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Parameters influencing wet biofuel drying during combustion in grate furnaces
Fuel 265 (2020) 117013
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Full Length Article
Parameters influencing wet biofuel drying during combustion in grate furnaces
T
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L. Vorotinskienė, R. Paulauskas , K. Zakarauskas, R. Navakas, R. Skvorčinskienė, N. Striūgas Laboratory of Combustion Processes, Lithuanian Energy Institute, Breslaujos St. 3, Kaunas LT-44403, Lithuania
A R T I C LE I N FO
A B S T R A C T
Keywords: Wet biofuel Furnace Drying Primary air Thermal radiation Flue gas recirculation
Mainly grate-fired furnaces are used for heat production, as these systems are designed for high thermal efficiency with low emission levels of gaseous pollutants when firing good quality biomass with 30–45% moisture content. However, high demand for biomass during the winter makes it necessary to necessitates use wet biofuel of lower quality with varying moisture content that can reach up to 60% wt. causing the burning instability and incomplete combustion. These negative effects could be avoided by optimizing the furnace control to intensify biomass drying on the grates, but the parameters affecting wet biofuel drying must be known. In order to expand the knowledge of the wet biofuel drying process and establish the parameters that influence the drying process most significantly, a special rig based on characteristics of a 6 MW grate furnace was produced and drying experiments simulating the primary air flow in the furnace, the recirculation of the flue gases and the primary air flow together with exposure to thermal radiation were performed. The results revealed that the radiation from surfaces has the biggest impact on biomass drying when preheated air of the temperature of 200 °C is supplied and the moisture content of the biomass is reduced to 0.66 of its initial value. It was also determined that the drying intensity depends on the mixing period and the drying intensity can be increased by up to 15% by additional mixing. Influence of the drying agent temperature depending on the particle size was established as well.
1. Introduction Widespread and long-term utilization of fossil fuels for heat and power production doubled the global CO2 emissions over the past 40 years [1]. Emissions of these greenhouse gases resulted in global climate warming, which causes more frequent and unusual weather events [2]. To reduce the environmental pollution, energy production from renewable sources is considered a valid solution [3–5] and the European Union has decided to produce 27% of its energy from renewable sources by 2030 [6]. Besides, more stringent environmental regulations were introduced to decrease the emissions of CO2. Biomass is the most prospective alternative to fossil fuel in thermal energy production of all the renewable energy sources, therefore, biomass is also used for district heating in the cold climate regions [7,8]. The typical heating plant sizes vary from small and medium to large (0.1–50 MW) scale depending on the extent of a district heating network [9]. It is common to use grate-fired furnaces for heat production in small and medium scale heating as these systems are designed for a high thermal efficiency with low emissions of gaseous pollutants (CO, NOx, etc.) when firing good quality biomass with a 30–45% moisture
⁎
content [10–13]. However, the expanding utilisation of renewable energy sources for heat production leads to an increased demand and the price for biomass. High demand of these forest fuels during the winter causes an imbalance in the market even though wood chips are harvested and produced continuously throughout the year [14]. Consequently, moist biofuel of lower quality such as stem wood, coniferous bark, pine branches with varying moisture content that can reach up to 60% wt. or more are used for heat production [15]. Combustion on the grate of so moist fuel becomes complicated as the drying process occupies most of the space in the furnace and the combustion heat from the lower layer is consumed for drying up the upper fuel layer [16]. Moreover, moisture can cause the burning instability and incomplete combustion, which leads to increased emissions including those of CO2. Besides, the lower heating value (LVH) of moist fuel is only 6 MJ/kg, while the LVH of biofuel dried to the moisture content of 30% wt. is approximately twice as high – 12 MJ/kg [17]. In most cases, solid biofuel is additionally dried before supplying to the furnace to avoid the mentioned problems. Drying techniques include a superheated steam dryer [15], a rotary dryer [18,19], a bed dryer or a fluidized bed dryer [20,21]. In case of the superheated steam
Corresponding author. E-mail address: [email protected] (R. Paulauskas).
https://doi.org/10.1016/j.fuel.2020.117013 Received 28 June 2019; Received in revised form 5 December 2019; Accepted 2 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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increases from 70 to 90 °C. The authors determined that the optimal temperature of the drying air should be over 70 °C, but the process was not analysed at higher temperatures. Similar research at higher drying temperatures was performed in work [27]. The authors analysed the drying process of a thin layer of ground pine chips (3.2–25.4 mm) and pellets (3.2–12.7 mm) with moisture content of 50% at drying temperatures of 50, 100, 150 and 200 °C. Particles of 3.2 mm size were fully dried in time periods ranging from 31 to 3 min at drying temperatures ranging from 50 to 200 °C, respectively. Increasing the particle size to 25.4 mm led to increased drying time by 90% to 117% at drying temperatures from 50 to 200 °C, respectively. Besides, at high drying temperature (200 °C) the particles lost 3–7% of dry mass due to volatile release. Even though the biomass drying was widely analysed in reviewed works [21–27], there is no sufficient knowledge about the wet solid biofuel drying process in a furnace and the parameters influencing drying of biomass of this type. In order to expand the knowledge in this field, a special rig was constructed to simulate the characteristics of a 6 MW reciprocating grate furnace and drying experiments were performed, where preheated air simulated the primary air flow in the furnace, preheated air and steam mixture simulated the recirculation of the flue gases from the furnace, and infrared lamps imitated radiation of the furnace surfaces. For drying experiments, wood chips, urban park waste and forest harvest waste were used and drying differences between these samples were established. In additional experiments, biofuel was mixed by a screw mixer to simulate the grate movement in order to analyse the mixing effect on the intensification of biomass drying.
dryer, the wet biofuel is mixed with sufficient amount of superheated steam to fully dry out the material and produce saturated steam [22]. Even though this system has a high energy efficiency, low level of fire risk and better process control compared to other systems, there are some disadvantages as well. This dryer is not suitable for wood chips, it requires high investment costs and there are wastewater treatment issues [15,21,22]. The packed moving bed, rotary and pneumatic drying systems are simpler than a superheated steam drying system because the drying agent is heated air and the construction of the system is suitable for drying wood chips [21]. Still, there are some drawbacks: the packed moving bed dryer occupies a large area in the heating plant, while the rotary dryer operates at the drying temperature range from 200 to 500 °C resulting in a higher risk of fire [21]. Moreover, Danielsson and Rasmuson [23] investigated the release of volatile organic components during the drying process. The results showed that energy is partially lost when biofuel is being dried with air at the temperature exceeding 100 °C because of evaporation of monoterpenes. The reviewed works have shown that the drying setup must be selected precisely taking into account the energy efficiency, risk of fire and drying emissions in order to reach the desired moisture content of supplied biofuel. Taking into account the advantages and drawbacks, the additional drying systems for use exclusively for small and medium scale heating plants have limited applicability for drying wood chips of varying moisture content,. A more favourable solution for heating plants of this type is to intensify biomass drying on the grates in a furnace during combustion, which requires knowledge of the parameters affecting this process. Few works have focused on the effect of drying air amount and drying agent temperature on biomass drying. Yang et al. [24] have investigated the effect of primary air flow for intensification of biomass drying during combustion. The results showed that the increase of the air flow leads to increase of moisture evaporation rate until the former reaches a critical point at which the latter starts to drop. This effect was related to the heat transfer into the evaporation zone as low air flow enhances the transfer of the radiation energy to evaporation process, whereas a high air flow rate increases only the front flame temperature and heat is carried away. The effect of the drying agent temperature and the biomass particle fraction on biomass drying was studied by Holmberg et al. [25]. For the experiments, birch bark was dried at different temperatures (50, 70, 90 and 110 °C) with different bed heights (50, 150 and 250 mm). The authors observed that simultaneous evaporation and condensation occur at high drying temperature (110 °C) and the drying rate in the beginning of the process (?) is lower than that at the 90 °C drying temperature. This happens because water evaporates from the bottom layers due to high air temperature and condenses on the upper layers. This effect was not observed at low drying temperatures. Myllyma et al. [26] investigated the drying time dependency on the drying temperature by conducting biomass drying experiments in a fixed-bed batch-dryer. The results revealed that the drying time for the bed height of 300 mm decreases in half when the drying air temperature increases from 50 to 70 °C, whereas the drying time changes only slightly when the temperature
2. Materials and methods 2.1. Biomass characteristics Small and medium scale heating plants usually utilise waste biofuel of various origins (waste materials from wood harvesting, wood processing and urban cleaning), therefore, particle size and moisture content can differ noticeably. These characteristic biomass parameters influence both the drying and combustion processes. In order to account for this fact, three different types of biofuels such as wood chips of various species of tree branches with bark (WB), chips of broad-leaved tree branches with leaves (WL), and chips of conifer branches with needles (WN) (Fig. 1), were selected for the stationary drying experiments. The ultimate and proximate analyses of biomass were determined using an IKA C5000 calorimeter and a Flash 2000 CHNS analyser in accordance with: LST EN 14774-1 (moisture content), LST EN 14918 (HHV), LST EN 14775 (ash content), LST EN 15148 (volatile content) and LST EN 15104 (CHNS content). The obtained characteristics are presented in Table 1. The ultimate and proximate analysis of different biofuel mixtures (Table 1) have shown that the analysed biofuel is similar in its
Fig. 1. Photo of used solid biofuels. 2
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thermocouples installed upstream and downstream the electrical heaters. The effect of hot recirculation flue gases on biomass drying was studied using air-steam mixture. The absolute humidity of the drying agent was increased from 5 to 17 g/m3 by injecting steam into the preheated air stream while maintaining the constant flow rate. As in the previous case, the preheated air-steam flow rate was 17.7 ± 0.5 m3/h. In addition, to simulate the thermal radiation from the incandescent surfaces of the furnace, two infrared lamps were installed above the solid biofuel bed and drying air with constant flow rate (17.7 ± 0.5 m3/h) was supplied from the bottom of the drying chamber. The infrared lamps (2 kW each with an efficiency of 85%) corresponded to the furnace radiation of 50 kW/m2 [11]. The effect of fuel mixing on enhancing the wood chips drying rate was additionally analysed. For this purpose, a special screw mixer, which imitates the movement of the reciprocating grates in the 6 MW furnace, was installed in the drying chamber (Fig. 3). Using the screw mixer, the bottom layer of the biofuel was lifted upwards and the top layer was moved downwards. Such movements were repeated every 150, 300 and 600 s. The procedure of experiments was identical in all the cases. Before performing the drying experiments, solid biofuel was loaded to a vessel filled with water and left to soak in the vessel for 16 h. After that the biofuel was lifted out and left to drain for 2 h in the ambient temperature, in order to obtain approx. 60% wt. moisture content of the biomass. Meanwhile, the drying chamber was preheated to the appropriate temperature by supplying drying air. When the drying chamber reached the desired temperature, the steel crate with length of 0.335 m, width of 0.235 m and height of 0.28 m was filled with the prepared biofuel sample. The bed height was 0.23 m, which corresponds to the height of the solid biofuel layer on the grates of the 6 MW furnace. The thermocouples were embedded at various heights (T1, T2, T3 and T4 respectively at 3, 10, 12 and 22 cm from the bottom) of the bed to determine the drying zone progression through the bed during the drying process (Fig. 3). All thermocouples were connected to a data logger PICO TC-08 for data collection. Humidity of exhaust air from drying chamber was measured by the Testo 454 analyser equipped with a relative humidity (RH) sensor. Typically, the total burnout of wet solid biofuel in the reciprocating grate furnace is completed in 30 min. The drying zone occupies 1/3 of the furnace length, therefore, the actual time spent in this zone is approximately 10 min. According to above mentioned conditions, the experimental duration was set for 30 min.
