Investigation of the removal efficiency of an ESP in a residential biomass boiler

Transcription

1 Investigation of the removal efficiency of an ESP in a residential biomass boiler ROBERTAS POSKAS, ARUNAS SIRVYDAS, JURGIS JANKAUSKAS Nuclear Engineering Laboratory Lithuanian Energy Institute 3 Breslaujos, LT Kaunas LITHUANIA rposkas@mail.lei.lt Abstract: - The aim of this work is to evaluate the emissions of solid particles from a modern domestic power boiler and to investigate the properties of an electrostatic precipitator to capture solid particles emitted during incineration of wood pellets from such type of boiler. Wood pellets were incinerated in an automatic class 3 boiler with the rated thermal output of 50 kw. During investigations the concentrations of solid particles were measured using an infrared particle sizer. Results showed that emissions of solid particles were well below recommended limit values. Investigations of solid particles precipitation were performed at different supply voltages, polarity and different number of collecting electrodes of the electrostatic precipitator. This allowed determining the performance of the electrostatic precipitator in various operating modes. In general, total collection efficiency of % was achieved. Key-Words: - electrostatic precipitator, residential boiler, wood pellets, particulate matters, polarity, number of electrodes, collection efficiency 1 Introduction Biofuel has a considerable potential as a fuel source and a reasonable cost level in comparison to other types of fuel. It is widely used in many countries for heat and electricity production [1]. Biofuel (e.g. wood, straw, grains, etc.) is also a renewable energy source [2] making the least negative impact on the environment, therefore its use has been increasing in Europe over the last years [3]. Currently, it is also a popular type of fuel used in small scale household furnaces. Concerning particulate emissions, especially emissions of fine particulates, the use of biofuel is still a disadvantage compared to oil and natural gas fired systems. However, the constructions of modern boilers intended for biofuel combustion allow reducing these emissions. The study performed by Obernberger et al. revealed that compared with old systems the particulate emissions of modern small-scale biomass furnaces (power 40 kw) are significantly lower [4]. Emissions from commercial residential boilers fired with wood logs and wood pellets were also compared in [5]. The comparison was made between new and old-type boilers. In general, oldtype wood boilers caused considerably higher emissions than modern wood and pellet boilers. The mass concentration of particles was almost 200 times larger in the worst old-type case compared to the best modern case (wood pellets). Study of the emissions of solid particles from a circulating fluidized bed power plant [6] showed that in general particle emissions from biomass combustion (forest residue and willow) consist of two fractions: the aerosols (particles < 1 µm) and coarse particles (particles > 1 µm). It was obtained that size distribution of particles might be in two ranges the first is between ~ µm and the second is between ~1 100 µm. Similar ranges were presented in the report [7], analyzing emissions of particles from large scale biomass combustion units. In Ehrlich s study [8], it was obtained that the peak diameters of coarse particles for different boiler types (from 450 MW to 6 kw) were between 2 8 µm. The peak diameter of the particles depends on boiler type (i. e. combustion peculiarities firing on grates, in stove, etc.) and also on fuel type. The effect from long term exposure to coarse (>1 µm), fine (0.1 1 µm) and ultra fine (< 0.1 µm) particulate matters (PM) causes health problems, mainly pneumonia, cardio-vascular morbidity and premature mortality [9], [10]. In order to avoid negative effects on health, EU Council directive 1999/30/EC defines the necessity to take certain measures to ensure that concentrations of PM10 and PM2.5 in ambient air ISBN:

2 should not exceed the limiting values. Usually, before discharge into the atmosphere, flue gases in power plants are filtered (cleaned) in multicyclones or fiber filters. However, such types of filters are not very effective in the case of fine or ultrafine particulates, therefore for that purpose electrostatic precipitators are used [11]. Due to this, a lot of studies are devoted to performance, operation and improvement of industrial precipitators. Since the use of biofuel has been increasing not only in the industrial area but also in private houses, the generation of solid particles as well as gaseous emissions also increases. As the environmental protection problems are becoming more relevant, the requirements for production of small combustion appliances (with a nominal heat output up to 500 kw) are currently being coordinated by the EN303-5:2012 standard, issued in Although there are advanced electrostatic precipitators already available on the market for small scale combusting units, their performances are not adequately analyzed and the reports/papers on pollution control devices for small-scale biomassfired units are very rare [12]. In the study [12] a simple, compact and cost-effective electrostatic precipitator was designed and constructed for removal of particulate matter from biomass burning in small combustors. It was found that over 70 % overall collection efficiency can be achieved with a relatively simple design. In another study [13] it is indicated that in Sweden the number of small bio-fuelled plants is increasing, and there is a need for cost effective means to precipitate the formed ultrafine particles. One of such techniques may be electrostatic precipitation, yet the economy of scale is a constraining factor for the systems commercially available today. In their paper, the authors show that collection efficiency of the ESP was determined to be between 30 % and 40 % in the µm range for a single filter, and between 55 % and 65 % for dual filters in the same size range. Schmatloch and Rausch [14] emphasize that for application in small heating systems, a compact and inexpensive solution that still offers a significant reduction of particle emissions is required. In [14] a design of the electrostatic precipitator was presented and its collection efficiency and electrical characteristics were evaluated. It was determined that the investigated precipitator resulted in collection efficiencies of more than 80 %. By improving the shape of the electrode, a collection efficiency of about 90 % was achieved. This reduction of particle emissions was much higher than what is possible by further improvements of combustion quality. So, literature review shows that publications related to comprehensive investigations/operatibility of ESPs for small scale combustion appliances are rather limited. In the present study, the emissions of solid particles from a class 3 modern small power boiler (50 kw nominal capacity) are studied and impact of the number and polarity of the electrodes on collection efficiency of the simple design electrostatic precipitator are evaluated. The obtained data partially fills the gap related to the lack of comprehensive investigations/operatibility of ESPs for small scale combustion appliances. 2 Experimental methods 2.1 Experimental setup Experimental setup used to evaluate the emissions from the boiler and characteristics of the electrostatic precipitator (ESP) is presented in Fig.1. Fig.1. Experimental setup (not to scale): 1 boiler; 2 flue gas analyser; 3 electrostatic precipitator; 4 flue gas pipe (chimney); 5 infrared particle sizer; 6 flue gas exhauster. ISBN:

3 Flue gases were generated by incinerating wood pellets in a boiler. The commercial boiler is designed to meet class 3 requirements according to EN303-5:2012. It is an automatic device with a rated power of 50 kw and can be used for incineration of pellets, wood, pea-coal and cereals. The fuel supply and air supply for combustion are regulated basing on the set boiler power. During experiments flue gas temperature exhausted from the boiler was about 150 C and the chimney draft ~ 25 Pa (the draft value was automatically kept by the flue gas exhauster). Such draft gives a flue gas velocity of approx. 1.9 m/s in the flue gas pipe of 180 mm diameter (flow rate ~177 m 3 /h). The total height of the insulated flue gas pipe was about 15 m. From the boiler, flue gases enter the ESP, where the solid particles are captured and then clean flue gases are exhausted to the outside. The ESP (see Fig.2) used in the experiments was designed at Lithuanian Energy Institute. Its frame was made of carbon steel angle bars with the external dimensions of ~530x370x1890 mm. The inlet to the ESP was made in the upper part and the outlet was positioned in the lower part. Inside the frame, three pairs of stainless steel pipes (i.e. in total six collecting electrodes) with a diameter of 120 mm (each) and a length of ~ 1000 mm were installed between two holding planes. A single nichrome wire (i.e. discharge