and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 7 (2008), p-51-62
Comparison of thermal and flow boiler parameters and pollutant
concentrations from waste wood incineration in two heating
boilers- working at low thermal output
Juszczak M.
Politechnika Poznańska, Instytut Inżynierii Środowiska, ul. Piotrowo 3,60-965 Poznań,
e-mail: marekjuszczak8@wp.pl
Streszczenie
Porównanie cieplnych i przepływowych parametrów kotłów oraz stężeń zanieczyszczeń z termicznego przekształcania drewna odpadowego w dwóch kotłach grzewczych- pracujących z małą wydajnością
Mierzono parametry cieplne i przepływowe dwóch kotłów centralnego ogrzewania (nie pracujących jednocześnie, każdy o mocy 25 kW), zasilanymi ręcznie w paliwo oraz stężenia tlenku węgla, tlenku azotu, tlenków azotu NOx i pyłu w spalinach za tymi kotłami. Konstrukcje każdego z tych kotłów były różne lecz termiczne przekształcanie odpadów drzewnych i drewna odpadowego przebiegało w dwóch fazach: gazyfikacji drewna i spalania gazu drzewnego. W czasie eksperymentów prowadzonych w kotłowni, komory spalania kotłów nie były wypełnione całkowicie drewnem odpadowym, temperatura w strefie spalania była zbyt niska i dlatego stężenie tlenku węgla było bardzo wysokie a stężenie tlenków azotu niskie, (taka sytuacja pojawia się bardzo często w domowych kotłowniach, jeśli paliwo nie jest podawane do kotła na czas i prawie cała jego masa ulega wypaleniu). Kotły były połączone z zasobnikiem ciepła za pomocą urządzenia mieszająco-pompowego. Z tego powodu strumień objętości wody kotłowej był prawie stały lecz temperatura wody kotłowej zmieniała się z powodu zmiennej intensywności procesu termicznego przekształcania (typowej w kotłach przeznaczonych do spalania polan drzewnych). Przedstawiono eksperymenty a stężenia zanieczyszczeń jak również parametry cieplne i przepływowe kotłów zestawiono w tabeli dla porównania. Poddano dyskusji zmienność (przedstawionych na rysunkach) wybranych parametrów kotłów oraz stężeń zanieczyszczeń.
Abstract
Thermal and flow boiler parameters as well as carbon monoxide, nitric oxide, nitrogen oxides NOx and dust concentrations were measured downstream two manual fuel supplied central heating boilers each of 25 kW thermal output (working not in the same time).
Construction of these two uncontrolled boilers was not the same, but in both cases wooden waste and waste wood incineration was performed in two steps: wood gasification and wooden gas combustion. During experiments performed in heat station , boiler combustion chambers were not filled completely with waste wood, temperature in combustion region was to low and therefore carbon monoxide concentration was very high and nitrogen oxides concentration was low (this situation appears very often in domestic heat stations, if fuel is not supplied to a boiler in time and almost all its mass is burnt out before the next part is added). The boilers were connected with water heat storage by help of mixing and piping device. Therefore the boiler water stream rate was almost constant but boiler water temperature was changing caused by unstable incineration intensity (typical for log- wood boilers). Experiments were presented and pollutant concentration values were gathered in the table for comparison as well as thermal and flow boiler parameters. Variation of selected (presented in figures) boiler parameters and pollutant concentrations, was discussed.
1. Introduction
Waste wood and wooden waste are incinerated in industrial and domestic heating boilers. It is possible to do it in heating boilers if wooden waste doesn’t contain chemical components. In this case wooden waste can be called biomass and consider as a fuel [1]. Generally, incineration of contaminated wooden waste is possible only in special incineration chambers at high temperature (850 or 1100°C depending to organic chlorine content in waste), in high and controlled oxygen concentration (above 6%) and at process time (above 2 seconds) [2]. It is now also permitted to burn in boiler combustion chamber only one example of wooden waste with chemical components: pieces of particle board [3], although this is not consider as biomass and in boiler combustion chamber must appear the same conditions as in special incineration chambers [2]. In small domestic central heating boilers we cannot obtain this conditions presented in [2], so we can to incinerate only waste wood or wooden waste without chemical components.
