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EFFECT OF PARTICLE SIZE ON THE BURNOUT AND EMISSIONS OF PARTICULATE MATTER FROM BIOMASS COMBUSTION IN A DROP TUBE FURNACE

Vera Sofia Branco Lopes (vera.branco@tecnico.ulisboa.pt)

Department of Mechanical Engineering, Instituto Superior Técnico, University of Lisbon

ABSTRACT

Milling and grinding biomass fuels for pulverized combustion in industrial furnaces can be very expensive. This study aims to evaluate the influence of the particle size on the burnout and emissions of particulate matter from biomass (wheat straw and rice husk) combustion in a drop tube furnace. To this end, three narrow size classes were established for each biomass fuel; specifically, 100-200 µm, 400-600 µm e 800-1000 µm. Subsequently, all biomass fuels, including the original biomass particle size distributions, were burnt in a drop tube furnace at 1100 ºC. The results reported include profiles of temperature, burnout and particulate matter (PM) concentration and size distribution measured along the DTF. In addition, selected PM samples for all biomass fuels were examined in a scanning electron microscope. The main conclusions from this study are: i) PM emissions are higher for wheat straw than for rice husk, being the secondary particle fragmentation more evident on small to intermediate particle size classes (100-200 µm and 400-600 µm), ii) Ca and P tend to be retained in larger particles, while K and Cl present higher concentration in the fine PM, and iii) from combustion efficiency and PM emissions point of view there is no benefit to separate pulverized fuels in narrow particle size classes.

KEYWORDS

Drop tube furnace, biomass, burnout, particle size, particle fragmentation

INTRODUCTION

During combustion, several phenomena may occur that contribute to the release of particles of different sizes. Particles can undergo aggregation by coagulation, condensation and particle nucleation, which are mainly responsible by the formation of new inorganic particles or by the increase of the size of a particle. However, particle elimination phenomena, caused by deposit formation on the furnace walls, and phenomena of size reduction, by evaporation or particle fragmentation, may also occur [1]. In general, coarse particles are generated by char fragments and fine particles by soot formation and the vaporization of inorganic matter that consequently can grow by mechanisms of agglomeration, nucleation and condensation. The phenomena regarding reduction of particle size by char fragmentation are of particular importance because during the combustion process a particle can generate two or more ultrafine particles that can contribute in a large scale to the ultrafine PM emissions, highly harmful to the environment and therefore to the human health. Due to its reduced size, the capture of these PM represents a challenge with conventional technology available for flue gas cleaning. PM emissions could be defined with several criteria. The aerodynamic diameter is one of the main criterion to describe its transport ability in the atmosphere and its ability to be inhaled by the respiratory system of an organism. The most accepted particle size classification is the one that divide PM in to two categories: ultrafine particles, with particle aerodynamic diameter lower than 2.5 µm (PM2.5) and the coarse particles with particle aerodynamic diameter higher than 10 µm (PM10) [2]. Although there are only few studies focused on the processes and the governing factors that promote particle fragmentation in biomass combustion, a general definition derived from studies on coal combustion and fragmentation can be applied to define the process [3]. Primary fragmentation occurs during heating up and devolatilization stages when the particles are fed into the furnace, as a consequence of internal overpressures associated with volatile matter release and possibly, thermal shock. Secondary fragmentation refers to char fragmentation as combustion weakens the bridges within the char particles. As secondary fragmentation is closely related to combustion, the combustion progress affects the fragmentation extent. In particular, carbon burnout is usually used as an indicator of the combustion degree [4]. Biomass solid fuels contain considerable amounts of ash forming inorganic elements responsible for ash production during the combustion process that are released in the flue gas. Potassium, sulphur and

