and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 8 (2008), p-63-68
Investigations of heat station with the boiler utilizing pellets
burner
Juszczak M.
Politechnika Poznańska, Instytut Inżynierii Środowiska Streszczenie
Badanie kotłowni z kotłem wykorzystującym palnik na pellety
Palnik na pellety umieszczono w kotle grzewczym o mocy 20 kW na polana drzewne i przeprowadzono wstępne badanie. Stanowisko badawcze zlokalizowano w Politechnice Poznańskiej, w laboratoryjnej kotłowni należącej do Instytutu Inżynierii Środowiska. Ta kotłownia może być traktowana jako model fizyczny małego systemu ciepłowniczego, ponieważ jest połączona (za pomocą małej sieci cieplnej ułożonej pod ziemią) z węzłem cieplnym zlokalizowanym w małym badawczym domu rodzinnym. Mierzono stężenie tlenku węgla, dwutlenku węgla, tlenu, tlenku azotu NO, tlenków azotu NOx, (przeliczone na NO2), i pyłu, jak również parametry cieplne i przepływowe kotła: moc cieplną, tempera-turę i strumień objętości wody, opór hydrauliczny, ilość ciepła uzyskanego przez wodę. Średnie stężenia zanieczyszczeń (odniesione do poziomu 10% stężenia tlenu w spalinach), określone po wstępnym pomiarze trwającym ok. 1,5 godz.. wynosiły: CO – 531 mg/m3, NO -135 mg/m3, NOx - 219 mg/m3, pył - 86 mg/m3. Zmienność parametrów została przed-stawiona na wykresie. Niestety pył we worku filtracyjnym pyłomierza miał kolor czarny, co oznacza pojawienie się dużej ilości produktów niezupełnego spalania. Pojawienie się tych substancji jest prawdopodobnie spowodowane natychmiastowym kontaktem spalin z palnika z zimną ścianą kotłowego wymiennika ciepła. Kocioł nie współpracował z odpy-laczem, lecz dużo pyłu osadzało się na powierzchniach wymiany ciepła kotła.
Abstract
Pellets burner was situated in log- wood heating boiler of 20 kW thermal output and pre-liminary investigation was performed. Experimental set-up was located in Poznan Univer-sity of Technology in laboratory heat station belonged to Institute of Environmental Engi-neering. This heat station can be considered as part of physical model of small district heat-ing system, because is connected ( by help of small underground water heat network) with heat transfer unit located in small research family house. Carbon monoxide, carbon dioxide, oxygen, nitrogen oxide NO, nitrogen oxides NOx ( calculated to NO2) and dust concentra-tions were measured in flue gas as well as boiler flow and thermal parameters: thermal output, water temperature and volume stream rate and hydraulic pressure loss, heat quantity obtained by water. Pollutant concentration medium values ( normalized to 10% oxygen
concentration in flue gas) were estimated after preliminary measurement, which lasted about 1.5 hour: CO - 531 mg/nm3, NO - 135 mg/nm3, NOx - 219 mg/m3, dust - 86 mg/m3. Parameters variations during measurement period were presented in the diagram. Unfortu-nately, the dust in the filter bag of dust meter had black color that indicates appearance significant amount of incompletely combustion products. Appearance of these substances is caused probably by immediate, contact of flue gas from the burner with cold wall of the boiler heat exchanger. The boiler was working without dust collector , but a lot of the dust was collected on heat exchange surface of the boiler.
1. Introduction
The simplest and cheapest biomass domestic heating boilers are uncontrolled log-wood boilers. Unfortunately, these boilers are fuel manual supplied and therefore are not comfor-table for living people. Every logs incineration period (if logs fill whole pyrolisis chamber) of these 25 kW heating boilers lasts between 3 and 4 hours but next part of this fuel is often added after 6, 8 or even 10 hours when the people return home after work. This is the re-ason, that heat station with log-wood boiler has to have also big water heat storage (for 25 kW boiler about 1200l. capacity). Uncontrolled boiler works without automatic device for air (used to combustion) stream regulation. That means that there is no oxygen probe (lam-ba sensor) located in stack gas downstream the boiler, connected with automatic device and the fan. The stack heat loss is then higher and boiler heat efficiency smaller than possible. Incineration process of logs consists of two stages: gasification and wooden gas combu-stion. Efficiency of the process depends most on temperature and oxygen concentration in gasification and combustion regions. Temperature in gasification region ought to be above 350°C and in the gas flame above 650°C [1] to obtain relatively low carbon monoxide concentration in stack gas. Unfortunately, incineration process in the boilers this type is not stable, also the drying of wet logs has to perform first decreasing the temperature in gasifi-cation and combustion zone at the beginning of every incineration period. The problems with operation of log-wood boilers in domestic heat stations, (described above) caused introduction of continuously fuel supplied heating boilers to domestic practice. Now one of the most popular boilers in domestic heat stations, is the boiler with pellets burner located in the furnace.
