experimental determination of bed temperatures during wood pellet

Article pubs.acs.org/EF

Experimental Determination of Bed Temperatures during Wood Pellet Combustion Jozef Jandačka, Jozef Mičieta,* Michal Holubčík,* and Radovan Nosek Department of Power Engineering, Faculty of Mechanical Engineering, University of Zilina, Univerzitna 1, 010 26 Zilina, Slovakia ABSTRACT: Constant effort to reduce heating costs leads to production of not only high-quality pellets from pure wood but also cheaper vegetable pellets of inferior quality in terms of energy properties.1,2 The combustion of alternative pellets in small boilers causes many problems, mainly ash sintering, as a result of their low melting temperature.3,4 The paper focuses on experimental determination of bed temperatures in the burning layer during wood pellet combustion. The bed temperatures are important for prediction of combustion phases and better localization and description of sites that are critical for the sintering of ash particles as temperatures are close to the ash melting points. This experiment describes a method for measurement of the temperature field in the symmetry plane of the burner during wood pellet combustion in the retort burner. The measurement of temperature profiles at different locations in the burner shows that the combustion process in the retort burner is rather more horizontally stratified than vertically stratified. The main combustion process takes place in a ring-shaped space around combustion air inlets.

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INTRODUCTION Several institutions and authors, such as, for example, the Chinese Harbin Institute of Technology, deal with temperature distribution in the bed layer, combustion rate, or spread of the combustion border in the burning bed. Their experimental combustion reactor consists of a vertical cylinder suspended on the scales. A 1.3 m high combustion chamber with the diameter of 180 mm is axially symmetric and insulated by 50 mm thick refractory lining. The stainless-steel grate at the bottom of the combustion chamber consists of a mesh with 7 mm holes, which occupies about 15% of the grate area.5,6 Another device is a cylindrical experimental reactor from the French institute ENSMA7 with a 200 mm chamber diameter. The combustion chamber height is 2000 mm, of which 800 mm forms the solid fuel section (primary part) and the rest is a zone for burning combustible gases (secondary and tertiary parts). The whole construction is double-insulated with 40 mm thick refractory lining and 40 mm rock wool insulation, which makes the combustion process a simplified one-dimensional process in the reactor axis direction. These and other similar devices8,9 provide very suitable combustion conditions, and combustion air is uniformly blown under the grate. The combustion process can be simply considered as stratified. It means that one can distinguish and describe the combustion process in layers. Having made this assumption, the combustion can be simplified to a onedimensional process in the vertical direction, where each other subprocess takes place in planar layers. For the combustion process, there are disclosed fairly detailed mathematical models, which are experimentally verified.10−12 During creation of a computational fluid dynamics (CFD) model, the appropriate step was to replace the burning bed layer, because there is no task to model detailed processes within it. Combustion processes in the mentioned papers were carried out under laboratory conditions. However, we assume different conditions from those already described and a case similar to the case of a retort burner, which has not been analyzed thus far. © 2017 American Chemical Society

EXPERIMENTAL SECTION

Within the framework of the project, a CFD model was created to experimentally verify the combustion model applied in the retort burner. Conditions for the retort are not significantly different, because the combustion air is supplied around the perimeter of the burner. Therefore, it was necessary to carry out measurements on the specific burner type. Another reason why we chose the retort burner was the fact that we wanted to use a cooled retort burner for the combustion of plant biomass. Experiments were performed on a semi-automatic commercial pellet boiler with the retort burner. Temperature profiles were measured on a well-burning bed layer. The used boiler with a common design has the advantage of having enough handling space around the burner in the boiler. During the experiment the pellets of ENplus A1 quality class produced by a certified producer were burned. The ENplus quality certification is based on the ISO 17225-2 standard and exactly defines threshold values of the most important pellet parameters, e.g., a net calorific value of more than 16.5 MJ kg−1, an ash content of up to 0.7 wt %, or a mechanical durability of more than 98 wt %. Ash melting temperatures of used pellets are ash deformation temperature (DT) = 1177 °C, ash sphere temperature (ST) = 1190 °C, ash hemisphere temperature (HT) = 1204 °C, and ash flow temperature (FT) = 1229 °C and were determined in a laboratory on the basis of standard STN ISO 540. The rise time lasted several hours to ensure a well-developed combustion process. Only then was the combustion considered as a stationary process. Used thermocouples were calibrated in boiled water when they measured identical temperatures. They were also tested at a high temperature in a propane−butane flame. All thermocouples measured approximately the same temperatures (±20 °C) at steady state. An auxiliary frame was added to the burner to adjust the thermocouple position and to avoid a necessary design change of the boiler body. At first, the experiments were performed using a suspended measurement assembly above the burner, which replaced an original hanging deflector, but it was not stable enough. A new experimental disassembling frame was made more robust and placed Received: November 19, 2016 Revised: January 27, 2017 Published: February 15, 2017 2919