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chlorine are the most relevant elements during the combustion of solid biomass fuels, whereas sodium and some easily volatile heavy metals like zinc provide minor contributions. The behavior of ash forming species is strongly dependent on the type of fuel, especially with the ash composition, combustion technologies and combustion conditions. In studies performed in fluidized bed combustion, the composition and presence of these elements varies within the particle size. The fine particles are composed mainly of K, Cl, S, Na and Ca, while the coarse particles have Ca, Si, Na, Al, P and Fe in its composition [5]. In fixed bed combustion conditions, a dependency of the particle composition on size can also be found. K, S and Cl are mainly found in submicron fraction of the PM, while the content of Ca is increasing with the increase on particle size. In general, the release of these elements are made in different stages and steps and have different impacts on the performance of the combustion system [6]. The presence of potassium at the emitted PM is highly influenced not only by the temperature but also by the presence of certain elements on ash composition such as silicon. If the ashes of the original biomass fuel contain considerable amounts of silicon, potassium can integrate the silicon matrix, making its vaporization difficult. The presence of chlorine can also leads to high release of potassium due to the formation of KCl [7]. Chlorine can react with metals such as K and Na, forming vapors and aerosols during the cooling process, leading to deposit formation on furnace walls [8]. The study of Steer et al. [9] focused his study on the effects of particle grinding on the burnout and surface chemistry during coal combustion, suggesting that the process of grinding alters the physical properties of the samples, so that in some cases, the larger size classification give improved combustion burnout profiles when compared with smaller sizes. One of most relevant evidences in this work is related with the higher burnout value obtained to many of the larger particle size classifications, suggesting that the larger sizes were more reactive than the smaller ones or that additional grinding was detrimental to the burnout of the smaller sizes. In this study, the effect of particle grinding on particle swelling and fragmentation were also evaluated. The trend is for larger size coals (< 1000 µm) to fragment and the smaller size coals (< 160 µm) to swell. The results for intermediate samples were inconclusive regarding the governing phenomena. Ninomiya et al. [10] also performed studies with different particle sizes of coal; although the particle size range used was narrow than the one used on the previous study. Three coals were divided into sizes of 125-250 µm, 63-125 µm and < 63 µm in order to evaluate the effect of the particle size on the emissions of particulate matter during combustion. The increasing of PM concentration with the decreasing of the particle size was explained later on Ninomiya’s study due to the presence of excluded minerals and their behaviour along the combustion process, performing some analyses on particle size distribution of raw minerals and fly ash of all the coals. The study of Jiménez [11] was only focused on one narrow particle size for all the tested samples of biomass solid fuels (250-300 µm for oak tree, chestnut and eucalyptus wood and 300-400 µm, for olive residue). The fine particles produced have been found to be composed by alkali sulphates and chlorines, in contrast with the coarse mode, which was composed mainly by silicon, calcium and iron. Jiménez suggested that the origin of larger particles was attributed to char fragmentation and coalescence of minerals, retaining most of the original fuel mineral composition, while most of the fine particles were generated by condensable mineral species. The results of Korbee work [12], although focused on a wide range of particle sizes (<1000 µm) are vital for the understanding of char conversion phenomena, devolatilization but, above all, of particle fragmentation and the influence of mineral matter on its composition. Four biomass fuels (wood chips, waste wood, olive residue and straw) and two coals (UK coal and Polish coal) were burnt, during different residence times between 200 ms and 1300 ms, in a Labscale Combustion Simulator (LCS) equipped with a flat flame gas burner, to ensure initially high heating rates and temperatures. , Korbee states that fragmentation looks to be dependent on fuel chemical conversion and devolatilization. For wood chips and wastes, and olive residue, with high volatile matter content, devolatilization results in a fast process than for the other fuels. The same was verified regarding char conversion. The higher levels of char conversion leads to higher levels of fragmentation. Apparently, particle fragmentation is also promoted by the low content of some mineral matter on the original fuel like silicon, aluminium and sulphur. At the maximum residence time of 1300 ms, it was observed for biomass fuels like olive residues and straws a presence of potassium and chlorine, promoting an increase of aerosol particles formation. Other important studies are available on literature regarding char fragmentation and particulate matter and inorganic matter emissions, both in biomass and coal [13–17].

