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CO2 co-gasification of coal and algae in a downdraft fixed bed gasifier: effect of CO2 partial pressure and blending ratio Nasim M.N. Qadi, Ilmannuran Zaini, Fumitake Takahashi, and Kunio Yoshikawa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03148 • Publication Date (Web): 01 Feb 2017 Downloaded from http://pubs.acs.org on February 7, 2017
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Energy & Fuels
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CO2 co-gasification of coal and algae in a downdraft fixed bed gasifier:
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effect of CO2 partial pressure and blending ratio
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Nasim M N Qadi*, Ilman Nuran Zaini, Fumitake Takahashi, Kunio Yoshikawa
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Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science
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and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama,
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Kanagawa 226-8502, Japan
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Corresponding author Tel: +81-45-924-5507. Fax: +81-45-924-5518. E-mail:
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[email protected] 9 10
Abstract
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In this study, Newlands coal, spirulina microalgae samples and mixtures of them were gasified in a
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fixed-bed downdraft reactor by CO2 at the atmospheric pressure and in the temperature range of
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950-1000 ºC. The effects of the reaction temperature, the CO2 partial pressure and the blending ratio
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on the syngas yield were studied. Results showed that the CO2 partial pressure didn’t affect the gas
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production yield until its value exceeded 0.05MPa. The co-gasification experimental results showed
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higher values than the predicted ones in terms of the gas production yield especially H2 and CO
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components when the blending ratio of algae is 50%wt. This synergetic effect was mainly attributed
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to the catalytic activity of the high content of alkali and alkaline metals in algae. The increase of the
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reaction temperature led to a higher gas production yield as the Boudouard reaction is an
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endothermic reaction that went to a higher extent with the temperature increase.
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Keywords: co-gasification; microalgae; coal; synergetic; syngas
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1. Introduction
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Utilizing coal without adding to global carbon dioxide level is a major challenge which is being
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addressed. However, the gasification technology is one of the cleanest ways to make energy from
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coal. A typical coal plant emits more sulfur dioxide and nitrogen oxide in a few weeks than a
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state-of-the-art gasification plant produces in a year 1.
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Gasification of solid fuel is the transformation of combustible substance into gaseous fuel as a result
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of the reaction with the gasifying medium at a high temperature and under atmospheric or increased
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pressure. Operational mediums for gasification include air, steam, carbon dioxide and mixtures of
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them. The air gasification leads to a syngas production with relatively low heating value, which
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influences its potential value due to the dilution effect of nitrogen. However, nitrogen-free syngas is
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preferable for synthesis of liquid fuel and chemicals 2. Indeed, the subcritical steam gasification
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provides an effective means of renewable hydrogen production and offers products with minimal
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environmental effect 3. However, the overall thermal efficiency of the steam gasification is decreased
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due to the heat loss of the unreacted steam 4. The CO2 gasification is the slowest reaction among all
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gasification agents. However, introducing CO2 as a gasifying agent leads to a higher thermal
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efficiency through the substantial increase in the final mass conversion by the Boudouard reaction
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and helps to reduce the tar content by means of the expedited cracking 5. Moreover, applying CO2 as
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a gasifying agent in gasification processes provides a reliable pathway for converting CO2 into clean
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fuel and helps to mitigate the accumulation of CO2 in the atmosphere 6.
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In typical steam or CO2 gasification processes, which are usually referred as indirect gasification
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processes, fuel undergoes two successive steps. Firstly, pyrolysis where the volatile species like CO
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and CH4 gases and tar are released, followed by secondary reactions which cause H2 and light tar to
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increase
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gasifying agent and produces CO through the Boudouard reaction as in the CO2 gasification. The
7,8
. Secondly, the remaining solid residue, that contains mainly carbon, reacts with the
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char gasification step represents the reaction limiting stage in the thermochemical conversion of
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biofuels 9. Moreover, the char gasification reaction either with CO2 or H2O is characterized as an
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endothermic reaction that requires external heating source and high operating temperatures
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Therefore, minimizing the energy consumption during the indirect gasification process is essential.
