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Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system
Applied Energy xxx (2017) xxx–xxx
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system Arif Darmawan a,b,⇑, Dwika Budianto b, Muhammad Aziz c, Koji Tokimatsu a a
Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan Agency for the Assessment and Application of Technology (BPPT), Puspiptek Serpong, Tangerang Selatan 15314, Indonesia c Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b
h i g h l i g h t s Cofiring of hydrothermally-treated empty fruit bunch with coal is analyzed. Computational fluid dynamics is performed to clarify the cofiring behavior. Integrated system covers coal drying, HT treatment, cofiring and power generation. High power generation efficiency, about 40%, is achieved.
a r t i c l e
i n f o
Article history: Received 15 January 2017 Received in revised form 20 March 2017 Accepted 27 March 2017 Available online xxxx Keywords: Cofiring Hydrothermal treatment Empty fruit bunch Computational fluid dynamics Integrated system Energy efficiency
a b s t r a c t High-potential biomass residues from the palm oil industry such as palm kernel shells and empty fruit bunch (EFB) must be utilized with the appropriate technology to optimize its economic benefit and minimize the environmental impacts. In this study, the cofiring behavior of hydrothermally treated EFB (HT-EFB) with coal is analyzed in terms of thermal behavior including temperature distribution and the composition of gases produced (CO and CO2) through computational fluid dynamics. Several HT-EFB mass fractions are evaluated, i.e., 0%, 10%, 25%, and 50%. To complement this research, an experimental study is conducted to validate the simulation results. In general, an HT-EFB mass fraction in the range of 10–25% seems to be the most preferable cofiring condition. In addition, an integrated system is also proposed and evaluated including coal drying, HT treatment of EFB, cofiring, and power generation. Very low energy consumption during coal drying and HT treatment of EFB can be achieved. Finally, the net power generation efficiency of the proposed integrated system is approximately 40% including coal drying and HT treatment of EFB processes. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The demand for energy sustainability has encouraged researchers to study the use of renewable energy sources in replacement of fossil fuel. In Indonesia, among the numerous available energy sources, biomasses including agricultural wastes play a very important role in the energy matrix. Recently, palm plantations have been expanding significantly due to the high demand for palm oil products throughout the world [1]. According to data from the Indonesian Ministry of Agriculture, the total area of palm tree
⇑ Corresponding author at: Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. E-mail addresses: [email protected], [email protected] (A. Darmawan).
plantations was approximately 8 104 km2 in 2015 or twice as much as in 2000 (4 104 km2). This number is projected to increase to 1.3 105 km2 by 2020 [2]. Annual production of crude palm oil in Indonesia was 27.78 Mt in 2013. This production is expected to reach 37 Mt in 2019 with an annual growth rate of 4.59% [3]. Palm oil production is mainly located in Sumatera (70%) and the Kalimantan (30%) islands [4]. The massive increase of palm oil production has led to the production of a significant amount of agricultural waste. It is assumed that approximately 90% of an entire palm tree has no significant utilization, including the empty fruit bunch (EFB), palm kernel shell (PKS), and fiber [5]. This leads to many problems associated with improper disposal practices of the palm oil wastes. Among these wastes, EFB has the largest share, representing approximately 24.82 Mt per year, and has the lowest economic value due to its characteristics [3]. Advanced utilization of EFB, including
http://dx.doi.org/10.1016/j.apenergy.2017.03.122 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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energy harvesting, is urgently required from both the economic and environmental viewpoints. In the effort to harvest the energy from EFB, cofiring with coal has been identified as one of the least expensive and most efficient technologies for converting these palm oil wastes to electricity. According to the Electricity Power Supply Business Plan (RUPTL) 2016–2025 issued by State-owned Power Utility (PLN) [6] to meet the rapid increase in industrial demand for electricity due to accelerated economic growth, coal-fired power plants have become the major electricity supplier and will expand significantly until 2030. Unfortunately, although Indonesia produces coal, coal reserves are limited [7]. In addition, environmental awareness has encouraged citizens and policy makers to use environment-friendly energy resources and technologies. Cofiring of biomass and coal is believed to be an excellent solution for answering these problems as well as extending power plant lifetimes and coal reserves. Unfortunately, cofiring generally requires biofuels with a uniform quality and high energy density to allow for processing in the fuel handling and combustion equipment of existing coal-fired power plants. Different to PKS, raw EFB has drawbacks of high moisture content, up to 70 wt% on a wet basis (wb) and low bulk density [5]. New techniques have been studied to increase the cofiring rates to desired levels for EFB including drying [8], hydrothermal (HT) [9], carbonization [10], and pelletization [11]. Among them, HT treatment, which is performed as a pretreatment process prior to the thermo-chemical conversion of biomass, offers significant merits such as high conversion efficiency, elimination of the energy-intensive drying process, and relatively low operation temperatures as compared to other thermal processes [12,13]. Recently, researchers have performed studies and proposed the utilization of systems of wastes from palm oil milling, especially EFB, for energy production. Ninduangdee and Kuprianov [14] studied the combustion of EFB using fluidized bed technology with different bed materials. Diego et al. [15] conducted an experimental study of ethanol production using the EFB. Their results have shown that using only an alkaline pretreatment for the EFB is not a feasible technology. In terms of cofiring or co-combustion, Yan et al. [16] analyzed the influence of particle size distribution of coal on the co-combustion performance of sewage sludge and coal. They confirmed that decreasing the particle size of the fuel improves the combustion performance. Luk et al. [17] proposed an integrated drying and power generation using EFB. Unfortunately, there was no significant effort to minimize the exergy loss in their system, hence, the energy efficiency was very low. Furthermore, Aziz et al. [18] developed a combined utilization of the EFB and palm oil mill effluent through gasification and digestion, respectively for power generation using a gas engine. Although their system looks feasible for application, it is designed as a standalone system at a milling site, thereby requiring new construction of a gasification system that leads to higher capital costs. Considering the high potential of EFB and the number of coalbased power plants, especially in Indonesia, EFB utilization through HT treatment and cofiring with existing coal-fired power plants or ones being constructed or planned becomes very important. However, to the best of the authors’ knowledge, studies dealing with the effort to evaluate the effect of hydrothermally treated EFB (HT-EFB) cofiring to coal-fired combustors are difficult to find. A new approach is urgently required to be developed in order to estimate the potential for retrofitting an existing power plant to cofire with HT-EFB. Therefore, this study focuses on two important issues with the objective of proposing efficient EFB utilization, especially for cofiring with coal. First, coal-cofiring behaviors with HT-EFB in a drop tube furnace (DTF) are modeled and analyzed through computa-
tional fluid dynamics (CFD) in terms of thermal behaviors including temperature profiles and composition of exhausted gases (CO and CO2). Secondly, an efficient integrated system model is developed to combine the CFD simulations with the entire plant processes including hydrothermal treatment, combustion system, and steam cycle. 2. Coal-cofiring of HT-EFB Fig. 1 shows the conceptual diagram of an integrated coalcofiring system with HT-EFB for power generation. Solid and dotted lines represent material and energy (electricity, heat) streams, respectively. Raw EFB is initially shredded to a smaller size before being hydrothermally treated. Generally, HT is performed under the subcritical region of water with a relatively low temperature of less than 250 °C [19]. Some researchers have evaluated the application of HT on several biomass processes to produce hydrochar [20,21]. To eliminate the drying process after HT, continuous HT using temperatures slightly higher than saturated one is adopted. Therefore, moisture inside the EFB is evaporated and exhausted together with the steam required for HT. The combined HT-EFB mixture is exhausted from the reactor with relatively low moisture content. In parallel, coal is initially ground and dried to the lower moisture content before being mixed with HT-EFB. The mixed HT-EFB and coal is then cofired in a combustor producing hightemperature heat for steam generation using a boiler. Then, the generated steam expands in the steam turbine generating the necessary electricity. In addition, the exhausted flue gas from boiler is utilized mainly for HT and coal drying. 3. Numerical modeling and calculation for cofiring of HT-EFB and coal 3.1. Schematic diagram of coal cofiring with HT-EFB To observe the cofiring performance and its feasibility when using a mixture of coal-HT-EFB, cofiring using a DTF is modeled and evaluated in terms of temperature distribution and composition of the produced gases. Fig. 2 shows the schematic diagram of a coal-cofiring system with HT-EFB in a DTF. The raw, wet coal is initially dried to a specified low moisture content and ground to achieve small and uniformly sized particles. Raw EFB is hydrothermally treated before being ground and fed together with the dried coal to the DTF. Hydrothermal treatment was based on the experimental results that were conducted at pressures of 2.4 MPa for 60 min [22]. In this study, four mass fractions of HT-EFB to total mass of mixed fuel are evaluated, i.e., 0 (100% coal), 10, 25, and 50%. 3.2. DTF and materials Currently, despite the inherent differences in combustion behavior between a DTF and realistic combustion conditions, the DTF is still considered an effective component for evaluating certain combustion behaviors of fuel [23]. DTF is capable for facilitating an environment that simulates industrial conditions such as a fast heating rate, short residence time, and dilute particle phases [24]. In general, the DTF consists of a fuel feeding system, reactor, and particle collection system at the outlet side. HT-EFB is mixed initially with coal before being introduced into the feeding system. In simulation, a laboratory scale DTF, which is a vertical tubular furnace, having dimensions of 1.5 m in height and 0.07 m in diameter and a capacity of 1 kWth is used. Detailed geometry of the DTF is shown in Fig. 3. In addition, to facilitate isothermal conditions
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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EFB
Hydrothermal treatment
Grinding
Mixing Wet coal
Grinding
Cofiring
Power
Electricity
Drying
Fig. 1. Basic schematic diagram of HT-EFB and coal-cofiring system.