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MATERIALS AND METHODS

Both raw biomass fuels were pulverized in a laboratory-scale mill to 1 mm size. The narrow particle size classes were obtained with the aid of a SS-15 Gilson Economy 203 mm Sieve Shaker with different sieve sizes to obtain the three different particle size classes used in this study namely 100-200 µm, 400-600 µm and 800-1000 µm. Table 1 shows the properties of both the raw and the particle size classes of each biomass fuel, including the ash composition. Figure 1 shows schematically the Drop Tube Furnace (DTF) and the auxiliary equipment used during this study, which is described in great detail elsewhere [18]. The combustion chamber is a cylindrical electrically heated ceramic tube, with a total length of 1300 mm and an inner diameter of 38 mm. The furnace wall temperatures are continuously monitored using eight thermocouples (type-K) along the combustion chamber. The feeding system is composed by a water-cooled injector, placed at the top end of the DTF and it is used to feed the fuel and the oxidizer to the combustion chamber. A twin-screw volumetric feeder transfers the biomass to an ejector system from which the particles are gas-transported to the water-cooled injector.

Figure 1 Schematic of the Drop Tube Furnace and auxiliary equipment

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The local mean temperature measurements along the axis of the furnace were obtained using 76 µm diameter fine wire type-R thermocouples (platinum/platinum-13% rhodium). The thermocouple hot junction was installed and supported by 350 µm supporting wire of platinum/platinum-13% rhodium located in a twin-bore alumina sheath with 5 mm of external diameter. The temperature measured with an exposed thermocouple is the gas temperature biased by the furnace wall temperature so that the real temperature was estimated based on an energy balance on the thermocouple bead [19], based on the following equations:

𝑄𝑐𝑎𝑡 + 𝑄𝑐𝑜𝑛𝑑 + 𝑄𝑟𝑎𝑑

+ 𝑄𝑐𝑜𝑛𝑣 = 0 (1)

𝜀𝑡𝜎(𝑇𝑡4 − 𝑇𝑤𝑎𝑙𝑙

4 ) + ℎ(𝑇𝑡 − 𝑇𝑔) = 0 𝑤𝑖𝑡ℎ ℎ =𝑁𝑢. 𝑘

𝑑

(2)

Based on equation 1 and its simplified form on equation 2, the maximum uncertainty of the temperature measurements was around 10%. The particle (char) sampling along the furnace were performed with the aid of the 1.5 m long, water-cooled stainless steel probe on five axial positions along the axis of the reactor (300 mm, 500 mm, 700 mm, 900 mm and 1100 mm). The probe comprised a centrally located 3 mm inner diameter tube, through which quenched samples were evacuated with the aid of a pump. The quenching of the chemical reactions on the surface of the particles was achieved by nitrogen direct injection jets in the main gas stream through small holes near to the probe tip. On leaving the probe, the solid char samples were collected in a Tecora total filter holder equipped with a 47 mm-diameter quartz microfiber filter. After the sampling, the solid char samples were placed in an oven to eliminate the remained moisture content, at approximately 105 ºC. In order to understand if the moisture content had been completely removed from the solid char samples, repeated drying and weighting of the samples were made until the measured mass remained constant. The ash content in the solid samples was evaluated following the procedures described in the standard CEN/TS 14775. Particle burnout data were calculated using the following equation:

𝜓(%) = 1 −

𝜔𝑓

𝜔𝑥

1 − 𝜔𝑓

× 100

(3)

where 𝜓 is the particle burnout, 𝜔𝑓 is the ash weight fraction in the input biomass fuel, and 𝜔𝑥 is the ash

