and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 5 (2007), p-91-98
Thermal utilization of poplar chips in the central heating boiler
Juszczak M.
Poznan University of Technology, Institute of Environmental Engineering, Streszczenie
Termiczne przekształcanie zrębków topoli w kotle centralnego ogrzewania
Przeprowadzono badania termicznej utylizacji zrębków topoli, pozyskanych z sanitarnej wycinki gałęzi drzew w mieście. Obiektem badanym był kocioł o mocy 25 kW i pojemności wodnej ok. 200l, współpracujący z zasobnikiem ciepła 900l. Taki układ zapewniał warunki pracy zbliżone do stacjonarnych. Termiczna utylizacja przebiegała dwustopniowo: piroliza oraz spalanie gazu drzewnego. Dokonując odczyty, co 3 minuty mierzono: temperaturę w strefie pirolizy oraz w strefie spalania; w kominie: temperaturę spalin, stężenie tlenku węgla, tlenku azotu, współczynnik nadmiaru powietrza - za pomocą termopar oraz analizatora gazów z celami elektrochemicznymi. Przy użyciu ultradźwiękowego licznika ciepła dokonano pomiaru temperatury i strumienia objętości wody kotłowej, mocy cieplnej kotła oraz ciepła przekazywanego do wody. Obliczono sprawność cieplną kotła. Badania prowadzono dla kilku porcji paliwa, dozowanego ręcznie do kotła, po jego uprzednim rozgrzaniu.
Abstract
Thermal utilization of poplar chips, obtained from tree branches sanitary cutting in the city, was conducted in the boiler of 25 kW heat power, which was working with the help of the 900 heat storage. The thermal utilization was carrier out as a two step process: pyrolisis and combustion of wooden gas. Following parameters were measured every three minutes: temperature in pyrolisis zone of the combustion chamber and in the region where wooden gas was burnt; temperature, carbon monoxide, nitric oxide and dust concentrations and air excess rate in flue gases in the stack. Thermocouples and gas analyzer with electrochemical cells were used for this purpose. Temperature and volume stream rate of the boiler water, heat power and heat carried by boiler water, were measured using ultrasonic heat meter. Heat efficiency was calculated. Experiments were performed for several portions of the fuel, added manually to the boiler, which was previously preheated.
1. Introduction
In Poznan University of Technology, small district heating system was built to perform the investigations of wooden waste and other biomass thermal utilization in the heating boilers in conditions similar to the real ones. The district heating system seems to be a physical
model of the heating system in the city. It is possible to measure the concentration of carbon monoxide, nitric oxide, dust and so on, in flue gases obtained from thermal utilization, in the situation when the heat demand for heating and heat tap water is changing in daytime and boilers are working with or without the help of the heat store. This small heating system was finished in April of 2005 year, on the basis of two water boilers each of 25 kW heat power. Every boiler has a combustion chamber of different construction. The boilers were connected with 900 l water heat storage with the help of ‘LADDOMAT 21’ (pump and valves in one body, Swedish production, for proper circulation between the boilers and the heat storage). The first boiler was situated in the heat station in March of 2004 year and connected to the insulated steel stack , 200mm inner diameter, 7.5 m high [1]. The second boiler was connected to the same stack and its height rose to 8.5 m. The water heat storage with ‘LADDOMAT 21’ and a newly constructed underground water network with two insulated pipes, each 35 mm inner diameter about 40m long were connected to the boilers at the same time . The hot water flows (and returns) in the pipes from the heat station to the building type DREWBUD (fig.1.1). With the help of the heat transfer unit (fig.1.1), the heat coming from hot water flowing in the network transfers to the heating installation of the building type DREWBUD.
Fig.1.1. Experimental small district heating water network with two underground insulated pipes (in construction); the house type DREWBUD, heat transfer unit inside the house In the year 2004, the heat exchanger with fan was situated on the roof of the heat station near the stack. The heat coming from the boilers is transferred directly to the atmosphere (fig.1.2).