EFB
Hydrothermal treatment
Grinding
Mixer
Wet coal
Grinding
DTF
Drying
Fig. 2. Basic schematic diagram of coal-cofiring system with HT-EFB.
Fig. 3. Geometry of the DTF used for the simulation and experimental validation.
along the furnace, electric heaters are installed and divided into three heating zones. The combustion process takes place inside the tubular furnace and in the downward direction. Combustion air supply is divided into primary and secondary air with volumetric flow rates of 3 and 4 L min1, respectively. The former is utilized to feed the fuels to the combustor, while the latter covers the rest of the demand for combustion. Mixed fuel particles are fed at 45–60 g h1 through an injection probe mounted at the top of the DTF. The produced gas is exhausted from the bottom of the DTF. The coal used in the simulation is originated from Kalimantan, Indonesia. This coal is classified as a low-rank coal having a high moisture content and low calorific value. Table 1 shows the compositions of used coal and HT-EFB including proximate and ultimate analyses. 3.3. Computational modeling CFD is an effective tool to model and calculate fluid flow, heat and mass transfers, and chemical reactions as well as interaction of solids and fluids [26]. CFD modeling methods for the combustion
of biomass particles is challenging work. However, compared to physical investigation through experiment, CFD modeling is significantly more effective from a time and cost perspective as well as safe and easy for scaling up. Hence, it is usually adopted before performing an experimental study. Related to cofiring using CFD analysis, it is expected that the combustion performance for all stages of the combustion including the combustion temperatures, kinetics behavior, and concentration of the produced gases can be clarified. In this simulation, a commercial CFD software ANSYS DesignModeler and Fluent ver. 16.2 (ANSYS Inc.) are used to build a 3D combustor model and analyze the cofiring behavior. Cofiring simulation considers dynamical equations, conservation of mass (continuity), momentum and enthalpy, turbulence (k-e turbulence model), radiation heat transfer (P-1 model), and reactions in both particle (Eulerian-Lagrangian model) and gas (global 2-steps reactions) phases [27]. Additional boundary conditions are the following: (1) fuel and air inlet flow rates are 1.38 105 and 1.6 104 kg s1 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) the feeding wall is considered isothermal at 300 K. In the conserved form, the continuity, momentum, and total energy conservations can be represented as Eqs. (1)–(5), respectively [28].
@q VÞ ¼ 0 þ r ðq~ @t
ð1Þ
@ðquÞ @p @ sxx @ syx @ szx VÞ ¼ þ þ r ðqu~ þ þ þ qf x @t @x @x @y @z
ð2Þ
! @ðqv Þ @p @ sxy @ syy @ szy þ r ðqv V Þ ¼ þ þ þ þ qf y @t @y @x @y @z
ð3Þ
@ðqwÞ @p @ sxz @ syz @ szz þ r ðqw~ VÞ ¼ þ þ þ þ qf z @t @z @x @y @z
ð4Þ
" !# " !# @ V2 V2 ~ q eþ þr q eþ V @t 2 2 @ @T @ @T @ @T @ ðupÞ þ þ k k k ¼ qq_ þ @x @x @y @y @z @z @x @ ðv pÞ @ ðwpÞ @ ðusxx Þ @ usyx @ ðuszx Þ @ v sxy þ þ þ þ @y @x @y @z @x @z @ v syy @ v szy @ ðwsxz Þ @ wsyz @ ðwszz Þ þ f ~ V þ þ q~ þ þ þ @x @z @y @z @y ð5Þ where q, V, p, f, s, k, and q_ are the density, velocity, pressure, body force per unit mass, shear force, thermal conductivity, and heat
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Table 1 Material composition. Component
Raw coal [25]
Dried coal [25]
Raw EFB [1]
HT-EFB [22]
Proximate analysis Fixed carbon (wt% wb) Volatile matter (wt% wb) Moisture (wt% wb) Ash (wt% wb)
24.93 25.76 48.76 0.56
40.23 41.57 17.30 0.90
3.71 34.84 60.00 1.46
28.62 62.57 3.00 5.82
Ultimate analysis C (wt% wb) H (wt% wb) O (wt% wb) N (wt% wb) S (wt% wb) Calorific value (MJ kg1)
35.30 2.29 11.23 1.75 0.11 13.84
56.98 3.69 18.13 2.83 0.17 22.34
17.97 2.49 17.60 0.47 0.01 17.02
52.92 5.35 32.06 0.85 0.00 22.22
transferred through thermal conduction per unit time per unit area, respectively. The values u, v, and w are the velocity components in each x, y, and z directions, respectively. In addition, e, V 2 =2, and e þ V 2 =2 are the internal energy, kinetic energy per unit mass, and total energy, respectively. The turbulence is approximated by using a standard k-e model, in which the transport equations for each k and can be represented as Eqs. (6) and (7), respectively [29].
@ @ @ ðqkÞ þ ðqkui Þ ¼ @t @xi @xj @ @ @ ðqui Þ ¼ ðqÞ þ @t @xi @xj
lt @k þ 2lt Eij Eij q rk @xj
lþ
ð6Þ
lt @ 2 þ C 1 2lt Eij Eij qC 2 r @xj k k ð7Þ
where k, , ui, and Eij are the turbulent kinetic energy, turbulent dissipation, velocity component in the corresponding direction, and deformation rate component, respectively. Furthermore, the turbulent viscosity, lt, is calculated using Eq. (8).
lt ¼ q C l
k
2
ð8Þ
In addition, in this study, the values of some of the required constants are defined as: C1e = 1.44; C2e = 1.92; Cl = 0.09; rk = 1.0; and re = 1.3. These values are based on the recommendation of Launder and Spalding [29] for free turbulent flows. Regarding radiation heat transfer, the following P-1 radiation model, which is based on the first order of the spherical harmonic expansion of the radiation intensity, is adopted for this study.
qrad ¼ Crad rG
ð9Þ
rðCrad rGÞ aG þ 4arT 4 ¼ SG
ð10Þ
1 3ða þ rs Þ C rs
ð11Þ
Crad ¼
where qrad, G, a, r, rs, SG, and C are the radiation flux, incident radiation, absorption coefficient, Stefan-Boltzmann constant (5.68 108 W m2 K4), scattering coefficient, user-defined radiation source, and linear-anisotropic phase function coefficient, respectively. The reactions in both particle and gas phases for coal cofiring with HT-EFB is shown as the following reactions: (i) Char of coal
C þ 0:5O2 ! CO
ð12Þ
(ii) Char of HT-EFB
C þ 0:5O2 ! CO
ð13Þ
(iii) Volatile matter of coal
CH0:39 O0:24 N0:043 S0:0011 þ 0:48O2 ! CO þ 0:195H2 O þ 0:0011SO2 þ 0:022N2
ð14Þ
(iv) Volatile matter of HT-EFB
CH1:21 O0:45 N0:014 þ 0:5775O2 ! CO þ 0:605H2 O þ 0:007N2
ð15Þ
CO þ 0:5O2 ! CO2
ð16Þ
3.4. Model validation To validate the model, comparison between the simulation and experiment-based validation test results of coal combustion using DTF is performed. Fig. 4 shows the geometry of the experimental model used in this study for validation. In the validation test, coal is fed using a screw feeder equipped with a vibrator. A water-cooling system is also mounted outside the feeder to avoid any initial combustion before exiting the feeder. The combustion process takes place inside the tubular furnace and in the downward direction. Combustion air supply is divided into primary and secondary air with volumetric flow rates of 3 and 4 L min1, respectively. The airflow rate is determined by stoichiometric calculation with air excess of combustion set at 20%. The former is utilized to feed the fuels to the furnace, while the latter covers the rest of the demand required for combustion. Fuel particles are fed at 45 g h1 through the injection probe mounted at the top of the DTF. The mean particle size of the coal is 75 mm. The produced gas is exhausted from the bottom of the DTF. Furthermore, the exhaust gases are cooled and the composition is analyzed using a gas analyzer. Fig. 5(a) compares the predicted temperature distribution with the experimental values for coal combustion. The orange and red lines represent the predicted and the measured temperature distribution along the combustor, respectively. The results show that the simulation model for combustion in the DTF is very accurate. The validation study also examines gas composition (CO and CO2) generated during combustion (Fig. 5(b) and (c)). To validate the gas emissions in the simulation, the gas composition in the experimental study is also measured. The dots in Fig. 5(b) and (c) show the CO and CO2 pollutants observed during the experimental study at the corresponding height. Meanwhile, the red line indicates the CO and CO2 emissions based on the simulation model. The emission values in the simulation model appear to be in good agreement with the experimental data.
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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5
Fig. 4. Experimental model of DTF for model validation.
Based on these results, it is confirmed that the developed model has a very high validity to predict the combustion behavior of the fuel inside the DTF. Since HT-EFB and coal is treated as single fuel after being mixed, the developed model can be adopted to analyze the combustion behavior of coal cofiring with HT-EFB. Nevertheless, some attention should be paid primarily to the particle size and shape due to different characteristics between coal and biomass.