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However, the presence of alkali and alkaline earth metals can greatly enhance the char gasification
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reactivity 11. Also, it can reduce the tar formation by means of catalyzing tar cracking or stopping tar
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formation 12.
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In coal gasification under CO2 atmosphere, a high operating temperature is needed, but from the
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economical point view, lower temperature is desired
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gasification reaction are often found in the inherent alkali content of the biomass 14. Co-gasification
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technology of coal-biomass blend is promising and offers the potential to create synergetic effects
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that help to overcome some impediments which arise when individual coal or biomass is gasified.
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(1) The efficiency and the energy balance of coal gasification can be enhanced through the catalytic
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effect that comes from higher amounts of alkali and alkaline earth metals in biomass
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Utilization of biomass with coal can help to overcome the limited availability of biomass due to its
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seasonal nature and so a stable supply of co-gasification materials can be guaranteed
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elevated co-gasification temperature assists to reduce tar formation from biomass 16.
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Terrestrial biomass, like rice straw, sawdust and cedar wood, has been widely co-gasified with coal.
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As another option, unconventional biomass like the so called algae is gaining increasing interest as a
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feedstock for sustainable fuel production 17. Algae is considered to be a cost-competitive biofuel due
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to its faster growth, higher yield per area and higher CO2 capture and photosynthesis
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algae has high ash content which contains larger amounts of alkali and alkaline earth metals than
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most of land-based biomass, thus better performance can be foreseeable
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content algae is the main technical drawback of algae utilization since it is a high energy consuming
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.
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. Cheaper and excellent promoters of the
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18
15
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. (2)
. (3) The
. Moreover,
. Drying high water
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process
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including conventional heat recovery-based technologies 21. Incorporation of a heat recovery process
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will help to mitigate the energy penalty of algae drying, and the drying effect becomes less
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influential when limited amount of algae is used in the co-gasification process.
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CO2 was tested as gasifying agent mainly at the TGA scale. Individual biomass gasification and coal
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gasification were investigated mainly to model the gasification kinetics
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co-gasification kinetics of coal-biomass blend under CO2 atmosphere either under isothermal
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conditions
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gasifying agent at the lab scale. Prabowo et al. 4 examined CO2 gasification of biomass at a bench
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scale experiment and studied the effect of co-existence of CO2 and H2O on the biomass gasification.
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Sadhawni et al.
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explored the effect of the reaction temperature and the role of the CO2 partial pressure on the output
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syngas.
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Some researchers have involved algae in the gasification process. Kirtania et al. 26 studied the effect
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of different pyrolysis conditions on the gasification reactivity of algae char and woody biomass char.
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Sanchez-Silva et al.
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However, algae co-processing still scarce in literature. Zhu et al.
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Australian brown coal with algae in a fluidized bed reactor, and found that more syngas is produced
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and agglomeration problems appear when high ash content algae specie is used. Alghurabie et al. 29
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investigated the fluidized bed gasification of Kingston coal with marine microalgae in a spouted bed
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reactor, and they found the high salt content in the algae leads to operational problems such as
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agglomeration and fouling.
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In light of the above-mentioned literature survey and the increasing interest of employing CO2 as a
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gasifying agent, this study undertakes the sole CO2 co-gasification of coal-algae blend without
. However, various energy-efficient thermal drying technologies have been developed,
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or under non-isothermal conditions
25
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22,23
. Some studies explored
. However, limited studies tackled with CO2 as
has investigated biomass gasification using CO2 in a fluidized bed reactor and
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studied the steam gasification of Nannochloropsis gaditana microalgae. 28
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studied the co-gasification of
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combining the effects of other oxidizers. In this work we firstly present the gas evolution behavior of
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algae and coal samples. The role of the CO2 partial pressure on the gas production yields was
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investigated. Finally, the effects of the blending ratio and the reaction temperature on the syngas
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production yield were clarified.