DOI: 10.1021/acs.energyfuels.6b03071 Energy Fuels 2017, 31, 2919−2926

Article

Energy & Fuels on the cast iron collar of the burner. The new experimental assembly was much more resistant against thermal loading. Manual adjustment of the thermocouple location was performed by a centering adjusting screw, which was fixed on the steel plate above the burner. Figures 1 and 2 show, apart from a pair of thermocouples for temperature measurement in the burning bed, two thermocouples for

was based on our experience gathered from previous measurements. The accumulation of pellets is slightly higher at the distant edge, where the burning surface of the bed layer is higher. Furthermore, the screw conveyor extends inside the area directly below the burning bed. It limits the available depth for the measurement in the conveyor axis. For the purpose of component assembly, it was necessary to take into account the adjustment of the device, space for thermocouple wiring, and necessary handling space. For these reasons, we chose a vertical plane in the axis of the burner (when the boiler door opens, the plane was parallel to directly look at the burner) as the reference plane for temperature measurements in Scheme 2. In this direction, pellet feeding and distribution of the burning layer should be steadier and more balanced than in the direction of the conveyor. As a result of the geometric symmetry, we assume the approximate temperature symmetry along the burner. This assumption allowed us to measure two temperatures at one position. It is possible to place more thermocouples into the holder. On the other hand, the more thermocouple probes are in the burning bed, the more influence they have on the combustion. Thermocouple probes (tubes with thermocouples) fill space of the burning layer and have a significant effect on the combustion process because the heat from the hottest part of the burning bed is taken away more intensively. However, stainless-steel probes are necessary to protect the thermocouple wire and also to keep it in the required position. An acceptable compromise between temperature measurements of speed and quality is to use simultaneously not more than one pair of thermocouples. We additionally wrapped the thermocouple wire with Sibral insulation (ceramic fibers + aluminum foil tape + wound tantalum wire). There can be more layers if necessary as it is exposed to radiation and flames (Figure 3).

Figure 1. Auxiliary frame with the thermocouple pair above the installed burner.

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RESULTS A pair of temperatures (front and back) for the compilation of temperature profiles was measured only in the reference plane and at the positions shown in Scheme 2. The measurement time of one position depended upon the stabilization of combustion. In some measurements, after having set the required position, we had to wait for 1 h. Otherwise, the steady time within which the results were considered is shorter. On the basis of the visual judgment, the period of pellet feeding was adjusted to 6 s and the steady operation period was adjusted to 4 s. This adjustment was maintained during all of the measurements. The bed movement might be neglected because it is only about 2−3 mm in each axis. Overall, more than 100 measurements were performed, from which about 80 pairs of temperatures were selected. For illustration, Charts 1−6 show one temperature measurement in location A at a depth of 80 mm below the collar edge. Chart 4 shows significant decreases in the air flow supply, and Chart 5 presents increases in the chimney draft. The cause of these fluctuations lies in the regulation by the boiler control system. Despite a manual mode, the control system influenced the fan speed. These anomalies sometimes occurred in several minute periods, but there were irregularities in their occurrence during individual measurements. They do not have significant impact on the temperature, but at that moment, they only proportionately affect the carbon emission. Chart 6 shows progress of the flue gas composition during one of the measurements. Average flue gas emissions in other measurements had a very similar composition: 10.9% CO2, 810 ppm of CO, 37% NO, and 720 ppm of NO2 considered in relation to 10% O2. The combustion incorporated high excess air during the experiment because an excess air ratio achieved the average value of 3.0, which is not optimal for the semi-automatic pellet boiler. The adiabatic flame temperature of used fuel at a stoichiometric air ratio is about 1870 °C, but for very high air

