Journal of Mechanical Science and Technology 27 (6) (2013) 1701~1706

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-013-0419-x

Development of an air fuel control system for a domestic wood pellet boiler†

Sae Byul Kang1,*, Jong Jin Kim1, Kyu Sung Choi1, Bong Suk Sim1,2 and Hong Young Oh1 1Energy Efficiency Department, Korea Institute of Energy Research, Daejeon, Korea 2 School of Mechanical Engineering, Kyungpook National University, Daegu, Korea

(Manuscript Received November 23, 2012; Revised February 27, 2013; Accepted March 4, 2013)

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Abstract Wood pellets are a kind of solid biomass energy and a renewable energy source. Made by compressing sawdust, wood pellets have a

higher energy density than split firewood and wood chips. In 2007, the new and renewable energy (NRE) portion was 2.4% with respect to total primary energy in Korea. The Korean government wants to increase the new and renewable energy (NRE) portion up to 6.1% by 2020 [1]. To achieve this target, the government has been establishing some policies, such as incentive policy, NRE mandatory use for public building and renewable portfolio standard (RPS) and so on. To supply wood pellets as fuel for the combustion chamber of a wood pellet boiler, most domestic wood pellet boilers put a constant volume by using an auger type fuel feed system. In an auger system as fuel feeding, there is the possibility of changing energy input due to the different density of wood pellets even in a constant volume flow rate of wood pellets. If fuel input rate is changed without any correction of air flow rate for combustion, the condition of combustion in a wood pellet boiler can be deteriorated. We have developed an air-fuel control system for a domestic wood pellet boiler by using flue gas oxygen concentration measurement and a PID controller. To measure O2 concentration of flue gas, a wide band O2 sensor was adopted. We changed fuel input from 100% to 50% by artificial manipulation to confirm the control system. The O2 concentration in flue gas can be controlled to be 8.5% ± 1% without significant change of CO and NOx concentration.

Keywords: Air fuel ratio; Boiler; Control; Renewable energy; Wood pellet

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

With increase of the fossil fuel prices, the heating costs of household and industry increase rapidly. Many countries are interested in using renewable energy to decrease the consump-tion of fossil fuel. Wood pellets are a kind of solid biomass energy and one of renewable energy sources. In 2008, the new and renewable energy portion is 2.4% with respect to the total primary energy in Korea. Most of renewable energy portion of Korea has been from waste energy, up to 78%. The other re-newable energy sources are very low, such as bioenergy por-tion is 7.3%, wind energy 1.6%, solar power 1.0%, and solar heat 0.5% and so on (Table 1) [1]. A wood pellet is made by compressing sawdust. A major

defect of solid biomass from wood is low energy density compared to fossil fuel. Density of a split log is 310 ~ 460 kg/m3 and that of wood chips is about 220 kg/m3. Furthermore, the density of sawdust is 170 kg/m3. That means woody bio-mass fuel has low density, so energy density (density / heating value) of woody biomass fuel must be low. Logistical cost of

woody biomass fuel can be increased compared to fossil fuel. But woody biomass fuels are categorized as renewable energy, so woody biomass usage is increasing nowadays to raise the portion of renewable energy. To overcome the defect of woody biomass, its bulky volume, a wood pellet can be used as heating fuel. Wood pellets can be produced by compressing sawdust about one-third. So the particle density of a wood pellet is more than 1100 kg/m3 and bulk density is more than 600 kg/m3. The wood pellet density is nearly triple compared to wood chip. There are standards of wood pellets as fuel in the EU and Korea [2, 3]. Especially, Austria has a wood pellet standard ÖNorm M 7135 with more than 15 requirements of wood pellet as fuel, such as diameter, length, density, ash con-tent and so on [2]. Korea also has a wood pellet standard di-vided into four grades, as established by the Korea Forest Research Institute in 2009 (Table 2) [3]. Another reason to use wood pellets as heating fuel in Korea

is to save heating cost. In Korea, there are several fuels for industrial use, such as heating oil, light oil, LNG, wood pellet and wood chip. Heating oil is one of major heating fuel in industrial or farming area. But the price of heating oil is soar-ing recently. In January 2010, the heating oil price was about 1000 KRW (0.87 USD, 1150 KRW/USD 2012.6). But by June 2012, heating oil price was about 1400 KRW (1.22