4. Integrated system of HT, cofiring, and combined cycle 4.1. Process modeling and calculation In the existing system, a coal-fired power system mainly comprises of drying, combustion, and a power generation module. The wet coal is ground to smaller size particles before being fed
to the dryer system. Practically, the grinding of coal particles is performed in two steps: before drying and after drying. The former is important to achieve uniform and small particle size (about 1–2 mm) to facilitate an excellent heat transfer and moisture removal rate during drying. The latter is performed to achieve smaller particle size, below 0.15 mm (pulverized coal), with the aim of achieving excellent combustion performance. Dried coal is then flown to the combustor chamber utilizing air as the feeding gas. As the combustion occurs inside the combustor, the heat generated from combustion is recovered by a superheater and economizer to generate steam for the steam turbine. In this conventional system, no effort is made to minimize the exergy destruction or improve the total energy efficiency. Fig. 6 shows the schematic process flow diagram of the integrated system consisting of HT treatment of the EFB, coal drying, cofiring, and power generation, developed based on the principles of enhanced process integration (EPI) technology. The integration
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Height of Furnace (m)
1.60 1.40 1.20 1.00
Experimental result (coal)
0.80 0.60 300
500
700
900
1100
1300
1500
Temperature (K)
(a) 1.80
P
1.60
Height of Furnace (m)
Therefore, drying after HT treatment can be eliminated and the produced HT-EFB can be directly cofired with coal after being ground to produce pulverized HT-EFB. Coal drying is also performed using superheated steam with vapor recompression technology. Dried coal and HT-EFB is premixed with a specified mass ratio before being fed to the combustor. The reaction during combustion follows the reaction discussed in Section 3.3 (Reactions (12)–(16)). The required drying temperature to achieve the target moisture content of 17.30 wt% wb is 113 °C according to [36]. Other assumed conditions are shown in Table 2. The calculation is conducted using the process simulator Aspen Plus ver. 8.8 (Aspen Technology, Inc.). The flow rates of raw EFB and wet coal are 12.5 and 35 kg s1, respectively. Therefore, the HT-EFB mass fraction during cofiring is about 19.5%. The net power generation efficiency (g) is obtained by the following equation
P
gross int g¼ _ _ EFB CV EFB Þ ðmcoal CV coal Þ þ ðm
ð17Þ
1.40
_ coal , m _ EFB , CV coal , and CV EFB are the power output where Pgross , Pint , m (gross generated power), internal energy consumption (compressor work, pump, blower, etc.), mass flow rate of coal, mass flow rate of EFB, calorific value of wet coal, and calorific value of the raw EFB, respectively.
1.20
1.00
experimental result (coal) 0.80
0.60 0.12
5. Results and discussion 0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
5.1. Temperature distribution
CO2
(b) 1.80
Height of Furnace (m)
1.60
1.40
1.20
1.00
Experimental result (coal) 0.80
0.60
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
(c) Fig. 5. Measured and predicted results in coal combustion: (a) flame temperature, (b) produced CO2, and (c) produced CO.
of the involved processes is carried out by implementing the principles of EPI technology. As an effort to optimize the heat circulation and minimize the exergy loss throughout the integrated system, EPI has been explained sufficiently in [30,31] and applied in some fields including coal conversion [32], biomass gasification [33], and algae-based power generation [34]. Raw EFB is initially shredded to smaller and more uniform size before being fed to the HT reactor. For the HT treatment, a continuous autoclave with a tube heat exchanger installed inside is adopted as the main reactor. The continuous autoclave has already been developed and applied in industry [35] for this type of purpose. Tube heat exchangers are installed to facilitate heat exchange between the compressed evaporated moisture and the EFB leading to self-heat exchange. HT treatment is performed in superheated conditions slightly higher than saturation point, hence the moisture in the EFB is in vapor condition and the produced HT-EFB has low moisture content.
The HT-EFB after hydrothermal treatment is found to become more uniform and coal-like. Hydrothermal treatment also can improve the drying and dehydration performance; thus, the moisture content of the HT-EFB decreases to approximately 3%. This characteristic is very important in the combustion system. Fig. 7 shows the temperature distribution along the axis of the DTF under different cofiring mass fractions. The figure excludes the axis of the DTF at a high of 0–0.6 m considering that there is no substantial change in the bottom of the DTF and can be neglected. The dots in Fig. 7 correspond to the measured results obtained from experimental validation for coal. In general, a higher HT-EFB mass fraction will increase the temperature inside the combustor. HT-EFB mass fractions of 50% results in the highest outlet temperature (maximum of 1536 K). For comparison, the highest outlet temperature in the case of no HT-EFB is 1347 K. The change in the HT-EFB mass fraction leads to a change in the heating value of the mixture; therefore, the flame shape and temperature profile within the combustor changes accordingly. Fig. 8 shows the temperature distribution across the combustor under different HT-EFB mass fractions. HT-EFB contains higher volatile matter content but it has a lower fixed carbon content than coal, as can be observed in Fig. 8. The location of the hightemperature region corresponds to the combustion location of the volatile matter and the oxygen availability. The fuel combustion process involves three basics stages: devolatilization, volatile combustion, and char oxidation. Compared to the main combustion area, a lower temperature is observed in the upper combustor where the devolatilization process occurs. The mixture is pyrolyzed and then evolves as volatile matter. The devolatilization of HT-EFB particles occurs earlier and in a shorter time than the coal because of the lower moisture content of the HT-EFB and higher volatile matter content. On the other hand, since coal has significantly higher moisture content, its particles require a longer time for drying once devolatilization occurs. Therefore, for a high HT-EFB mass fraction, the flame temperature remains high and is distributed more uniformly, although
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Wet coal
Compressed steam
Compressor
Grinder Separator
Preheater
Dryer
HRSG High pressure steam
Flue gas
Combustor Dried coal Economizer
Air Dried HT EFB
Steam turbine
Condenser
Grinder
Wet raw EFB
Pump
Superheater Splitter Shredder
Cooler Blower (Make-up waater)
Preheater
Blower
Feeder
Recycled steam Condensate
HT treatment reactor
Fig. 6. Schematic process flow diagram of integrated HT treatment of EFB, coal drying, cofiring, and power generation.
Table 2 Assumptions given during the calculation of the integrated system. Parameter(s)
Value
Adiabatic efficiency of compressor and blower (%) Minimum approach of heat exchangers (°C) HRSG outlet pressure (MPa) Steam turbine isentropic efficiency (%) Minimum outlet vapor quality (%) Discharge pressure of steam turbine (MPa)
87 30 20 90 0.9 0.13
1.80
Height of Furnace (m)
1.60
1.40
coal fully coal + HT EFB 10%
1.20
coal + HT EFB 25% coal + HT EFB 50%
1.00
Experimental result (coal 100%) 0.80
0.60 300
500
700
900
1100
1300
1500
Temperature (K) Fig. 7. Temperature distributions inside the combustor of HT-EFB cofiring.
it is located in the lower part of the combustor. In addition, since the HT-EFB has lower moisture content than coal, a high HT-EFB mass fraction leads to a lower total moisture content for the mixed fuel in the combustor system. Finally, this condition affects the flame temperature due to the high heat capacity of water.
Fig. 8. Temperature distribution (K) of coal and HT-EFB cofiring across the combustor: (a) coal fully, (b) HT-EFB 10%, (c) HT-EFB 25%, and (d) HT-EFB 50%.
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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In contrast to the full coal combustion, HT-EFB cofiring has increased CO and NO concentrations in the combustion. Figs. 9 and 10 provide additional information regarding the concentrations of CO and CO2 gases during cofiring. Regarding the produced CO concentration, a higher mass fraction of HT-EFB leads to an increase in the CO mass fraction during the initial combustion reaction. The volatile matter, especially from HT-EFB, is oxidized under high combustion temperatures that allow the formation of CO. Afterward, CO reacts with O2 (air) in the combustor forming CO2. In addition, coal cofiring with HTEFB results in lower CO2 concentration following an increase in the HT-EFB mass fraction (Fig. 9). The dots in Fig. 10 show the CO2 pollutant observed during the experimental study, while the lines show CO2 emissions based on the simulation model.
1.60
Height of Furnace (m)
5.2. Distribution of produced CO and CO2
1.40
coal fully 1.20
coal + HT EFB 10% coal + HT EFB 25%
1.00
coal + HT EFB 50% 0.80
experimental result (coal 100%) 0.60
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
CO2 Fig. 10. CO2 mass fraction along the combustor height under different HT-EFB mass fractions.
5.3. Integrated system performance Fig. 11 and Table 3 show the detailed schematic process flow diagram of coal drying and the condition of each stream, respectively. The energy required for coal drying with exergy recovery is 2.48 MW, mainly for compression work. This compression is performed to increase the steam pressure to 0.2 MPa to facilitate selfheat exchange between the compressed steam and wet coal. Through compression, the saturation point of the steam increases accordingly that leads to the possibility of coupling the latent heat. In this process, wet coal (1) with moisture content of 48.76 wt% wb is preheated up to 90 °C before flowing to the dryer using flue gas originally exhausted from the gas turbine. Furthermore, to further minimize the exergy destruction throughout the integrated system, the condensed, compressed steam (8) is utilized in the HT treatment of the EFB for preheating. Fig. 12 and Table 4 show the detailed schematic process flow diagram of the HT treatment of the EFB and stream properties, respectively. Wet raw EFB (11) is initially cut and shredded to a smaller size before being preheated and fed to the HT treatment reactor. The steam evaporated during HT treatment (17) is exhausted and split into recycled steam (22) and superheated steam (18). The superheated steam is superheated using the hot gas produced from the combustor for heat combination, which increases the exergy rate of the steam. Therefore, this steam can be recycled back as the heat source for subsequent HT treatment (20). The amount of heat to superheat the steam is 20.1 MW. This energy requirement potentially decreases the possible generated power in the event that no further effort regarding heat recovery is carried out. To recover the waste heat from the HT treatment
1.80
1 (Wet coal)
Flue gas
Grinder
7 Compressed steam 6
5
9
Compressor
Separator
2 3 Preheater
Dryer 10
Cold flue gas
8
Dried coal 4
As EFB preheater Fig. 11. Schematic diagram of a coal drying process employing EPI.