Figure 2. Three-dimensional burner model with the auxiliary frame for adjustment of the thermocouple location during the experiments. informative temperature measurement in the combustion chamber. During the experiments, we measured the flow rate and temperatures of heating water, velocity, temperature and humidity of combustion air, chimney draft and flue gas temperature, and actual weight of the boiler with fuel and analyzed the composition of the combustion gases (O2, CO2, CO, NO, NO2, and SO2). The measuring assembly with sensors and devices was arranged as shown in Scheme 1. The performance, combustion speed, and flow rate of combustion air were calculated on the basis of experimental data. The density of the return water was calculated according to the equation for the temperature variation of water. The specific heat capacity was calculated from the average temperature of boiler water. The elbow of the retort burner influences the distribution of pellets significantly, and thus, the pressing of pellets behind the elbow is not uniform in the entire burner mouth. The choice of measurement plane 2920


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Energy & Fuels Scheme 1. Experimental Scheme during the Measurement of the Temperature Field in the Pellet Burner

Scheme 2. Cross Section with Thermocouple Locations of Measured Temperature Profiles

excess, which is typical for this experiment, it is only about 742 °C. The key for relevant interval selection was, besides time selection, also performance over a specific threshold limit. We did not consider results below this limit. The bed surface height was always steady at different levels (in the range of up to 20 mm); therefore, we divided all of the measured data for better visibility into two groups with lower (Chart 7) and higher

(Chart 8) constant burning surface of the bed. Both groups have a similar course shifted by 15 up to 20 mm. The temperature profile near the retort wall is steadier than the temperature profile closer to the axis, which is shown in Chart 9. Aside from a few values, temperatures did not significantly differ from other bed surface heights. The reason for steady-state temperatures is that the inlet nozzles for the combustion air are placed around the burner edge (formed by 2921


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included low-melting particles, although stuck particles might also be residuals from a previous operation.

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DISCUSSION The solution for heat-resistant insulation of the thermocouple wire was only temporary, and the long-term and appropriate application would be a solid ceramic insulation put on the thermocouple wire to have enough protection against the radiation and flue gas stream and allow for bending of the insulated wire. The robust auxiliary frame reduces the space of the combustion chamber and forms a solid barrier having a significant impact on the combustion, which contributed to the reduction of the current performance compared to the nominal performance. It was necessary to interrupt the steady combustion to adjust a new position and wait for stable measured quantities again. This process causes significant interference with the combustion conditions. It is reflected on the fact that it is impossible to achieve identical conditions in each of other measurements. Mainly, the burning bed cannot be stabilized in the same position as the results confirmed. The experiment is burdened by this method error that had the greatest but acceptable impact on the results. A good solution for this would be the outer frame for vertical position adjustment. In such a case, the immersion depth of a thermocouple would be adjusted without a direct intervention with combustion. Through the casing in the chamber, only a vertically adjustable pole holder with thermocouples would be guided, but this requires design modifications of the boiler. The results obtained by this way would represent the actual temperature field more accurately, because the stabilized combustion process would run continuously when the positions are changed. On the other hand, the measurement method represents different and random conditions during common operation more accurately. Overall, this error did not devaluate measurements, because parameters that can be constant are kept at the same level and during more experiments. The boiler with pellets was placed on the scales but not as one measurement complex because of flexible connections to the refrigeration system, the chimney section, and the measurement devices, which are located outside the scales. These facts affect the measured weight loss. However, the biggest problem is weight steadiling after the adjustment of the thermocouple position and impact of the fans on the weight deviation, as seen in Charts 3 and 4. Measured values fluctuated a lot, and therefore, the burning rate determination was more

Figure 3. Measurement equipment during the experiment.

2−3 cm annulus), so that the excess air will provide appropriate conditions for the most intensive combustion. This fact is documented in normal operation of the burner by Figure 4, when glowing pellets are at the edge of the burner and both smoldering and burning of pellets are in the middle (Figure 5). Experience with the boiler operation and measured temperature profiles at different positions across the burner show that the retort burner does not have a vertically stratified combustion process but rather more likely a horizontally stratified combustion. Essentially, burning pellets are not fed beneath the burning bed as we could assume, but they are poured from the middle to the burner edge. Pellets are burned there, and porosity is increased at the burner edge. Thus, pellet combustion is the most intensive along the burner edge, as shown in Chart 7. Upon closer observation, the edge area appears to be like a fluidized bed, in which the pellets are raised and burned in the flow of flue gases and air. The measurements lasted a few days, and there was a slight decrease in the performance during the combustion at the same setting. At first, we assigned this to the interference of the holder in the burning bed and to the influence of the auxiliary frame with the combustion. However, after several days, we noticed in the burner, visible dark agglomerations that were not carbon. It was the result of particles gradually sticking together and creating fragile but solid clusters, as shown in Figure 6. Sticking particles were also on the burner walls, as illustrated in Figure 7. Deposits caused the reduction of both the free burner cross section and the reaction space of the burning bed. During the measurements, we used pellets of the highest quality. However, their color was visibly darker because they also