Table 1 The characteristics of biomass samples. Parameter
WB
WL
WN
Standard deviation
Ultimate analysis wt% Carbon Hydrogen Oxygen (diff.) Nitrogen Sulphur
48.6 5.80 41.49 0.60 < 0.01
49.13 5.97 39.87 0.52 < 0.01
48.53 5.39 38.47 0.79 < 0.01
1.17 0.49 0.15 0.04 –
36 3.50 18,998
33 4.50 18,630
14 7.60 19,785
0.10 0.05 52.30
Proximate analysis wt% Moisture Ash Higher heating value (HHV) kJ/kg
elemental composition, however, different moisture and ash contents indicate different nature of biofuel. Also, to estimate the influence of the particle size to the drying process, the granulometry of dry biomass samples was determined experimentally using sieves with mesh sizes ranging from 1 to 10 mm. The average particle sizes of three samples were determined from the obtained particle size distributions (Fig. 2) using the method described in [28].The characteristic particle length of the sample WB is 13 mm, that of WL – 9 mm, and that for WN is 7 mm. The major part (35–53%) of biofuel samples used in the experiments consisted of particles larger than 10 mm in length. In this case, WN consisted of the smallest particles of the sample. A removal of moisture from the fine particles is assumed to be more intense. To validate this assumption, a few different samples of biomass were selected.
2.2. Experimental methodology The experimental setup of solid biofuel drying used in this study is shown in Fig. 3. The rig consists of four main components: a drying chamber, an air supply system equipped with electrical heaters, a steam generator, and two infrared lamps. The rig parameters such as dimensions of the drying chamber, the primary air flow, the amount of steam and the power of heat radiation were selected according to the real working parameters of a 6 MW reciprocating grate furnace. A more detailed information on the mentioned furnace is provided in the previous work [11]. The rig was adapted to analyse the influence and effects of preheated primary air, hot recirculation flue gases and the furnace radiation on biomass drying. Three separate drying methods were analysed. In the case of drying by preheated primary air, the drying agent was supplied from the compressed air system through two heating coils used to preheat air to the desired temperatures of 50, 100, 150 and 200 °C, while the flow rate was kept constant at 17.7 ± 0.5 m3/h. The temperature of hot air was measured and controlled by means of the K-type
2.3. Data analysis The main parameters characterising fuel drying were determined by the indirect method, i.e., by estimating the solid biofuel moisture content from the exhaust products. More detailed information about the
Fig. 2. Characteristic particle length by mass of different wood chips samples. 3
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Fig. 3. Scheme of experimental rig for biomass drying.
the sample mass at the previous time moment:
indirect determination of the moisture content is presented in the previous work [11]. From the absolute humidity of exhaust products as measured by the Testo 454 and its temperature, the changes in moisture content in exhaust flow was calculated at the appropriate time moments, which determines the biofuel water loss rate g H2 O by the following equation:
g H2 O = hg ∙
Vair + VH2 O(g ) 3600
(
273 + Tg 273 + Tamb
);
m H2 O (ti) = g H2 O (ti ) ∙Δt
where Δt = ti − ti − 1 is the sampling interval of the absolute humidity in the exhaust flow. Taking into account that the processes during biofuel drying are long lasting and not very intense, the data collection interval was selected to be 5 s. When normalising the moisture content in the biofuel samples, the maximum ratio of the biofuel samples is assigned the value of 1 and is calculated from Eq. (5):
(1)where Vair is the drying air
supply rate in m3/h; VH2 O(g ) – water flow rate in gaseous states, m3/h; hg – the absolute humidity of the exhaust products g/m3; Tg – the temperature of the exhaust products in °C; Tamb – the ambient temperature °C. The flow rate of water in gaseous states are calculated by:
VH2 O(g ) = VH2 O(fuel) + VH2 O(rec . pr )
Ym = 1 − (ΔY(max ) − ΔY(min) )
3. Results and discussion
where VH2 O(fuel) is the water flow rate from fuel in m /h; VH2 O(rec . pr .) – the water flow rate, in m3/h, resulting from injecting steam at the rate of 17 g/m3 into the preheated air stream, imitating the recirculation flue gases.. The initial sample mass was different in all the experiments, to keep the height of the biofuel bed the same. The initial moisture content in all the samples varied as well (60 ± 3%), therefore, the moisture content in fuel was normalised to the largest (maximal) ΔY(max ) and smallest (minimal) ΔY(min) value of the ratio between the moisture content and the dry sample mass throughout the entire 30 min duration of the experiment:
= ( max min )
3.1. Effect of preheated primary air, flue gas recirculation and thermal radiation to the solid biofuel drying The first set of experiments was conducted to evaluate the influence of preheated primary air, air-steam mixture and air coupled with thermal radiation on the wet WB drying. In order to understand the ongoing processes, temperatures at four locations in the biofuel bed (Fig. 2) were recorded and the obtained temperature profiles are presented in Fig. 4. Moreover, the water loss rate was also estimated in order to predict the biofuel drying trends. The normalised moisture content (Ym) and the water loss rate (Fig. 5) were calculated from the moisture content of the exhaust gas using Eqs. (1)–(6).
m (ti) − mdry mdry
(3)
here, mdry is the dry sample mass g; m (ti) is the sample mass g at the time moment ti. The sample mass g at the time moment ti is calculated by subtracting the water mass in the exhaust products m H2 O (ti) at the same time moment from the sample mass at the previous time moment:
m (ti) = m (ti − 1) − m H2 O (ti)
(6)
(2) 3
ΔY
(5)
3.1.1. Preheated primary air The curves of the temperature changes in different layers of the WB bed using the preheated air of 50 °C indicated that the heat transfer is insignificant (Fig. 4A). Even after 30 min of drying, the highest temperature in the entire biofuel bed was only approximately 25 °C (Fig. 4 A1) resulting in the constant water loss rate of 0.09 g/s for the remaining time of the experiment (Fig. 5A2). When the temperature of the drying agent was increased to 100 °C, the bottom of the biofuel bed (T1) reached only 38 °C per 30 min (Fig. 4A2). Meanwhile, the middle
(4)
where m H2 O (ti) is the water mass in the exhaust products g, m (ti= 0) is the weighted mass of the sample at the beginning of the experiment. The water mass in the exhaust products m H2 O (ti) is calculated using 4
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Fig. 4. Temperature profiles at different layers of WB bed varying the temperature of drying agent.
content in 30 min (Fig. 5A1). The most intense drying process using preheated air was observed at the drying agent temperature of 200 °C (Fig. 4A4). Due to the fast heat-up of the bed, the water loss rate (Fig. 5A2) stabilizes after approximately 18 min from the start of the experiment indicating a shift to uniform drying. In the end of the experiment, the normalized moisture content in the sample was 0.87 (Fig. 5A1). The highest temperature of biofuel was exposed using the preheated air of 200 °C and approximately 66 °C was reached in the layer T1 after 10 min and T1 = 127 °C after 30 min (Fig. 4A4). The similar values of moisture content were established using the air-steam mixture as the drying agent, but the trend of the water loss rate differed (Fig. 5B). From the start of the experiment, a stagnation period (non-drying) was observed and duration of it depended on the
of the bed (T3) starts to heat up after 7 min and from this moment the water loss rate slowly increases till 0.18 g/s (Fig. 5A2). The trend of the bed heat up and water loss rate possibly indicates that water vapour condensed in the middle layer starts to evaporate from the lower layer, but a sluggish increase of water loss rate reveals that the condensation on the bed top is still going on. More intense drying of the bed bottom (T1) and heating of the middle (T3) were observed using drying agent of 150 °C (Fig. 4A3). In this case, the temperature in the middle of the bed (T3) and the water loss rate are increasing after 5 min from the beginning of the experiment (see Figs. 4A3 and 5A2). The water loss rate becomes constant (0.28 g/s) when the temperature of the bed top (T4) stabilizes and the sample dries by approximately 5% from the initial sample moisture 5
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Fig. 5. Normalized moisture content and water loss rate of wet WB by exposing with the preheated air (A1, A2), air-steam mixture (B1, B2) and air coupled with thermal radiation (C1, C2).
