Emissions and deposit properties from combustion of wood pellet with magnesium additives

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 41, Issue 5, May 2013 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2013, 41(5), 530−539

RESEARCH PAPER

Emissions and deposit properties from combustion of wood pellet with magnesium additives Tomas Persson1,*, Jochen Riedel1, Jonas Berghel2, Ulf Bexell1, Kaung Myat Win1 1

School of Technology and Business Studies, Dalarna University, 79188 Falun, Sweden

2

Department of Energy, Environmental and Building Technology, Karlstad University, 65188 Karlstad, Sweden

Abstract:

This work studies the amount of gaseous and particle emissions and deposits on heat exchanger surfaces in a boiler fired

with commercially available pellets and with pellets primed with magnesium oxide and magnesium hydroxide. The combustion experiments were performed on a residential boiler of 20 kW. Substrates placed in the heat exchanger was analysed with SEM-EDX-mapping to evaluate the chemical composition of the deposits. The results show that particle emissions (PM 2.5) using the additives increased by about 50% and the mass of the deposits in the flue gas heat exchanger (excluding loose fly ash) increased by about 25% compared to the combustion of pellets without additives. The amount of additives was found to be eight times higher than the amount of the main alkali metals potassium (K) and sodium (Na) which leads to the assumption that the additives were overdosed and therefore caused the problems reported. The SEM analysis of the substrates placed in the flue gas heat exchanger indicate that the deposits of sodium (Na), potassium (K), chlorine (Cl) and sulphur (S) decrease using the additives. If this was due to the expected chemical reactions or due to the loose fly ash covering the substrates after the test, could not be determined in this study. Keywords: softwood pellets; magnesium; additives; particle emissions; deposit formation

Deposit formation in boilers causing harmful particle emissions[1] and require regular cleaning, which is time consuming and costly. In addition deposits on the boiler convection surfaces decrease heat transfer and efficiency of boilers. In case of corrosive deposits including chloride and sulphur, also a reduced life time of the boiler is expected[2]. During combustion, alkali metals in the fuel such as potassium (K) and sodium (Na) are released and they may condensate on heat exchanger surfaces and build sticky layers together with chlorine (Cl), silicon (Si) and sulphur (S) which will develop more deposits and may also enhance corrosion[2–5]. Bäfver et al[6] and Carvalho[7] describe in what principal way additives are reacting with alkali compounds during combustion. The additives can react instead of the alkali and form substances with higher melting temperatures that do not build sticky layers; they can prevent the release of gaseous KCl or react with KCl into less corrosive components and a combination of these effects are also possible. Though the use of additives in combustion has been thoroughly investigated, experiments with magnesium additives in biomass fuels are scarce. This paper investigates the effect on emissions and deposit formation using softwood

pellet with magnesium additive in a 20 kW boiler. It is known from combustion of diesel[8] and different biomass like oat[4,5] that metal-based additives can reduce deposit formation as well as particle emissions. Magnesium hydroxide is currently used to control slagging, fouling and tube cracking in coal fired boilers[9]. Steenari et al[10] point out the importance of magnesium in reactions with kaolin for anti-sintering effects. It is expected that also the addition of magnesium oxide will form mixtures with much higher melting temperatures, which will decrease the amount of deposits in the boiler convection surfaces[11,12]. Khullar[12] reports experience based results by using magnesium additive, such as higher melting temperatures of ashes and improved combustion (less soot).

1

Experimental

Three different types of wood pellet were produced, one reference pellet without additives, one pellet with magnesium oxide (MgO), and one with magnesium hydroxide (Mg(OH)2). The pellet with additives and the reference pellet without the additives were produced from the same raw material. In addition a commercial pellet was tested.

Received: 23-Oct-2012; Revised: 17-Dec-2012 * Corresponding author. Tel: +46-23-778717; E-mail: [email protected] Copyright  2013, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Fig. 1

Pellet production line at Karlstad University

1: diagonal mixer; 2: conveyor screw; 3: inlet feeder; 4: pelletizing press with a flat die and a maximum output; 5: volumetric feeder

Table 1 Additive

Producing conditions for the fuel producing line

Pressure p/MPa

Temperature t/°C

Mass flow rate /(kg·min–1)

wb/%

/%

a

Humidity

Reference

0.0

101.1

104

12.4

1.5

MgO

0.8a

93.8

106

12.4

1.5

Mg(OH)2

0.8a

96.5

108

12.4

1.5

analysed amount of added magnesium is given in Table 2

Firing experiments at stationary operation for about six hours were performed in a pellet boiler with nominal power of 20 kW. Measurements of emissions (CO, NO, TOC, and PM 2.5) were performed and related to the fuel consumption and flue gas flow rate. Gravimetric measurements of ash deposition were conducted using sample rings in the flue gas heat exchanger tubes. Substrates in the heat exchanger tubes were also analysed with Scanning electron microscopes (SEM) and X-ray analysis to evaluate the different spices that agglomerates on the heat exchanger surfaces. EDX-mapping was used to estimate the amount of the different spices. 1.1