Table 3 Stream properties in the coal drying process. Stream
Phase
Mass flow (kg s1)
Temperature (°C)
Pressure (MPa)
1 2 3 4 5 6 7 8 9 10
Mixed Mixed Mixed Mixed Vapor Vapor Vapor Vapor-liquid Gas Mixed
35.00 35.00 35.00 21.69 13.31 13.31 13.31 13.31 276.39 276.39
25 25 90 113 113 113 210 120 120 102
0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.1 0.1
Height of Furnace (m)
1.60 1.40
coal fully
1.20
coal + HT EFB 10% 1.00
coal + HT EFB 25% coal + HT EFB 50%
0.80
Experimental result (coal 100%) 0.60 0.E+00
2.E-02
4.E-02
6.E-02
8.E-02
1.E-01
1.E-01
Fig. 9. CO mass fraction along the combustor height under different HT-EFB mass fractions.
reactor, high-temperature stream (21) is piped to the economizer acting as a preheater for the water recycled from the steam turbine cycle. Due to the physical and chemical changes of the HT-treated EFB and the performance of the heat exchangers, the heat involved in the HT treatment process cannot be fully recovered in the same process. However, by integrating with the other processes through EPI to minimize the exergy destruction, HT treatment only reduces the produced gross electricity by 1.6 MW. Fig. 13 shows the comparison of energy consumed during coal drying, HT treatment of EFB, and internal consumption in the power generation system. The internal consumption required during power generation, such as the high-pressure pump, represents the highest power consumption during the cycle. On the other
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Combustor
Grinder
Wet raw EFB 11
16 Dried HT EFB
15
From coal drying
20 Splitter 18 19 17 22 Blower 21
8 Shredder 12
Superheater
To economizer 13 14 Feeder
Preheater
Blower 23 Recycled steam
24 Condensate
HT treatment reactor
Fig. 12. Detailed schematic process flow diagram of the proposed HT treatment process of EFB employing EPI.
Table 4 Stream properties in HT treatment for EFB. Stream
Phase
Mass flow (kg s1)
Temperature (°C)
Pressure (MPa)
11 12 13 14 15 16 17 18 19 20 21 22 23 24
Mixed Mixed Mixed Mixed Mixed Mixed Vapor Vapor Vapor Vapor Liquid Vapor Vapor Liquid
12.50 12.50 12.50 12.50 5.24 5.24 12.26 7.26 7.26 7.26 7.26 5.00 5.00 13.31
25 25 80 80 230 230 230 230 230 465 183 230 230 86
0.1 0.1 0.1 2.4 0.1 0.1 2.4 2.4 2.41 2.41 2.41 2.41 2.41 0.2
Energy consumpon (MW)
14 12 10
Referring to Fig. 6, it is obvious that changes are needed primarily in the drying and hydrothermal treatment processes. The proposed system is applicable and replicable for various capacities. The proposed system is based on EPI technology, which is the integration of exergy recovery and process integration. Both technologies are currently available and have been performed by several researchers. However, as a methodological approach, EPI tries to efficiently combine both technologies. Therefore, they can be applied easily and effectively with optimum results in energy efficiency. Additional investment cost will be required for the retrofitting of the drying system, hydrothermal treatment system, and boiler, respectively. In the high-temperature heat exchanger, especially for the hydrothermal treatment process, more attention should be paid to the materials, thermal stresses during startup, and load fluctuations. To account for the high temperatures of the heat exchanger, materials that can be used include carbon fiber-SiC composite or various ceramics oxides. However, the use of these materials may contribute to a higher cost for a heat exchanger able to withstand temperatures above 700 °C. Furthermore, optimization of the design and operation of both the drying system and hydrothermal treatment system can lead to higher efficiencies.
8 6
6. Conclusion
4 2 0
Internal consump
n
Coal drying
HT treatment EFB
Fig. 13. Comparison of the energy demand in coal drying, HT treatment of EFB, and internal consumption for power generation.
hand, the proposed coal drying and HT treatment of the EFB show lower energy consumption due to very low exergy loss. The proposed integrated system with EPI can achieve a net power generation efficiency of 39.7% including the processes for coal drying and HT treatment of the EFB. A significant energy reduction in both coal drying and HT treatment of the EFB can contribute to the high net power generation efficiency of the integrated system.
Advanced utilization of EFB as an energy source has been proposed and studied. Initially, the cofiring behavior of coal with HT-EFB has been modeled and evaluated using CFD analysis under different mixing mass fractions. In general, a HT-EFB mass fraction of 10–25% appears to be the most preferable cofiring condition in terms of temperature and produced gas compositions. In addition, an innovative integrated system consisting of HT treatment for the EFB, coal drying, cofiring, and power generation has also been modeled. Very low energy consumptions during coal drying and HT treatment of the EFB, which are 2.48 and 1.6 MW, respectively, can be achieved. Finally, the net power generation efficiency of the proposed integrated system is approximately 40% including coal drying and HT treatment of the EFB. The application of the proposed system is believed to be able to increase the effective utilization of EFB as well as reduce the environmental impact and extend the life of current coal-fired power plants.
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Acknowledgments The authors greatly appreciate the support of The Indonesia Endowment Fund for Education (LPDP). The authors also thank Ms. Annisa Nurdiawati and Dr. M. Kunta Biddinika from the Tokyo Institute of Technology for their assistance and advice. References [1] Lam PS, Lam PY, Sokhansanj S, Lim CJ, Bi XT, Stephen JD, et al. Steam explosion of oil palm residues for the production of durable pellets. Appl Energy 2015;141:160–6. [2] Tarigan SD, Widyaliza S. Expansion of oil palm plantations and forest cover changes in Bungo and Merangin districts, Jambi Province, Indonesia. Proc Environ Sci 2015;24:199–205. [3] BPS. Statistik Kelapa Sawit Indonesia. Badan Pusat Statistik Indonesia; 2014. p. 96. [4] Aziz M, Oda T, Kashiwagi T. Design and analysis of energy-efficient integrated crude palm oil and palm kernel oil processes. J Jpn Inst Energy 2015;94:143–50. [5] Aziz M, Prawisudha P, Prabowo B, Budiman BA. Integration of energy-efficient empty fruit bunch drying with gasification/combined cycle systems. Appl Energy 2015;139:188–95. [6] Indonesia State-owned Power Utility (PLN). Electricity Power Supply Business Plan (RUPTL) 2016–2025.
[16] Yan YF, Zhang ZE, Zhang L, Zhang L. Influence of coal properties on the cocombustion characteristics of low-grade coal and city mud. Global NEST J 2014;16:330–9. [17] Luk HT, Lam TYG, Oyedun AO, Gebreegziabher T, Hui CW. Drying of biomass for power generation: a case study on power generation from empty fruit bunch. Energy 2013;63:205–15. [18] Aziz M, Oda T, Kashiwagi T. Advanced power generation using biomass wastes from palm oil mills. Appl Therm Eng 2017;114:1378–86. [19] Li W, Guo J, Cheng H, Wang W, Dong R. Two-phase anaerobic digestion of municipal solid wastes enhanced by hydrothermal pretreatment: viability, performance and microbial community evaluation. Appl Energy 2017;189:613–22. [20] Chiaramonti D, Prussi M, Buffi M, Rizzo AM, Pari L. Review and experimental study on pyrolysis and hydrothermal liquefaction of microalgae for biofuel production. Appl Energy 2017;185:963–72. [21] Roman S, Nabais JMV, Laginhas C, Ledesma B, Gonzales JF. Hydrothermal carbonization as an effective way of densifying the energy content of biomass. Fuel Process Technol 2012;103:78–83. [22] Novianti S, Biddinika MK, Prawisudha P, Yoshikawa K. Upgrading of palm oil empty fruit bunch employing hydrothermal treatment in lab-scale and pilot scale. Proc Environ Sci 2014;20:46–54. [23] Du SW, Chen WH, Lucas JA. Pulverized coal burnout in blast furnace simulated by a drop tube furnace. Energy 2010;35:576–81. [24] Manquais KL, Snape C, McRobbie I, Barker J, Pellegrini V. Comparison of the combustion reactivity of TGA and drop tube furnace chars from a bituminous coal. Energy Fuels 2009;23:4269–77. [25] Aziz M, Budianto D, Oda T. Co-firing of palm kernel shell into coal fired power plant. Energies 2016;9:137. [26] Ranade VV, Gupta DF. Computational modeling of pulverized coal fired boilers. Boca Raton, USA: CRC Press; 2015. [27] Budianto D, Aziz M, Cahyadi, Oda T. Numerical investigation of co-firing of palm kernel shell into pulverized coal combustion. J Jpn Inst Energy 2016;95:605–14. [28] Anderson JD. Governing equations of fluid dynamics. In: Wendt JF, editor. Computational fluid dynamics. Berlin, Germany: Springer-Verlag; 2009. p. 15–51. [29] Launder BE, Spalding DB. The numerical computation of turbulent flows. Comput Methods Appl Mech Eng 1974;3:269–89. [30] Aziz M, Oda T, Kashiwagi T. Integration of energy-efficient drying in microalgae utilization based on enhanced process integration. Energy 2014;70:307–16. [31] Kansha Y, Kotani Y, Aziz M, Kishimoto A, Tsutsumi A. Evaluation of a self-heat recuperative thermal process based on thermodynamic irreversibility and exergy. J Chem Eng Jpn 2013;46:87–91. [32] Aziz M, Juangsa FB, Kurniawan W, Budiman BA. Clean co-production of H2 and power from low rank coal. Energy 2016;116:489–97. [33] 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. [34] Aziz M. Power generation from algae employing enhanced process integration technology. Chem Eng Res Des 2016;109:297–306. [35] Catelli C, Notari R. Autoclave for the continuous cooking and sterilizing of food products in general. European Patent, EP0691271 B1; 2000. [36] Aziz M, Oda T, Kashiwagi T. Energy-efficient low rank coal drying based on enhanced vapor recompression technology. Dry Technol 2014;32:1621–31.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system Arif Darmawan a,b,⇑, Dwika Budianto b, Muhammad Aziz c, Koji Tokimatsu a a
Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan Agency for the Assessment and Application of Technology (BPPT), Puspiptek Serpong, Tangerang Selatan 15314, Indonesia c Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b
h i g h l i g h t s Cofiring of hydrothermally-treated empty fruit bunch with coal is analyzed. Computational fluid dynamics is performed to clarify the cofiring behavior. Integrated system covers coal drying, HT treatment, cofiring and power generation. High power generation efficiency, about 40%, is achieved.