(Fig. 4B3,4). Further it was followed by intense fuel drying (Fig. 5B). The water loss rate of the bed stabilised at the value of 0.55 g/s after 15 min using the air-steam mixture of 200 °C (Fig. 5B2).
drying agent temperature. It was assumed that the drying agent cools down below the dew point temperature as it flows around wet and cold solid biofuel and steam condenses on colder surfaces upwards in the bed. When the biofuel is heated up enough, the temperature in the middle of the bed (T3) starts to increase and the evaporation process intensifies (Fig. 4B).
3.1.3. Radiation from the hot furnace surfaces The most significant influence on biomass water loss rate was determined using the air coupled with thermal radiation (Fig. 5C). The moisture loss of the WB was observed instantly from the first seconds of experiments at all the drying agent temperatures. According to Fig. 5, it is assumed that evaporation from the bed bottom and condensation on the upper layers of the bed is going faster due to the top layer dried out by the infrared lamps and the further condensation on the top is avoided. In this way, the evaporation process surpass the condensation process resulting in continuously increasing water loss rate of the bed (see Fig. 4C) even though the temperature changes in the bottom and the middle of the bed did not show influence of thermal radiation to solid biofuel drying and are lower than those observed when using the previously described methods of drying (Fig. 5). When supplying drying air of 50 °C temperature, the normalized moisture content in wood
3.1.2. The flue gas recirculation products In case of drying with air-steam mixture of 50 °C, it was observed that the stagnation period lasted for about 12 min and after that the temperature of bed middle (T3) started to increase (Fig. 4 B1). It also correlated with the increase of the water loss rate but the moisture content in biofuel (WB) was still high (0.99 of the initial value) after 30 min (Fig. 5B). Using the air-steam mixture with higher temperature (150 °C), more intense drying was determined. The water loss rate increased to 0.42 g/s within 30 min while the moisture content decreased to 0.92. It was noticed that using the air-steam mixture of 150–200 °C for drying of WB, the stagnation period was reduced by half and lasted until the middle of the bed started to overheat (T4 starts to increase) 6
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Table 2 Water loss rate at different drying case. Drying agent temp °C
Preheated air
Air-steam mixture
After 10 min
After 30 min
Air coupled with thermal radiation
After 10 min
After 30 min
After 10 min
After 30 min
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
WB 50 100 150 200
0.09 0.12 0.15 0.27
16.10 21.20 30.74 41.82
0.09 0.19 0.28 0.42
18.9 31.05 38.27 41.8
0.12 0.16 0.22 0.35
18.53 27.73 38.09 46.42
0.18 0.29 0.43 0.56
29.65 37.24 43.3 46.8
0.26 0.29 0.31 0.44
19.20 31.30 34.02 42.70
0.53 0.63 0.69 1.11
19.62 31.61 38.96 42.88
WL 50 100 150 200
0.11 0.16 0.24 0.32
21.14 28.34 35.89 39.61
0.11 0.22 0.31 0.43
23.05 32.15 37.43 41.11
0.13 0.18 0.26 0.33
22.47 29.33 35.98 36.56
0.18 0.29 0.43 0.58
30.5 37.98 43.15 47.52
0.24 0.38 0.44 0.56
19.73 25.59 37.15 41.88
1.01 1.03 1.14 1.25
20.05 31.78 37.83 42.99
WN 50 100 150 200
0.09 0.15 0.23 0.36
16.72 28.27 32.26 40.15
0.09 0.21 0.28 0.44
19.12 29.16 38.4 39.66
0.12 0.18 0.36 0.42
22.53 36.62 40.44 47.32
0.18 0.31 0.42 0.55
30.29 38.61 43.4 47.56
0.33 0.42 0.44 0.43
20.79 28.96 38.65 38.95
1.06 1.27 1.25 1.35
21.2 31.36 38.77 43.79
results of this analysis demonstrate the necessity to apply the means for the process intensification.
chips was reduced to 0.84 after 30 min. The measured value was higher by 0.1 point than that in the case of drying methods without the thermal radiation (Fig. 4). In case of supplying the drying agent with the temperature of 100 °C and 150 °C, the moisture content in the bed was decreased by 0.19 and 0.21 after 30 min, respectively. The most intensive drying, at which the water loss rate increased to 1.12 g/s, was achieved at the drying agent temperature of 200 °C. In the end of the experiment, the normalized moisture content in the WB was determined to be 0.67 (decreased by 20% from the initial sample moisture content) (Fig. 4C). Considering the temperature changes (Fig. 4), the highest drying intensity in all the cases was recorded in the bottom of the bed (T1, T2) meanwhile the stagnation period was observed in the middle and the top of the bed (T3, T4). It can be assumed that the primary flow is saturated with moisture as it flows through wet biofuel. The moisture evaporated from the sample is transferred to higher layers of the bed. In this case, the condensation takes place as the temperature drops below the dew point. In order to clearly define the tendency of water loss and how the temperature changes in the middle layer T3 in different cases, the results were summarised in Table 2. It can be noticed that preheated air enrichment by steam in the case of drying with air-steam mixture led to the increased water loss rate thereby enhancing the condensation process. Increase of the T3 (Table 2) might indicates that the heat released during the condensation process was consumed heating the biomass bed (Fig. 4A,B). Besides, the drying agent containing more moisture (preheated air versus air-steam mixture) has a higher heat transfer coefficient approximately 20 times, which also influences faster overall heating of the bed as shown by T3 (Table 2). According to the obtained results, it could be assumed that supplying wet recirculation products (air-steam mixture) to the furnace increase the water loss rate, though the drying rate (Fig. 5B1) does not become more intense compared to the case when preheated air is used (Fig. 5A1). The most promising results were obtained (Fig. 5C) in the case of biomass drying using the air coupled with thermal radiation (Table 2). However, according to the temperature readings, it can be considered that T3 does not reach the level of intense drying which is necessary to attain the moisture content of biofuel in the drying zone (approximately 30%). It was also assumed that the drying processes are too slow that can create unfavorable conditions for fuel combustion as the drying zone occupies the largest part of the grate. Deviations from the desired furnace operation regimes lead to an inefficient energy production in the heating plant which does not meet the operation requirements. Therefore, the
3.2. Influence of biomass types on drying rate Identical sets of experiments were conducted with WL and WN as well to analyse the influence of biomass type and the fraction size on drying. The analysis reveals a near identical drying behaviour of WL and WN as in the case of WB (Table 2). In the previous case (see Section 3.1), the most promising results on drying were achieved at the drying agent temperature of 200 °C. For this reason, this temperature of drying agent was selected for further analysis in this work (Fig. 6). Comparing the results of drying involving preheated air and airsteam mixture (Fig. 6), it is seen that the drying intensity was rather stable. At the selected time moments, the drying rates varied insignificantly and independently from the used biomass type (Fig. 6). As in the case of the WB, the most intense drying of WL and WN was also observed using the air coupled with thermal radiation. In these cases, the drying rate increases with a decrease of the particle size fraction in the samples. It was most clearly noticed after 30 min since the start of the drying experiments, especially for WN. The drying rate for the WN was approximately 1.2 times higher than that for the WL (the WN contains the particle fraction that is 1.4 times smaller than that of the WL (Fig. 2)). Besides, an identical trend was observed comparing the results of the WB and WL as well. In order to gain a better understanding of the influence of the biomass type on drying with the presence of the simulated thermal radiation from the hot furnace surfaces, the moisture content and water loss rates of the WL and WN at different drying temperatures are presented in Fig. 7. It can be seen from Fig. 7, that WL and WN start to dry at once in the beginning of the experiment and the water loss rate increases linearly regardless of the drying agent temperature. Moreover, the trend of the water loss rate accentuates the differences arising from the differences in the solid biofuel fraction. During the drying process in low temperature (50 °C), the water loss rate from the WL and WN is higher by the factors of 1.01 g/s and 1.06 g/s, respectively, compared to the case of drying the WB (0.53 g/s). When drying both fuel types at 100 – 150 °C, it was determined that the normalized moisture contents of WL and WN were 0.60–0.58 and 0.48–0.46 of the initial values, respectively, after 30 min. It has also been determined that the changes of the moisture content in the WB were identical and the normalized 7
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Fig. 6. The influence of solid biofuel type on drying rate.