It is possible to change the thermal output of the exchanger, altering the volume stream rate: of the air and in the boiler water, simulating thermal demand variations. During the presented investigations, the water network was not working and the heat produced in the boiler went directly to the atmosphere. A study carried out by Swedish National Board of Industrial and Technical Development shows that biomass boilers in the thermal output range 0.5 to 10 MW emit disproportional amount of pollutants in form of product of incomplete combustion during low or varying thermal output [2].
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 5(2007) 93
Fig.1.2. Steel insulated stack , 200 mm inner diameter, 8.5 m high; heat exchanger with fan; poplar chips
2. Test facility, measuring equipment and investigation program
The tested boiler of about 200 l water capacity has the combustion chamber of 150 l capacity. In the upper part of the combustion chamber (fig.2.1) the pyrolisis process is performed.
Fig.2.1. View of the boiler 25 kW; view of the combustion chamber; view of the back of the boiler
Then the wooden gas flows through the non burnt fuel to the button part of the combustion chamber, where the secondary air is added and wooden gas is burnt. The temperature is measured in the upper and bottom part of the combustion chamber using two radiation shielded thermocouples PtRhPt. The air is supplied to the pyrolisis zone and to the bottom part of the combustion chamber with the help of the same fan. It is possible to change manually the angular fan velocity, in the range varying from 5 to 100 % of maximum value, with 5 % steps. During the experimental study, the following parameters were observed:
temperature, flame color in the combustion chamber and also air excess rate in flue gases, carbon monoxide and nitric oxide concentration in the stack. Dark red flame color suggested that the temperature was too low, air excess too low or too high and carbon monoxide concentration too high. In such situation it was necessary to deliver additional portion of fuel or (and) change the value of air supply to the combustion chamber. On the other hand, bright yellow or almost white flame color means that the temperature in the flame and combustion air excess were too high. It was then necessary to change this situation manually watching the carbon monoxide and nitric oxide values on the display of the gas analyzer. During the research dust concentration was also measured with the help of gravimetrical dust meter with izokinetical probe sampling. These experiments served to describe the performance of the poplar chips thermal utilization process, especially when the combustion chamber is not filled completely. We could also examine the possibility to use this fuel type in domestic boilers (with two step combustion and manual supplying of the fuel). It was interesting to compare the obtained values of pollutant concentrations with the rules [3, 4]. During the study, also the combustion air volume stream rate was measured using the orifice. The fuel was weighed and the calorific value and moisture were measured in Institute of Wood Technology in Poznan (17.1%; 17,88,kJ/kg). The wood contains about 48% of carbon and about 0.4% of nitrogen. The boiler was preheated using 11.1 kg of alder wood and the portions of poplar chips were added as it can be seen in table 2.1. The experiment was finished after 390 minutes. The total weigh of poplar chips was about 42.5 kg. Investigations were performed together with the student [5].
Table 2.1. Poplar chips thermal utilization process in the 25 kW boiler operation time (from the beginning
of the process) [min]
fuel portion kg
percentage of max angular fan velocity [%] 0 8.3 20-30 51 5.3 20 96 4.9 35 153 4.0 35 192 8.0 25 264 8.0 25 327 4.0 30 3.Results
The more important parameters of the thermal utilization were presented below (only 30-minute periods of the lowest and highest values of carbon monoxide concentrations). Table 3.1. Parameter values of measured during the poplar chips thermal utilization - the 30-minute period of the lowest carbon monoxide concentration