In Poland in small family houses are used mainly small uncontrolled heating boilers working often together with water heat storage, but not in all cases. Water heat storage price is almost as high as the price of fuel manual supplied uncontrolled water boiler. The device in controlled boilers used for air (for combustion) stream changing in relation to oxygen concentration downstream the boiler cost a lot, so the price of automatical controlled boilers used in family houses (10-30 kW), continuously supplied with fuel is several times higher than the price of manual supplied uncontrolled boilers. The price of these last (uncontrolled boilers) is in Poland comparable with price of gas boiler similar capacity.
Unfortunately, manual supplied boilers have to obtain next part of fuel after every 3 or 4 hours and combustion chamber has to be fulfilled with fuel to remain high temperature and completely combustion. In family houses are living working people and frequently next part of fuel is fed to the boiler after 8 and more hours (when people return home after work) and temperature in combustion chamber is often low. This situation of course causes incompletely combustion and very high emission of carbon monoxide and other products of incompletely combustion like soot and hydrocarbons. It is widely known, that if CO
concentration downstream the boiler is low then concentration other products of incompletely combustion are relatively low [4], so we can CO concentration consider as one of good combustion indicators. Uncontrolled, manually supplied wooden waste boilers are mainly used in Poland. The knowledge of real CO emission during these boilers operation is important, because the boilers producers give only the information about pollutant emissions obtained in stable conditions and controlled, selected thermal outputs. It is also very important to know the combustion process of wooden waste to design this process in the furnace. Devolatilization of wood chips starts after drying at low temperatures 160-200°C. Around 200°C, devolatilization is rapid and significant wood mass loss is recorded. This situation takes place up to temperature 500°C and then the weight of wood chips is almost constant.
2. Experimental set up and measuring equipment, program of experiments
Heat station belonged to Poznan University of Technology, Institute of Environmental Engineering was constructed in the years 2003 and 2004. First only one water boiler (fig.1) was situated and heat achieved during incineration was lost in water cooler placed in the roof of heat station near steel insulated stack, 200 mm inner diameter, 8.5 m. high.ultrasonic heat meter
mixing and piping device
additional circulation pump Fig.1 The log-wood boiler of 25 KW thermal output (first one), boiler schematic, mixing and piping device.
Then the second boiler (fig.2) was situated as well as 900l water heat storage.
Fig.2 The log-wood boiler of 25 kW thermal output (second one), combustion chaber.
The boilers (each of 25 kW thermal output but different construction) were connected with water heat storage with help of mixing and piping device and therefore temperature of water entered the boiler was above 60°C. In first minutes of incineration process water doesn’t flow through the boiler and is heated up to 62°C. Then the pump (of mixing and piping device) starts and water begins to flow only through the boiler but not through the water heat storage. Next, when boiler water reaches 72°C, begins to flow through water heat storage. This way of mixing and piping device work causes situation in which temperature of water returning to the boiler is as high as possible and not below 60°C. Incineration process in each mentioned boilers is performed in two stages: waste wood
gasification and wooden gas combustion. In the first boiler (fig. 1), gasification process is performed in gasification chamber and wooden gas is combusted in the nozzle located at the bottom of gasification chamber. Air to the process (gasification and wooden gas combustion) is supplied with use of one fan. Air volume stream rate in the case of the first boiler can be changed manually by use of chalking valve situated before the fan. In the second boiler changing of air volume stream rate can be done manually by increasing or decreasing of fan rotations (from 0 to 100%, step 5%). In the second boiler (fig.2), air to wooden gas combustion is supplied using two horizontal inlets located near the bottom of gasification chamber. The two boilers are log- wood boilers with fuel manual supply. They are uncontrolled, that means that there is no oxygen probe (lambda sensor) situated in flue gas downstream the boiler as a part of automatic device used for air stream changing. Two sight glasses are placed in front wall of each boiler to flame color observation. In gasification space flame can not appear on incandescent wooden waste and if that happens, it means that it is to much air. To obtain low pollutant emission, flame in wooden gas combustion region ought to have dark yellow color. If the flame has dark red color, then a