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Chemical Looping Gasification of Coal in a 5 kWth Interconnected Fluidized Bed with Two-stage Fuel Reactor Tianxu Shen, Jian Wu, Laihong Shen, Jingchun Yan, and Shouxi Jiang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017
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Chemical Looping Gasification of Coal in a 5 kWth Interconnected Fluidized Bed with Two-stage Fuel Reactor Tianxu Shen, Jian Wu, Laihong Shen, Jingchun Yan, Shouxi Jiang Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment,
Southeast University, Nanjing 210096, China
E-mail:
[email protected] Abstract Chemical looping gasification (CLG) is a novel technology using lattice oxygen in solid oxygen carrier (OC) for syngas production. The present work investigated the CLG performance of coal in a 5 kWth interconnected fluidized-bed, in which fuel reactor was designed as a two-stage based circulating bed. The fresh OC from air reactor entered into the upper stage of fuel reactor, in which it was reduced by syngas. Then, the partial reduction of OC was circulated to the bottom stage of fuel reactor to be reduced further to low valence metal oxide and catalyze the process of coal gasification. Afterwards, the reduced OC was transported to air reactor for regeneration. The influences of coal feeding rate, gasification temperature and gasification agent on the performance of CLG were evaluated in detail. The syngas yield and gasification efficiency were remarkably influenced by coal feeding rate with an optimal value of 487.5 g/h in the current unit. The high temperature promoted the coal gasification process with the maximum gasification efficiency and syngas yield corresponding to 75.2% and 0.97 Nm3/ (kg· coal) respectively at gasification temperature of 915 ℃. Steam, CO2 and mixture of them were used as gasifying agent separately. The experiment results demonstrated that carbon conversion, syngas yield and gasification efficiency increased with the proportion of steam in gasifying agent. The OC particles in the upper
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and bottom stages of fuel reactor were analyzed by XRD. The main phase of the OC particles in the bottom stage was FeO, which has a high catalytic activity on coal gasification and tar cracking. Serious particle agglomeration was observed under the low fluidization flow of 20 L/min and high temperature of 950 ℃ at which the system cannot be stable operated. Keywords: Chemical looping gasification; Two-stage circulating bed; Gasifying agent; Oxygen carrier
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1. Introduction The gasification is a significant approach on high efficient and clean utilization of carbonaceous fuel via transition fossil into renewable and sustainable energy
[1]
. Nevertheless,
there are some several challenges on traditional gasification mode including intensive capital investment and large operating cost [2]. Producing high-concentration oxygen and CO2 capture are the major trends to increase the cost and decrease the energy utilization efficiency [3].
Figure 1. Schematic of coal gasification in CLG process. Chemical looping gasification (CLG) has been identified as a cost-effective gasification way with efficient CO2 capture. The CLG employs lattice oxygen in metal oxide instead of gaseous molecular oxygen
[4]
. The schematic principle of CLG is shown in Figure 1, that the system is
comprised of air reactor (AR) and fuel reactor (FR). Based on the circulating oxygen carrier (OC) between two reactors, oxygen is transferred from air to meeting the oxygen demand of fuel gasification in FR. Fuel is isolated from air and the huge cost of producing pure oxygen has been avoided
[5]
. The weak oxidation of lattice oxygen is another advantage of CLG, which is prone to
partially oxidize fuel than gas-phase oxygen. The indirect combustion of CLG also can evidently promote yield of syngas and increase lower heating value (LHV) of gas production [6]. At the same time, the metal oxide as oxygen carrier has certain catalysis on the tar cracking and decomposing
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[7]
. One of the key parameter in chemical looping process is the oxygen carrier selection. Many
species of metallic oxide, for instance Fe, Cu, Mn and Co, are chosen as candidates. Fe-based oxygen carrier attracts much attention due to competitive prices, high wear resistance and environmental friendliness. The natural hematite has the concern of the lower costs and economic feasibility, which can be acquired directly from steel industry and pigment production. Song et al. evaluated the performance of hematite with bituminous and anthracite coal, the results of which indicates high reactivity in CLC process and inert material SiO2 alleviates sintering degree
[8]
. In
addition, the alkali metals are introduced into oxygen carrier as catalyzer to accelerate the char gasification rate; nevertheless, it would decrease the melting point and lead to the sintering of OC [9, 10]
. The low valence metal oxide has a significant result for catalyzing coal tar cracking. The
active metal has high activity for the decomposition of aromatic hydrocarbons, oxidation to crack aliphatic and cleavage of C–C and C–H bonds
[11-13]
. The design of reactor structure is another
vital influence factor of CLG performance. The great majority of oxygen carrier is belonged to Geldart B particle, which is prone to cause gas escaping and reduce gas-solid contact. The gas exchange rate between bubble and dense-phase is low and bubble formation is more often than other particles
[14]
. Penthor et al.