Pellet production and fuel analysis

Fresh sawdust of Norway spruce (Picea Abies) produced at a local sawmill with frame saws was used as raw material for the production of pellets. The pellets were produced in a production unit located at the Department of Energy, Environmental and Building Technology at Karlstad University, Sweden (see Figure 1). It consists of: a diagonal mixer, a conveyor screw, an inlet

feeder where conditioning takes place if needed, an Amandus Kahl C33-390 pelletizing press with a flat die and a maximum output of 300 kg/h, and a volumetric feeder for additives and a cooling tower. The wet sawdust was first conditioned in the diagonal mixer by adding water up to the point when the appropriate moisture content was obtained. The additive was supplied to the inlet screw feeder using a volumetric feeder. The flat die has nine holes radius with 52 holes in each row, totalling 468 holes. The die has a working width of 75 mm, a hole diameter of 8 mm, an effective compression length of 30 mm and a total thickness of 50 mm. The open area of the die is 64% of the working area. The pellet production plant was operated until stationary conditions were obtained. Before a sample was collected, there was a break-in period of 5 min with the current additive to insure stationary conditions. MgO and Mg(OH)2 in powder form were used as a pellet additive. The additive flow rate through the volumetric feeder was held constant at 0.8% based on weight produced pellets. The producing conditions can be seen from Table 1.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Fig. 2

Measurement set up

In addition to the three fuels produced by Karlstad University (Table 1) also a commercially produced softwood pellet was tested. The fuel compositions were analysed and the results are presented in Table 2. Samples of the different fuels that was used in the combustion tests were collected in a plastic bag and the moisture content was analysed according to EN 14774-1[13]. The actual humidity ratio was used to recalculate the heating value of the fuels depending on the humidity level. Analysis of the remaining ash according to EN 14775-1[14] was used to determine the amount of unburned carbon which was left in the ash box after each experiment. 1.2

Boiler setup

Figure 2 shows the experimental setup. The combustion took place in a 20 kW overfed horizontal pellet burner which was connected to a boiler with a horizontal two-pass-principle convective pass. An external-fuel-storage with a transport screw was used to feed the pellets into the combustion chamber. The fuel falls down through a shaft with a hose directly into the combustion chamber where the combustion is supported by a fan. The boiler was used for several months before these experiments took place. The boiler was connected to a heat exchanger and a three way mixing valve to obtain a constant inlet temperature of about 75°C. The fuel feeding ratio was adjusted to a combustion power of about 12 kW and the air flow rate was set as low as possible trying to keep the CO emission as low as possible.

Each combustion experiment (totally four different fuels) was run for two times 3 h including a 25 min break from the beginning of the stop sequence to the next start. The burner could be run for a maximum of 3 h due to a limit in the control unit. During each period stable conditions at a constant heat output were achieved. 1.3

Energy balance and fuel consumption

Water flow rate through the boiler are measured by flow sensor (VFS1) and temperature sensors (TS1, TS2) connected at the outlet and inlet of the boiler (Figure 2). Heat rate is calculated using temperature dependent density and heat capacity of water. In order to calculate the average boiler efficiency during the measurement period, the boiler was conditioned to 20°C before and after each test. Air temperature in the room was measured by TS3 and TS4 in Figure 2. Temperature was measured at the inlet and the middle of the heat exchanger (TS5, TS6); at the outlet of the boiler (TS7), in the chimney (TS8) and in the dilution duct (TS9). The boiler and the fuel storage were mounted on a scale WS1 (Figure 2) in order to measure the pellet consumption instantaneously. An external scale was also used to measure the fuel filled to double check the measurements. The store and the fuel feeder were completely emptied between the measurements of the different fuels. The volume flow rate of the flue gas was continuously measured using a multi-port averaging pitot tube (VFS2) calibrated in combination with pressure transducers specifically to the installed chimney.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Fig. 3

Stainless steel rings for measuring of deposited particles in the flue gas heat exchanger (middle part) (a): stainless steel ring for gravimetric measurement of deposits and substrate for SEM/EDS-analysis; (b): position of the stainless steel rings in the flue gas heat exchanger

Based on the measured flue gas flow rate, flue gas temperature, O2 concentration and the fuel properties, the instant and total fuel consumption was calculated and the total amount of fuel was used to double check the validity of the flue gas flow sensor. 1.4