big amount of incompletely combustion products can be expected. If flame has bright yellow color then the mass of incompletely combustion products can be smaller and nitrogen oxides mass can be bigger (caused by high temperature and oxygen concentration). Looking at the flame color, air excess rate value and pollutant concentrations at gas analyzer display and temperature values (measured with help of two radiation shielded thermocouples PtRhPt, situated in gasification and combustion regions), air volume stream rate can be changed manually for reduction of carbon monoxide concentration in stack gases. Hewever, it was not done often during measurement period, because in domestic central heating this way of regulation is almost impossible during the whole heating season. During investigations, pollutant concentrations in stack gases: carbon monoxide, nitric oxide, nitrogen oxides were measured using gas analyzer with electrochemical cells. Dust concentration was measured in the same place (in the stack) with help of gravimetrical dust meter with isokinetical probe sampling. Water temperature, volume stream rate, boiler thermal output (power), quantity of heat obtained by boiler water were measured by ultrasonic heat meter. Waste wood mass was weighted and its humidity measured (as water mass divided through waste and water mass) before every experiment. Calorific value was calculated in relationship to waste wood humidity (ever below 20%). Boiler thermal efficiency was calculated as heat quantity obtained by boiler water through waste wood mass multiplied by its calorific value. Pollutant indicators were calculated. During experiments gasification chamber was not filled completely with waste wood and temperatures in gasification and combustion regions were to low, carbon monoxide concentration was high and boiler heat efficiency was small, nitric oxide and nitrogen oxides concentrations were low. Situation in which combustion temperature is to low, is typical for log -wood boilers (even in good performed incineration process and gasification chamber fulfilled with logs) in time when the boiler starts to work or next part of waste wood is not added in time. These situations are very common in operation practice of central heating in Poland. Parameters were measured with accuracy: boiler water temperature – 1°C, temperature in gasification chamber and combustion zone – 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 measured value, dust concentration - 5mg/m3, air excess rate 0.01.
Table 1. Permitted values of carbon monoxide, organic carbon and dust concentrations* for manual supplied biomass boiler of 25 kW thermal output.
Heat efficiency η %
Carbon monoxide Concentration
mg/m3
Total organic carbon
concentration mg/m3 Dust concentration mg/m3 η > 75.4; (67+6log25) 5000 (6818) 150 (205) 150 (205) η >65.4; (57+6log25) 8000 (10909) 300 (409) 180 (245) η >55.4; (47+6log25) 25000 (34091) 2000 (2730) 200 (273)
*normalized to 10% oxygen concentration, dry gas.
Achieved results were gathered in table 2 and compared with Polish Rules [5] - tab.1, and regulations [6], (in which we can notice, that to obtain by boiler ecological certificate it is necessary to maintain nitrogen oxides concentration in stack gases below 400 mg/m3, calculated as nitrogen dioxide - normalized to 10% oxygen concentration in stack gases, that equals 545 mg/m3- normalized to 6% oxygen concentration). The permitted pollutant concentration values in stack gases depend on boiler heat efficiency, declared by boiler producer [5]. Experiments were carried out together with the students [7, 8].
The values in parenthesis are concentrations normalized to 6 % oxygen concentration in flue gas.
3. Results and discussion
When (in table 2) more then one value per cell is presented, they stand for: medium value, values interval.
Looking at the figure 3, it is possible to see that temperature in flame region (in the nozzle) only in first minutes of measurement period was above value 580-600°C, (even above 800°C) and CO concentration was the smallest, obtaining value between 1100 and 3200
mg/nm3. Temperature in gasification region rised from 240 to 360°C, through value 280°C
(limit of good gasification condition). Below this temperature: 280°C, CO concentration value was rising quickly. Then, from about the 20th minute of measurement up to 56th minute -gasification temperature was below 280°C and CO concentration was rising not so quickly although the flame temperature decreased below 580°C (limit of CO good combustion). In 56th minute, gasification temperature was below 280°C and CO concentration was rising quickly. The whole time flame temperature in the nozzle was decreasing continuously and in 56th minute was even below 500°C (CO combustion was not good). After 100th minute we could see that CO concentration was decreasing and in 116th minute was obtained about 5500 mg/nm3. Then CO concentration was rising, although in one moment small decrease could be seen (it is rather strange, perhaps caused by small wooden waste movement during combustion in gasification chamber).