*Corresponding author. Tel.: +82 42 860 3321, Fax.: +82 42 860 3098 E-mail address: byulkang@kier.re.kr

† This paper was presented at the ISFMFE 2012, Jeju, Korea, October 2012. Rec-ommended by Guest Editor Hyung Hee Cho

© KSME & Springer 2013

1702 S. B. Kang et al. / Journal of Mechanical Science and Technology 27 (6) (2013) 1701~1706

USD), that is, an increase of about 40% compared to in 2010. But wood pellet cost is the same or decreased compared to in 2010. In 2010 wood pellet for 1 ton was about 350000 KRW (304 USD), but in 2012 the cost was about 320000 KRW (261 USD). Wood pellet cost decreased by 10% for 2 years. In Table 3, heating costs of heating oil and wood pellet are esti-mated. In this estimation, we assume that boiler thermal effi-ciency for heating oil is 90% and that of wood pellet is 80%. In Table 3, we calculate the price per lower heating value of

each fuel. For heating oil, price per 1 liter is 1.22 USD and its lower heating value is 34.3 MJ/L. Price per lower heating value of heating oil is 0.0356 USD/MJ. The final data, price index, can be obtained by dividing boiler efficiency. As a re-sult, wood pellet heating can lower heating cost 47% com-pared to heating oil. Solid fuel combustion boiler has a disadvantage to feed fuel

into combustor. Usually, a wood pellet boiler has an auger

(screw) to feed wood pellets from the storage tank to the boil-er combustor. The auger system gives a constant volume of wood pellet,

not a constant mass flow rate of wood pellet. Each wood pellet can have a different bulk density. So there is a possibility to have different mass flow rate with each wood pellet product. For any reason, if the mass flow rate of wood pellet is changed, combustion condition can be differ from boiler initial setting, which is usually an optimum condition. On the contrary, gas and liquid fuel boiler put relatively constant fuel flow rate into boiler. If mass flow rate is changed, O2 concentration in flue gas might be changed. In EU, there are many kinds of domes-tic wood pellet boilers which can control the air fuel ratio of a boiler by measuring O2 concentration in flue gas. Fig. 1 shows domestic wood pellet boiler specification in EU [4]. The last column represents whether air fuel control system (lambda sensor, lambdasonde) is or not. So, many european domestic wood pellet boiler manufacturers have the technology to con-trol air fuel ratio by using a lambda sensor. But in Korea, there is no domestic wood pellet boiler which can control air fuel ratio by measuring O2 concentration in flue gas [5]. Kristensen and Kristensen developed and tested a small

scale batch-fired straw boiler which had air fuel control sys-tem by measuring flue gas temperature and O2 content [6]. They insisted that carbon monoxide could be reduced by using insulation, more efficient technique for supplying air, and optimization of the electronic control unit for the air supply. Žandeckis et al. developed a lambda control algorithm based on the measurements of the actual O2 concentration in the flue

Table 1. The renewable energy portion of each energy sources in 2008. Source: Korea Energy Management Co. [1].

Waste Hydro Biofuel Wind Solar cell

Solar heat Geothermal

78.0 11.3 7.3 1.6 1.0 0.5 0.3

Table 2. Wood pellet standard in Korea (Korea Forest Research Insti-tute in 2009) [3].

Parameter Unit 1st 2nd 3rd 4th

Diameter mm 6~8 6~8 6~8 6~25

Length mm ≤32 ≤32 ≤32 ≤32

Bulk density kg/m3 ≥640 ≥600 ≥550 ≥500

Water content % ≤10 ≤10 ≤15 ≤15

Ash content % ≤0.7 ≤1.5 ≤3.0 ≤6.0

Fines % < 1.0 < 1.0 < 2.0 < 2.0

Durability kcal/kg (MJ/kg)

≥97.5 ≥97.5 ≥95 ≥95

Heating value kcal/kg (MJ/kg)

≥4 300 (18.0)

≥4 300 (18.0)

≥4 040 (16.9)

≥4 040 (16.9)

S % < 0.05 < 0.05 < 0.05 < 0.05

Cl % < 0.05 < 0.05 < 0.05 < 0.05

N % < 0.3 < 0.3 < 0.3 < 0.3

others % < 2.0 < 2.0 < 2.0 < 2.0

Table 3. Heating costs estimation for heating oil and wood pellet (in Korea June 2012).