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 5(2007) 95 τ min N kW Tz K Tp K Ts K Ta K Tb K ˘λ - CO 10%O2 mg/nm3 NO 10%O2 mg/nm3 NO→NO2 10%O2 mg/nm3 135 37.5 355 329 385 483 893 2.50 2527 136 208 138 37.2 354 328 393 493 923 2.49 6484 134 205 141 30.0 352 334 392 503 913 2.39 6122 127 195 144 1-.5 347 340 391 503 893 1.92 5003 137 210 147 5.3 346 342 389 503 823 1.63 4264 155 238 150 9.3 348 341 386 503 763 1.56 4074 165 253 153 16.1 350 339 379 503 703 1.55 3067 167 256 156 22.5 351 335 390 463 693 1.81 4703 178 273 159 31.2 353 331 399 473 923 2.01 4782 167 256 162 31.1 353 331 399 493 928 2.27 2217 169 259 A 23.1 351 335 390 492 846 2.01 4321 154 236 B 31.1 353 331 399 493 928 2.27 2217 169 259 C 37.2 354 328 393 493 923 2.49 6484 134 205 The dust concentration, measured during the whole thermal utilization process was 38 mg/nm3, normalized to 10 % oxygen, dry gas. The humidity of the stack gases was measured by the psychrometer (part of the dust meter). The boiler producer declared heat efficiency of the boiler above 75 %. In this case, for the 25 kW heat power boiler, the values of carbon monoxide, nitric oxide (calculated to nitric dioxide) and dust concentrations (normalized to 10% oxygen) should be lower than: 5000, 400, 150 mg/nm3 respectively, and: 8000, 400, 180 mg/nm3 respectively, if declared heat efficiency is above 65.4 % [3,4]. The value of the heat efficiency, calculated using direct method, was 66.7%. Great and unpredictable variability of the heat power is a result of unsteady work of device ‘LADDOMAT 21’
Table 3.2. Parameter values measured during the poplar chips thermal utilization - the 30-minute period of the highest carbon monoxide concentration
τ min N kW Tz K Tp K Ts K Ta K Tb K ˘λ - CO 10%O2 mg/nm3 NO 10%O2 mg/nm3 NO→NO2 10%O2 mg/nm3
213 7.1 347 342 379 423 713 4.94 8257 139 213 216 20.2 352 336 381 443 823 5.96 5778 174 267 219 30.1 353 333 366 438 643 5.40 13871 172 264 222 27.6 352 334 368 443 673 4.42 11297 150 230 225 12.4 349 334 366 443 673 3.19 8240 108 166 228 4.0 346 343 376 453 783 2.09 5452 116 178 231 8.7 352 342 376 453 788 1.88 4870 110 169 234 14.5 350 340 374 458 773 1.04 5050 93 173 237 23.7 352 336 373 468 783 1.98 5145 142 217 240 26.5 352 333 370 473 728 2.00 5193 108 166 A 17.5 351 337 373 450 738 3.29 7315 131 201 B 8.7 352 342 376 453 788 1.88 4870 110 169 C 8.1 353 333 366 438 643 5.40 13871 172 264 D 13.3 349 340 360 462 603 3.00 6530 123 189
Symbol explanation for table 3.1 and 3.2: τ – time from the beginning of the measurement period, N – heat power, Tz – boiler water temperature (supply), Tp – boiler
water temperature (return), Ts – flue gas temperature in the stack, Ta - temperature in the
combustion chamber (pyrolisis zone), Tb –temperature in the combustion chamber (flame
zone), 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, B – values for minimum CO concentration, C – values for maximum CO concentration, D – medium values from the whole measurement period that lasted 390 min
4.Summary
The thermal utilization of the poplar chips, made from tree branches, was possible and the values of carbon monoxide, nitric oxide and dust concentrations were acceptable. The carbon monoxide concentration was a little bit too high and the heat efficiency too small,
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 5(2007) 97 but it is possible to change this situation . The boiler with manual fuel supply is not so good for thermal utilization of the low density wooden waste because the fuel has to be added almost every one or two hours.
References
[1] Juszczak M., Badania stężenia tlenku węgla i tlenku azotu w gazach odlotowych z termicznej utylizacji odpadów drzewnych w kotle’ VI Międzynarodowe Forum Gospodarki Odpadami: Efektywność gospodarowania odpadami, PZITS , Licheń 29.0 5-1.06. 2005-09-05.
[2] Lundgren J., Hermansson R., Dahl J., Experimental studies of a biomass boiler suitable for small district heating systems, Biomass and Bioenergy 26, 2004, 443-453
[3] 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.
[4] 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.
[5] Iwanucha J., Badania zamian stężenia tlenku węgla, tlenków azotu i pyłu z kotła centralnego ogrzewania w czasie spalania biomasy, praca magisterska, Politechnika Poznańska, 2005
