Cofiring Assessment of Hydrothermally-treated Empty Fruit Bunch and Low Rank Coal in a Drop Tube Furnace

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 105 (2017) 1545 – 1550

The 8th International Conference on Applied Energy – ICAE2016

Cofiring assessment of hydrothermally-treated empty fruit bunch and low rank coal in a drop tube furnace Arif Darmawana,b*, Dwika Budiantob, Muhammad Azizc, Koji Tokimatsua a

Dept. Transdisciplinary Science and Engineering, Tokyo Institute of Technology, Tokyo 226-8503, Japan Balai Besar Teknologi Konversi Energi (B2TKE-BPPT) Kawasan PUSPIPTEK Serpong, Tangerang 15314, Indonesia c Institute of Innovative Research, Tokyo Institute of Technology, Tokyo 152-8550, Japan

b

Abstract Currently there is an effort to reduce operating cost, increase the use of renewable energy and improve energy security by fuel-switching technique in existing boiler. The main objective of this study is to investigate the Cofiring behaviour of hydrothermally-treated empty fruit bunch (HT-EFB) and coal in the existing coal power plant. The research was conducted in a drop tube furnace (DTF) through computational fluid dynamics (CFD) simulation. The key performance parameters such as temperature distribution, fluid flow velocity, concentration of produced gases (CO2, CO, SO2and NO) were evaluated under various HT-EFB mass fractions: 0%, 10%, 25% and 50%. The results indicate that hydrothermal treatment process prior to the combustion is required to improve EFB quality as fuel. HT-EFB can enhance the ignition characteristics of low rank coal due to high volatile matter content in HT-EFB. Higher HT-EFB mass fraction increases the combustion temperature, therefore influences the fluid velocity. The cofiring can reduce both CO2 and SO2 amounts during combustion. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-reviewofunder responsibility of of ICAE Peer-review under responsibility the scientific committee the 8th International Conference on Applied Energy.

Keywords: co-firing; hydrothermal treatment; empty fruit bunch; computational fluid dynamics (CFD); drop tube furnace; simulation

1. Introduction In Indonesia, to fulfil the electricity demand due to high economic growth rate, the government has issued the policy to increase the share of coal-fired power plants. The country is rich of coal resources which is about 1% of the global coal reserves (about 120 Gt). In addition, it is projected that future coal production is dominated by low rank coal including lignite and brown coal [1]. Unfortunately, the

* Corresponding author. Tel.: +8180-8191-8922. E-mail address:[email protected], [email protected]

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.473

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expansion of fossil fuels- based thermal power plants is in conflict with the need to reduce CO2 emissions, which is one of the main causes of global climate change. One option to reduce emissions of coal-fired power plants is substituting the coal proportion by biomass (cofiring). Biomass cofiring can increase the domestic energy security as well as improve and extending the utilization of power plants which were initially designed for coal. Moreover, the use of renewable resources for sustainable energy supply and environmental issues motivate researchers to study the biomass-to-energy conversion [2]. Indonesia is also rich in biomass resources including wood, agricultural and industrial wastes. Among the available biomasses, empty fruit bunch (EFB) receives very high attention due to its huge availability but high environmental problems. Hence, EFB utilization for energy including cofiring is believed to be able to increase the economic value and reduce the environmental problems. Parshetti et al. investigated co-combustion characteristics of blended hydrothermally-treated empty fruit bunch (HT-EFB) and coal using a TGA-FTIR to examine the emission characteristics of gaseous pollutants[3]. Unfortunately, the detailed physical behaviour of cofired HT-EFB in a combustor is not yet completely understood. This work aims to investigate the opportunity of cofiring biomass with coal in an existing coal-fired power plant. The study will examine combustion behaviour of cofiring of HT-EFB with coal in a drop tube furnace (DTF) using computational fluid dynamics (CFD) simulation. The key performance parameters such as temperature distribution, fluid flow velocity, the concentration of produced gas (CO 2, CO, SO2 and NO) are evaluated for various cofiring ratios. 2. Overall proposed system Fig1 shows the conceptual diagram of the overall proposed cofiring system of hydrothermally treated EFB and coal. Raw EFB from palm mill is initially hydrothermally treated converting the lignocellulosic material into solid carbon which has lower moisture content and characteristics near to the coal. Hydrothermally treated EFB is then ground to achieve smaller and uniform size of particles. On the other hand, coal particles are ground initially before being fed to the dryer for water removal. Both hydrothermally treated EFB and dried coal particles are then fed together to combustor for cofiring. The heat pro cofiring is utilized to generate steam in boiler which is further utilized for power generation. The rest of heat from cofiring is utilized for hydrothermal treatment. On the other hand, a part of generated electricity is consumed internally for drying (steam compression) and others, while the larger part of electricity can be sold to the grid.