which the average particle size is 9 mm, is slower by 7% on average than in the case of drying WN with the particle size of 7 mm (Fig. 2). According to the obtained results, it is considered that more intense heat transfer processes take place due to smaller particles (higher area of the surface), which influence the solid biofuel drying intensity.
moisture content differs by only 0.01 (Fig. 7A1). The same effect was observed in the case of drying WB as well (see Fig. 4C). This possibly indicates that the drying process proceeds mainly due to the thermal radiation and the moisture evaporation process depends insignificantly on the temperature of air supplied from below in the temperature range of 100–150 °C. Increasing the drying agent temperature to 200 °C results in more intensive drying and the normalized moisture content is 0.10–0.13 lower than that at the drying agent temperature of 100–150 °C in both cases (Fig. 7A, B). The estimated values of normalized moisture content show that the water loss rate (Table 2, Fig. 7) depends on the particle fraction in bed as the drying process of WL, in
3.3. Influence of mixing on the solid biofuel drying The performed experiments of drying solid biofuels revealed the influence of preheated air, air-steam mixture and air coupled with thermal radiation on drying. The most intense biofuel drying was
1.0
1.4 A1
Ym
0.8 0.7 0.6 0.5 0.4
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0
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30
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1 B1
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0.6 0.5 0.4
1 0.8 0.6 0.4 0.2 0
0.3 0
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15 20 Time min
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Fig. 7. Normalized moisture content and water loss rate of WL (A1, A2) and WN (B1, B2) at different drying temperatures in the case of air coupled with thermal radiation. 8
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0.3 0.25
T600 After 10 min T300 T150
T600 T300 T150
T600 T300 T150
After 20 min
After 30 min
Fig. 8. Fuel mixing influence on ΔYm at different drying temperatures.
0.2 0.15 0.1 0.05 0 50 100 150 200 Drying agent temperature °C
50 100 150 200 Drying agent temperature °C
50 100 150 200 Drying agent temperature °C
4. Conclusions
identified in the presence of the simulated thermal radiation from the furnace surfaces. But to increase the drying and combustion intensity, additionally reciprocating grates are used in furnaces. To gain data for this particular case, an additional set of experiments for drying the WB was performed. In order to simulate a grate motion in the furnace, fuel was mixed additionally. Three cases, when the solid biofuel bed was mixed every 600 s, 300 s and 150 s, were analysed. The obtained results were compared to those without mixing and the determined change in biofuel normalized moisture content depending on the drying agent temperature and the mixing period are presented in Fig. 8. It has been established that during 10 min of drying, the bed mixing frequency has only negligible influence to the drying intensity and even at the drying agent temperature of 200 °C, the normalized moisture content decreases by 0.02 when mixing every 150 s (Fig. 8). Further results revealed that at low drying agent temperature (50 °C), the normalized moisture content of WB is reduced by about 0.05 after 20 min and by 0.1 after 30 min regardless of the mixing period. Analysis of the mixing results after 20 min revealed that the mixing of wet solid biofuel intensifies the drying process. The drying intensity increases when the drying agent of higher temperature (100–150 °C) is supplied from below, but the drying rate depends insignificantly (by up to 2%) on the mixing intensity (Fig. 8). After increasing the drying agent temperature to 200 °C, the effect of mixing to the biofuel drying intensity becomes lower compared to the cases when colder drying agent is used (at 100 and 150 °C). This effect becomes more pronounced when analysing the drying results after 30 min (Fig. 8). The initial experiments have shown that high temperatures of the drying agent (200 °C) lead to the most intense drying of solid biofuel in the entire bed, therefore, additional mixing has lesser effect on the bed drying compared to other cases (100, 150 °C). However, in the case of the drying agent temperature of 200 °C, the influence of the mixing period (150, 300 and 600 s) is significant. The change of the normalized moisture content increases linearly with the increasing mixing period. From Fig. 8 could be pointed out that the influence of thermal radiation to the drying rate is significant when the drying agent temperature is high (200 °C). It can be assumed that in the presence of mixing, the drying process is influenced by both convection and thermal radiation in the entire volume, therefore, solid biofuel dries out most intensely. The efficiency of mixing depending on the drying agent temperature is revealed by the curves in Fig. 8. It has been determined that the influence of mixing is highest for the drying agent temperature of 150 °C, and drying is intensified by the largest factor when the mixing period is 150 s. This is evident by the change in the residual normalized moisture content, which amounts to 0.24 compared to the results in the absence of mixing (Fig. 8).
In this work, the parameters influencing the drying intensity of wet solid biofuel during combustion in grate furnaces were evaluated by performing the drying experiments with the simulated primary air flow in the furnace, simulated recirculation of the flue gases and the simulated primary air flow with simultaneous exposure to radiation from the hot furnace surfaces. For the drying experiments, wood chips, urban park waste and forest harvest waste were used and the differences in drying intensities between these samples were established. As a result, the following conclusions were made: 1. The results revealed that during the drying process of wet solid biofuel with supply of the primary air or air-steam mixture in the simulated furnace conditions, the initial moisture release from the bed stagnates. It is assumed that moisture from the lower layers is transferred to the middle of the bed and condenses on cold surfaces. Applying higher temperature (150–200 °C) of the drying agent, the condensation rate decreases. Taking into account the obtained results, it is recommended to avoid recirculation of flue gas with the temperature of 50–100 °C to the drying zone of the furnace when firing biofuel with the moisture content around 60%. 2. In the case of air coupled with thermal radiation at temperature of 200 °C, the highest decrease in the moisture content during drying was determined for WN. After 10 min, the normalized moisture content decreased from 1 to 0.93 i.e., only by 4% from the initial moisture content, and after 30 min – from 1 to 0.35, i.e., from 60% of initial sample moisture content till 20%. These results demonstrate that biofuel will not dry out per 10 min to the desired requirements of the furnace, therefore, in order to reach and maintain the proper dryness of biofuel within 10 min, more efficient methods/techniques must be applied. 3. Among of the studied intensification methods (preheated primary air, air–steam mixture, air coupled with thermal radiation and mixing), the mixing of solid biofuel affected the drying process positively and the moisture content decreased linearly with the increasing mixing intensity. In the case of preheated air with temperature 150 °C, the drying intensity increased by up to 15% due to biofuel mixing with period of 150 s. The moisture content in the WB decreased (from 1 to 0.79 relative units) by approximately 13% from the initial WB moisture content while without mixing, the moisture content decreased (from 1 to 0.54) by approximately 28% from the initial WB moisture content. 4. In order to attain the moisture content of solid biofuel in the drying zone of the furnace (approximately 30%) resulting in a sustained flame propagation in the fuel, the furnace control must be optimised. This could be achieved by applying combined methods, such as recirculation of hot (˃200 °C) combustion products from the
9
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furnace combustion zone to the drying zone; a creation of the thermal radiation wall; or, blocking the products of recirculation with a high moisture content from entering to the drying zone.
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CRediT authorship contribution statement L. Vorotinskienė: Conceptualization, Data curation, Investigation, Methodology, Validation, Writing - original draft, Writing - review & editing, Formal analysis. R. Paulauskas: Writing - original draft, Writing - review & editing, Visualization. K. Zakarauskas: Methodology, Investigation, Validation. R. Navakas: Software, Writing - review & editing. R. Skvorčinskienė: Visualization, Writing - original draft, Writing - review & editing. N. Striūgas: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was conducted within the framework of COST Action CM1404328 “Chemistry of Smart Energy Carriers and Technologies (SMARTCATS)”. References [1] Jiang X, Guan D. Determinants of global CO2 emissions growth. Appl Energy 2016;184:1132–41. https://doi.org/10.1016/j.apenergy.2016.06.142. [2] Höök M, Tang X. Depletion of fossil fuels and anthropogenic climate change—a review. Energy Policy 2013;52:797–809. https://doi.org/10.1016/j.enpol.2012.10. 046. [3] Benoist A, Dron D, Zoughaib A. Origins of the debate on the life-cycle greenhouse gas emissions and energy consumption of first-generation biofuels – a sensitivity analysis approach. Biomass Bioenergy 2012;40:133–42. https://doi.org/10.1016/j. biombioe.2012.02.011. [4] Dhillon RS, von Wuehlisch G. Mitigation of global warming through renewable biomass. Biomass Bioenergy 2013;48:75–89. https://doi.org/10.1016/j.biombioe. 2012.11.005. [5] Anselmo Filho P, Badr O. Biomass resources for energy in North-Eastern Brazil. Appl Energy 2004;77:51–67. https://doi.org/10.1016/S0306-2619(03)00095-3. [6] European Commission. 2030 Climate and Energy Policy Framework 2014;2014:1–6. [7] Rezaie B, Rosen MA. District heating and cooling: review of technology and potential enhancements. Appl Energy 2012;93:2–10. https://doi.org/10.1016/j. apenergy.2011.04.020. [8] Hendricks AM, Wagner JE, Volk TA, Newman DH, Brown TR. A cost-effective evaluation of biomass district heating in rural communities. Appl Energy
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Full Length Article
Parameters influencing wet biofuel drying during combustion in grate furnaces
T
⁎
L. Vorotinskienė, R. Paulauskas , K. Zakarauskas, R. Navakas, R. Skvorčinskienė, N. Striūgas Laboratory of Combustion Processes, Lithuanian Energy Institute, Breslaujos St. 3, Kaunas LT-44403, Lithuania
A R T I C LE I N FO
A B S T R A C T
Keywords: Wet biofuel Furnace Drying Primary air Thermal radiation Flue gas recirculation
Mainly grate-fired furnaces are used for heat production, as these systems are designed for high thermal efficiency with low emission levels of gaseous pollutants when firing good quality biomass with 30–45% moisture content. However, high demand for biomass during the winter makes it necessary to necessitates use wet biofuel of lower quality with varying moisture content that can reach up to 60% wt. causing the burning instability and incomplete combustion. These negative effects could be avoided by optimizing the furnace control to intensify biomass drying on the grates, but the parameters affecting wet biofuel drying must be known. In order to expand the knowledge of the wet biofuel drying process and establish the parameters that influence the drying process most significantly, a special rig based on characteristics of a 6 MW grate furnace was produced and drying experiments simulating the primary air flow in the furnace, the recirculation of the flue gases and the primary air flow together with exposure to thermal radiation were performed. The results revealed that the radiation from surfaces has the biggest impact on biomass drying when preheated air of the temperature of 200 °C is supplied and the moisture content of the biomass is reduced to 0.66 of its initial value. It was also determined that the drying intensity depends on the mixing period and the drying intensity can be increased by up to 15% by additional mixing. Influence of the drying agent temperature depending on the particle size was established as well.