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2. Methods
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2.1 Samples
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Australian Newlands coal (NC) sample was supplied by Central Research Institute of Electrical
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Power Industry (CRIEPI) in Japan. Spirulina algae (ALG) cultivated in a fresh water pond was
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supplied by Indonesia Islamic University. The proximate analysis was carried out by the
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thermogravimetric analyzer (DTG-50, Shimadzu Inc.). The ash composition was analyzed by the
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CHN analyzer. The ultimate analysis was performed using Vario Micro Cube Elemental Analyzer
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(Elementar, Germany). The proximate, ultimate and ash analyses of these samples are given in Table
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1.
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Table 1 Proximate, ultimate and ash analysis results of the samples Coal
Algae
(wt%)
(wt%)
volatile matter (Vmd)
28.70
77.17
fixed carbon (Cd)
57.50
13.33
ash (Ad)
13.80
9.50
C
82.50
44.77
H
5.11
6.46
N
1.36
9.09
O
10.61
39.69
S
0.42
0.56
49.90
8.73
Proximate analysis
Ultimate analysis(d.a.f)
Ash analysis SiO2 5
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Fe2O3
4.40
2.88
CaO
1.78
9.42
MgO
0.72
6.47
SO3
0.77
18.80
P2O5
1.10
18.70
Na2O3
0.26
12.30
K2O
0.57
21.70
Fuel calorific values
HHV[MJ/kg]
NC
33.60
ALG
16.04
112 113 114
2.2 Experimental set up
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Figure 1 shows the experimental set-up of the co-gasification test that performed in this study. It
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consists of a vertically adjusted quartz tube with 30 mm inner diameter, 34 mm outer diameter and
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510 mm length. Fixed quartz tube is covered by the cylindrical electrical heater with 45 mm inner
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diameter, 52 mm outer diameter and 950 mm length. A temperature controller was connected to the
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electrical heater equipped with a K-type thermocouple to control the reactor temperature. A cleaning
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system for the syngas was composed of isopropanol impingers (IPA), a cotton filter, a char coal filter
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and a silica gel tube. A micro-gas chromatograph (Micr-GC) was used to analyze the syngas
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composition.
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Figure 1 Downdraft fixed bed gasifier set-up
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2.3 CO2 gasification lab-scale installation
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In this study, a batch type gasification tests were performed. In each run, either the sole gasification
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of each fuel or the blended sample gasification, 500 mg of oven-dried sample was loaded in the
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adjusted tube and kept in the upper position in the cold zone until it was purged by N2 with the flow
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rate of 400 ml/min for 30 minutes to ensure an oxygen-free atmosphere inside the reactor. In
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addition, the fixed quartz tube was purged with N2 as well for the same reason. After reaching the
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target temperature and completing the purging step with N2, the gasifying agent (CO2) was supplied
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and the flow rate of N2 was decreased to 200 ml/min. Then the adjusted tube which contained the
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sample was moved down to the heated zone to start the gasification reaction. A temperature drop of
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about 40ºC was noticed during the first 5 minutes after the reaction started. The syngas was analyzed
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until there was no any syngas components are detected by the Micro-GC and the experiment was
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assumed to be completed. The CO2 partial pressure was changed by changing the CO2 flow rate of
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100, 200, 300 and 400 ml/min with the fixed N2 flow rate of 200 ml/min. The co-gasification
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experiment was conducted under 200 ml/min CO2 and 200 ml/min N2 mixture. Each experiment was
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repeated at least twice to ensure the reproducibility of the data. The results shown represent the
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average data. Similar methodology has been employed in 4.
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2.4 Methods of data analysis
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The composition of the syngas is analyzed using a micro-GC (Varian CP-4900). For each gas sample,
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the concentrations of H2, O2, N2, CO, CO2, C2H4 and C2H6 were measured.
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In the co-gasification experiment, the predicted gas production yield of each gas component was
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calculated according to eq 1:
= × +
(1)
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represents the calculated (predicted) value of the gas production yield, and are the
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gas production yields from algae and coal when they were individually gasified, respectively.
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and are the mass fraction of algae and coal in the blend, respectively.