weight fraction in the char sample. Uncertainties in char burnout calculations based on the use of the ash as a tracer are related to ash volatility at high heating rates and temperatures and ash solubility in water. Based on these factors, the best estimates of this study indicated that the uncertainties in char burnout calculations were negligible [20]. PM concentration and size distributions resulted from the process of combustion were obtained with the aid of a low pressure three-stage cascade impactor (LPI, TCR Tecora) at the last stages of the DTF (x = 700 mm, x = 900 mm and x = 1100 mm) and, subsequently a more detailed study was performed at x = 1100 mm, with the aid of a thirteen-stage low pressure cascade impactor (DLPI, Dekati Ltd.). The LPI used allowed collecting three PM cut sizes during the same measurement: PM with diameters above 10 µm (PM10), PM with diameters between 2.5 µm and 10 µm (PM2.5-10), and PM with diameters below 2.5 µm (PM2.5). PM was collected on quartz microfiber filters of 47 mm diameter, which were dried in an oven at 105 ºC and weight before and after each test. The DLPI allowed to classify particle diameters below 10 µm. PM was collected on aluminium substrates of 25-mm diameter. In order to avoid condensation along the line connecting the probe outlet to the impactor inlet and also inside the impactor, a heating jacket (model Winkler WOXT1187) was used during sampling. The data repeatability was, on average, within 15% of the mean value. Finally, the PM collected for all biomass fuels at the exit of the DTF, x = 1100 mm, was examined in a scanning electron microscope equipped with an energy dispersive X-ray spectroscopy detector (SEM-EDS) – Hitachi S2400, which allows the quantification of the ultimate composition of a sample with a resolution of about 1 µm2. Measurements were carried out for all biomass fuels for DTF wall temperatures of 1100 ºC. The solid fuels feed rate was approximately 23 g/h and the total flow rate was 4l/min. The sampling flow rate was 10 l/min, and the initial particle velocity was estimated to be ~0.3 m/s.

RESULTS AND DISCUSSION

Temperature and burnout

Burnout and temperature profiles of wheat straw samples and rice husk samples are shown in Figure 2 and Figure 3, respectively. It is possible to observe that in regions near the burner there are no significant

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variations in the temperature profiles with the particle size of the sample. Variations start to become noticeable near the middle of the reactor, as the particles approach the reactor exit. Regarding the burnout profiles, significant differences between both wheat straw and rice husk should be pointed out. On wheat straw samples (figure 2 a)), the effect of initial particle size is remarkable. Overall, larger particles require longer residence times than smaller particles, so that the burnout can be higher. Samples have residence times necessary to the completion of the combustion process in the following ascending order: 100-200 µm, 400-600 µm, raw and 800-1000 µm. On rice husk samples (figure 2 b)), the effect of particle size on char burnout is slighter when compared with wheat straw. Given the fact that differences between burnout profiles are smaller, when compared with wheat straw, it can be concluded that in the case of rice husk, the effect of particle size does not have significant impact on combustion behaviour. The residence time necessary to achieve the nearly complete combustion state has a slight variation between all the rice husk samples, suggesting that the reactivity of the samples are very similar.

Table 1 Properties of biomass solid fuels

Sample

Rice Husk Wheat Straw

100-200

400-600

800-1000

Original 100- 200

400-600

800-1000

Original

Proximate Analysis (wt,% as received)

Volatiles 64,5 66,2 64,9 65,0 64,9 65,2 64,8 63,8

Fixed Carbon

12,7 13,6 14,3 13,3 12,4 15,2 15,4 14,9

Ash 12,8 11,1 11,6 12,2 14,7 11,4 11,5 13,0

Moisture 10,0 9,1 9,2 9,5 8,0 8,2 8,3 8,3

Ultimate analysis (wt,%, as dry ash

free)

Carbon 42,4 44,4 43,7 44,0 41,1 43,0 42,6 42,0

Hydrogen 5,6 5,5 5,8 5,6 5,3 5,4 5,4 5,4

Nitrogen 0,8 0,4 0,4 0,6 0,7 0,6 0,6 0,6

Sulphur <0,2 <0,2 <0,2 <0,2 <0,2 <0,2 <0,2 <0,2

Oxygen 51,1 49,4 50,0 49,6 52,6 50,8 51,2 51,8

Heating Value

(MJ/kg)