[15]
proposed to arrange flow obstacles along the height of fuel
reactor to ameliorate gas-solid contact. To enhance fuel conversion, Werther et al.
[16]
designed a
25 kWth chemical looping system with a two-stage fuel reactor. In the study of biomass gasification using chemical looping (BGCL), Huang
[17]
selected
Fe-Ni bimetallic oxygen carrier and found evident facilitation of metal particles on biomass gasification. Ge et al.
[18]
investigated the performance of BGCL in a 25 kWth prototype and
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explored the effects of gasification temperature, steam-to-biomass (S/B) ratio and hematite mass percentage. Also Song
[19]
carried out experiment on hydrogen production from biomass
gasification in a laboratory scale apparatus of interconnected fluidized beds. At current, there is rare researches on coal chemical looping gasification in dual circulating fluidized beds. The most studies are carried out on thermo gravimetric analysis (TGA) and batch fluidized bed reactors. Therefore, exploration about the feature of coal gasification with continuous operation has a vital significance in chemical looping process. The research on coal gasification use chemical looping was investigated in a 5 kWth prototype, in which fuel reactor was divided into two-stage circulating fluidized beds by an internal air distributor as flow obstacle. The configuration of two-stage fuel reactor was to increase residence time and utilize catalysis of low valence metal oxide. The arrangement of internal air distributor was to enable uniform gas distribution and strengthen gas-solid mixture. The impact of operational parameters as coal feeding rate, gasification temperature and gasifying agent were investigated in the experiments.
2. Experiment 2.1 Experimental setup and procedure The schematic diagram of interconnected circulating fluidized-bed is shown in Figure 2. The system is composed of two interconnected circulating fluidized beds (air and fuel reactor), two cyclones (AR and FR cyclones), a siphon and two loop-seals (AR and FR loop-seals). Air reactor (AR) is a fast fluidized bed with 50 mm inner diameter and 2000 mm in height. The fuel reactor (FR) is a rectangular spout-fluid with 100*50 mm2 cross section and 1250 mm height. FR is
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divided into bottom stage (FR-I) and upper stage (FR-II) by an internal air distributor, which is arranged in a height of 1 m. The height of internal air distributor is to prevent OC particles in FR-I entrained into FR-II. A gas collecting pipe is used to sample furnace gas in FR-I, which is installed below the internal air distributor. The oxygen carrier from AR is transferred to the FR-II firstly and reacted with the syngas from coal pyrolysis and gasification. The partial reduced OC in FR-II is circulated to the FR-I through the FR cyclone and FR loop-seal. The OC is reduced further to low valence metallic oxide in FR-I, which is participated in the coal pyrolysis and gasification reactions as catalytic agent. The major of unreacted tiny char in FR-I is carried by fluidizing agent and syngas into FR-II through the internal air distributor, but the large diameter OC is unable to be entrained to the certain height. The reduced oxygen carrier can only be transferred to AR throughout the siphon and then is oxidized by air.
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Figure 2. Schematic diagram of the interconnected circulating fluidized-bed with two-stage circulating bed fuel reactor system for coal chemical looping gasification. The reactors were electrically heated in an oven which supplies heat for start-up and compensates heat loss during operation. The temperature of different system part was measured by thermocouple and the data was recorded by manual work. Points from 1 to 6 outlined in Figure 2 were temperature measuring points. Points 1, 2 and 3 were located in AR from bottom up respectively, and the distribution of points 4, 5 and 6 were from lower to upper section of FR. The average temperature of each measuring points of FR was set as the gasification temperature in experiments. The gas sample collection was started after a steady operation of the system. The stable system operation was judged by that operation parameters including pressure curve and temperature keep relatively stable over half an hour. The furnace gas of FR-I and outlet gases of