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The heating value of each fuel was calculated according to the Dulong-Berthelot formula 30:
HHV = 0.3414C + 1.4445 (H – (N+O-1)/8) + 0.093S
(2)
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Where HHV denotes the high heating value. C, H, N, O and S are the respective carbon, hydrogen,
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nitrogen, oxygen and organic sulfur contents of the fuel (all were calculated on the dry, ash-free
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basis).
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The higher heating value of the syngas (HHV) was calculated according to eq 3:
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HHV = ((12.76 × ×H2 %) + (12.63 × × CO %)) *
(3)
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Where V [m3] is the volume of the syngas production yield from 1 kg sample.
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The thermal conversion efficiency (η) refers to the ratio of the energy content in the produced syngas
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to the energy content of the fuel and it was calculated according to eq 4:
× η = × 100% ! " $ × 1 # #
(4)
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3. Results and discussion
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3.1 Feedstock characteristics
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The proximate, ultimate and ash analyses for Newlands coal and spirulina algae are shown in Table 1.
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The ash content of most woody biomass is less than the ash content of this algae (9.5%), thus better
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performance can be expected in terms of the catalytic activity during the coal co-gasification process,
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because most of the ash compositions in algae are alkali and alkaline metals, where K and Ca are
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presented in considerable amounts in algae and they can act as catalysts to improve the gasification
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reaction reactivity
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confirmed its deactivation effect on the gasification reaction 9.
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. On the other hand, the coal sample contains more Si content which was
171 172
3.2 Effect of CO2 partial pressure on the gasification process of coal
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In this study, to obtain the highest syngas production yield, an optimized value of CO2 partial
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pressure that was introduced to the gasifier was experimentally determined. The main reaction
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involved in the gasification process when CO2 is the only gasifying agent is the Boudouard reaction
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(C + CO2
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order to produce 2 moles of CO. In order to eliminate the effects of other parameters on the role of
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the CO2 partial pressure, only sole coal gasification experiments were conducted. Experiments with
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the CO2 flow rates of 100, 200, 300 and 400 ml/min and the fixed N2 flow of 200 ml/min were
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employed. In each run, 500 mg of Newlands coal was gasified at the fixed temperature of 1000 ºC.
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Figures 2 (a) and (b) show the H2 and CO evolution rates for different CO2 partial pressure values
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corresponding to the aforementioned CO2 flow rates. Both gas components exhibited relatively
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different evolution profiles especially different peak values and final reaction time periods. This
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suggest the important role of the CO2 partial pressure on the amount of gas production yield, while
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the intrinsic surface reaction rate of the char gasification is known to be largely independent of total
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pressure 6. Figure 3 shows the effect of the CO2 partial pressure on the CO production yield. It can
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be seen that, the CO production yield didn’t change until the value of the CO2 partial pressure
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exceeded 0.05 MPa. This behavior can be explained by suppression of the mass transfer and the
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decrease of the CO2 diffusivity at a high pressure 32. Therefore, the value of the CO2 partial pressure
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that enables a higher syngas production yield shouldn’t exceed 0.05 MPa. Thus, the CO2 flow rate of
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200 ml/min that corresponds to the partial pressure of 0.05 MPa was employed for coal-algae
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co-gasification experiments.
2CO 171.1kJ/mol), which needs 1 mole of CO2 to react with 1 mole of carbon in
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Figure 2(a) Hydrogen evolution rate from coal gasification at 1000 ºC under different CO2 partial
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pressures
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Figure 2(b) Carbon monoxide evolution rate from coal gasification at 1000 ºC under different CO2
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partial pressures
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Figure 3 CO production yield from coal gasification at 1000 ºC under different CO2 partial pressures
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Figure 4 Gas production yield from sole gasification experiment
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Figure 5 Gas evolution profiles of sole gasification experiment for algae and coal samples at 950 ºC
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and 1000 ºC.