In Institute of Environmental Engineering (Poznan University of Technology), in laboratory heat station, two log-wood boilers were situated. First boiler has about 25 kW nominal thermal output, the second one about 20 kW. The construction of each boiler was not the same. The whole experimental set-up also consist of insulated underground pipeline and heat transfer unit located in small research family house (therefore this experimental set-up can be considered as physical model of small district heating system). In the year 2007 small pellet burner produced in Denmark was introduced to combustion chamber of 20 kW log-wood boiler and some experiments were performed to estimate the concentrations of: carbon monoxide, nitric oxide NO, nitrogen oxides NOx and dust in stack gas downstream the boiler, as well as thermal and flow boiler parameters. Total organic carbon was not measured. The firsts results were presented below.
2. Experimental set-up, measurement equipment, program of investigation The boiler has no automatic device (connected with the burner) with oxygen probe (lambda sensor) located downstream the boiler for air (used to combustion) stream regulation. The boiler with pellets burner utilized to the investigation is presented in fig.1.
Pellets are taken from storage of fuel (with help of screw conveyor) and then they fell gravitationally to the burner. The burner has also its own horizontal screw conveyor to put pellets into combustion region, fig 2.
Air to burning zone is supplied by fan of the burner. The flame of burning pellets was pre-sented on fig. 2 and had dark yellow color. It was possible to see the flame through sight glass. Observation of flame color is now often introduced to big, energetic boilers operation for combustion air regulation and pollutant emission reduction [2], parallel to observation of pollutant concentrations (on gas analyzer display). Flame color observation was not used during presented investigation. The burner has its own ignition device. Electrically heated air causes pellets ignition. The burner has three speeds of pellets supply (thermal outputs), but in practice the burner works with first and second speed only few minutes in the begin-ning and then works with the third speed almost the whole burbegin-ning period. The burner stops if boiler water temperature reaches 85°C. The investigated boiler is connected with water heat storage by help of mixing and piping device. To obtain high temperature of combu-stion chamber wall (as high as possible – for good burning conditions), water flows through the boiler only when its temperature is above 62°C. When temperature of water leaving the boiler is between 62 and 72°C then water flows only through the boiler and mixing and piping device (short circuit). Above the temperature of 72°C a part of water begins to flow also through water heat storage. Then the whole amount of water leaving the boiler comes to water heat storage. Water entered the boiler ought to have temperature above 60°C. Bo-iler flow and thermal parameters: water temperature and volume stream rate, heat quantity obtained by water, boiler, thermal output were measured by ultrasonic heat meter and ga-thered in computer memory every 30 seconds. Concentrations of: oxygen, carbon dioxide, nitric oxide NO, nitrogen oxides NOx (calculated to nitrogen dioxide) were measured dow-nstream the boiler using gas analyzer with electrochemical cells. Dust concentration was measured in stack using gravimetrical dust meter with isokinetical probe sampling. The flue gas humidity was measured using two thermometers: dry and wet (Asmann hydrometer)and dust concentration was calculated for dry gas. Temperature at the end of flame was measu-red with help of radiation shielded thermocouple PtRhPt and gathemeasu-red by computer also every 30 seconds. This time interval of measurements was good enough, because measured values were not changing very quickly. Temperature of the water inside the boiler was measured by boiler resistant thermometer and values were presented on boiler display. Experiment lasted about 90 minutes. Pellet parameters were presented by pellets producer [3], such as: calorific value- 18677 kJ/kg, dust content 1.21 %, moisture 6.11 %, nitrogen continent 0.1 %. We knew also pellets mass stream rate, because we were measured during half a hour the weight of pellets supplied by screw conveyer to the bucket situated on ba-lance. Heat efficiency was calculated dividing heat quantity obtained by boiler water through pellets mass multiplied by their calorific value.
Fig.1. The boiler of 20 kW thermal output with pellet burner, pellet storage and conveyer.
Fig.2. Pellet burner during work (boiler door are opened for a moment to present pellets burner and flame).
Parameters were measured with accuracy: boiler water temperature – 1°C, temperature at the end of flame – 5°C, temperature of flue gas in stack – 1°C, boiler water volume stream rate-0.001 m3/h, boiler thermal output-0.1 kW, oxygen and carbon dioxide concentration - 0.2%, nitric oxide, nitrogen oxides, carbon monoxide concentrations-10 ppm + 5% of me-asured value, dust concentration-5mg/m3, fuel mass 10g, dust mass in dust meter-1mg, air excess rate - 0.01.
3. Results
The first experiment was presented below in graphical form in the figure 3.