Table 2. Results of waste wood incineration in two central heating boilers (each of 25 kW nominal thermal output), working with low thermal output in the heat station situated in Poznan University of Technology
First boiler Second boiler
wooden waste incineration
parameters unit poplar
chips oak logs alder logs
apple tree
branches oak logs
poplar chips
wood mass kg 19.50 11.75 17.75 36.10 15.50 42.50
measurement time min 150 141 135 204 150 390
thermal output, mean, range kW 9.95 2.50-30.2 13,50 5.20-22.00 24.10 16.50-39.00 18.80 8.3-26.6 12.8 5.1-23.5 13.3 3.9-37.5 heat efficiency % 50 57 71 78 55 69 gasification temperature °C 200 110-335 275 225-380 371 260-465 300 185-485 259 200-310 189 170-230 flame temperature °C 429 150-850 436 280-605 728 500-850 648 530-795 183 130-285 329 210-650 air excess rate in
stack --- 6.5 2.2-14.3 3.8 2.5-4.5 2.0 1.5-2.8 4.8 2.5-6.8 3.9 2.8-5.3 3.0 1.5-4.4 O2 concentr. % 16.0 11.3-19.4 14.9 12.4-16.8 9.8 7.1-13.3 16.5 12.4-17.8 15.4 13.3-16.9 14.0 7.1-16.1 CO concentr* mg/nm3 17275 3090-30987 8217 2117-16054 2755 362-7108 3817 1469-12333 11583 4787-15991 8908 1188-18302 NO concentr.* mg/nm3 214 90-440 149 76-284 166 115-207 105 43-223 62 18-126 168 127-243 NOx concentr.* mg/nm3 338 158-712 240 124-456 268 187--333 266 144-242 100 28-204 270 204-390 dust concentr.* mg/nm3 92 43 21 97 37 34 CO emission indicator g/MJ g/kg 23.88 1.61 55.61 3.60 26.98 1.75 38.12 2.48 34.80 2.4 2 85.00 5.57 NO emission indicator g/MJ g/kg 0.30 0.01 1.01 0.07 1.62 0.11 1.73 0.12 1.02 0.07 1.14 0.08 NOx emission indicator g/MJ g/kg 0.47 0.03 1.62 0.11 2.62 0.17 2.83 0.19 0.30 4.62 1.83 0.13 dust emission indicator g/MJ g/kg 0.13 0.01 0.29 0.02 0.21 0.01 0.97 0.07 0.11 0.01 0.10 0.01 flue gas temp. in
stack °C 90 67-122 119 76-161 137 113-155 139 108-155 73 54-93 83 63-127 boiler water temperature supply °C 73 71-79 76 75-79 80 76-83 77 75-80 76 74-79 76 72-81
0 100 200 300 400 500 600 700 800 900 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0 5000 10000 15000 20000 25000 30000 35000 Water temperature - supply [°C] Water temperature - return [°C]
Gasification temperature [°C] Temperature in nozle [°C] Flue gases temperature [°C] CO, concentration (6%O2) [mg/m3] NO concentration (6%O2) [mg/m3] NOx concentration (6%O2) [mg/m3]
T e m e p e ra tu re [ °C ]
Measurement time [min] CO
, N O , N O x c o n c e n tr a ti o n n o rm a liz e d t o 6 % O 2 [ m g /n m 3] 0 5 10 15 20 25 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0 2 4 6 8 10 12 14 16 18 20
Boiler power [kW] Air excess rate Lambda [-] Oxgen concentration O2 [%]
B o ile r p o w e r [k W ], L a m b d a [ -]
Measurement time [min]
O x g e n c o n c e n tr a tio n O 2 [ % ]
Fig. 3. Variation of selected boiler parameters obtained during oak logs incineration in the heating boiler of 25 kW thermal output (presented in fig.1).