1. Introduction Widespread and long-term utilization of fossil fuels for heat and power production doubled the global CO2 emissions over the past 40 years [1]. Emissions of these greenhouse gases resulted in global climate warming, which causes more frequent and unusual weather events [2]. To reduce the environmental pollution, energy production from renewable sources is considered a valid solution [3–5] and the European Union has decided to produce 27% of its energy from renewable sources by 2030 [6]. Besides, more stringent environmental regulations were introduced to decrease the emissions of CO2. Biomass is the most prospective alternative to fossil fuel in thermal energy production of all the renewable energy sources, therefore, biomass is also used for district heating in the cold climate regions [7,8]. The typical heating plant sizes vary from small and medium to large (0.1–50 MW) scale depending on the extent of a district heating network [9]. It is common to use grate-fired furnaces for heat production in small and medium scale heating as these systems are designed for a high thermal efficiency with low emissions of gaseous pollutants (CO, NOx, etc.) when firing good quality biomass with a 30–45% moisture
⁎
content [10–13]. However, the expanding utilisation of renewable energy sources for heat production leads to an increased demand and the price for biomass. High demand of these forest fuels during the winter causes an imbalance in the market even though wood chips are harvested and produced continuously throughout the year [14]. Consequently, moist biofuel of lower quality such as stem wood, coniferous bark, pine branches with varying moisture content that can reach up to 60% wt. or more are used for heat production [15]. Combustion on the grate of so moist fuel becomes complicated as the drying process occupies most of the space in the furnace and the combustion heat from the lower layer is consumed for drying up the upper fuel layer [16]. Moreover, moisture can cause the burning instability and incomplete combustion, which leads to increased emissions including those of CO2. Besides, the lower heating value (LVH) of moist fuel is only 6 MJ/kg, while the LVH of biofuel dried to the moisture content of 30% wt. is approximately twice as high – 12 MJ/kg [17]. In most cases, solid biofuel is additionally dried before supplying to the furnace to avoid the mentioned problems. Drying techniques include a superheated steam dryer [15], a rotary dryer [18,19], a bed dryer or a fluidized bed dryer [20,21]. In case of the superheated steam
Corresponding author. E-mail address: [email protected] (R. Paulauskas).
https://doi.org/10.1016/j.fuel.2020.117013 Received 28 June 2019; Received in revised form 5 December 2019; Accepted 2 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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increases from 70 to 90 °C. The authors determined that the optimal temperature of the drying air should be over 70 °C, but the process was not analysed at higher temperatures. Similar research at higher drying temperatures was performed in work [27]. The authors analysed the drying process of a thin layer of ground pine chips (3.2–25.4 mm) and pellets (3.2–12.7 mm) with moisture content of 50% at drying temperatures of 50, 100, 150 and 200 °C. Particles of 3.2 mm size were fully dried in time periods ranging from 31 to 3 min at drying temperatures ranging from 50 to 200 °C, respectively. Increasing the particle size to 25.4 mm led to increased drying time by 90% to 117% at drying temperatures from 50 to 200 °C, respectively. Besides, at high drying temperature (200 °C) the particles lost 3–7% of dry mass due to volatile release. Even though the biomass drying was widely analysed in reviewed works [21–27], there is no sufficient knowledge about the wet solid biofuel drying process in a furnace and the parameters influencing drying of biomass of this type. In order to expand the knowledge in this field, a special rig was constructed to simulate the characteristics of a 6 MW reciprocating grate furnace and drying experiments were performed, where preheated air simulated the primary air flow in the furnace, preheated air and steam mixture simulated the recirculation of the flue gases from the furnace, and infrared lamps imitated radiation of the furnace surfaces. For drying experiments, wood chips, urban park waste and forest harvest waste were used and drying differences between these samples were established. In additional experiments, biofuel was mixed by a screw mixer to simulate the grate movement in order to analyse the mixing effect on the intensification of biomass drying.
dryer, the wet biofuel is mixed with sufficient amount of superheated steam to fully dry out the material and produce saturated steam [22]. Even though this system has a high energy efficiency, low level of fire risk and better process control compared to other systems, there are some disadvantages as well. This dryer is not suitable for wood chips, it requires high investment costs and there are wastewater treatment issues [15,21,22]. The packed moving bed, rotary and pneumatic drying systems are simpler than a superheated steam drying system because the drying agent is heated air and the construction of the system is suitable for drying wood chips [21]. Still, there are some drawbacks: the packed moving bed dryer occupies a large area in the heating plant, while the rotary dryer operates at the drying temperature range from 200 to 500 °C resulting in a higher risk of fire [21]. Moreover, Danielsson and Rasmuson [23] investigated the release of volatile organic components during the drying process. The results showed that energy is partially lost when biofuel is being dried with air at the temperature exceeding 100 °C because of evaporation of monoterpenes. The reviewed works have shown that the drying setup must be selected precisely taking into account the energy efficiency, risk of fire and drying emissions in order to reach the desired moisture content of supplied biofuel. Taking into account the advantages and drawbacks, the additional drying systems for use exclusively for small and medium scale heating plants have limited applicability for drying wood chips of varying moisture content,. A more favourable solution for heating plants of this type is to intensify biomass drying on the grates in a furnace during combustion, which requires knowledge of the parameters affecting this process. Few works have focused on the effect of drying air amount and drying agent temperature on biomass drying. Yang et al. [24] have investigated the effect of primary air flow for intensification of biomass drying during combustion. The results showed that the increase of the air flow leads to increase of moisture evaporation rate until the former reaches a critical point at which the latter starts to drop. This effect was related to the heat transfer into the evaporation zone as low air flow enhances the transfer of the radiation energy to evaporation process, whereas a high air flow rate increases only the front flame temperature and heat is carried away. The effect of the drying agent temperature and the biomass particle fraction on biomass drying was studied by Holmberg et al. [25]. For the experiments, birch bark was dried at different temperatures (50, 70, 90 and 110 °C) with different bed heights (50, 150 and 250 mm). The authors observed that simultaneous evaporation and condensation occur at high drying temperature (110 °C) and the drying rate in the beginning of the process (?) is lower than that at the 90 °C drying temperature. This happens because water evaporates from the bottom layers due to high air temperature and condenses on the upper layers. This effect was not observed at low drying temperatures. Myllyma et al. [26] investigated the drying time dependency on the drying temperature by conducting biomass drying experiments in a fixed-bed batch-dryer. The results revealed that the drying time for the bed height of 300 mm decreases in half when the drying air temperature increases from 50 to 70 °C, whereas the drying time changes only slightly when the temperature
2. Materials and methods 2.1. Biomass characteristics Small and medium scale heating plants usually utilise waste biofuel of various origins (waste materials from wood harvesting, wood processing and urban cleaning), therefore, particle size and moisture content can differ noticeably. These characteristic biomass parameters influence both the drying and combustion processes. In order to account for this fact, three different types of biofuels such as wood chips of various species of tree branches with bark (WB), chips of broad-leaved tree branches with leaves (WL), and chips of conifer branches with needles (WN) (Fig. 1), were selected for the stationary drying experiments. The ultimate and proximate analyses of biomass were determined using an IKA C5000 calorimeter and a Flash 2000 CHNS analyser in accordance with: LST EN 14774-1 (moisture content), LST EN 14918 (HHV), LST EN 14775 (ash content), LST EN 15148 (volatile content) and LST EN 15104 (CHNS content). The obtained characteristics are presented in Table 1. The ultimate and proximate analysis of different biofuel mixtures (Table 1) have shown that the analysed biofuel is similar in its
Fig. 1. Photo of used solid biofuels. 2
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thermocouples installed upstream and downstream the electrical heaters. The effect of hot recirculation flue gases on biomass drying was studied using air-steam mixture. The absolute humidity of the drying agent was increased from 5 to 17 g/m3 by injecting steam into the preheated air stream while maintaining the constant flow rate. As in the previous case, the preheated air-steam flow rate was 17.7 ± 0.5 m3/h. In addition, to simulate the thermal radiation from the incandescent surfaces of the furnace, two infrared lamps were installed above the solid biofuel bed and drying air with constant flow rate (17.7 ± 0.5 m3/h) was supplied from the bottom of the drying chamber. The infrared lamps (2 kW each with an efficiency of 85%) corresponded to the furnace radiation of 50 kW/m2 [11]. The effect of fuel mixing on enhancing the wood chips drying rate was additionally analysed. For this purpose, a special screw mixer, which imitates the movement of the reciprocating grates in the 6 MW furnace, was installed in the drying chamber (Fig. 3). Using the screw mixer, the bottom layer of the biofuel was lifted upwards and the top layer was moved downwards. Such movements were repeated every 150, 300 and 600 s. The procedure of experiments was identical in all the cases. Before performing the drying experiments, solid biofuel was loaded to a vessel filled with water and left to soak in the vessel for 16 h. After that the biofuel was lifted out and left to drain for 2 h in the ambient temperature, in order to obtain approx. 60% wt. moisture content of the biomass. Meanwhile, the drying chamber was preheated to the appropriate temperature by supplying drying air. When the drying chamber reached the desired temperature, the steel crate with length of 0.335 m, width of 0.235 m and height of 0.28 m was filled with the prepared biofuel sample. The bed height was 0.23 m, which corresponds to the height of the solid biofuel layer on the grates of the 6 MW furnace. The thermocouples were embedded at various heights (T1, T2, T3 and T4 respectively at 3, 10, 12 and 22 cm from the bottom) of the bed to determine the drying zone progression through the bed during the drying process (Fig. 3). All thermocouples were connected to a data logger PICO TC-08 for data collection. Humidity of exhaust air from drying chamber was measured by the Testo 454 analyser equipped with a relative humidity (RH) sensor. Typically, the total burnout of wet solid biofuel in the reciprocating grate furnace is completed in 30 min. The drying zone occupies 1/3 of the furnace length, therefore, the actual time spent in this zone is approximately 10 min. According to above mentioned conditions, the experimental duration was set for 30 min.