Fuel Price (A)

Lower heating value, LHV

(B)

Price/LHV (C, A/B)

Boiler Efficiency

(D)

Price index (C/D)

Heating oil

1.22 USD/ℓ

34.3 MJ/ℓ

0.0356 90 0.0395 (100)

0.30 USD/kg

0.0180 80 0.0225 (57) Wood

pellet 0.28 USD/kg

16.7 MJ/kg

0.0167 80 0.0210 (53)

Fig. 1. Domestic wood pellet boiler specification sold in EU (Lamb-dasonde = Lambda sensor, ja = yes, nein = no) [4].

S. B. Kang et al. / Journal of Mechanical Science and Technology 27 (6) (2013) 1701~1706 1703

gas by a lambda sensor. By control of the air blower, they could achieve a constant O2 concentration in flue gas with stepwise heating output mode of a 35 kW wood pellet boiler [7]. In this research, we installed a domestic wood pellet boiler

(25 kW heating capacity) and selected the proper O2 detection sensor to control air blower for a constant O2 concentration in the flue gas. We considered five kinds of O2 detecting sensors, such as galvanic cell type, semi-conductor type, narrow band lambda sensor, lean lambda sensor and wide band lambda sensor (Bosch). To determine which sensor was suitable for a domestic wood pellet boiler, a pre-test was conducted. With a selected O2 sensor, we did a performance test for a condition of changing with wood pellet feeding rate.

2. Methodology

2.1 O2 detection sensors

There are several O2 detection sensors, such as galvanic cell type, semi-conductor type, narrow band lambda sensor, lean lambda sensor and wide band lambda sensor (Bosch). Galvanic cell type O2 detection sensor is shown in Fig. 2(a).

This type of sensor can detect O2 from 0 ~ 100% and output is 6 mV in atmosphere, which needs to be amplified. Tempera-ture range of galvanic cell type O2 sensor is -10 ~ 50℃. If this

galvanic cell type O2 sensor is to be installed for flue gas O2 detection, a cooling system is needed. The semi-conductor type sensor is very cheap and small as

shown in Fig. 2(b). But this sensor detects CO concentration instead of O2 concentration. To control air fuel ratio, one can use a CO sensor. In general, when combustion is in optimum condition for a domestic wood pellet boiler, the O2 range is about 5 ~ 8% and CO concentration will be minimum value. So we can control air flow rate by using the CO detection sensor to be combustion stable. The output of the semi-conductor type CO detector is its resistance change. One can detect the resistance change by using a voltage divider. But this semi-conductor type CO sensor can be usable in less than 60℃ environment. There are three kinds of lambda sensor that can be used in

automobile exhaust gas O2 detection: 1) narrow band lambda sensor, 2) lean lambda sensor (usually rarely used in automo-biles), and 3) wide band lambda sensor. These sensors use zirconia membrane to measure O2 concentration. Narrow band lambda sensor and lean lambda sensor have only one zirconia membrane, and the wide band lambda sensor has two zirconia membranes. The output of the narrow band lambda sensor is 0.2 ~ 0.8 V voltage output. But in wood pellet boiler operating region that is O2 concentration 5 ~ 10%, the voltage output of narrow band is very low and unstable. So it is very difficult to use a narrow band lambda sensor for a domestic wood pellet boiler. Fig. 3 shows lean lambda sensor (LSM11, Bosch) volt-age output with respect to O2 concentration. O2 concentration of the wood pellet boiler is about 5 ~ 10%, so voltage output of the sensor is 5 ~ 20 mV. In this research we chose a lean lambda sensor, LSM11. The wide band lambda sensor has two zirconia membranes which can use wide band of O2 de-tection range. This sensor is the most accurate and suitable O2

(a) Galvanic cell type (b) Semi-conductor type

(c) Lean lambda sensor (LSM11, Bosch)

(d) Wide band lambda sensors (LSU, Bosch)

Fig. 2. Figures of O2 detection sensor obtainable in Korea.

Fig. 3. Lean lambda sensor output characteristic (LSM11, Bosch).

1704 S. B. Kang et al. / Journal of Mechanical Science and Technology 27 (6) (2013) 1701~1706

detection sensor.