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3.3 CO2 gasification of individual coal and algae
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Figure 4 illustrates the gas production yield produced from each fuel sample when coal and algae
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were gasified individually. All samples have been studied at the temperatures of 950 ºC and 1000 ºC.
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At lower temperatures, it was difficult for the coal sample to start the reaction which implies the low
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reactivity of coal at lower temperatures. In this experiment, we assumed that the experiment was
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completed when the concentration of CO gas in the gas sample being analyzed reached zero. Big
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differences in the syngas amounts between both fuel samples can be noticed, which is due to the fact
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that much more char can be produced from coal (as shown in the proximate analysis presented in
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Table 1) and the main reaction involved in this process was the gas-solid Boudouard reaction which
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was promoted by the high concentration of CO2 gas in the gasifying agent. The H2 yield from the
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coal gasification was also higher than the algae gasification. The possibility of the water gas shift
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reaction, which is the main contributor for H2 production, was very low because the unavailability of
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H2O in the system and the high concentration of CO2 in the reactor led to suppress the water gas
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shift reaction. However, one of the highly possible mechanisms that might occur is that, the
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formation of free radicals on the char surface after the rapid pyrolysis step 33. These free radicals can
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be extracted by other free radicals and have a greater tendency to form the molecular hydrogen 25.
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This combination of more char produced from coal and possible more free radicals increased the
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hydrogen yield from the coal gasification. Moreover, the dry reforming reaction might take place
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and help to convert the hydrocarbons into H2 and CO 4. The possibility of high hydrocarbons
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production from the coal gasification can explain the higher yield of H2.
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Figure 5 introduces the gas evolution profiles of each fuel sample until no more CO gas was detected.
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H2 gas exhibited relatively low evolution rate, this may resulted from the high concentration of CO2
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in the gasifying agent which weakened the role of the water gas shift reaction and H2 was consumed
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through the backward of the water gas shift reaction 4. Faster algae gasification completion was
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achieved because less char was produced in the algae sample and the chemical composition of algae
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was simple which was mainly consisted of carbohydrates (sugar), protein and simple lipids
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Moreover, larger amount of alkali and alkaline compounds, especially K and Ca compounds, was
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contained in the algae sample. Many researchers have reported the promoting effects of these
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compounds for the char gasification reaction. It works as a catalyst and plays an important role in the
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formation of new active sites for the surface carbon gasification reaction
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minimize the formation of tar either by stopping the tar formation or by catalyzing the tar
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decomposition
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. Also, it helps to
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3.4 CO2 co-gasification of coal-algae blend
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3.4.1 Effect of the blending ratio
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In order to investigate the role of algae weight ratio in the CO2 co-gasification with coal, different
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mass ratios of algae have been investigated.10, 30 and 50% w/w algae were blended with coal and it
249
was shaken for several minutes using electrical shaker in order to assure the homogeneity of the
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mixture. The particle size of both fuels was kept around 100 microns.
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Figure 6 shows the effect of the algae blending ratio on the syngas production yields from the CO2
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co-gasification of coal-algae blend at the reaction temperature of 950 ºC. Only CO and H2 are shown
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in this figure since the rest of syngas components produced was in very low concentrations and
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could not be detected by the Micro-GC in most cases. The calculated values are the ones obtained by
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eq 1 on the dry-ash-free basis assuming that all algae and coal samples in the blend were gasified
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completely. It can be seen that, by increasing the algae weight ratio in the blend, the syngas
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production yield decreased, as a result of less char amount produced from the fuel blend which is the
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main contributor to the solid-gas reaction in the gasification process. For all blending ratios, the
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experimental values of the H2 yield were slightly higher than the calculated values, which show the
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synergetic effect that took place during the co-gasification process. One possibility for this
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synergetic effect is the catalytic effect of alkali and alkaline contents (especially K2O) in the algae
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sample, which helped to convert tar in the coal sample into gases. Also, the radicals produced from
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the algae sample promoted the heavy tare conversion into light tar and gases 38. However, the effect
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of the blending ratio can’t be seen clearly for the H2 yield due to small differences between the
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experimental and calculated H2 yields. Here, CO is the main product of this co-gasification process
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due to the high concentration of CO2 in the gasification agent which assisted the Boudouard reaction
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to be dominant, especially when the reaction temperature was high enough to ensure Boudouard
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reaction activation. Meanwhile, the synergetic effect in the CO yield showed a gradual increase with
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the increase of the algae weight ratio in the blend. The observed synergetic effect in the
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co-gasification process could be attributed to the algae’s alkali and alkaline contents which worked
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as reaction promoters and enhanced the reactivity of the coal char particles.