Table 1 The characteristics of biomass samples. Parameter
WB
WL
WN
Standard deviation
Ultimate analysis wt% Carbon Hydrogen Oxygen (diff.) Nitrogen Sulphur
48.6 5.80 41.49 0.60 < 0.01
49.13 5.97 39.87 0.52 < 0.01
48.53 5.39 38.47 0.79 < 0.01
1.17 0.49 0.15 0.04 –
36 3.50 18,998
33 4.50 18,630
14 7.60 19,785
0.10 0.05 52.30
Proximate analysis wt% Moisture Ash Higher heating value (HHV) kJ/kg
elemental composition, however, different moisture and ash contents indicate different nature of biofuel. Also, to estimate the influence of the particle size to the drying process, the granulometry of dry biomass samples was determined experimentally using sieves with mesh sizes ranging from 1 to 10 mm. The average particle sizes of three samples were determined from the obtained particle size distributions (Fig. 2) using the method described in [28].The characteristic particle length of the sample WB is 13 mm, that of WL – 9 mm, and that for WN is 7 mm. The major part (35–53%) of biofuel samples used in the experiments consisted of particles larger than 10 mm in length. In this case, WN consisted of the smallest particles of the sample. A removal of moisture from the fine particles is assumed to be more intense. To validate this assumption, a few different samples of biomass were selected.
2.2. Experimental methodology The experimental setup of solid biofuel drying used in this study is shown in Fig. 3. The rig consists of four main components: a drying chamber, an air supply system equipped with electrical heaters, a steam generator, and two infrared lamps. The rig parameters such as dimensions of the drying chamber, the primary air flow, the amount of steam and the power of heat radiation were selected according to the real working parameters of a 6 MW reciprocating grate furnace. A more detailed information on the mentioned furnace is provided in the previous work [11]. The rig was adapted to analyse the influence and effects of preheated primary air, hot recirculation flue gases and the furnace radiation on biomass drying. Three separate drying methods were analysed. In the case of drying by preheated primary air, the drying agent was supplied from the compressed air system through two heating coils used to preheat air to the desired temperatures of 50, 100, 150 and 200 °C, while the flow rate was kept constant at 17.7 ± 0.5 m3/h. The temperature of hot air was measured and controlled by means of the K-type
2.3. Data analysis The main parameters characterising fuel drying were determined by the indirect method, i.e., by estimating the solid biofuel moisture content from the exhaust products. More detailed information about the
Fig. 2. Characteristic particle length by mass of different wood chips samples. 3
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Fig. 3. Scheme of experimental rig for biomass drying.
the sample mass at the previous time moment:
indirect determination of the moisture content is presented in the previous work [11]. From the absolute humidity of exhaust products as measured by the Testo 454 and its temperature, the changes in moisture content in exhaust flow was calculated at the appropriate time moments, which determines the biofuel water loss rate g H2 O by the following equation:
g H2 O = hg ∙
Vair + VH2 O(g ) 3600
(
273 + Tg 273 + Tamb
);
m H2 O (ti) = g H2 O (ti ) ∙Δt
where Δt = ti − ti − 1 is the sampling interval of the absolute humidity in the exhaust flow. Taking into account that the processes during biofuel drying are long lasting and not very intense, the data collection interval was selected to be 5 s. When normalising the moisture content in the biofuel samples, the maximum ratio of the biofuel samples is assigned the value of 1 and is calculated from Eq. (5):
(1)where Vair is the drying air
supply rate in m3/h; VH2 O(g ) – water flow rate in gaseous states, m3/h; hg – the absolute humidity of the exhaust products g/m3; Tg – the temperature of the exhaust products in °C; Tamb – the ambient temperature °C. The flow rate of water in gaseous states are calculated by:
VH2 O(g ) = VH2 O(fuel) + VH2 O(rec . pr )
Ym = 1 − (ΔY(max ) − ΔY(min) )
3. Results and discussion
where VH2 O(fuel) is the water flow rate from fuel in m /h; VH2 O(rec . pr .) – the water flow rate, in m3/h, resulting from injecting steam at the rate of 17 g/m3 into the preheated air stream, imitating the recirculation flue gases.. The initial sample mass was different in all the experiments, to keep the height of the biofuel bed the same. The initial moisture content in all the samples varied as well (60 ± 3%), therefore, the moisture content in fuel was normalised to the largest (maximal) ΔY(max ) and smallest (minimal) ΔY(min) value of the ratio between the moisture content and the dry sample mass throughout the entire 30 min duration of the experiment:
= ( max min )
3.1. Effect of preheated primary air, flue gas recirculation and thermal radiation to the solid biofuel drying The first set of experiments was conducted to evaluate the influence of preheated primary air, air-steam mixture and air coupled with thermal radiation on the wet WB drying. In order to understand the ongoing processes, temperatures at four locations in the biofuel bed (Fig. 2) were recorded and the obtained temperature profiles are presented in Fig. 4. Moreover, the water loss rate was also estimated in order to predict the biofuel drying trends. The normalised moisture content (Ym) and the water loss rate (Fig. 5) were calculated from the moisture content of the exhaust gas using Eqs. (1)–(6).
m (ti) − mdry mdry
(3)
here, mdry is the dry sample mass g; m (ti) is the sample mass g at the time moment ti. The sample mass g at the time moment ti is calculated by subtracting the water mass in the exhaust products m H2 O (ti) at the same time moment from the sample mass at the previous time moment:
m (ti) = m (ti − 1) − m H2 O (ti)
(6)
(2) 3
ΔY
(5)
3.1.1. Preheated primary air The curves of the temperature changes in different layers of the WB bed using the preheated air of 50 °C indicated that the heat transfer is insignificant (Fig. 4A). Even after 30 min of drying, the highest temperature in the entire biofuel bed was only approximately 25 °C (Fig. 4 A1) resulting in the constant water loss rate of 0.09 g/s for the remaining time of the experiment (Fig. 5A2). When the temperature of the drying agent was increased to 100 °C, the bottom of the biofuel bed (T1) reached only 38 °C per 30 min (Fig. 4A2). Meanwhile, the middle
(4)
where m H2 O (ti) is the water mass in the exhaust products g, m (ti= 0) is the weighted mass of the sample at the beginning of the experiment. The water mass in the exhaust products m H2 O (ti) is calculated using 4
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Fig. 4. Temperature profiles at different layers of WB bed varying the temperature of drying agent.
content in 30 min (Fig. 5A1). The most intense drying process using preheated air was observed at the drying agent temperature of 200 °C (Fig. 4A4). Due to the fast heat-up of the bed, the water loss rate (Fig. 5A2) stabilizes after approximately 18 min from the start of the experiment indicating a shift to uniform drying. In the end of the experiment, the normalized moisture content in the sample was 0.87 (Fig. 5A1). The highest temperature of biofuel was exposed using the preheated air of 200 °C and approximately 66 °C was reached in the layer T1 after 10 min and T1 = 127 °C after 30 min (Fig. 4A4). The similar values of moisture content were established using the air-steam mixture as the drying agent, but the trend of the water loss rate differed (Fig. 5B). From the start of the experiment, a stagnation period (non-drying) was observed and duration of it depended on the
of the bed (T3) starts to heat up after 7 min and from this moment the water loss rate slowly increases till 0.18 g/s (Fig. 5A2). The trend of the bed heat up and water loss rate possibly indicates that water vapour condensed in the middle layer starts to evaporate from the lower layer, but a sluggish increase of water loss rate reveals that the condensation on the bed top is still going on. More intense drying of the bed bottom (T1) and heating of the middle (T3) were observed using drying agent of 150 °C (Fig. 4A3). In this case, the temperature in the middle of the bed (T3) and the water loss rate are increasing after 5 min from the beginning of the experiment (see Figs. 4A3 and 5A2). The water loss rate becomes constant (0.28 g/s) when the temperature of the bed top (T4) stabilizes and the sample dries by approximately 5% from the initial sample moisture 5
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Fig. 5. Normalized moisture content and water loss rate of wet WB by exposing with the preheated air (A1, A2), air-steam mixture (B1, B2) and air coupled with thermal radiation (C1, C2).