0 100 200 300 400 500 600 0 :0 0 0 :0 4 0 :0 7 0 :1 1 0 :1 4 0 :1 8 0 :2 1 0 :2 5 0 :2 8 0 :3 2 0 :3 5 0 :3 9 0 :4 2 0 :4 6 0 :4 9 0 :5 3 0 :5 6 1 :0 0 1 :0 3 1 :0 7 1 :1 0 1 :1 4 1 :1 7 1 :2 1 1 :2 4 1 :2 8 1 :3 1 0 200 400 600 800 1000 1200 O2 [%] T flam e [°C]
Boiler therm al output CO/m g/m3, 10% O2
NO/mg/m 3, 10% O2 NOx/mg/m 3, 10% O2 F la m e t e m p e ra tu re [ °C ], B o ile r p o w e r [k W ], O x y g e n c o n c e n tr a tio n [ % ] C O , N O , N O x , c o n c e n tr a tio n m g / m 3, 1 0 % O 2 [ % ]
Measurement tim e [h:m in]
Fig. 3. Parameter variation of 20 kW heating boiler with pellet burner, during measurement period
Nitric oxide, nitrogen oxides, carbon monoxide, dust concentrations were normalized to 10% O2 concentration in flue gas.
It was the first measurement in the day and in the first minutes of measurement boiler water was cold, therefore minimum values of boiler parameters were very low.
Table 1. Minimum, medium and maximum values (estimated during measurement period):
Selected parameters Unit min medium max
Oxygen concentration % 6,8 9,7 15,7
Carbon dioxide concentration % 5,1 10,9 13,8 Carbon monoxide concentration mg/m3 367 531 1155 Nitric oxide (NO) concentration mg/m3 99 135 151 Nitrogen oxide (NOx) concentration mg/m3 160 219 245 Dust concentration mg/m3 86
Temperature at the end of flame °C 504 518 527 Boiler water temperature, supply °C 43 73 83 Boiler water temperature, return °C 27 59 68
Boiler thermal output kW 1,6 15,7 51,3
Thermal efficiency of the boiler % 83 Hydraulic pressure loss Pa about 5000 4. Discussion
In Polish Rules [4] admissible values of pollutant concentrations (carbon monoxide, total organic carbon, dust) for solid fuel boilers of thermal output of 300kW or below depend on boiler thermal output and heat efficiency value declared by boiler producer. The investiga-ted boiler with the pellets has nominal thermal output of 20 kW and permitinvestiga-ted concentra-tion values were calculated and presented in the table 2 [4]:
Table 2. Admissible values of carbon monoxide, total organic carbon and dust concentra-tions in flue gas from 20 kW biomass boiler, automatically supplied* [4]
Heat efficiency η %
Carbon monoxide Concentration mg/m3
Total organic carbon concentration mg/m3 Dust concentration mg/m3 η > 74.8; (67+6log20) 3000 100 150 η >64.8; (57+6log20) 5000 200 180 η >54.8; (47+6log20) 15000 1750 200 *normalized to 10% oxygen concentration, dry gas.
In order to receive by the boilers an ecological certificate, the nitrogen oxides concentration value (calculated to nitrogen dioxide), must be below 400 mg/m3, dry gas - normalized to 10% oxygen concentration in flue gas [5].
Average pollutant concentration values (measured during whole period of the investiga-tion), presented in table 1, were compared with the permitted values, located in polish rules [4].
Total organic carbon concentration was not measured. It is widely known that if carbon monoxide emission is reduced, the emission of other products of incomplete combustion (e.g. total organic carbon) are also reduced, therefore the emission of carbon monoxide can be considered as good indicator of combustion quality [6]. However, if the flue gas in the furnace touches cold surface of boiler heat exchanger almost at once. Significant amount of soot can appear. In my experiments a significant amount of dark substance was observed in dust meter filter bag.
5. Conclusions
Introduction of pellet burner to combustion chamber of log-wood boiler is a very good idea, although significant mass of dark substances (product of incompletely combustion) can appear in flue gas downstream the boiler. To avoid this situation it is better to situate this burner in combustion chamber another construction (in which the flue gas has not contact with cold wall of boiler heat exchanger at once).
References
[1] Juszczak M., Investigations of oak-wood logs thermal utilization in the heating dome-stic boiler, Proceedings of The Eleven International Symposium: Heat Transfer and Renewable Sources of Energy, PAN, Szczecin University of Technology, Łeba, 13-16.09.2006, p. 83-89.
[2] Flymn T, Bailey R., Fuller T., Daw C.,S., Finney C., Stallings J., Flame Monitoring Enhances Burner Menagement, Power Engineering, February 2003, p.50-54
[3] Lundgren J., Hernansson R., Dahl J., Experimental studies of biomass boiler suitable for small district heating systems, Biomass and Bioenergy, (26) 2004, p.443-453 [4] PN-EN 3003: Heating boilers, part 5. Solid fuel heating boilers, manual and automatic
supplied of thermal output 300kW or below. Nomenclature, requirement, investiga-tions, signature, (in polish)
[5] Kubica K., Certificate establish for ecological security, Institute of Coal Transforma-tion, 1999, (in polish)
[6] Pellet investigations, information of producer, Zacisze, www.zacisze-pellety.pl, (in polish)