2.2 Test facility

Fig. 4 is schematic diagram of the experimental facility [8, 9]. A 23 kW domestic wood pellet boiler was installed to evaluate the air fuel ratio control system. LSM11 lambda sen-sor was adopted to measure O2 concentration in the flue gas, and a PID controller was used to control the air blower by using LSM11 sensor output. The PID controller receives volt-age output of LSM11 and control power into air blower to make LSM11 sensor output voltage constant, which means constant O2 concentration of the flue gas. A flue gas analyzer (TESTO 340) was used to evaluate the performance of the air fuel control system. To measure wood pellet consumption rate, an electric scale (FG-60 KA(H), AND) was used under the wood pellet storage tank and its resolution was 5 g. Usually, the domestic wood pellet boiler installed consumes about 5.2 kg/h wood pellet at nominal power output.

3. Results

3.1 Without air fuel control

We tested the wood pellet boiler with conventional method, that is, without air fuel ratio control system. To simulate an exceptional situation, excessive wood pellets were supplied to the boiler with respect to air flow rate. In other words, the fuel feeding rate (wood pellet feeding rate) of the installed boiler was about 5.2 kg/h. With this fuel feeding rate, air blower was set to be optimum combustion condition so that combustion was stable and steady state and CO concentration of the flue gas was minimum. With this combustion condition, if fuel feeding rate is increased up to 5.4 kg/h intentionally without adjusting air flow rate, wood pellets are heaped up gradually in the combustor. As a result, O2 concentration of the flue gas of the test is smaller than that of normal condition. Fig. 5 shows flue gas O2 and CO concentration result in the

condition of without air fuel control. As seen in the figure, up to 2 hours, the CO concentration was very low and O2 concen-

tration slowly decreased. But after 2 hours from the start, CO concentration increased rapidly. The reason is as follows. Air flow rate is set to be an optimized condition at wood pellet feeding rate of 5.2 kg/h. Suddenly, the wood pellet feeding rate increased up to 5.4 kg/h, then unburned wood pellets were piled up slowly in the combustor. With this phenomenon, combustion was getting worse as time went by and CO con-centration might increase. We could see a pile of unburned wood pellets in the combustor. At test time 3:30, we inten-tionally put more air flow rate into the combustor. Then O2 concentration of the flue gas increased and CO concentration decreased. With this result, if there is no air fuel control sys-tem and wood pellet feeding rate is changed with any reason, combustion of wood pellet boiler will deteriorate.

3.2 With air fuel control

To confirm the air fuel control system installed in the wood pellet boiler, a similar experiment in chapter 3.1 was per-formed. The test result shows in Fig. 6 with O2, CO and NOx concentration in the flue gas. The test time was more than 9 hours and during all test periods the O2 concentration was

A 24kW domestic

Wood pellet boiler

137V 8.2%

PID Controller

122.7℃

Air blower

LSM11 lambda sensor

Sensor

output signal

Air blower

control signal

Flue gas analyzer

8.2

Fig. 4. Schematic diagram of the experimental facility.

Fig. 5. Flue gas O2 and CO concentration result – without air fuel control.

Fig. 6. Flue gas O2 and CO concentration result – with air fuel control.

S. B. Kang et al. / Journal of Mechanical Science and Technology 27 (6) (2013) 1701~1706 1705

constant near 8%. With this result, CO concentration also was constant at about 37 ppm. For more confirmation of performance of the air fuel con-

trol system, wood pellet feeding rate was intentionally changed as shown in Fig. 7. At the interval of 30 minute, pel-let feeding rate was changed abruptly such as 5.5, 2.8, 4.0, 2.9, 5.4 and 4.2 kg/h. Fig. 8 shows the result of voltage input of the air blower. The trend of the air blower voltage follows the wood pellet feeding rate. As pellet feeding rate abruptly de-creased, the air blower voltage slowly went down. But at 12:00 in Figs. 7 and 8, the wood pellet feeding rate suddenly increased; there was an overshoot of the air blower voltage andthen voltage decreased a little. If the wood pellet feeding rate is increasing, wood pellets piles up in the combustor in a short time due to relatively low air flow rate. So, more air is needed in order to burn the piled wood pellets in the combus-tor. Fig. 9 shows the result of O2, CO and NOx concentration for

this test, that is, the wood pellet feeding rate change test. Seen in the figure, O2 concentration remains constant during the test.