Looking at the figure 4 (related to second boiler) it was possible to notice, that when that boiler was working with low thermal output and combustion chamber was not completely filled with fuel, air excess rate reached high values (about 4.0) and flame grate combustion was performed (instead of two stages incineration: wood gasification and wooden gas combustion). Temperature values in the region which ought to be “gasification region” (but was in fact the flame region), were higher than in the bottom of combustion chamber (in the region which ought to be flame combustion region). CO concentration was changing very
much and obtained high values. Temperature values in combustion chamber were low, below 400°C. 0 100 200 300 400 500 600 700 800 0 20 40 60 80 100 120 140 160 180 200 220 240 260 -1000 1000 3000 5000 7000 9000 11000 13000 15000 17000 W ater temperature - supply [°C] W ater temperature - return [°C]
Gasification temperature [°C] Combustion region temperature [°C] Flue gases temperature [°C] CO, concentration (6%O2) [mg/m3] NO concentration (6%O2) [mg/m3] NOx concentration (6%O2) [mg/m3]
T e m e p e ra tu re [ °C ]
Measurement time [min]
C O , N O , N O x c o n c e n tr a ti o n n o rm a liz e d t o 6 % O 2 [ m g /n m 3] 0 5 10 15 20 25 0 20 40 60 80 100 120 140 160 0 2 4 6 8 10 12 14 16 18
Boiler power [kW ] Air excess rate Lambda [-] Oxgen concentration O2 [%]
B o ile r p o w e r [k W ], L a m b d a [ -]
Measurement time [min]
O x g e n c o n c e n tr a ti o n O 2 [ % ]
Fig.4. Variation of selected boiler parameters obtained during oak logs incineration in the heating boiler of 25 kW thermal output (presented in fig.2).
Comparison of selected results were presented in the table 2. Looking at that table, figure 3 and 4 and thinking about some notices obtained from investigations, it was possible to say that:
– gasification chambers of each boiler were not completely filled with waste and frequently flame appeared,
– temperature in gasification chamber and combustion region obtained sometimes high values, only in case of the first boiler, in the case of the second one values were to low and flame appeared very often in gasification region,
– the two boilers were uncontrolled and air volume stream rate had almost constant value
during combustion period, air excess rate was changing therefore in wide range and only in short time at the beginning had satisfied level (1.5-2.5),
– chips (i.e.poplar chips) are not good waste type for incineration in log-wood boiler and in this case high carbon monoxide concentration values were obtained,
– the first boiler was better then the second one for waste wood incineration, from ecological and thermal points of view.
4. Conclusions
To obtain low carbon monoxide concentration values, (below permitted values in the Rules) it would be better to use, in small family houses, automatically controlled boilers (and with continuously fuel supply). Unfortunately, controlled boilers, with oxygen probe located downstream the boiler for air regulation, are several times more expensive than investigated boilers. When uncontrolled and fuel manual supplied boiler is used, then gasification chamber has to be filled completely with wooden waste (to avoid flame combustion in gasification zone and obtain there sufficient temperature values) and next part of waste wood has to be added after 3 or 4 hours (to maintain continuous combustion) and not after 6 or seven hours.
References
[1] Disposal of Environment Minister from the 4 th of August 2003 in the matter of emission standards, Journal of Laws from the 18th of September 2003 (in polish) [2] Disposal of Economy Minister from the 21th of March 2002 in the matter of
requirements of waste thermal incineration process, Journal of Laws no. 37, pos. 339 (in polish)
[3] Disposal of Economy Minister from the 29th January 2002 in the matter of the waste other than dangerous and types of devices and installations, in which thermal incineration is permitted, Journal of Laws no. 8, pos.176 (in polish)
[4] 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 [5] PN-EN-303-5, Heating boilers, part 5. Solid fuel heating boilers, manual and automatic
supplied of thermal output 300kW and below. Nomenclature, requirement, investigations, signature (in polish)
[6] Kubica K., Energetistic and ecological criterions of low thermal output and solid fuel boilers used in town. Certificate establish for criterions security, Institute of Coal Transformation, 1999 (in polish)
[7] Iwanucha J., Investigations of carbon monoxide, nitrogen oxides and dust concentration changes from the heating boiler during biomass combustion, master thesis Poznan University of Technology, 2005 (in polish)
[8] Kałużna M., Investigations of carbon monoxide, nitrogen oxides and dust concentrations from heating boiler obtained during wooden waste combustion, master thesis, Poznan University of Technology, 2005 (in polish)