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273 274
Figure 6 Effect of the algae blending ratio on the syngas production yield at 950 ºC
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3.4.2 Effect of the reaction temperature on co-gasification of coal-algae blend
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Figure 7 shows the effect of the algae blending ratio on the syngas production yields at 1000 ºC. For
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all blending ratios, the experimental values of 10% w/w algae blend showed lower values than the
279
calculated ones, suggesting the happening of the inhibition effect. This phenomena could happen
280
because of a low concentration of the mobile alkali that provided from the algae ash content. Also,
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the elements that characterized by its deactivation nature like Si presented in high amounts in the
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coal sample which sequestrated the mobile alkali from algae and caused the inhibiting behavior 39.
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However, by increasing the algae ratio in the blend, the experimental values became higher than the
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predicted ones for both H2 and CO, suggesting that the synergy effect occurred due to the catalytic
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activity of the algae alkali and alkaline contents. Furthermore, the results showed that the gas
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production yields increased with the increase of the temperatures, and this is an expected behavior
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according to the fact that the Boudouard reaction is an endothermic reaction that can be enhanced by
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the temperature rise. Moreover, temperature rising enhances the catalytic prosperities of the ash and
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helps to improve the formation of the micro-porous network in the char particles which in turn
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provides access for the gasifying agent to reach the active sites in the char, and thus enhances the
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catalytic property of the char 4.
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Figure 7 Effect of the algae blending ratio on the syngas production yield at 1000 ºC
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3.4.3 Effect of the co-gasification process on the thermal conversion efficiency
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One important aspect to be quantified about the syngas production yield from the co-gasification
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process of coal-algae blend is that, the syngas calorific value. Previous study unveiled that higher
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biomass content in the co-gasification process leads to minimize the energy efficiency
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higher heating value of coal and algae were calculated according to eq 2 as shown in
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Table 1. While the syngas heating value was calculated according to eq 3. Fig. 8
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. The
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demonstrates the heating values of the syngas produced from both fuel samples and from their
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blends at 950 ºC and 1000 ºC temperatures. Results showed that, syngas heating value produced
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from the individual coal gasification is the highest while for sole algae gasification exhibited the
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lowest value. The heating value of the blended samples decreased as the algae weight ratio presented
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in the blend increased. This behavior is not surprising as, more carbon content is contained in the
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coal sample. Furthermore, algae is characterized by its high moisture content even a dry sample is
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used and has low energy density that may affect the algae gasification process. Fig. 9 shows the
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thermal conversion efficiency index (η) results. The lower energy efficiency of sole algae
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gasification was confirmed by means of low thermal conversion efficiency index. However, thermal
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conversion efficiency index of coal-algae blends co-gasification are equivalent to the respected
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values of sole coal gasification. Thus, blending co-gasification process has improved thermal
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conversion index of algae gasification.
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316 317
Figure 8 Produced syngas heating values
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4. Conclusion
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Using a lab-scale downdraft fixed bed reactor, the role of CO2 partial pressure on the coal
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gasification was studied, and the effect of the algae blending ratio in the co-gasification experiment
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with coal and under CO2 atmosphere was investigated. Results showed that the gas production yields
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from co-gasification experiment were higher than the predicted ones especially for a higher blending
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ratio suggesting the occurrence of the synergetic effect. The thermal conversion efficiency of algae
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gasification was enhanced when it was utilized in the co-gasification process with coal. It was
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demonstrated that the co-gasification is a promising pathway for effective utilization of algae.
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