a r t i c l e
i n f o
Article history: Received 15 January 2017 Received in revised form 20 March 2017 Accepted 27 March 2017 Available online xxxx Keywords: Cofiring Hydrothermal treatment Empty fruit bunch Computational fluid dynamics Integrated system Energy efficiency
a b s t r a c t High-potential biomass residues from the palm oil industry such as palm kernel shells and empty fruit bunch (EFB) must be utilized with the appropriate technology to optimize its economic benefit and minimize the environmental impacts. In this study, the cofiring behavior of hydrothermally treated EFB (HT-EFB) with coal is analyzed in terms of thermal behavior including temperature distribution and the composition of gases produced (CO and CO2) through computational fluid dynamics. Several HT-EFB mass fractions are evaluated, i.e., 0%, 10%, 25%, and 50%. To complement this research, an experimental study is conducted to validate the simulation results. In general, an HT-EFB mass fraction in the range of 10–25% seems to be the most preferable cofiring condition. In addition, an integrated system is also proposed and evaluated including coal drying, HT treatment of EFB, cofiring, and power generation. Very low energy consumption during coal drying and HT treatment of EFB can be achieved. Finally, the net power generation efficiency of the proposed integrated system is approximately 40% including coal drying and HT treatment of EFB processes. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The demand for energy sustainability has encouraged researchers to study the use of renewable energy sources in replacement of fossil fuel. In Indonesia, among the numerous available energy sources, biomasses including agricultural wastes play a very important role in the energy matrix. Recently, palm plantations have been expanding significantly due to the high demand for palm oil products throughout the world [1]. According to data from the Indonesian Ministry of Agriculture, the total area of palm tree
⇑ Corresponding author at: Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. E-mail addresses: [email protected], [email protected] (A. Darmawan).
plantations was approximately 8 104 km2 in 2015 or twice as much as in 2000 (4 104 km2). This number is projected to increase to 1.3 105 km2 by 2020 [2]. Annual production of crude palm oil in Indonesia was 27.78 Mt in 2013. This production is expected to reach 37 Mt in 2019 with an annual growth rate of 4.59% [3]. Palm oil production is mainly located in Sumatera (70%) and the Kalimantan (30%) islands [4]. The massive increase of palm oil production has led to the production of a significant amount of agricultural waste. It is assumed that approximately 90% of an entire palm tree has no significant utilization, including the empty fruit bunch (EFB), palm kernel shell (PKS), and fiber [5]. This leads to many problems associated with improper disposal practices of the palm oil wastes. Among these wastes, EFB has the largest share, representing approximately 24.82 Mt per year, and has the lowest economic value due to its characteristics [3]. Advanced utilization of EFB, including
http://dx.doi.org/10.1016/j.apenergy.2017.03.122 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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energy harvesting, is urgently required from both the economic and environmental viewpoints. In the effort to harvest the energy from EFB, cofiring with coal has been identified as one of the least expensive and most efficient technologies for converting these palm oil wastes to electricity. According to the Electricity Power Supply Business Plan (RUPTL) 2016–2025 issued by State-owned Power Utility (PLN) [6] to meet the rapid increase in industrial demand for electricity due to accelerated economic growth, coal-fired power plants have become the major electricity supplier and will expand significantly until 2030. Unfortunately, although Indonesia produces coal, coal reserves are limited [7]. In addition, environmental awareness has encouraged citizens and policy makers to use environment-friendly energy resources and technologies. Cofiring of biomass and coal is believed to be an excellent solution for answering these problems as well as extending power plant lifetimes and coal reserves. Unfortunately, cofiring generally requires biofuels with a uniform quality and high energy density to allow for processing in the fuel handling and combustion equipment of existing coal-fired power plants. Different to PKS, raw EFB has drawbacks of high moisture content, up to 70 wt% on a wet basis (wb) and low bulk density [5]. New techniques have been studied to increase the cofiring rates to desired levels for EFB including drying [8], hydrothermal (HT) [9], carbonization [10], and pelletization [11]. Among them, HT treatment, which is performed as a pretreatment process prior to the thermo-chemical conversion of biomass, offers significant merits such as high conversion efficiency, elimination of the energy-intensive drying process, and relatively low operation temperatures as compared to other thermal processes [12,13]. Recently, researchers have performed studies and proposed the utilization of systems of wastes from palm oil milling, especially EFB, for energy production. Ninduangdee and Kuprianov [14] studied the combustion of EFB using fluidized bed technology with different bed materials. Diego et al. [15] conducted an experimental study of ethanol production using the EFB. Their results have shown that using only an alkaline pretreatment for the EFB is not a feasible technology. In terms of cofiring or co-combustion, Yan et al. [16] analyzed the influence of particle size distribution of coal on the co-combustion performance of sewage sludge and coal. They confirmed that decreasing the particle size of the fuel improves the combustion performance. Luk et al. [17] proposed an integrated drying and power generation using EFB. Unfortunately, there was no significant effort to minimize the exergy loss in their system, hence, the energy efficiency was very low. Furthermore, Aziz et al. [18] developed a combined utilization of the EFB and palm oil mill effluent through gasification and digestion, respectively for power generation using a gas engine. Although their system looks feasible for application, it is designed as a standalone system at a milling site, thereby requiring new construction of a gasification system that leads to higher capital costs. Considering the high potential of EFB and the number of coalbased power plants, especially in Indonesia, EFB utilization through HT treatment and cofiring with existing coal-fired power plants or ones being constructed or planned becomes very important. However, to the best of the authors’ knowledge, studies dealing with the effort to evaluate the effect of hydrothermally treated EFB (HT-EFB) cofiring to coal-fired combustors are difficult to find. A new approach is urgently required to be developed in order to estimate the potential for retrofitting an existing power plant to cofire with HT-EFB. Therefore, this study focuses on two important issues with the objective of proposing efficient EFB utilization, especially for cofiring with coal. First, coal-cofiring behaviors with HT-EFB in a drop tube furnace (DTF) are modeled and analyzed through computa-
tional fluid dynamics (CFD) in terms of thermal behaviors including temperature profiles and composition of exhausted gases (CO and CO2). Secondly, an efficient integrated system model is developed to combine the CFD simulations with the entire plant processes including hydrothermal treatment, combustion system, and steam cycle. 2. Coal-cofiring of HT-EFB Fig. 1 shows the conceptual diagram of an integrated coalcofiring system with HT-EFB for power generation. Solid and dotted lines represent material and energy (electricity, heat) streams, respectively. Raw EFB is initially shredded to a smaller size before being hydrothermally treated. Generally, HT is performed under the subcritical region of water with a relatively low temperature of less than 250 °C [19]. Some researchers have evaluated the application of HT on several biomass processes to produce hydrochar [20,21]. To eliminate the drying process after HT, continuous HT using temperatures slightly higher than saturated one is adopted. Therefore, moisture inside the EFB is evaporated and exhausted together with the steam required for HT. The combined HT-EFB mixture is exhausted from the reactor with relatively low moisture content. In parallel, coal is initially ground and dried to the lower moisture content before being mixed with HT-EFB. The mixed HT-EFB and coal is then cofired in a combustor producing hightemperature heat for steam generation using a boiler. Then, the generated steam expands in the steam turbine generating the necessary electricity. In addition, the exhausted flue gas from boiler is utilized mainly for HT and coal drying. 3. Numerical modeling and calculation for cofiring of HT-EFB and coal 3.1. Schematic diagram of coal cofiring with HT-EFB To observe the cofiring performance and its feasibility when using a mixture of coal-HT-EFB, cofiring using a DTF is modeled and evaluated in terms of temperature distribution and composition of the produced gases. Fig. 2 shows the schematic diagram of a coal-cofiring system with HT-EFB in a DTF. The raw, wet coal is initially dried to a specified low moisture content and ground to achieve small and uniformly sized particles. Raw EFB is hydrothermally treated before being ground and fed together with the dried coal to the DTF. Hydrothermal treatment was based on the experimental results that were conducted at pressures of 2.4 MPa for 60 min [22]. In this study, four mass fractions of HT-EFB to total mass of mixed fuel are evaluated, i.e., 0 (100% coal), 10, 25, and 50%. 3.2. DTF and materials Currently, despite the inherent differences in combustion behavior between a DTF and realistic combustion conditions, the DTF is still considered an effective component for evaluating certain combustion behaviors of fuel [23]. DTF is capable for facilitating an environment that simulates industrial conditions such as a fast heating rate, short residence time, and dilute particle phases [24]. In general, the DTF consists of a fuel feeding system, reactor, and particle collection system at the outlet side. HT-EFB is mixed initially with coal before being introduced into the feeding system. In simulation, a laboratory scale DTF, which is a vertical tubular furnace, having dimensions of 1.5 m in height and 0.07 m in diameter and a capacity of 1 kWth is used. Detailed geometry of the DTF is shown in Fig. 3. In addition, to facilitate isothermal conditions
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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EFB
Hydrothermal treatment
Grinding
Mixing Wet coal
Grinding
Cofiring
Power
Electricity
Drying
Fig. 1. Basic schematic diagram of HT-EFB and coal-cofiring system.
EFB
Hydrothermal treatment
Grinding
Mixer
Wet coal
Grinding
DTF
Drying
Fig. 2. Basic schematic diagram of coal-cofiring system with HT-EFB.
Fig. 3. Geometry of the DTF used for the simulation and experimental validation.
along the furnace, electric heaters are installed and divided into three heating zones. The combustion process takes place inside the tubular furnace and in the downward direction. Combustion air supply is divided into primary and secondary air with volumetric flow rates of 3 and 4 L min1, respectively. The former is utilized to feed the fuels to the combustor, while the latter covers the rest of the demand for combustion. Mixed fuel particles are fed at 45–60 g h1 through an injection probe mounted at the top of the DTF. The produced gas is exhausted from the bottom of the DTF. The coal used in the simulation is originated from Kalimantan, Indonesia. This coal is classified as a low-rank coal having a high moisture content and low calorific value. Table 1 shows the compositions of used coal and HT-EFB including proximate and ultimate analyses. 3.3. Computational modeling CFD is an effective tool to model and calculate fluid flow, heat and mass transfers, and chemical reactions as well as interaction of solids and fluids [26]. CFD modeling methods for the combustion
of biomass particles is challenging work. However, compared to physical investigation through experiment, CFD modeling is significantly more effective from a time and cost perspective as well as safe and easy for scaling up. Hence, it is usually adopted before performing an experimental study. Related to cofiring using CFD analysis, it is expected that the combustion performance for all stages of the combustion including the combustion temperatures, kinetics behavior, and concentration of the produced gases can be clarified. In this simulation, a commercial CFD software ANSYS DesignModeler and Fluent ver. 16.2 (ANSYS Inc.) are used to build a 3D combustor model and analyze the cofiring behavior. Cofiring simulation considers dynamical equations, conservation of mass (continuity), momentum and enthalpy, turbulence (k-e turbulence model), radiation heat transfer (P-1 model), and reactions in both particle (Eulerian-Lagrangian model) and gas (global 2-steps reactions) phases [27]. Additional boundary conditions are the following: (1) fuel and air inlet flow rates are 1.38 105 and 1.6 104 kg s1 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) the feeding wall is considered isothermal at 300 K. In the conserved form, the continuity, momentum, and total energy conservations can be represented as Eqs. (1)–(5), respectively [28].
@q VÞ ¼ 0 þ r ðq~ @t
ð1Þ
@ðquÞ @p @ sxx @ syx @ szx VÞ ¼ þ þ r ðqu~ þ þ þ qf x @t @x @x @y @z
ð2Þ
! @ðqv Þ @p @ sxy @ syy @ szy þ r ðqv V Þ ¼ þ þ þ þ qf y @t @y @x @y @z
ð3Þ
@ðqwÞ @p @ sxz @ syz @ szz þ r ðqw~ VÞ ¼ þ þ þ þ qf z @t @z @x @y @z
ð4Þ
" !# " !# @ V2 V2 ~ q eþ þr q eþ V @t 2 2 @ @T @ @T @ @T @ ðupÞ þ þ k k k ¼ qq_ þ @x @x @y @y @z @z @x @ ðv pÞ @ ðwpÞ @ ðusxx Þ @ usyx @ ðuszx Þ @ v sxy þ þ þ þ @y @x @y @z @x @z @ v syy @ v szy @ ðwsxz Þ @ wsyz @ ðwszz Þ þ f ~ V þ þ q~ þ þ þ @x @z @y @z @y ð5Þ where q, V, p, f, s, k, and q_ are the density, velocity, pressure, body force per unit mass, shear force, thermal conductivity, and heat
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Table 1 Material composition. Component
Raw coal [25]
Dried coal [25]
Raw EFB [1]
HT-EFB [22]
Proximate analysis Fixed carbon (wt% wb) Volatile matter (wt% wb) Moisture (wt% wb) Ash (wt% wb)
24.93 25.76 48.76 0.56
40.23 41.57 17.30 0.90
3.71 34.84 60.00 1.46
28.62 62.57 3.00 5.82
Ultimate analysis C (wt% wb) H (wt% wb) O (wt% wb) N (wt% wb) S (wt% wb) Calorific value (MJ kg1)
35.30 2.29 11.23 1.75 0.11 13.84
56.98 3.69 18.13 2.83 0.17 22.34
17.97 2.49 17.60 0.47 0.01 17.02
52.92 5.35 32.06 0.85 0.00 22.22
transferred through thermal conduction per unit time per unit area, respectively. The values u, v, and w are the velocity components in each x, y, and z directions, respectively. In addition, e, V 2 =2, and e þ V 2 =2 are the internal energy, kinetic energy per unit mass, and total energy, respectively. The turbulence is approximated by using a standard k-e model, in which the transport equations for each k and can be represented as Eqs. (6) and (7), respectively [29].
@ @ @ ðqkÞ þ ðqkui Þ ¼ @t @xi @xj @ @ @ ðqui Þ ¼ ðqÞ þ @t @xi @xj
lt @k þ 2lt Eij Eij q rk @xj
lþ
ð6Þ
lt @ 2 þ C 1 2lt Eij Eij qC 2 r @xj k k ð7Þ
where k, , ui, and Eij are the turbulent kinetic energy, turbulent dissipation, velocity component in the corresponding direction, and deformation rate component, respectively. Furthermore, the turbulent viscosity, lt, is calculated using Eq. (8).
lt ¼ q C l
k
2
ð8Þ
In addition, in this study, the values of some of the required constants are defined as: C1e = 1.44; C2e = 1.92; Cl = 0.09; rk = 1.0; and re = 1.3. These values are based on the recommendation of Launder and Spalding [29] for free turbulent flows. Regarding radiation heat transfer, the following P-1 radiation model, which is based on the first order of the spherical harmonic expansion of the radiation intensity, is adopted for this study.
qrad ¼ Crad rG
ð9Þ
rðCrad rGÞ aG þ 4arT 4 ¼ SG
ð10Þ
1 3ða þ rs Þ C rs
ð11Þ
Crad ¼
where qrad, G, a, r, rs, SG, and C are the radiation flux, incident radiation, absorption coefficient, Stefan-Boltzmann constant (5.68 108 W m2 K4), scattering coefficient, user-defined radiation source, and linear-anisotropic phase function coefficient, respectively. The reactions in both particle and gas phases for coal cofiring with HT-EFB is shown as the following reactions: (i) Char of coal
C þ 0:5O2 ! CO
ð12Þ
(ii) Char of HT-EFB
C þ 0:5O2 ! CO
ð13Þ
(iii) Volatile matter of coal
CH0:39 O0:24 N0:043 S0:0011 þ 0:48O2 ! CO þ 0:195H2 O þ 0:0011SO2 þ 0:022N2
ð14Þ
(iv) Volatile matter of HT-EFB
CH1:21 O0:45 N0:014 þ 0:5775O2 ! CO þ 0:605H2 O þ 0:007N2
ð15Þ
CO þ 0:5O2 ! CO2
ð16Þ
3.4. Model validation To validate the model, comparison between the simulation and experiment-based validation test results of coal combustion using DTF is performed. Fig. 4 shows the geometry of the experimental model used in this study for validation. In the validation test, coal is fed using a screw feeder equipped with a vibrator. A water-cooling system is also mounted outside the feeder to avoid any initial combustion before exiting the feeder. The combustion process takes place inside the tubular furnace and in the downward direction. Combustion air supply is divided into primary and secondary air with volumetric flow rates of 3 and 4 L min1, respectively. The airflow rate is determined by stoichiometric calculation with air excess of combustion set at 20%. The former is utilized to feed the fuels to the furnace, while the latter covers the rest of the demand required for combustion. Fuel particles are fed at 45 g h1 through the injection probe mounted at the top of the DTF. The mean particle size of the coal is 75 mm. The produced gas is exhausted from the bottom of the DTF. Furthermore, the exhaust gases are cooled and the composition is analyzed using a gas analyzer. Fig. 5(a) compares the predicted temperature distribution with the experimental values for coal combustion. The orange and red lines represent the predicted and the measured temperature distribution along the combustor, respectively. The results show that the simulation model for combustion in the DTF is very accurate. The validation study also examines gas composition (CO and CO2) generated during combustion (Fig. 5(b) and (c)). To validate the gas emissions in the simulation, the gas composition in the experimental study is also measured. The dots in Fig. 5(b) and (c) show the CO and CO2 pollutants observed during the experimental study at the corresponding height. Meanwhile, the red line indicates the CO and CO2 emissions based on the simulation model. The emission values in the simulation model appear to be in good agreement with the experimental data.
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5
Fig. 4. Experimental model of DTF for model validation.
Based on these results, it is confirmed that the developed model has a very high validity to predict the combustion behavior of the fuel inside the DTF. Since HT-EFB and coal is treated as single fuel after being mixed, the developed model can be adopted to analyze the combustion behavior of coal cofiring with HT-EFB. Nevertheless, some attention should be paid primarily to the particle size and shape due to different characteristics between coal and biomass.
4. Integrated system of HT, cofiring, and combined cycle 4.1. Process modeling and calculation In the existing system, a coal-fired power system mainly comprises of drying, combustion, and a power generation module. The wet coal is ground to smaller size particles before being fed
to the dryer system. Practically, the grinding of coal particles is performed in two steps: before drying and after drying. The former is important to achieve uniform and small particle size (about 1–2 mm) to facilitate an excellent heat transfer and moisture removal rate during drying. The latter is performed to achieve smaller particle size, below 0.15 mm (pulverized coal), with the aim of achieving excellent combustion performance. Dried coal is then flown to the combustor chamber utilizing air as the feeding gas. As the combustion occurs inside the combustor, the heat generated from combustion is recovered by a superheater and economizer to generate steam for the steam turbine. In this conventional system, no effort is made to minimize the exergy destruction or improve the total energy efficiency. Fig. 6 shows the schematic process flow diagram of the integrated system consisting of HT treatment of the EFB, coal drying, cofiring, and power generation, developed based on the principles of enhanced process integration (EPI) technology. The integration
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Height of Furnace (m)
1.60 1.40 1.20 1.00
Experimental result (coal)
0.80 0.60 300
500
700
900
1100
1300
1500
Temperature (K)
(a) 1.80
P
1.60
Height of Furnace (m)
Therefore, drying after HT treatment can be eliminated and the produced HT-EFB can be directly cofired with coal after being ground to produce pulverized HT-EFB. Coal drying is also performed using superheated steam with vapor recompression technology. Dried coal and HT-EFB is premixed with a specified mass ratio before being fed to the combustor. The reaction during combustion follows the reaction discussed in Section 3.3 (Reactions (12)–(16)). The required drying temperature to achieve the target moisture content of 17.30 wt% wb is 113 °C according to [36]. Other assumed conditions are shown in Table 2. The calculation is conducted using the process simulator Aspen Plus ver. 8.8 (Aspen Technology, Inc.). The flow rates of raw EFB and wet coal are 12.5 and 35 kg s1, respectively. Therefore, the HT-EFB mass fraction during cofiring is about 19.5%. The net power generation efficiency (g) is obtained by the following equation
P
gross int g¼ _ _ EFB CV EFB Þ ðmcoal CV coal Þ þ ðm
ð17Þ
1.40
_ coal , m _ EFB , CV coal , and CV EFB are the power output where Pgross , Pint , m (gross generated power), internal energy consumption (compressor work, pump, blower, etc.), mass flow rate of coal, mass flow rate of EFB, calorific value of wet coal, and calorific value of the raw EFB, respectively.
1.20
1.00
experimental result (coal) 0.80
0.60 0.12
5. Results and discussion 0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
5.1. Temperature distribution
CO2
(b) 1.80
Height of Furnace (m)
1.60
1.40
1.20
1.00
Experimental result (coal) 0.80
0.60
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
(c) Fig. 5. Measured and predicted results in coal combustion: (a) flame temperature, (b) produced CO2, and (c) produced CO.
of the involved processes is carried out by implementing the principles of EPI technology. As an effort to optimize the heat circulation and minimize the exergy loss throughout the integrated system, EPI has been explained sufficiently in [30,31] and applied in some fields including coal conversion [32], biomass gasification [33], and algae-based power generation [34]. Raw EFB is initially shredded to smaller and more uniform size before being fed to the HT reactor. For the HT treatment, a continuous autoclave with a tube heat exchanger installed inside is adopted as the main reactor. The continuous autoclave has already been developed and applied in industry [35] for this type of purpose. Tube heat exchangers are installed to facilitate heat exchange between the compressed evaporated moisture and the EFB leading to self-heat exchange. HT treatment is performed in superheated conditions slightly higher than saturation point, hence the moisture in the EFB is in vapor condition and the produced HT-EFB has low moisture content.
The HT-EFB after hydrothermal treatment is found to become more uniform and coal-like. Hydrothermal treatment also can improve the drying and dehydration performance; thus, the moisture content of the HT-EFB decreases to approximately 3%. This characteristic is very important in the combustion system. Fig. 7 shows the temperature distribution along the axis of the DTF under different cofiring mass fractions. The figure excludes the axis of the DTF at a high of 0–0.6 m considering that there is no substantial change in the bottom of the DTF and can be neglected. The dots in Fig. 7 correspond to the measured results obtained from experimental validation for coal. In general, a higher HT-EFB mass fraction will increase the temperature inside the combustor. HT-EFB mass fractions of 50% results in the highest outlet temperature (maximum of 1536 K). For comparison, the highest outlet temperature in the case of no HT-EFB is 1347 K. The change in the HT-EFB mass fraction leads to a change in the heating value of the mixture; therefore, the flame shape and temperature profile within the combustor changes accordingly. Fig. 8 shows the temperature distribution across the combustor under different HT-EFB mass fractions. HT-EFB contains higher volatile matter content but it has a lower fixed carbon content than coal, as can be observed in Fig. 8. The location of the hightemperature region corresponds to the combustion location of the volatile matter and the oxygen availability. The fuel combustion process involves three basics stages: devolatilization, volatile combustion, and char oxidation. Compared to the main combustion area, a lower temperature is observed in the upper combustor where the devolatilization process occurs. The mixture is pyrolyzed and then evolves as volatile matter. The devolatilization of HT-EFB particles occurs earlier and in a shorter time than the coal because of the lower moisture content of the HT-EFB and higher volatile matter content. On the other hand, since coal has significantly higher moisture content, its particles require a longer time for drying once devolatilization occurs. Therefore, for a high HT-EFB mass fraction, the flame temperature remains high and is distributed more uniformly, although
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Wet coal
Compressed steam
Compressor
Grinder Separator
Preheater
Dryer
HRSG High pressure steam
Flue gas
Combustor Dried coal Economizer
Air Dried HT EFB
Steam turbine
Condenser
Grinder
Wet raw EFB
Pump
Superheater Splitter Shredder
Cooler Blower (Make-up waater)
Preheater
Blower
Feeder
Recycled steam Condensate
HT treatment reactor
Fig. 6. Schematic process flow diagram of integrated HT treatment of EFB, coal drying, cofiring, and power generation.
Table 2 Assumptions given during the calculation of the integrated system. Parameter(s)
Value
Adiabatic efficiency of compressor and blower (%) Minimum approach of heat exchangers (°C) HRSG outlet pressure (MPa) Steam turbine isentropic efficiency (%) Minimum outlet vapor quality (%) Discharge pressure of steam turbine (MPa)
87 30 20 90 0.9 0.13
1.80
Height of Furnace (m)
1.60
1.40
coal fully coal + HT EFB 10%
1.20
coal + HT EFB 25% coal + HT EFB 50%
1.00
Experimental result (coal 100%) 0.80
0.60 300
500
700
900
1100
1300
1500
Temperature (K) Fig. 7. Temperature distributions inside the combustor of HT-EFB cofiring.
it is located in the lower part of the combustor. In addition, since the HT-EFB has lower moisture content than coal, a high HT-EFB mass fraction leads to a lower total moisture content for the mixed fuel in the combustor system. Finally, this condition affects the flame temperature due to the high heat capacity of water.
Fig. 8. Temperature distribution (K) of coal and HT-EFB cofiring across the combustor: (a) coal fully, (b) HT-EFB 10%, (c) HT-EFB 25%, and (d) HT-EFB 50%.
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In contrast to the full coal combustion, HT-EFB cofiring has increased CO and NO concentrations in the combustion. Figs. 9 and 10 provide additional information regarding the concentrations of CO and CO2 gases during cofiring. Regarding the produced CO concentration, a higher mass fraction of HT-EFB leads to an increase in the CO mass fraction during the initial combustion reaction. The volatile matter, especially from HT-EFB, is oxidized under high combustion temperatures that allow the formation of CO. Afterward, CO reacts with O2 (air) in the combustor forming CO2. In addition, coal cofiring with HTEFB results in lower CO2 concentration following an increase in the HT-EFB mass fraction (Fig. 9). The dots in Fig. 10 show the CO2 pollutant observed during the experimental study, while the lines show CO2 emissions based on the simulation model.
1.60
Height of Furnace (m)
5.2. Distribution of produced CO and CO2
1.40
coal fully 1.20
coal + HT EFB 10% coal + HT EFB 25%
1.00
coal + HT EFB 50% 0.80
experimental result (coal 100%) 0.60
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
CO2 Fig. 10. CO2 mass fraction along the combustor height under different HT-EFB mass fractions.
5.3. Integrated system performance Fig. 11 and Table 3 show the detailed schematic process flow diagram of coal drying and the condition of each stream, respectively. The energy required for coal drying with exergy recovery is 2.48 MW, mainly for compression work. This compression is performed to increase the steam pressure to 0.2 MPa to facilitate selfheat exchange between the compressed steam and wet coal. Through compression, the saturation point of the steam increases accordingly that leads to the possibility of coupling the latent heat. In this process, wet coal (1) with moisture content of 48.76 wt% wb is preheated up to 90 °C before flowing to the dryer using flue gas originally exhausted from the gas turbine. Furthermore, to further minimize the exergy destruction throughout the integrated system, the condensed, compressed steam (8) is utilized in the HT treatment of the EFB for preheating. Fig. 12 and Table 4 show the detailed schematic process flow diagram of the HT treatment of the EFB and stream properties, respectively. Wet raw EFB (11) is initially cut and shredded to a smaller size before being preheated and fed to the HT treatment reactor. The steam evaporated during HT treatment (17) is exhausted and split into recycled steam (22) and superheated steam (18). The superheated steam is superheated using the hot gas produced from the combustor for heat combination, which increases the exergy rate of the steam. Therefore, this steam can be recycled back as the heat source for subsequent HT treatment (20). The amount of heat to superheat the steam is 20.1 MW. This energy requirement potentially decreases the possible generated power in the event that no further effort regarding heat recovery is carried out. To recover the waste heat from the HT treatment
1.80
1 (Wet coal)
Flue gas
Grinder
7 Compressed steam 6
5
9
Compressor
Separator
2 3 Preheater
Dryer 10
Cold flue gas
8
Dried coal 4
As EFB preheater Fig. 11. Schematic diagram of a coal drying process employing EPI.