Fig 1. Conceptual diagram of the proposed cofiring system of hydrothermally treated EFB and coal

3. Computational modelling of cofiring of EFB and coal To observe the cofiring performance and its feasibility, cofiring using DTF is modelled and evaluated in terms of temperature distribution and produced gases composition. Fig 2 shows the schematic diagram

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of cofiring system of HT-EFB and coal in a DTF. The raw wet-coal is dried to certain moisture content and ground initially to small size particles. In addition, raw EFB is hydrothermally treated before being ground and fed together with the dried coal to the DTF. Hydrothermal treatment is conducted at temperature of 220 °C for 60 min. In this study, four mass fractions of HT-EFB to total mass of fuel are evaluated: 0% (100% coal), 10%, 25%, and 50%.

Fig 2. Basic schematic diagram of co-firing system

3.1. Drop tube furnace In simulation, a laboratory scale of DTF, which is a vertical tubular furnace, has a capacity of 1 kWth and dimensions of 1.5 m in height and 0.07 m in diameter is used. In addition, to facilitate an isothermal condition along the furnace, electric heaters are installed and divided into three heating zones. Coal and HT-EFB particles are mixed initially and fed using a screw feeder equipped with a vibrator. Water cooling system is also mounted outside the feeder to avoid any initial combustion before exiting from the feeder. The combustion process takes place inside the tubular furnace and in downward direction. Combustion air supply is divided into primary and secondary air with volumetric flow rates of 3 and 4 L min-1, respectively. The former is utilized to feed the fuels to furnace, while the latter covers the rest of the demand for combustion. Mixed fuel particles are fed at 45–60 kg h-1 through injection probe mounted at the top of DTF. The produced gas is exhausted from the bottom of DTF. Furthermore, the gases are cooled down and its compositions are analysed using gas analyser. Fig 3 shows the geometry of used DTF in simulation and its experimental model which is generally adopted. (a)

(b)

Pre-heater

System of coal and HT-EFB screw feeder

Electric heat furnace

Gas storage

Fig 3. The used DTF: (a) geometry of DTF in simulation, (b) experimental model of DTF

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3.2. Material The coal used in the simulation was originated from Kalimantan, Indonesia. This coal is classified as low rank coal having high moisture content and lower calorific value. Table 1 shows the compositions of used coal and EFB including both promixate and ultimate analyses. In simulation, dried coal and HT-EFB are mixed and fed together to the combustor. Table 1. Material composition of coal, EFB, and HT-EFB used in this study

Component Proximate analysis Fixed Carbon Volatile Matter Moisture Ash Ultimate analysis C H O N S Calorific Value (MJ kg-1)

Coal [4] Raw

Dried

Raw

EFB [5] HT-EFB

24.93 25.76 48.76 0.56

40.23 41.57 17.30 0.90

6.43 33.09 58.00 2.48

28.62 62.57 3.00 5.82

35.30 2.29 11.23 1.75 0.11 13.85

56.98 3.69 18.13 2.83 0.17 22.34

17.58 2.48 19.22 0.71 0.00 7.86

52.74 5.33 31.95 0.85 0.00 22.22

3.3. Simulation conditions In this simulation, a commercial CFD software ANSYS DesignModeller and Fluent ver. 16.2 (ANSYS Inc.)are used to build 3D combustor model and analyse the cofiring behaviour, respectively. The cofiringsimulation includes some considerations of governing dynamics equations (conservations of mass, momentum and enthalpy), turbulences (k-ε turbulence model), radiation heat transfer (P-1 model), particle and reaction inboth particle (Eulerian-Lagrangian model) and gas (global 2-steps reactions) phases. Some additional boundary conditions include: (1) fuel and air inlet flow rates are 1.38×10-5 kg s-1 and 1.6 ×10-4 kg s-1 at 300 K, (2) furnace wall temperature, wall roughness and internal emissivity are set to 1300 K (isothermal), 0.5 and 1, respectively, and (3) feeding wall is considered isothermal at 300 K. The combustion reactions for each coal char and HT-EFB char can be expressed as reactions (1) and (2), respectively. In addition, the reaction mechanisms for the volatile matter in the gas phase can be expressed as reactions (3–5). Chemical compositions of the fuel are assumed empirically from the mixture of coal and HT-EFB listed in Table 1. Moreover, formation of NOx due to high combustion temperature is also considered. Coal char (C) + 0.5 O2→CO

(1)

HT-EFB char (C) + 0.5 O2→CO

(2)

Coal volatile matter (CH0.39O0.24N0.043S0.0011) + 0.48O2→ CO + 0.195H2O + 0.0011SO2+ 0.022N2

(3)

HT-EFB volatile matter (CH1.21O0.45 N0.014) + 0.5775O2→ CO + 0.605H2O + 0.007N2

(4)

CO + 0.5 O2→ CO2

(5)

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4. Result and discussion 4.1 Temperature distribution Fig 4 shows the temperature distribution along the combustor under different cofiring mass fractions. In general, higher HT-EFB mass fraction increases the temperature inside the combustor. Mass fractions of 50% results in the highest outlet temperature (maximum of 1536 K). For comparison, the highest outlet temperature in case of no HT-EFB is 1347 K. The change in HT-EFB mass fraction leads to the change in heating value of the mixture, therefore the flame shape and temperature profile within the combustor changes accordingly. HT-EFB contains higher volatile matter and lower fixed carbon. The location of the high-temperature region corresponds to the combustion location of volatile matter. (a)