4. Conclusions

An air fuel control system was installed for a domestic wood pellet boiler by using a lambda sensor and a PID con-troller. To confirm the control system, tests with and without the air fuel control system were conducted. Conclusions of the research are as follow. (1) Lean and wideband lambda sensors are suitable for the

air fuel control system of domestic wood pellet boiler. (2) Without an air fuel control system, changes of wood pel-

let feeding rate can make the system become unstable. (3) With an air fuel control system, when wood pellets are

fed into the domestic wood pellet boiler from 50 to 100%, the O2 concentration in exhaust gas can be stable around 8.7% ± 1% and the concentration of CO in exhaust gas is also stable.

Acknowledgment

This research was funded by Korea Institute of Energy Re-search so the authors give thanks to KIER.

References

[1] Korea energy management corporation, Overview of New and Renewable Energy in Korea 2010 (2010).

[2] European committee for standardization, EN 303-5 Heating boilers for solid fuels, hand and automatically stoked, with a

nominal heat output up to 300 kW, European Standard (1991).

[3] Korea forest research institute, Wood pellet standard in Ko-rea (2009).

[4] Pellet-Zentralheisungen und pelletofen, Bundesmini-sterium fur Ernashrung, Landwirtschaft und Verb-raucherschutz.

[5] S. B. Kang, J. J. Kim and K. S. Choi, Performance test and flue gas characteristics of domestic wood pellet boilers, Pro-ceeding of SAREK 2009 Winter Annual Conference (2009) 569-573.

[6] E. F. Kristensen and J. K. Kristensen, Development and test

Fig. 7. Wood pellet feeding rate into the boiler.

Fig. 8. Voltage change of air blower as a result of air fuel control system.

Fig. 9. Flue gas O2 and CO concentration result with respect to abrupt-ly changing wood pellet feeding rate – with air fuel control.

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of small-scale batch-fired straw boilers in Denmark, Biomass & Bioenergy, 26 (2004) 561-569.

[7] A. Žandeckis, L. Timma, D. Blumberga, C. Rochas and M. Rosa, Solar and pellet combisystem for apartment buildings: Heat losses and efficiency improvements of the pellet boiler, Applied Energy (2012).

[8] Korean Agency for Technology and Standards, KS B 6205 Land boilers – heating balancing, Korea Standard (2008).

[9] Korean Agency for Technology and Standards, KS B 8109 Gas burning hot water boiler, Korea Standard (2011).

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Sae Byul Kang received Ph.D. degree in Mechanical Engineering from Seoul National University in 2003. Now he is working at Korea Institute of Energy Research (KIER) Energy Efficiency Department. Dr. Kang's research inter-ests are development of woody biomass boiler such as wood pellet and wood

chip boiler, combustion control of boiler for optimum com-bustion and program development for smart pad and smart phone.

Jong Jin Kim received master degree in Mechanical Engineering from Inha Uni-versity in 1981. Now he is working at Korea Institute of Energy Research (KIER) Energy Efficiency Department. Mr. Kim's research interests are devel-opment of woody biomass boiler such as wood pellet and wood chip boiler, in-

dustrial boiler system, calculation program development for industrial boiler and certification code development for solid biomass boiler.

Kyu Sung Choi received master de-gree in Mechanical Engineering from Kongju National University in 2003. Now he is working at Korea Institute of Energy Research (KIER) Energy Efficiency Department. Mr. Choi's research interests are development of woody biomass boiler such as wood

pellet and wood chip boiler, industrial boiler system and test for certification.

Bong Suk Sim received master degree in Mechanical Engineering from Kyungpook National University in 2012. Now he is working at Korea Institute of Energy Research (KIER) Energy Efficiency Department. Mr. Sim's research interests are develop- ment of woody biomass boiler such as

wood pellet and wood chip boiler, and CFD analysis for industrial boiler system.

Hong Young Oh received master degree in Environmental Engineering from Hanbat National University in 2011. Now he is working at Korea Institute of Energy Research (KIER) Energy Efficiency Department. Mr. Oh's research interests are develop-ment of woody biomass boiler such as

wood pellet and wood chip boiler, and control system de-velopment for wood pellet boiler system.