Table 3 Stream properties in the coal drying process. Stream
Phase
Mass flow (kg s1)
Temperature (°C)
Pressure (MPa)
1 2 3 4 5 6 7 8 9 10
Mixed Mixed Mixed Mixed Vapor Vapor Vapor Vapor-liquid Gas Mixed
35.00 35.00 35.00 21.69 13.31 13.31 13.31 13.31 276.39 276.39
25 25 90 113 113 113 210 120 120 102
0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.1 0.1
Height of Furnace (m)
1.60 1.40
coal fully
1.20
coal + HT EFB 10% 1.00
coal + HT EFB 25% coal + HT EFB 50%
0.80
Experimental result (coal 100%) 0.60 0.E+00
2.E-02
4.E-02
6.E-02
8.E-02
1.E-01
1.E-01
Fig. 9. CO mass fraction along the combustor height under different HT-EFB mass fractions.
reactor, high-temperature stream (21) is piped to the economizer acting as a preheater for the water recycled from the steam turbine cycle. Due to the physical and chemical changes of the HT-treated EFB and the performance of the heat exchangers, the heat involved in the HT treatment process cannot be fully recovered in the same process. However, by integrating with the other processes through EPI to minimize the exergy destruction, HT treatment only reduces the produced gross electricity by 1.6 MW. Fig. 13 shows the comparison of energy consumed during coal drying, HT treatment of EFB, and internal consumption in the power generation system. The internal consumption required during power generation, such as the high-pressure pump, represents the highest power consumption during the cycle. On the other
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122
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Combustor
Grinder
Wet raw EFB 11
16 Dried HT EFB
15
From coal drying
20 Splitter 18 19 17 22 Blower 21
8 Shredder 12
Superheater
To economizer 13 14 Feeder
Preheater
Blower 23 Recycled steam
24 Condensate
HT treatment reactor
Fig. 12. Detailed schematic process flow diagram of the proposed HT treatment process of EFB employing EPI.
Table 4 Stream properties in HT treatment for EFB. Stream
Phase
Mass flow (kg s1)
Temperature (°C)
Pressure (MPa)
11 12 13 14 15 16 17 18 19 20 21 22 23 24
Mixed Mixed Mixed Mixed Mixed Mixed Vapor Vapor Vapor Vapor Liquid Vapor Vapor Liquid
12.50 12.50 12.50 12.50 5.24 5.24 12.26 7.26 7.26 7.26 7.26 5.00 5.00 13.31
25 25 80 80 230 230 230 230 230 465 183 230 230 86
0.1 0.1 0.1 2.4 0.1 0.1 2.4 2.4 2.41 2.41 2.41 2.41 2.41 0.2
Energy consumpon (MW)
14 12 10
Referring to Fig. 6, it is obvious that changes are needed primarily in the drying and hydrothermal treatment processes. The proposed system is applicable and replicable for various capacities. The proposed system is based on EPI technology, which is the integration of exergy recovery and process integration. Both technologies are currently available and have been performed by several researchers. However, as a methodological approach, EPI tries to efficiently combine both technologies. Therefore, they can be applied easily and effectively with optimum results in energy efficiency. Additional investment cost will be required for the retrofitting of the drying system, hydrothermal treatment system, and boiler, respectively. In the high-temperature heat exchanger, especially for the hydrothermal treatment process, more attention should be paid to the materials, thermal stresses during startup, and load fluctuations. To account for the high temperatures of the heat exchanger, materials that can be used include carbon fiber-SiC composite or various ceramics oxides. However, the use of these materials may contribute to a higher cost for a heat exchanger able to withstand temperatures above 700 °C. Furthermore, optimization of the design and operation of both the drying system and hydrothermal treatment system can lead to higher efficiencies.
8 6
6. Conclusion
4 2 0
Internal consump
n
Coal drying
HT treatment EFB
Fig. 13. Comparison of the energy demand in coal drying, HT treatment of EFB, and internal consumption for power generation.
hand, the proposed coal drying and HT treatment of the EFB show lower energy consumption due to very low exergy loss. The proposed integrated system with EPI can achieve a net power generation efficiency of 39.7% including the processes for coal drying and HT treatment of the EFB. A significant energy reduction in both coal drying and HT treatment of the EFB can contribute to the high net power generation efficiency of the integrated system.
Advanced utilization of EFB as an energy source has been proposed and studied. Initially, the cofiring behavior of coal with HT-EFB has been modeled and evaluated using CFD analysis under different mixing mass fractions. In general, a HT-EFB mass fraction of 10–25% appears to be the most preferable cofiring condition in terms of temperature and produced gas compositions. In addition, an innovative integrated system consisting of HT treatment for the EFB, coal drying, cofiring, and power generation has also been modeled. Very low energy consumptions during coal drying and HT treatment of the EFB, which are 2.48 and 1.6 MW, respectively, can be achieved. Finally, the net power generation efficiency of the proposed integrated system is approximately 40% including coal drying and HT treatment of the EFB. The application of the proposed system is believed to be able to increase the effective utilization of EFB as well as reduce the environmental impact and extend the life of current coal-fired power plants.
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Acknowledgments The authors greatly appreciate the support of The Indonesia Endowment Fund for Education (LPDP). The authors also thank Ms. Annisa Nurdiawati and Dr. M. Kunta Biddinika from the Tokyo Institute of Technology for their assistance and advice. References [1] Lam PS, Lam PY, Sokhansanj S, Lim CJ, Bi XT, Stephen JD, et al. Steam explosion of oil palm residues for the production of durable pellets. Appl Energy 2015;141:160–6. [2] Tarigan SD, Widyaliza S. Expansion of oil palm plantations and forest cover changes in Bungo and Merangin districts, Jambi Province, Indonesia. Proc Environ Sci 2015;24:199–205. [3] BPS. Statistik Kelapa Sawit Indonesia. Badan Pusat Statistik Indonesia; 2014. p. 96. [4] Aziz M, Oda T, Kashiwagi T. Design and analysis of energy-efficient integrated crude palm oil and palm kernel oil processes. J Jpn Inst Energy 2015;94:143–50. [5] Aziz M, Prawisudha P, Prabowo B, Budiman BA. Integration of energy-efficient empty fruit bunch drying with gasification/combined cycle systems. Appl Energy 2015;139:188–95. [6] Indonesia State-owned Power Utility (PLN). Electricity Power Supply Business Plan (RUPTL) 2016–2025.
[16] Yan YF, Zhang ZE, Zhang L, Zhang L. Influence of coal properties on the cocombustion characteristics of low-grade coal and city mud. Global NEST J 2014;16:330–9. [17] Luk HT, Lam TYG, Oyedun AO, Gebreegziabher T, Hui CW. Drying of biomass for power generation: a case study on power generation from empty fruit bunch. Energy 2013;63:205–15. [18] Aziz M, Oda T, Kashiwagi T. Advanced power generation using biomass wastes from palm oil mills. Appl Therm Eng 2017;114:1378–86. [19] Li W, Guo J, Cheng H, Wang W, Dong R. Two-phase anaerobic digestion of municipal solid wastes enhanced by hydrothermal pretreatment: viability, performance and microbial community evaluation. Appl Energy 2017;189:613–22. [20] Chiaramonti D, Prussi M, Buffi M, Rizzo AM, Pari L. Review and experimental study on pyrolysis and hydrothermal liquefaction of microalgae for biofuel production. Appl Energy 2017;185:963–72. [21] Roman S, Nabais JMV, Laginhas C, Ledesma B, Gonzales JF. Hydrothermal carbonization as an effective way of densifying the energy content of biomass. Fuel Process Technol 2012;103:78–83. [22] Novianti S, Biddinika MK, Prawisudha P, Yoshikawa K. Upgrading of palm oil empty fruit bunch employing hydrothermal treatment in lab-scale and pilot scale. Proc Environ Sci 2014;20:46–54. [23] Du SW, Chen WH, Lucas JA. Pulverized coal burnout in blast furnace simulated by a drop tube furnace. Energy 2010;35:576–81. [24] Manquais KL, Snape C, McRobbie I, Barker J, Pellegrini V. Comparison of the combustion reactivity of TGA and drop tube furnace chars from a bituminous coal. Energy Fuels 2009;23:4269–77. [25] Aziz M, Budianto D, Oda T. Co-firing of palm kernel shell into coal fired power plant. Energies 2016;9:137. [26] Ranade VV, Gupta DF. Computational modeling of pulverized coal fired boilers. Boca Raton, USA: CRC Press; 2015. [27] Budianto D, Aziz M, Cahyadi, Oda T. Numerical investigation of co-firing of palm kernel shell into pulverized coal combustion. J Jpn Inst Energy 2016;95:605–14. [28] Anderson JD. Governing equations of fluid dynamics. In: Wendt JF, editor. Computational fluid dynamics. Berlin, Germany: Springer-Verlag; 2009. p. 15–51. [29] Launder BE, Spalding DB. The numerical computation of turbulent flows. Comput Methods Appl Mech Eng 1974;3:269–89. [30] Aziz M, Oda T, Kashiwagi T. Integration of energy-efficient drying in microalgae utilization based on enhanced process integration. Energy 2014;70:307–16. [31] Kansha Y, Kotani Y, Aziz M, Kishimoto A, Tsutsumi A. Evaluation of a self-heat recuperative thermal process based on thermodynamic irreversibility and exergy. J Chem Eng Jpn 2013;46:87–91. [32] Aziz M, Juangsa FB, Kurniawan W, Budiman BA. Clean co-production of H2 and power from low rank coal. Energy 2016;116:489–97. [33] 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. [34] Aziz M. Power generation from algae employing enhanced process integration technology. Chem Eng Res Des 2016;109:297–306. [35] Catelli C, Notari R. Autoclave for the continuous cooking and sterilizing of food products in general. European Patent, EP0691271 B1; 2000. [36] Aziz M, Oda T, Kashiwagi T. Energy-efficient low rank coal drying based on enhanced vapor recompression technology. Dry Technol 2014;32:1621–31.
Please cite this article in press as: Darmawan A et al. Retrofitting existing coal power plants through cofiring with hydrothermally treated empty fruit bunch and a novel integrated system. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.122