(b)

1.80 1.60

Height of Furnace (m)

1.40

coal fully

1.20

coal + HT EFB 10%

1.00

coal + HT EFB 25% 0.80

coal + HT EFB 50%

0.60

0.40 0.20

0.00 300

500

700

900

1100

1300

1500

Temperature (K)

(i)

(ii)

(iii)

(iv)

Fig 4. Simulation results: (a) temperature distribution, (b) combustion temperature profiles ((i) 0%, (ii) 10%, (iii) 25%, (iv) 50%)

4.2 Distribution of produced CO, CO2, SO2 and NO Fig 5 represents the concentration of produced gases during cofiring, including CO, CO 2, SO2 and NO. Regarding the produced CO concentration, higher mass fraction of HT-EFB leads to the increase of CO mass fraction during initial reaction of combustion (Fig 5(a)). The volatile matter, especially from HT-EFB, is oxidized under high combustion temperature forming CO. Afterward, CO reacts further with O2 (air) along the combustor forming CO2. In addition, cofiring with HT-EFB results in lower CO2 concentration following the increase of HT-EFB mass fraction (Fig 5(c)). On the other hand, the formation of NO is considered to be dominated by thermal NO due to high combustion temperature. Higher HT-EFB mass fraction results in significantly higher NO concentration, especially mass fraction of 50% (Fig 5(b)). In addition, SO2 compound is formed due to interaction between sulphur with air. Since HT-EFB contains no sulphur (Table 1), higher proposition of HT-EFB will impact in reduction of SO2 formation (Fig 5(c)). 5. Conclusion Cofiring of behaviour of coal and HT-EFB has been modelled and evaluated using CFD analysis under different mixing mass fractions. In general, HT-EFB mass fraction of 10 to 25% seems to be the most preferable cofiring condition in terms of temperature and produced gas compositions.

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(a)

(b)

1.80

1.60

1.60

1.40

1.40

Height of Furnace (m)

Height of Furnace (m)

1.80

1.20 1.00

coal fully

0.80

coal + HT EFB 10%

0.60

coal + HT EFB 25%

1.20 1.00

coal fully

0.80

coal + HT EFB 10% 0.60

coal + HT EFB 25%

0.40

0.40

coal + HT EFB 50%

0.20

0.20

0.00 0.E+00

0.00 0.E+00

2.E-02

4.E-02

6.E-02

8.E-02

1.E-01

1.E-01

coal + HT EFB 50%

1.E-09

2.E-09

(c)

3.E-09

4.E-09

5.E-09

NO Mass Fraction

CO Mass Fraction

(d)

1.80

1.80

1.60

1.60

1.40

1.40 Height of Furnace (m)

Height of Furnace (m)

coal fully 1.20

coal fully

1.00

coal + HT EFB 10% 0.80

coal + HT EFB 25%

0.60

coal + HT EFB 50%

0.40

1.20 1.00

0.80 0.60

coal + HT EFB 10% coal + HT EFB 25% coal + HT EFB 50%

0.40

0.20

0.20

0.00

0

0.02 0.04 0.06 0.08

0.1

0.12 0.14 0.16 0.18

0.2

0.22 0.24

0.00 0.E+00

5.E-05

1.E-04

2.E-04

CO2 Mass Fraction

2.E-04

3.E-04

3.E-04

4.E-04

4.E-04

SO2 Mass Fraction

Fig 5. Concentration of CO, NO, CO2 and SO2 along combustor

References [1]Pusdatin ESDM. Handbook of Energy & Economic Statistic of Indonesia 2014. Center of Data and Information (Pusdatin), Ministry of Energy and Mineral Resources Indonesia; 2014. [2] Prabowo B, Aziz M, Umeki K, Susanto H, Yan M, Yoshikawa K. CO2-recycling biomass gasification system for highly efficient and carbon-negative power generation. Appl Energy 2015; 158: 97–106 . [3] Parshetti GK, Quek A, Betha R, Balasubramanian R. TGA-FTIR investigation of co-combustion characteristics of blends of hydrothermally carbonized oil palm biomass (EFB) and coal. Fuel Process Technol, 2014; 118:228–234. [4] Aziz M, Budianto D, Oda T.Computational fluid dynamic analysis of co-firing of palm kernel shell and coal. Energies 2016; 9:137. [5] Novianti S, Biddinika MK, Prawisudha P, Yoshikawa K. Upgrading of palm oil empty fruit bunch employing hydrothermal treatment in lab-scale and pilot scale. Procedia Environ Sci 2014;20:46–54. [6] Yin C. Modeling of pulverized coal and biomass co-firing in a 150 kW swirling stabilized burner and experimental validation. Proc Int Conf Powder Eng (ICOPE 2009) 2009, pp. 305-310. Arif Darmawan He was born in Yogyakarta, Indonesia, in 1988. He received the B.E. degree in Engineering Physics from the Universitas Gadjah Mada, Indonesia, in 2012. He is graduate student in Department Transdisciplinary Science and Engineering, Tokyo Institute of Technology.