(Fig. 4B3,4). Further it was followed by intense fuel drying (Fig. 5B). The water loss rate of the bed stabilised at the value of 0.55 g/s after 15 min using the air-steam mixture of 200 °C (Fig. 5B2).
drying agent temperature. It was assumed that the drying agent cools down below the dew point temperature as it flows around wet and cold solid biofuel and steam condenses on colder surfaces upwards in the bed. When the biofuel is heated up enough, the temperature in the middle of the bed (T3) starts to increase and the evaporation process intensifies (Fig. 4B).
3.1.3. Radiation from the hot furnace surfaces The most significant influence on biomass water loss rate was determined using the air coupled with thermal radiation (Fig. 5C). The moisture loss of the WB was observed instantly from the first seconds of experiments at all the drying agent temperatures. According to Fig. 5, it is assumed that evaporation from the bed bottom and condensation on the upper layers of the bed is going faster due to the top layer dried out by the infrared lamps and the further condensation on the top is avoided. In this way, the evaporation process surpass the condensation process resulting in continuously increasing water loss rate of the bed (see Fig. 4C) even though the temperature changes in the bottom and the middle of the bed did not show influence of thermal radiation to solid biofuel drying and are lower than those observed when using the previously described methods of drying (Fig. 5). When supplying drying air of 50 °C temperature, the normalized moisture content in wood
3.1.2. The flue gas recirculation products In case of drying with air-steam mixture of 50 °C, it was observed that the stagnation period lasted for about 12 min and after that the temperature of bed middle (T3) started to increase (Fig. 4 B1). It also correlated with the increase of the water loss rate but the moisture content in biofuel (WB) was still high (0.99 of the initial value) after 30 min (Fig. 5B). Using the air-steam mixture with higher temperature (150 °C), more intense drying was determined. The water loss rate increased to 0.42 g/s within 30 min while the moisture content decreased to 0.92. It was noticed that using the air-steam mixture of 150–200 °C for drying of WB, the stagnation period was reduced by half and lasted until the middle of the bed started to overheat (T4 starts to increase) 6
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Table 2 Water loss rate at different drying case. Drying agent temp °C
Preheated air
Air-steam mixture
After 10 min
After 30 min
Air coupled with thermal radiation
After 10 min
After 30 min
After 10 min
After 30 min
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
Water loss rate g/s
T3 °C
WB 50 100 150 200
0.09 0.12 0.15 0.27
16.10 21.20 30.74 41.82
0.09 0.19 0.28 0.42
18.9 31.05 38.27 41.8
0.12 0.16 0.22 0.35
18.53 27.73 38.09 46.42
0.18 0.29 0.43 0.56
29.65 37.24 43.3 46.8
0.26 0.29 0.31 0.44
19.20 31.30 34.02 42.70
0.53 0.63 0.69 1.11
19.62 31.61 38.96 42.88
WL 50 100 150 200
0.11 0.16 0.24 0.32
21.14 28.34 35.89 39.61
0.11 0.22 0.31 0.43
23.05 32.15 37.43 41.11
0.13 0.18 0.26 0.33
22.47 29.33 35.98 36.56
0.18 0.29 0.43 0.58
30.5 37.98 43.15 47.52
0.24 0.38 0.44 0.56
19.73 25.59 37.15 41.88
1.01 1.03 1.14 1.25
20.05 31.78 37.83 42.99
WN 50 100 150 200
0.09 0.15 0.23 0.36
16.72 28.27 32.26 40.15
0.09 0.21 0.28 0.44
19.12 29.16 38.4 39.66
0.12 0.18 0.36 0.42
22.53 36.62 40.44 47.32
0.18 0.31 0.42 0.55
30.29 38.61 43.4 47.56
0.33 0.42 0.44 0.43
20.79 28.96 38.65 38.95
1.06 1.27 1.25 1.35
21.2 31.36 38.77 43.79
results of this analysis demonstrate the necessity to apply the means for the process intensification.
chips was reduced to 0.84 after 30 min. The measured value was higher by 0.1 point than that in the case of drying methods without the thermal radiation (Fig. 4). In case of supplying the drying agent with the temperature of 100 °C and 150 °C, the moisture content in the bed was decreased by 0.19 and 0.21 after 30 min, respectively. The most intensive drying, at which the water loss rate increased to 1.12 g/s, was achieved at the drying agent temperature of 200 °C. In the end of the experiment, the normalized moisture content in the WB was determined to be 0.67 (decreased by 20% from the initial sample moisture content) (Fig. 4C). Considering the temperature changes (Fig. 4), the highest drying intensity in all the cases was recorded in the bottom of the bed (T1, T2) meanwhile the stagnation period was observed in the middle and the top of the bed (T3, T4). It can be assumed that the primary flow is saturated with moisture as it flows through wet biofuel. The moisture evaporated from the sample is transferred to higher layers of the bed. In this case, the condensation takes place as the temperature drops below the dew point. In order to clearly define the tendency of water loss and how the temperature changes in the middle layer T3 in different cases, the results were summarised in Table 2. It can be noticed that preheated air enrichment by steam in the case of drying with air-steam mixture led to the increased water loss rate thereby enhancing the condensation process. Increase of the T3 (Table 2) might indicates that the heat released during the condensation process was consumed heating the biomass bed (Fig. 4A,B). Besides, the drying agent containing more moisture (preheated air versus air-steam mixture) has a higher heat transfer coefficient approximately 20 times, which also influences faster overall heating of the bed as shown by T3 (Table 2). According to the obtained results, it could be assumed that supplying wet recirculation products (air-steam mixture) to the furnace increase the water loss rate, though the drying rate (Fig. 5B1) does not become more intense compared to the case when preheated air is used (Fig. 5A1). The most promising results were obtained (Fig. 5C) in the case of biomass drying using the air coupled with thermal radiation (Table 2). However, according to the temperature readings, it can be considered that T3 does not reach the level of intense drying which is necessary to attain the moisture content of biofuel in the drying zone (approximately 30%). It was also assumed that the drying processes are too slow that can create unfavorable conditions for fuel combustion as the drying zone occupies the largest part of the grate. Deviations from the desired furnace operation regimes lead to an inefficient energy production in the heating plant which does not meet the operation requirements. Therefore, the
3.2. Influence of biomass types on drying rate Identical sets of experiments were conducted with WL and WN as well to analyse the influence of biomass type and the fraction size on drying. The analysis reveals a near identical drying behaviour of WL and WN as in the case of WB (Table 2). In the previous case (see Section 3.1), the most promising results on drying were achieved at the drying agent temperature of 200 °C. For this reason, this temperature of drying agent was selected for further analysis in this work (Fig. 6). Comparing the results of drying involving preheated air and airsteam mixture (Fig. 6), it is seen that the drying intensity was rather stable. At the selected time moments, the drying rates varied insignificantly and independently from the used biomass type (Fig. 6). As in the case of the WB, the most intense drying of WL and WN was also observed using the air coupled with thermal radiation. In these cases, the drying rate increases with a decrease of the particle size fraction in the samples. It was most clearly noticed after 30 min since the start of the drying experiments, especially for WN. The drying rate for the WN was approximately 1.2 times higher than that for the WL (the WN contains the particle fraction that is 1.4 times smaller than that of the WL (Fig. 2)). Besides, an identical trend was observed comparing the results of the WB and WL as well. In order to gain a better understanding of the influence of the biomass type on drying with the presence of the simulated thermal radiation from the hot furnace surfaces, the moisture content and water loss rates of the WL and WN at different drying temperatures are presented in Fig. 7. It can be seen from Fig. 7, that WL and WN start to dry at once in the beginning of the experiment and the water loss rate increases linearly regardless of the drying agent temperature. Moreover, the trend of the water loss rate accentuates the differences arising from the differences in the solid biofuel fraction. During the drying process in low temperature (50 °C), the water loss rate from the WL and WN is higher by the factors of 1.01 g/s and 1.06 g/s, respectively, compared to the case of drying the WB (0.53 g/s). When drying both fuel types at 100 – 150 °C, it was determined that the normalized moisture contents of WL and WN were 0.60–0.58 and 0.48–0.46 of the initial values, respectively, after 30 min. It has also been determined that the changes of the moisture content in the WB were identical and the normalized 7
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Fig. 6. The influence of solid biofuel type on drying rate.