electrode) of 0.2 mm was stretched between the special holders inside the centres of each stainless steel pipe. The distance between the discharge electrodes in each pair was about 160 mm. The tension of the electrodes was ensured by steel springs. In order to avoid flue gas leakages between the holding plates and the collecting electrodes, the contact perimeters between them were insulated. During experiments each pair of the discharge electrodes was connected to separate Glassman Series FR high voltage power supply units with adjustable output voltage in the range between 0 and 30 kv. So in total three high voltage supply units were used. Experiments were carried out using negative as well as positive potential of high voltage power supply units. To reveal the peculiarities of collecting efficiency of the ESP, different number of the collecting electrodes (1, 2 and 6) were used during the experiments. When one collecting electrode was used, the other tubes with five electrodes were closed using special plates and the flue gases could flow only via the open pipe. When two electrodes were used, the other four were closed, and when six electrodes were used, all electrodes were opened. The current-voltage characteristics (high voltage supply units used in the experiments had a feature enabling them to measure the current in the electrode depending on the supplied voltage) of this type of the electrodes are shown in Fig.3. From the figure (see curve 1) it is evident that for negative potential of high voltage power supply unit the current was registered starting from the voltage of ~6 kv and for positive potential from ~10 kv (Fig.3, curve 2). Fig.3. Current-voltage characteristics of the ESP electrodes: 1 negative potential; 2 positive potential. Fig.2. Electrostatic precipitator (not to scale): 1 frame; 2 stainless steel pipes (collecting electrodes); 3 discharge electrodes; 4 planes for holding the pipes; 5 high voltage supply unit. 2.2 Measuring techniques An infrared particle sizer (IPS) was installed in the chimney segment between the electrostatic precipitator and flue gas exhauster (Fig.1, pos. 5). IPS is an instrument for direct measurements of total ISBN:

4 dust concentration and concentration of dust fractions in flue gas channels. It is composed of a measuring head and an electronic computercontrolled measurement unit. Operation of IPS is based on the principle of light scattering. Infrared light is scattered by particles moving through the measurement zone. Each particle moving through the measurement zone generates an electric pulse proportional to the diameter of the spherical particle. For non-spherical particles, the pulse amplitude depends on how the particle is oriented in the measurement zone. So, this device ensures only 1D measurements and it may be used for measurements of particles in the range from 0.4 to 300 µm (this range is divided into 4 subranges). IPS also measures flue gas temperature, flue gas velocity and flow rate. The software provided for IPS management automatically calculates particle concentrations in cubic meter of flue gases. To ensure that the stream of flue gases flowing into the intake nozzle of the IPS measuring head is not disturbed, the suction of the gases is isokinetic. The measuring head is also equipped with special holder for the filter which can be used for the determination of the concentration of solid particles by gravimetric method. by ~85-87 %, while for six collecting electrodes it was decreased to ~95 %. Further increase of voltage till kv gave only slight decrease of the concentration to ~97-99 % and the results showed that decrease of the concentration did not depend on the number of the collecting electrodes at voltage above 10 kv. 3 Results 3.1 Collection efficiency of ESP First of all, concentrations of solid particles (PM) in the flue gases were measured without ESP using IPS (see Fig.1, pos. 5), and the result based on gravimetric method showed that concentration of the solid particles normalised for the reference O 2 value was rather small ~13 mg/nm 3. It was much smaller than the recommended limiting values provided in EN303-5:2012. After that the experiments were performed with ESP. The obtained relative distribution of particles concentration dependence on the supplied voltage and the number of collecting electrodes is presented in Fig.4. The results are based on IPS calculations of particle concentrations in cubic meter of flue gases. For negative corona (Fig.4, a) and one collecting electrode, the initial increase of