Chart 1. Temperatures in the Burning Bed and in the Combustion Chamber during One of the Measurements

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Energy & Fuels Chart 2. Air and Water Temperatures during One of the Measurements

Chart 3. Performance and Weight Loss during One of the Measurements

Chart 4. Air Velocity and Water Flow during One of the Measurements

Chart 5. Chimney Draught and Temperature during One of the Measurements

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Energy & Fuels Chart 6. Emissions Considered in Reference to 10% Oxygen during One of the Measurements

Chart 7. Temperatures in A_Front Location at the Lower Bed Surface (Location A, Front Thermocouple)

Chart 8. Temperatures in A_Front Location at the Higher Bed Surface (Location A, Front Thermocouple)

Chart 9. Temperatures in the A_Back Location (Location A, Back Thermocouple)

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Figure 4. Surface of burning bed immediately after turning the air supply fan off.

Figure 7. Deposits from the inside of the burner wall.

phytomass pellets are combusted, there will be a higher amount of agglomerates and deposits. Therefore, the long-term operation would not be possible without regular cleaning, which is necessary to do at least 1−2 times per week. The retort-type burner does not remove agglomerates automatically, and it is a compelling reason why it is not suitable for phytomass combustion.

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CONCLUSION The temperature analysis of the burning bed provided us with not only measured temperature profiles but also a better understanding of the combustion process in this burner type. The aim of this experiment was to determine the thickness of each zone (drying, devolatilization, combustion of fixed carbon, etc.). However, with respect to the character of the burning bed, it did not make sense to measure the zone height. The remedial action was to measure the temperature field in the reference plane, but there would be a necessary steady bed layer. The measuring method does not enable it because combustion conditions are neither continuous nor identical. Thermocouples, which were used for the temperature measurement in the burning bed layer, stuck out of the metal tubes only a few millimeters. Even though the tip was directly in the bed, we assumed that the measured temperatures were lower than the actual temperatures because the material of probes affected the local measuring by conductivity and their volumes. The main combustion process takes place at the edge of the burner. The temperatures in this area exceed 1000 °C up to nearly 1200 °C thanks to the excellent heat accumulation of cast iron parts, which created favorable conditions for rapid heating of the new pellets and did not cool burned pellets excessively. On the basis of the measurements, we deduced the conclusions. If the designed water-cooled burner walls are used, the combustion will be disturbed significantly. This fact would at least lead to either increased CO emissions or even mostly incomplete combustion. We therefore do not consider the idea of a water-cooled retort burner as a viable solution for a small boiler for households.

Figure 5. Burning bed in the underfed burner during the experiment.

Figure 6. Agglomerates from the burner.

for information only, even though the feeding was adjusted to the same operating and rest time. Another problem arose from the combustion of low-quality pellets in terms of the calorific value, ash content, certain elements, and other combustion properties. Incombustible particles in pellets were not blown out of the burner, but they agglomerated and cumulated in the burner. Finally, they created large agglomerates and deposits on the burner wall. If

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 2925


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Energy & Fuels ORCID

Michal Holubčík: 0000-0001-9400-0842 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Projects VEGA 1/0548/15 “The Impact of Bark Content and Additives on Mechanical, Energy and Environmental Characteristics of Wood Pellets”, KEGA 046ZU-4/2016 “Unconventional Systems Using Renewable Energy”, and APVV-15-0790 “Optimization of Biomass Combustion with Low Ash Melting Temperature”.

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NOMENCLATURE CFD = computational fluid dynamics DT = ash deformation temperature ST = ash sphere temperature HT = ash hemisphere temperature FT = ash flow temperature

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REFERENCES

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experimental determination of bed temperatures during wood pellet

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