Low 14,6 14,6 14,4 14,5 13,0 13,8 13,6 13,2

High 15,8 15,8 15,6 15,7 14,1 14,9 14,8 14,3

Ash Analysis (wt,% dry

basis)

SiO2 86,6 91,1 92,3 90,4 42 40.5 43.5 42.5

Al2O3 1,2 0,7 0,9 1,0 8.7 8.6 8.7 8.5

Fe2O3 0,5 0,3 0,3 0,4 5 4.8 4.9 4.9

CaO 1,5 1,8 1,6 1,6 28 27.9 25.3 26.9

SO3 0,4 0,3 0,3 0,3 1 1.1 1 1.1

MgO 1,0 0,7 0,7 0,9 3.7 4.6 4 4

P2O5 2,2 1,3 0,6 1,1 2.6 2.7 2.6 2.9

K2O 3,6 2,0 1,6 2,5 6.9 8.1 7.7 7.2

Na2O 0,3 0,3 0,3 0,3 0.6 0.5 0.9 0.7

Cl 0,5 0,5 0,6 0,5 0.6 0.4 0.5 0.6

Other Oxides

2,2 1 0,8 1 0.9 0.8 0.9 0.7

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Particulate matter emissions

Figure 4 shows the PM emissions and char burnout for three axial positions along the axis of the DTF, namely 700 mm, 900 mm and at 1100 mm from the top bottom of the DTF, for all the samples of wheat straw (left) and rice husk (right) studied. For wheat straw samples (left), some evidences of char fragmentation can be seen between x = 900 mm and x = 1100 mm, on small and intermediate particle size classes of 100-200 µm and 400-600 µm, where both char burnout and the total concentration of particles are almost constant, varying only the relative concentration of PM10, which has a considerable reduction, in contrast with the increase of PM below 10 µm (PM2.5 and PM2.5-10). For 100-200 µm particle size class (figure 4 a)) particle burnout varies between 92.2% and 93.6%, for x = 700 mm and x = 1100 mm, respectively, while the concentration of particles with size below 10 µm (PM2.5 and PM2.5-10) increase from 97.8% to 99.3%, between x = 700 mm and x = 1100 mm, and the concentration of particles with size above 10 µm (PM10) decreases from 2.2% to almost 0%. For 400-600 µm particle size class (figure 4 b)) particle burnout varies between 84.7% and 91.4%, for x = 700 mm and x = 1100 mm, while the concentration of particles with size below 10 µm (PM2.5 and PM2.5-10) increase from 89.9% to 98.3%, between x = 700 mm and x = 1100 mm, and the concentration of particles with size above 10 µm (PM10) decreases from 10.1% to 1.7%.

Figure 2 a) Burnout profile and b) temperature profile for wheat straw samples

Figure 3 a) Burnout profile and b) temperature profile for rice husk samples

For rice husk samples (right), some evidences of char fragmentation can been seen between x = 900 mm and x = 1100 mm for small particle size class of 100-200 µm, where a sudden increase on PM2.5, followed by the decrease of PM2.5-10 and PM10 is verified. For this condition (figure 4 e)), particle burnout varies between 96.9% and 97.8%, for x = 700 mm and x = 1100 mm, respectively, while the concentration of particles with size below 10 µm (PM2.5 and PM2.5-10) increase from 97.8% to 99.0%, and the concentration of particles with size above 10 µm (PM10) decreases from 2.2% to 1%. Figure 5 shows the particle size distributions obtained with the aid of a DLPI of 13 stages, at x = 1100 mm, for wheat straw (figure 5 a)) and rice husk (figure 5 b)). For wheat straw samples (figure 5 a)), the samples with particle size between 100-200 µm, 400-600 µm and with particle size below 1000 µm shows a bimodal particle size distribution, contrasting with the unimodal behaviour of the 800-1000 µm particle size class. The fine mode for all conditions is centred on 0.3 µm and the intermediate mode on 0.7 µm. It is possible to identify zones of coarse particles

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between 1 and 10 µm. The behaviour of the rice husk (figure 5 b)) regarding the particle size distribution is rather different than that observed for the wheat straw. All the rice husk samples revealed a unimodal behaviour. The fine mode is centred on 0.3 µm. It is possible to identify zones of coarse particles between 1 and 10 µm.

Figure 4 PM emissions and char burnout for wheat straw (a) 100-200 µm b) 400-600 µm c) 800-1000 µm d) raw)

rice husk (e) 100-200 µm f) 400-600 µm g) 800-1000 µm h) raw) samples

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Figure 5 Particle size distribution for a) wheat straw b) rice husk

Figure 6 Chemical composition obtained on SEM-EDS for wheat straw (a) 100-200 µm b) 400-600 µm c) 800-

1000 µm d) raw) rice husk (e) 100-200 µm f) 400-600 µm g) 800-1000 µm h) raw) samples

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Chemical composition and morphology

Figure 6 shows the chemical composition of 4 selected subtracts, namely 9, 7, 4 and 5, corresponding to particle diameter cut offs of 0.205 µm, 0.320 µm, 0.785 µm and 1.951 µm, respectively, from the 13-stages DLPI, for wheat straw (left) and rice husk (right). The most relevant elements are potassium (K) and chlorine (Cl), having higher concentrations on subtracts with particles with lower diameters, on both biomass fuels. On rice husk samples, sulphur (S) shows in larger quantities than in wheat straw samples. This fact could be explained by the ash composition of rice husk samples, which are very rich in silicon. The presence of calcium (Ca) and phosphorus (P) should also be noted, especially on subtracts containing large particle sizes, on both biomass fuels analysed. The concentration of both elements is directly proportional to the initial particle size of the samples. Figure 7 shows the images obtained from SEM of all substrates previously selected for wheat straw. In general, it is possible to observe some differences between substrates. In the case of the sample 100-200 µm (figure 7 a)), it is possible to see on substrates containing larger particles, the existence of spherical and prismatic structures, surrounded by char particles, which confirms the trend shown previously of the existence of significant inorganic matter among char particles. These structures are also visible on 400-600 µm (figure 7 b)) and raw samples (figure 7 d)). On 800-1000 µm (figure 7 c)), it is not possible to visualize clearly the shape of the fine particles in substrats 4 and 5, due to the occurrence of water condensation. . Other important observation relates to the size of the spherical and prismatic structures found in the original biomass (figure 7 d)), which tend to be substantially larger compared to the other biomass size classes. This fact is confirmed by the results obtained by EDS analysis, which reveals higher amounts of certain inorganic elements. Figure 8 shows the images obtained from the SEM of all substrates previously selected for rice husk samples. It is possible to observe some differences between substrates. For example, the 100-200 µm size class (figure 8 a)) has residual crystalline structures. In the substrats containing larger particles (7 and 9), however, as the initial particle size increases, the spherical structures become more evident. In substrats containing small particles, namely substrats 4 and 5 (figure 8 c) and figure 8 d)), spherical structures are replaced by prismatic structures with considerable size.

CONCLUSIONS

This study intended to understand the effect of the initial particle size on char burnout and particulate matter emissions. Therefore, two biomass solid fuels were burnt at 1100 ºC in a DTF. The major conclusions are as follows:

i) The effect of initial particle size is more accentuated in the case of wheat straw then in the

case of rice husk

ii) PM emissions are higher for wheat straw than for rice husk, being the secondary particle

fragmentation more evident on small to intermediate particle size classes (100-200 µm and

400-600 µm)

iii) Ca and P tend to be retained in larger particles, while K and Cl present higher concentration

in the fine PM

iv) From combustion efficiency and PM emissions point of view there is no benefit to separate

pulverized fuels in narrow particle size classes.

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Figure 7 SEM images for wheat straw samples: a) 100-200 µm b) 400-600 µm c) 800-1000 µm d) raw

Figure 8 SEM images for rice husk samples: a) 100-200 µm b) 400-600 µm c) 800-1000 µm d) raw

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