Emission measurements

Continuous measurement of gaseous emissions; oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxide (NO) and total organic carbon (TOC) are carried out by extracting a flue gas sample from the chimney through a heated filter and a heated tube (180°C). The analytical principles of the gas analysers are non-dispersive infrared (CO2, CO, NO), paramagnetic gas analyser for O2 and flame ionization detection (TOC, propane equivalent). The measured concentrations are related to the measured flue gas flow rate (VFS2 in Figure 2) to calculate the amount (mass flow rate) of the gaseous emissions. Continuous samples from particle emissions are taken from the dilution duct (non-isokinetic sample) and a second stage dilution with clean and dry air of 20°C is applied before measurements with an Electrical Low Pressure Impactor (ELPI). Number concentration and size distribution of particulate matters are measured in the range of 7 nm to 10 μm and the flow rate of particle emissions (characterized for PM 2.5) are calculated based on the total dilution ratio and the volume flow rate in the dilution duct.

remove all combustible particles. The rings were weighted before and after the measurement to the nearest 0.01 mg to measure the amount of deposits. As the rings were placed in a horizontal tube, fly ash that was not fixated to the surface was removed by carefully blowing the substrates. The amount of deposits is given as a specific value by dividing the amount of deposits by the time of combustion and the area of the measured ring. Finally an average value of the weight based on all four measured samples was calculated and distributed by the measurement time and exposed surface to get the unit kg/(m2·h). All the sample surfaces were exposed for 6.37 h except of the Mg(OH)2-case due to an unexpected boiler shut down that occurred during the measurement. Therefore the Mg(OH)2 sample was exposed only for 5.42 h. 1.6 SEM analysis determining the chemical composition of the deposits Four substrates (15 mm by 15 mm) from the same material used in the gravimetric measurements were placed close to the rings for gravimetric measurements (Figure 3). The samples were analysed with a SEM-instrument Jeol820 with an ISIS EDS-system to determine the chemical composition of the deposits and to estimate the amount of the surface covered with each substance.

2 2.1

1.5

Results and discussion Fuel and ash analysis

Gravimetrical measurements of deposited particles

To collect deposits in the flue gas heat exchanger of the boiler, four stainless steel rings (material number: 1.4432) made of X2CrNiMo17-12-3 were used. Two of the rings were placed in the beginning of the flue gas heat exchanger tubes (position A in Figure 2) and two of them in the middle of the flue gas heat exchanger (position B in Figure 2). The rings were cleaned in an ultrasonic bath with isopropanol and then placed in an oven according to EN 14775-1[14] at 550°C to

The elementary analysis and the heating values of the tested pellets are given in Table 2. The main difference in composition between the four samples could be found in the amount of silicon (Si), magnesium (Mg), nitrogen (N) and the net calorimetric value. The amount of silicon and nitrogen were highest for the commercial pellets and the amount of magnesium in the pellets with additives was about eight times higher than the main alkali metals potassium (K) and sodium (Na). The amounts of the alkali metals were about the same for all tested fuels.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539 Table 2

Elementary analysis and heating values of the used fuels analysed by an accredited laboratory MgO additive

Mg(OH)2 additive

Carbon (C)

50.9

50.7

50.3

50.0%

6.1

6.2

6.2

6.1%

Nitrogen (N)

0.27

0.07

0.09

0.07%

Oxygen (O)

42.2

42.7

42.5

42.8%

Sulphur (S)

0.007

0.009

0.010

0.008%

Chlorine (Cl)

0.01

0.01

0.01

0.01%

Net cal. value (dry)

19.26

18.94

18.96

18.83 MJ/kg

Moisture, delivered

6.8

7.1

8.6

7.7%

a

6.3

6.9

8.2

7.4%

Net cal. value, test cond.

17.89

17.47

17.22

17.26 MJ/kg

Ash content

0.4

0.4

1.1

1.0%

Silicon (Si)

308

178

140

136 mg/kg

Aluminium (Al)

55.5

30.3

28.5

26.4 mg/kg

Calcium (Ca)

773

1020

907

910 mg/kg

Iron (Fe)

48.8

66.6

46.6

42.4 mg/kg

Potassium (K)

433

422

406

450 mg/kg

Magnesium (Mg)

170

112

3580

3850 mg/kg

Manganese (Mn)

100

119

114

116 mg/kg

Sodium (Na)

35.5

37.6

41.4

38.9 mg/kg

Phosphorus (P)

28.9

50.6

58.5

59.5 mg/kg

Titanium (Ti)