which the average particle size is 9 mm, is slower by 7% on average than in the case of drying WN with the particle size of 7 mm (Fig. 2). According to the obtained results, it is considered that more intense heat transfer processes take place due to smaller particles (higher area of the surface), which influence the solid biofuel drying intensity.
moisture content differs by only 0.01 (Fig. 7A1). The same effect was observed in the case of drying WB as well (see Fig. 4C). This possibly indicates that the drying process proceeds mainly due to the thermal radiation and the moisture evaporation process depends insignificantly on the temperature of air supplied from below in the temperature range of 100–150 °C. Increasing the drying agent temperature to 200 °C results in more intensive drying and the normalized moisture content is 0.10–0.13 lower than that at the drying agent temperature of 100–150 °C in both cases (Fig. 7A, B). The estimated values of normalized moisture content show that the water loss rate (Table 2, Fig. 7) depends on the particle fraction in bed as the drying process of WL, in
3.3. Influence of mixing on the solid biofuel drying The performed experiments of drying solid biofuels revealed the influence of preheated air, air-steam mixture and air coupled with thermal radiation on drying. The most intense biofuel drying was
1.0
1.4 A1
Ym
0.8 0.7 0.6 0.5 0.4
1.0 0.8 0.6 0.4 0.2 0.0
0
5
10 15 20 Time min
25
30
0
5
10
15 20 Time min
25
30
10 15 20 Time min
25
30
1.4
1 B1
B2
1.2 Water loss rate g/s
0.9 0.8 0.7 Ym
A2
1.2 Water loss rate g/s
0.9
0.6 0.5 0.4
1 0.8 0.6 0.4 0.2 0
0.3 0
5
10
15 20 Time min
25
0
30
5
Fig. 7. Normalized moisture content and water loss rate of WL (A1, A2) and WN (B1, B2) at different drying temperatures in the case of air coupled with thermal radiation. 8
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0.3 0.25
T600 After 10 min T300 T150
T600 T300 T150
T600 T300 T150
After 20 min
After 30 min
Fig. 8. Fuel mixing influence on ΔYm at different drying temperatures.
0.2 0.15 0.1 0.05 0 50 100 150 200 Drying agent temperature °C
50 100 150 200 Drying agent temperature °C
50 100 150 200 Drying agent temperature °C
4. Conclusions
identified in the presence of the simulated thermal radiation from the furnace surfaces. But to increase the drying and combustion intensity, additionally reciprocating grates are used in furnaces. To gain data for this particular case, an additional set of experiments for drying the WB was performed. In order to simulate a grate motion in the furnace, fuel was mixed additionally. Three cases, when the solid biofuel bed was mixed every 600 s, 300 s and 150 s, were analysed. The obtained results were compared to those without mixing and the determined change in biofuel normalized moisture content depending on the drying agent temperature and the mixing period are presented in Fig. 8. It has been established that during 10 min of drying, the bed mixing frequency has only negligible influence to the drying intensity and even at the drying agent temperature of 200 °C, the normalized moisture content decreases by 0.02 when mixing every 150 s (Fig. 8). Further results revealed that at low drying agent temperature (50 °C), the normalized moisture content of WB is reduced by about 0.05 after 20 min and by 0.1 after 30 min regardless of the mixing period. Analysis of the mixing results after 20 min revealed that the mixing of wet solid biofuel intensifies the drying process. The drying intensity increases when the drying agent of higher temperature (100–150 °C) is supplied from below, but the drying rate depends insignificantly (by up to 2%) on the mixing intensity (Fig. 8). After increasing the drying agent temperature to 200 °C, the effect of mixing to the biofuel drying intensity becomes lower compared to the cases when colder drying agent is used (at 100 and 150 °C). This effect becomes more pronounced when analysing the drying results after 30 min (Fig. 8). The initial experiments have shown that high temperatures of the drying agent (200 °C) lead to the most intense drying of solid biofuel in the entire bed, therefore, additional mixing has lesser effect on the bed drying compared to other cases (100, 150 °C). However, in the case of the drying agent temperature of 200 °C, the influence of the mixing period (150, 300 and 600 s) is significant. The change of the normalized moisture content increases linearly with the increasing mixing period. From Fig. 8 could be pointed out that the influence of thermal radiation to the drying rate is significant when the drying agent temperature is high (200 °C). It can be assumed that in the presence of mixing, the drying process is influenced by both convection and thermal radiation in the entire volume, therefore, solid biofuel dries out most intensely. The efficiency of mixing depending on the drying agent temperature is revealed by the curves in Fig. 8. It has been determined that the influence of mixing is highest for the drying agent temperature of 150 °C, and drying is intensified by the largest factor when the mixing period is 150 s. This is evident by the change in the residual normalized moisture content, which amounts to 0.24 compared to the results in the absence of mixing (Fig. 8).
In this work, the parameters influencing the drying intensity of wet solid biofuel during combustion in grate furnaces were evaluated by performing the drying experiments with the simulated primary air flow in the furnace, simulated recirculation of the flue gases and the simulated primary air flow with simultaneous exposure to radiation from the hot furnace surfaces. For the drying experiments, wood chips, urban park waste and forest harvest waste were used and the differences in drying intensities between these samples were established. As a result, the following conclusions were made: 1. The results revealed that during the drying process of wet solid biofuel with supply of the primary air or air-steam mixture in the simulated furnace conditions, the initial moisture release from the bed stagnates. It is assumed that moisture from the lower layers is transferred to the middle of the bed and condenses on cold surfaces. Applying higher temperature (150–200 °C) of the drying agent, the condensation rate decreases. Taking into account the obtained results, it is recommended to avoid recirculation of flue gas with the temperature of 50–100 °C to the drying zone of the furnace when firing biofuel with the moisture content around 60%. 2. In the case of air coupled with thermal radiation at temperature of 200 °C, the highest decrease in the moisture content during drying was determined for WN. After 10 min, the normalized moisture content decreased from 1 to 0.93 i.e., only by 4% from the initial moisture content, and after 30 min – from 1 to 0.35, i.e., from 60% of initial sample moisture content till 20%. These results demonstrate that biofuel will not dry out per 10 min to the desired requirements of the furnace, therefore, in order to reach and maintain the proper dryness of biofuel within 10 min, more efficient methods/techniques must be applied. 3. Among of the studied intensification methods (preheated primary air, air–steam mixture, air coupled with thermal radiation and mixing), the mixing of solid biofuel affected the drying process positively and the moisture content decreased linearly with the increasing mixing intensity. In the case of preheated air with temperature 150 °C, the drying intensity increased by up to 15% due to biofuel mixing with period of 150 s. The moisture content in the WB decreased (from 1 to 0.79 relative units) by approximately 13% from the initial WB moisture content while without mixing, the moisture content decreased (from 1 to 0.54) by approximately 28% from the initial WB moisture content. 4. In order to attain the moisture content of solid biofuel in the drying zone of the furnace (approximately 30%) resulting in a sustained flame propagation in the fuel, the furnace control must be optimised. This could be achieved by applying combined methods, such as recirculation of hot (˃200 °C) combustion products from the
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furnace combustion zone to the drying zone; a creation of the thermal radiation wall; or, blocking the products of recirculation with a high moisture content from entering to the drying zone.
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CRediT authorship contribution statement L. Vorotinskienė: Conceptualization, Data curation, Investigation, Methodology, Validation, Writing - original draft, Writing - review & editing, Formal analysis. R. Paulauskas: Writing - original draft, Writing - review & editing, Visualization. K. Zakarauskas: Methodology, Investigation, Validation. R. Navakas: Software, Writing - review & editing. R. Skvorčinskienė: Visualization, Writing - original draft, Writing - review & editing. N. Striūgas: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was conducted within the framework of COST Action CM1404328 “Chemistry of Smart Energy Carriers and Technologies (SMARTCATS)”. References [1] Jiang X, Guan D. Determinants of global CO2 emissions growth. Appl Energy 2016;184:1132–41. https://doi.org/10.1016/j.apenergy.2016.06.142. [2] Höök M, Tang X. Depletion of fossil fuels and anthropogenic climate change—a review. Energy Policy 2013;52:797–809. https://doi.org/10.1016/j.enpol.2012.10. 046. [3] Benoist A, Dron D, Zoughaib A. Origins of the debate on the life-cycle greenhouse gas emissions and energy consumption of first-generation biofuels – a sensitivity analysis approach. Biomass Bioenergy 2012;40:133–42. https://doi.org/10.1016/j. biombioe.2012.02.011. [4] Dhillon RS, von Wuehlisch G. Mitigation of global warming through renewable biomass. Biomass Bioenergy 2013;48:75–89. https://doi.org/10.1016/j.biombioe. 2012.11.005. [5] Anselmo Filho P, Badr O. Biomass resources for energy in North-Eastern Brazil. Appl Energy 2004;77:51–67. https://doi.org/10.1016/S0306-2619(03)00095-3. [6] European Commission. 2030 Climate and Energy Policy Framework 2014;2014:1–6. [7] Rezaie B, Rosen MA. District heating and cooling: review of technology and potential enhancements. Appl Energy 2012;93:2–10. https://doi.org/10.1016/j. apenergy.2011.04.020. [8] Hendricks AM, Wagner JE, Volk TA, Newman DH, Brown TR. A cost-effective evaluation of biomass district heating in rural communities. Appl Energy
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