voltage till 2 kv resulted in negligible concentration decrease only about 1 %, while using two or six electrodes, concentration decrease was much higher and in both cases almost the same about %. At the voltage of the corona discharge (6 kv) and in the case of one and two collecting electrodes, the concentration of particles was already decreased Fig.4. Variation of relative concentration of particles with supplied voltage to ESP for the different number of collecting electrodes in the case of negative (a) and positive (b) potential. In the case of positive corona (Fig.4, b) no sharp decrease of concentration was observed with the increase of voltage in the range from 0 till 6 kv as it was observed for negative corona. The results showed that at 6 kv, one collecting electrode was able to reduce concentration only by few percents; two collecting electrodes reduced concentration to about 10 % and six electrodes reduced concentration only to ~25 %. So there was no noticeable concentration decrease for negative corona at the same voltage. This was due to the fact that more energy is needed to ionize gases as negative corona ionization energy depends on electron affinity, while positive corona energy depends on gas positive ionization energy. At the voltage of the ISBN:

5 corona discharge 10 kv (Fig.4, b), the results of particles concentrations were scattered over a wide range. It is clearly seen that distribution of particle concentrations depended very much on the number of the collecting electrodes used. For this case one collecting electrode allowed reducing concentration of particles to about 15 %, two electrodes to ~80 % and six electrodes even to ~98 %. Further increase of voltage showed that two and six collecting electrodes were able to reduce concentration till the same level of ~98 %. However a single collecting electrode at the highest voltage presented in Fig.4, b was able to reduce concentration only to ~92 %. Thus in general, it can be concluded that the use of ESP with negative corona in the analysed range of voltages allowed obtaining higher concentration decrease of solid particles in comparison with positive corona. Variation of total collection efficiency of particles with power of the ESP is presented in Fig.5. Fig.5. Variation of total collection efficiency of particles with power of the ESP in case of negative (a) and positive (b) potential. Until corona discharge (6 kv), although power was ~0 W (i. e. current is so small that it is practically non-measurable), the maximum total collection efficiency of the ESP increased slightly and in the case of negative corona and one collecting electrode (Fig.5, a), it reached approx. 45 %. At the corona discharge (6 kv), the power was about 0.04 W, and the total collection efficiency was almost 80 %. Slight increase of power above voltage of the corona discharge gave collection efficiency of ~94 %. Further increase of power gave only small total collection efficiency increase. When the power was >3.5 W, the total collection efficiency was the highest ~99 %. In the case of two collecting electrodes, power at the voltage of the corona discharge was practically the same as for the case with one collecting electrode; however collection efficiency was higher about 85 %. With further increase of power, collection efficiency was only negligibly higher in comparison to one collecting electrode; even when the power was >3 W, it did not exceed 99 %. When the experiments were conducted with six collecting electrodes of the ESP, the highest collection efficiency achieved before voltage of the corona discharge was about 45 %, while at the corona discharge the efficiency was ~97 %. In comparison with one or two collecting electrodes, in this case the power of the ESP was three times higher, but the collection efficiency was higher only for few percents. Further increase of power resulted in the same total collection efficiency of ~98 %. Experiments with positive corona and one collecting electrode (Fig.5, b) showed that the highest total collection efficiency before voltage of corona discharge (10 kv) was only ~17 % and in the case of two collecting electrodes - about 40 % (i.e. achieved efficiencies in this case are less in comparison with the negative corona). At the voltage of the corona discharge, the use of one collecting electrode allowed achieving the total efficiency of ~70 % and in the case of two collecting electrodes, the efficiency was ~80 %. Further increase of power revealed that one collecting electrode, even at power of ~1.8 W, gave collection efficiency of ~90 %. Further steep increase of power resulted only in negligible efficiency increase and when the power was more than 25 W (not shown in Fig.5, b), the total collection efficiency was about 99 %. Use of two collecting electrodes showed better performance of the ESP and when the power was about 3 W it resulted in collection efficiency of ~98 %. Further steep increase of power did not show any marked results and the efficiency was at the same level it did not increase more than 98 %. When the six collecting electrodes were operating at voltage lower than the voltage of the ISBN:

6 corona discharge (when power was <0.45 W), they resulted in the collection efficiency of ~90 %. At the voltage of the corona discharge, when the power was 0.45 W, the collection efficiency was ~96 %, which only slightly increased with further power increase of the ESP (till ~98 % and remained almost in the same level). 4 Conclusions After measurements of emissions of solid particles in the flue gases from a class 3 boiler in the case of biofuel combusting and application of the ESP in different operating modes, the following conclusions can be made: 1. Without the ESP, emissions of solid particles from the boiler normalized for the reference O 2 value were ~13 mg/nm 3 and it is much lower value than the recommended limiting values. 2. The corona discharge for negative potential was fixed at 6 kv and for positive potential at 10 kv. 3. It was defined that before corona discharge, collection efficiency of the ESP depends on the number of the collecting electrodes and it was more expressed for the positive potential. 4. After corona discharge, for both potentials the collection efficiency practically did not depend on the number of the collecting electrodes used. 5. The total collection efficiency of the ESP was about % even at low power (~4 W). References: [1] Demibras A., Combustion Systems for Biomass Fuel, Energy Sources, Vol. 29, No. 4, 2007, pp [2] Manual for biofuel users, Tallinn University of Technology, [3] European bioenergy outlook Statistical report, Aebiom, [4] Obernberger I., Brunner T and Bärnthaler G., Fine particulate from modern Austrian smallscale biomass combustion plants. Proceedings of the 15th European biomass conference and exhibition, Berlin, Germany [5] Johansson L.S., Leckner B., Gustavsson L., Cooper D., Tullin C. and Potter A., Emission characteristics of modern and old-type residential boilers fired with wood logs and wood pellets, Atmospheric Environment, Vol. 38, 2004, pp [6] Maenhaut W., Fernández-Jiménez M.-T., Lind T., Kauppinen E.I., Valmari T., Sfiris G. and Nilsson K., In-stack particle size and composition transformations during circulating fluidized bed combustion of willow and forest residue, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Vol. 150, No. 1-4, 1999, pp [7] Aerosols in fixed-bed biomass combustion formation, growth, chemical composition, deposition, precipitation and separation from flue gas. Final report. Graz University of Technology, [8] Ehrlich C., Noll G., Kalkoff W.-D., Baumbach G. and Dreiseidler A., Emissions from industrial plants - Results from measurement programmes in Germany, Atmospheric environment, Vol. 41, No 29, 2007, pp [9] Zelikoff J.T., Chen L.C., Cohen M.D., Fang K., Gordon T., Li Y., Nadziejko C. and Schlesinger R.B., Effects of inhaled ambient particulate matter on pulmonary antimicrobial immune defense, Inhallation Toxicology, Vol. 15, No. 2, 2003, pp [10] de Hartog J.J., Hoek G., Peters A., Timonen K.L., Ibald-Mulli A., Brunekreef B., Heinrich J., Tiittanen P., van Wijnen J.H., Kreyling W., Kulmala M. and Pekkanen J., Effects of fine and ultrafine particles on cardiorespiratory symptoms in elderly subjects with coronary heart disease: the ULTRA study, American Journal of Epidemiology, Vol. 157, No. 7, 2003, pp [11] Parker K.L., Applied electrostatic precipitation, London: Chapman & hall, [12] Intra P., Limueadphai P. and Tippayawong P., Particulate emission reduction from biomass burning in small combustion systems with a multiple tubular electrostatic precipitator, Particulate Science and Technology, Vol. 28, No. 6, 2010, pp [13] Pettersson J., Strand M. and Lin L., Chargingand removal efficiency of an ESP in a 250 kw biomass boiler in Proc. of the XII International Conference on Electrostatic Precipitation, ICESP Nürnberg [14] Schmatloch V. and Rausch S., Design and characterisation of an electrostatic precipitator for small heating appliances, Journal of Electrostatics, Vol. 63, No. 2, 2005, pp ISBN: