c o n c e n tr a ti o n [ m g /n m 3] te m p . in th e co m b us tion c ha m b er te m p . of flue g as se s in s ta ck N O x - 10 % O 2 [m g /n m 3 ] N O - 1 0% O 2 [m g /n m 3 ] +2 73 ,1 3
Fig.3.2. NO and NOx concentrations, flue gas temperature in the stack , temperature of the flame in the nozzle
Rys.3.2. Stężenia NO and NOx, temperatura spalin w kominie, temperature płomienia w dyszy
4.Summary
A special kind of thermal utilization process was investigated, which sometimes appears in practice in domestic boilers when manually supplied fuel is not added in time. In such cases the temperature of the thermal utilization process decreases and the carbon monoxide concentration increases. For small density wooden waste, it would be better to carry out the thermal utilization process in domestic boilers with automatic fuel supply.
References
[1] PN-EN 303-5, Kotły grzewcze. Część 5: Kotły grzewcze na paliwa stałe z ręcznym i automatycznym zasypem paliwa o mocy nominalnej do 300 kW. Terminologia, wymagania, badania i oznakowanie, 2002.
[2] Kubica K., Kryteria efektywności energetyczno-ekologicznej kotłów małej mocy i paliw stałych dla gospodarki komunalnej, Certyfikacja na znak bezpieczeństwa ekologicznego, I. Ch..P W. 1999.
[3] Flymn T., Bailey R., Fullert T., Daw C.S., Finney C., Stallings J; Flame Monitoring Enhances Burner Management, Power Engineering, February, 50-54, 2003.
[4] Marks P., Laboratoryjne badanie emisji zanieczyszczeń przy termicznej utylizacji biomasy,praca magisterska, Politechnika Poznańska, 2005,
[5] Juszczak M., Badania stanów niestacjonarnych kotła centralnego ogrzewania zasilanego drewnem dębowym, XI Międzynarodowa konferencja: Air Conditioning , Air Protection and District Heating, Politechnika Wrocławska, PAN, Wrocław- Szklarska Poręba, 23-26.06.2005, 241-246