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FR-II and AR were sampled by gas bags for offline analysis. The sampling gases is entered into a scrubber for water cooling and ash disposal before collecting into gas bags. A NGA2000 type gas analyzer (EMERSON Company, USA) was used to measure the gas composition content of O2, CO, CO2, CH4 and H2. Nitrogen was used as the fluidization agent for FR, siphon and two loop-seals and the air was used for AR. Coal was fed into the bottom section of FR-I by a screw conveyor. 2.2Material preparation The lignite coal from Xiaolongtan China was used for experiments. The coal particles were crushed and sieved into a size range of 0.3-0.45 mm. The proximate and ultimate analysis are shown in Table 1. The lower heating value was 15.68 MJ/kg. Table 1. The proximate and ultimate analysis of coal (wt.%). Proximate analysis
Ultimate analysis
M
V
FC
A
C
H
O
N
S
15
35.3
34.6
15.1
45.39
2.85
16.95
0.98
1.45
The natural hematite was selected as oxygen carrier, which was supplied by Nanjing steel manufacturing company. The size range of oxygen carrier was also from 0.3 to 0.45 mm and the accumulation bulk density was 2440 kg/m3. The OC particles were calcinated in a muffle oven at 900 °C for 3h to improve the mechanical strength. The elemental composition of hematite based on the XRF analysis is illustrated in Table 2. The natural hematite consisted of active phase Fe2O3 and some inert materials such as SiO2 and Al2O3 which were proved to improve the resistance to sintering [20, 21]. During the experiments, the hematite was blended with a percentage of silica sand. The addition of silica sand in admixture can transfer enough heat from AR to FR and improve
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fluidization quality simultaneously. The size range of silica sand was the same with hematite from 0.3 to 0.45 mm, and the accumulation bulk density was 1660 kg/m3. Table 2. Elemental composition of Hematite (wt.%). Fe2O3
SiO2
Al2O3
P2O5
CaO
SO3
others
83.21
7.06
5.13
0.38
0.24
0.21
3.77
2.3 Thermodynamics analysis Owing to the existence of oxygen carrier and gasifying agent, the reaction process of CLG is complicated [22]. There are several competing reactions in CLG, comprising (a) coal pyrolysis, (b) coal gasification reaction, (c) combustion reaction and (d) regeneration reaction of reduced oxygen carrier. In FR-I, the coal pyrolysis (R1), gasification reactions (R2-R5) took place firstly and a portion of combustion reaction was also occurred. The syngas and fine char particle were transported into the FR-II throughout the internal distributor. The reactions in FR-II were combustion reaction (R6-R11) and part of gasification reaction including water gas, Boudouard and water gas shift (WGS) reactions. The reduced oxygen carrier can be regenerated by reactions (R12, R13) with air in AR. It should be noted that reactions in fuel reactor are intensive endothermic reactions in general and the oxidation reaction in air reactor is exothermic process. (a) Coal pyrolysis: Coal→char+
tar+
syngas
(CO;
H2;
CO2;
CH4;
CnH2m)
(R1) (b) Coal gasification: Water Gas: C+H2O(g)→CO+H2 Boudouard:
(R2) C+CO2→2CO
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(R3) Water-gas shift: CO+H2O(g)→H2+CO2
(R4)
Steam reforming: CH4+H2O(g)→CO+3H2
(R5)
(c) Combustion reactions: CO+3Fe2O3→2Fe3O4+CO2
(R6)
CO+Fe2O3→2FeO+CO2 (R7) H2+3Fe2O3→2Fe3O4
+2H2O
(R8) H2+Fe2O3→2FeO+H2O
(R9)
CH4+3Fe2O3→2Fe3O4
+2H2+CO
(R10) CH4+4Fe2O3→8FeO+H2O+CO2 (R11) (d)Regeneration reactions: 4Fe3O4+O2→Fe2O3 (R12) 4FeO+O2→2Fe2O3 (R13) 2.4 Data evaluation The evaluating parameters of chemical looping gasification including carbon conversion efficiency (E1), syngas yield (E4) and gasification efficiency (E5) is referenced from former
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researches [17-19]. (1) Carbon conversion efficiency The carbon conversion efficiency ( ) is defined by the ratio of utilizing carbon in system to the total carbon of coal fed into system. The utilizing carbon in system is calculated by the concentrations of carbonaceous gases (CO, CO2, and CH4) including AR and FR.
=
, ,
(E1)
,
, = , ×
, , ,
(E2)
( , , , , )
, = ( , × 0.79 + , ) (
,
(E3)
, , )
The , and , are the molar flow of carbonaceous gases from the outlet gases of fuel and air reactor respectively. The ,$%& is the total carbon molar quantities of coal fed into the system, which is calculated by coal proximate analysis and coal feeding quantity. The ' are the concentration of corresponding outlet gases of O2, CO2, CO, CH4 and H2 from the fuel and air reactor respectively. The , is the fluidizing gas N2 flow of fuel reactor. The , and
, are the inlet air flow of air reactor and the nitrogen flow of siphon entering into air reactor, respectively. (2) Syngas yield The syngas yield (Nm3/kg· coal) is the ratio of effective synthesis gas consisting of CO, H2 and CH4 in the FR outlet to the quantity of coal fed into the system.
Syngas yield = , ×
, , , ( , , , , )
×
3456 7 89:; ?3
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(E5)
@ ×3456 7 89:;