22.4

1.38

1.56

1.5 mg/kg

Arsenic (As)

0.146

<0.1

<0.1

<0.09 mg/kg

Barium (Ba)

9.35

19.3

18

17.3 mg/kg

Lead (Pb)

0.284

0.232

0.206

0.208 mg/kg

Boron (B)

1.55

1.77

1.82

1.8 mg/kg

Cadmium (Cd)

0.148

0.0802

0.0725

0.0736 mg/kg

Cobalt (Co)

0.0606

0.0488

0.0457

0.0424 mg/kg

Copper (Cu)

1.32

1.08

0.943

0.856 mg/kg

Chromium (Cr)

0.954

0.354

0.704

0.253 mg/kg

Mercury (Hg)

<0.01

<0.01

0.0142

<0.01 mg/kg

Molybdenum (Mo)

0.0418

0.0396

<0.05

<0.05 mg/kg

Nickel (Ni)

0.132

0.214

0.205

0.207 mg/kg

Vanadium (V)

0.0392

0.0362

0.042

0.0364 mg/kg

Zink (Zn)

25.4

14.9

14.5

14.0 mg/kg

sample from test day analysed according to EN 14774-1

2.2

Reference-pellet

Hydrogen (H)

Moisture, test cond.

a

Commercial pellet

[13]

Emissions

During stationary combustion CO emissions were slightly lower for the pellets with the Mg(OH)2-additive compared to the reference pellets, but lowest for the commercial pellets (Figure 4). However, the differences are almost within the uncertainty of the measurements. The TOC emissions show the same tendency as the CO-emissions. The NO-emissions

were the same for the three laboratory manufactured pellets and much higher for the commercial pellets. This is related to the higher nitrogen content of the commercial fuel (Table 2). The particle emissions were about 50% higher for the fuels with additives compared to the reference pellets. The particle emissions from the commercial pellets were about 40% lower compared to the reference pellets.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Fig. 4

Gaseous and particle emissions from combustion of the different fuels (stationary periods) TOC-emissions were presented as methane equivalents (C3H8)

To summarize it can be seen that the additive of MgO reduce the number of the smallest particles, and the Mg(OH)2-additive increase the number of the smallest particles, however the total mass of particles increase using the additives, as can be seen in Figure 4. 2.3

Fig. 5

Number size distribution of particles, normalized to 10% O2

Fig. 6

Average weight gain of the sample rings collecting deposits

Figure 5 shows the average number-size distribution of particulate matter as a function of particle diameter during stationary periods for the four different pellet samples. Most particles occurred below 1 µm with a local peak at around 0.1 µm for all four cases. The highest number of small particles was found for the fuel with Mg (OH)2-aditive, followed by the commercial pellets and the reference pellets. The number of particles larger than 0.1 µm was smallest for the commercial pellets but quite similar for the other pellet samples.

Gravimetric measurements

Figure 6 shows the average collected deposits on the four sample rings. The amount of deposits was about 25% higher for the pellets with additives compared to the reference pellets and the commercial pellets. The fly ash production was much higher in the cases with additives compared to the reference case, as the heat exchanger tubes were covered with a white layer of fly ash particles (Figure 7). The increased amount of deposits on the surface is the reverse to the desired results and may reduce the efficiency of the boiler; however, the measurement time was too short to evaluate the influence on boiler efficiency. The measured boiler efficiency was slightly reduced in the additive cases compared to the reference case; however, it can as well be explained by a slightly higher air factor in the reference case and the measurement uncertainty. The amount of magnesium in the tested pellets was almost eight times higher than the amount of the main alkali metals potassium (K) and sodium (Na) (Table 2). The Magnesium content was also much higher than the dosage recommended by[12] which is roughly between 0.02% and 0.05% by weight. This leads to the conclusion that the additives were overdosed and therefore caused the problems reported, however, it is possible that for fuels with such low amount of alkali as soft wood pellets, even with a proper amount of additive, they may cause more problems than they solve. 2.4

SEM analyses

Figures 8 and 9 show the SEM analyses with EDX mapping for the different fuels and the two different positions (A and B) in the boiler where the temperature in position A was around 460°C and the temperature in position B was around 260°C.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Fig. 7

Flue gas heat exchanger tubes after the experiment with the reference pellets and the pellets with additives of MgO (a): reference pellets; (b): MgO-pellets

Fig. 8

SEM and EDX qualitative maps of the samples placed at the high temperature position A (a): commercial pellet; (b): reference pellet; (c): MgO-pellet; (d): Mg(OH)2-pellet

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Fig. 9

SEM and EDX qualitative maps of the samples placed at the low temperature position B (a): commercial pellet; (b): reference pellet; (c): MgO-pellet; (d): Mg(OH)2-pellet

The appearance of a specific substance will appear as white spots and a dark surface indicates no appearance of the substance. The elements iron, chromium and nickel in steel are shown in the first three maps in the upper row of the EDX mappings. If there is a high intensity of these elements it means that the area has little ash deposit. The last two maps on the first row and all the maps in the lower row show the ash elements sodium (Na), potassium (K), chlorine (Cl), sulphur (S), silicon (Si) and in the additive cases also magnesium (Mg). In general it can be seen from the figures that the deposited

particles from the samples placed at the high temperature position “A” was bigger and more irregular than in the colder part of the flue gas heat exchanger “B”, where the deposits was smaller and more punctual shaped. It is likely that the bigger irregular shaped particles are coarse fly ash and the smaller particles are inorganic aerosols. The samples showing the highest amount of particle deposits is the reference pellets, especially in position “A” (Figure 8), where the intensity of the substrate, Fe, Cr and Ni was low. From the SEM analysis it seems that there are fewer deposits for the pellet with additives compared to the reference pellet.

Tomas Persson et al. / Journal of Fuel Chemistry and Technology, 2013, 41(5): 530−539

Figure 10 also gives the estimated amount of ash that was not collected. This is based on the amount of pure ash collected and the calculated amount based on the used fuel and the elementary analysis of the fuels. The amount of ash that was not collected was highest for the commercial pellets, indicating that more ash was leaving the boiler to the chimney or the ambient. A majority of these particles will be much larger than PM 2.5 and therefore not registered by the particle measurements.

3 Fig. 10

Total amount of collected ash from the ash box, the amount

of unburned carbon in the ash and the amount of ash that was not collected

This is especially visible in the low temperature position B (Figure 9). The deposits of sodium (Na), potassium (K), chlorine (Cl) and sulphur (S) decrease using the additives. The high amount of loose fly ash (that was removed before the analysis) may protect the substrates from further particles being accumulated on the substrate; however, this may not be the case for the gravimetric measurements, where at least half of the rings also cover the upper part of the heat exchanger, where less fly ash was accumulated. The gravimetric measurements in Figure 6 indicate increased amount of deposits using the magnesium additives. Thus reliable conclusions from the SEM-analysis cannot be drawn. On all four substrates the main anticipating particles are potassium, chlorine and sulphates, which indicate that there are potassium chlorides and potassium sulphates. Small amounts of silicon indicate that there may also be potassium silicates. 2.5

Ash in the ash box

Figure 10 shows the total amount of collected ash and the amount of unburned carbon for each type of fuel. While the total amount of ash was increased by adding MgO and Mg(OH)2, the fraction of unburned carbon was more than halved compared to the reference cases. The ash from the combustion of the reference and the commercial pellets was black, whereas the ash from the additive pellets was white. Using the assumption that all unburned particles are carbon the amount of energy remaining in the ash box is 0.6% and 0.7% for the commercial and the reference pellets respectively. For the ash from pellet with additive of MgO and Mg(OH)2, the amount of energy corresponds to 0.3% and 0.2% respectively. It seems that the additive improve the combustion and reduce the amount of unburned carbon in the ash, however the energy saving potential is small as the remaining energy in the ash box was below 1% in all cases.

Conclusions

The additives of MgO and Mg(OH)2 increased the amount (mass) of particle emissions by about 50% and the mass of the deposits in the flue gas heat exchanger excluding loose fly ash increased by about 25% compared to the combustion of pellets without additives. The amount of loose fly ash caught in the flue gas heat exchanger was also much higher using the additives. The amount of additives was almost eight times higher than the amount of the main alkali metals potassium (K) and sodium (Na) which leads to the assumption that the additives were overdosed and therefore caused the problems reported. The addition of Mg(OH)2 increase the emissions of small particles below 0.1 µm and the addition of MgO reduce the number of small particles, however, this has not much influence on the total mass of PM 2.5 particle emissions. Both types of additives caused a small increase in particle numbers in the range from 0.1 to 2.5 µm. The SEM analysis of the substrates placed in the flue gas heat exchanger indicate that the deposits of sodium (Na), potassium (K), chlorine (Cl) and sulphur (S) decrease using the additives. If this was due to the expected chemical reactions or due to the loose fly ash covering the substrates after the test, could not be determined in this study. The additives reduce the amount of carbon in the ash box, however the energy saving potential is small as the remaining amount of energy in the ash box was below 1% for all tested pellets.

Acknowledgements This work was performed within the project SWX-Energi and financed by the European Union, Region Dalarna, Region Gävleborg, and Dalarna University.

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