Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 303 – 308
The 8th International Conference on Applied Energy – ICAE2016
Enhanced process Integration of entrained flow gasification and combined cycle: modeling and simulation using Aspen Plus Arif Darmawana,b , Flabianus Hardia, Kunio Yoshikawaa, Muhammad Azizc, Koji Tokimatsua* b
a Dept. Transdisciplinary Science and Engineering, Tokyo Institute of Technology, Tokyo 226-8503, Japan Agency for Assessment and Application of Technology (BPPT), Puspiptek Serpong, Tangerang 15314, Indonesia c Institute of Innovative Research, Tokyo Institute of Technology, Tokyo 152-8550, Japan
Abstract Energy recovery from black liquor can be performed through gasification process at temperatures above the melting point of the inorganic chemicals. Complementing the experimental research, this study was conducted in Aspen Plus software to simulate thermodinamic modeling of detail process for gasification and combined cycle in an integrated system power plant. Mass and energy balances were examined to quantify process performance. The unrecoverable energy in a single process will be utilized in other processes. The combination of these technologies is expected minimizing the total exergy destruction the throughout system. Kraft black liquor was used as sample during process calculation. The proposed integrated-system shows a high energy efficiency. A significant positive energy harvesting from black liquor can be achieved for further development. © 2017 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-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: black liquor gasification; energy recovery; modeling; integrated system
1. Introduction Energy recovery from biomass can be performed through thermochemical or biochemical route. Generally, thermochemical conversion, especially gasification, shows faster conversion rate and higher
* Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address:
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1876-6102 © 2017 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.2017.03.318
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conversion efficiency [1]. The production of charred matter prior gasification always involves a thermochemical conversion process. It is an essential reaction step in any combustion or gasification process. Furthermore, to achieve high total power generation efficiency from biomass, an integrated gasification and combined cycle (IGCC) has been developed with some advantages of high carbon conversion, high power generation efficiency and low environmental influence [2]. Since the power block of an IGCC plant is similar to that of a natural gas combined cycle (NGCC) plant, the efficiency of the latter is a natural reference for the IGCC plant. Currently, NGCC efficiencies are approaching 60%. Depending on configuration, some of the produced heat may or may not be recovered. Either way, a significant efficiency penalty or exergy loss arises because heat is a lower quality energy form than chemical energy. Since black liquor is obtained from existing kraft process, an approach is needed to produce the desirable black liquor. One method to produce black liquor is using hydrothermal liquefaction (HTL) prior biomass gasification. This study is a preliminary effort to investigate the feasibility of an integrated system for power generation from gasification and combined cycle employing an existing black liquor as fuel resource. 2. Overall proposed system 2.1. Overview Fig.1 shows the conceptual diagram of the overall proposed integrated system. The proposed integrated processes consist of gasification and combined cycle. The black liquor is flown to gasification module which are converted to syngas consisting of hydrogen, methane, carbon monoxide, etc. The produced syngas flows to the combined-cycle-based power generation comprising combustor, gas turbine, heat recuperator and steam turbine. Moreover, a high temperature raw syngas from the gasifier module is recycled and utilized for pre-heating in combined cycle process.
Fig.1. Conceptual diagram of the proposed integrated system
In the gasifier, black liquor pyrolysis, volatile combustion and char gasification reactions take place subsequently to produce the syngas. Hot syngas flows to heat recovery module to preheat the steam for combined cycle process. Subsequently, syngas will be cleaned up, removing the particulates and Sulphur. The clean syngas is then used as fuel for combustion, creating a high temperature pressurized gas to rotate the gas turbine. As the temperature of the flue gas from the gas turbine is still high, the rest of the heat will be utilized basically to generate steam in heat recovery steam generator (HRSG) which is used to rotate the steam turbine to generate the electricity. 2.2. Aspen plus modeling The simulations of the integrated system were based on the mass-energy balance and chemical equilibrium. The integrated plant design is modeled with Aspen Plus V8.8, a simulation tool which can calculate material flow and energy balances. The stream for biomass were specified as a non-conventional
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stream and the ultimate and proximate analyses were entered based on experimental data (see fig 3). Another assumption in this research are: (1) The operation is simulated in standard conditions for temperature and pressure (250C and 1 atm). (2) The model is steady state, kinetic free and isothermal (3) Chemical reactions take place at an equilibrium state, and there is no pressure loss (4) The composition of black liquor and syngas product are based on experimental data. (5) All gases are ideal gases. 3. Gasification condition 3.1. Physical properties In this model, HCOALGEN and DCOALIGT models are used to calculate the enthalpy and density of non-conventional components, respectively. The HCOALGEN model requires these three component attributes for nonconventional components: proximate analysis results (denoted as PROXANAL in Aspen Plus), ultimate analysis results (denoted as ULTANAL in Aspen Plus), and sulfur analysis results (denoted as SULFANAL in Aspen Plus). Table 1. Black Liquor (BL) Elemental Analysis (on a Dry Basis) and Heating Values [3]
% mass
C
H
N
Cl
Na
K
S
O
27.50
3.75
0.07
0.16
19.85
3.12
6.20
39.35
Dry solid 72.53%
HHV (MJ/kg) 12.13
The proximate analysis gives the weight contents of moisture, fixed carbon, volatile matter, and ash. The ultimate analysis gives the weight composition of coal in terms of ash, carbon, hydrogen, nitrogen, chlorine, sulfur, and oxygen. The sulfur analysis gives the weight fractions of sulfur divided into pyritic, sulfate, and organic sulfur. For the DCOALIGT model, it requires only these two component attributes: ULTANAL and SULFANAL. Table 1 shows the component attributes of coal used in our model, which are from the literatures. Based on these analysis results, the enthalpy and density of coal are calculated, respectively. 3.2. Reaction in the gasifier
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Reactions of black liquor gasification: It was simply divided as four stages of drying, pyrolysis, combustion and reduction. These reactions are based on experimental study for black liquor gasification [4]. In gasifier, the temperature is typically higher than 1000ºC. When coal is fed into the gasifier, it first undergoes the pyrolysis process to decompose to volatile matter and char, as shown in Eq. (1). In our model, volatile matter includes Na2CO3, CO, H2, H2O, CO2, CH4, and H2S. NaCaHbOcSe Æ i C + k Na2CO3 + l CO2 + m CO + n H2 + o CH4 + p H2S + q H2O
(1)
Inside the gasifier, the pyrolysis products such as small molecules CO, H 2, CH4 and H2S diffused to the outer space of particles and combusted with oxygen. H2 + 0.5 O2 Æ H2O CO + 0.5 O2 Æ CO2 CH4 + 1.5 O2 Æ CO2 + H2O H2S + 1.5 O2 Æ SO2 + H2O C + O2 Æ CO2
(2) (3) (4) (5) (6)
After the combustion reaction (oxygen depleted), mixtures of water vapor, CO 2, H2, SO2 and charcoal stayed in the gasifier. At high temperature, the charcoal could react with those water vapor, CO 2 and H2 and SO2 and CO2 could be reacted with H2. Reactions in the in the reduction stage were: C + H2O Æ CO + H2 C + 2H2O Æ CO2 + 2H2 C + CO2 Æ 2CO C + 2H2 Æ CH4 SO2 + 3H2 Æ H2S + 2H3O CO2 + 4H2 Æ CH4 + 2H2O
(7) (8) (9) (10) (11) (12)
In the model, coal pyrolysis process is simulated with two RYield reactors. The first reactor is used to simulate the coal pyrolysis at 1 atm based on the results of the pyrolysis experiment. The second is used to make a pressure correction for the yield of each component generated in the first reactor. Since the reaction rate of volatile combustion is generally fast and the combustible gases can be considered to be consumed up in a short time, the kinetics of volatile combustion process is neglected in the model. An RStoic reactor, is used to simulate the volatile combustion process. In the model, the reduction stage is modelled with an RPlug reactor.
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Table 1. Assumed gasification conditions used during calculation including syngas composition inside the gasifier. Components
Value
Gasification temperature (K)
1309
Syngas composition
Main components; H2 (35.2%), CO (25.7%), CO2 (35.7%), Minor components; CH4 (0.47%), H2S (1.49%)
Black liquor input (kg/s)
3.03
Carbon Conversion Efficiency (%)
99.00
4. Result and discussion Fig. 2 shows the hot raw syngas >10000C contains sensible heat which may be recovered in heat exchangers to produce steam for the steam turbine. The use of syngas coolers for this purpose increases efficiency, but will add capital costs. In the gas clean up process, particles, sulfur and other impurities are removed. Because of the high partial pressures of the species and the low volume flow of syngas, the gas clean up process is very efficient and low cost compared to traditional flue gas cleaning. The clean syngas is then fed to the gas turbine for production of electricity. Gas turbines for syngas operation are commercially available. Compared to natural gas operation, some minor modifications in combustors and operating conditions are required.
Fig.2. Diagram of the IGCC system
Electrical efficiencies around 49.5% have been achieved in this model. Because the power block of an IGCC plant is similar to that of a natural gas combined cycle (NGCC) plant, the efficiency of the latter is a natural reference for the IGCC plant. Currently, NGCC efficiencies are approaching 60%. The
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efficiency penalty of an IGCC compared to an NGCC is mainly explained by effects in the gasification process. Conclusion and recommendation An integrated system for energy harvesting from black liquor based on combination of exergy recovery and process integration has been evaluated and seems to be promising. A significant high energy efficiency could be achieved with total power generation efficiency of about 49.5%. For further development, integration of black liquor production system, entrained flow gasification and combined cycle should be studied. Currently, Experimental of black liquor production through hydrothermal liquefaction (HTL) and gasification process is being investigated in experimental study. References [1] [2] [3] [4]
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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.
