New Process of High-Quality Syngas Production through Sequential

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A New Process of High-quality Syngas Production through Sequential Oxidation-Reduction Cycles of Pulverized Coals Dehong Xia, Qiang Zhang, Hongbin Wu, and Weiwei Xuan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02417 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016

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Energy & Fuels

A New Process of High-quality Syngas Production through Sequential Oxidation-Reduction Cycles of Pulverized Coals Dehong Xia*, Qiang Zhang, Hongbin Wu, Weiwei Xuan School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China Keywords: High-quality syngas, Pulverized coal gasification, Oxidation-reduction Cycles

ABSTRACT

A new process is proposed to prepare high-quality syngas (H2+CO) for the production of dimethyl ether, methanol and other chemical products at atmospheric pressure, during which the pulverized coal is used as raw material. The process consists of two zones: reduction and oxidation. Steam is injected into the reduction zone, which then reacts endothermically with coke to produce syngas. The raw material (pulverized coal) is fed to the oxidation zone, and then reacts with air and releases heat. Circulating coal ash and char are heated in the oxidation zone, which provide heat for the reduction zone to maintain the endothermic reactions. The coke just reacts with the steam in the reduction zone, and the gas composition mainly contains H2 and CO. Therefore, high quality syngas is produced. In order to acquire the operating parameters and

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provide reference for the industrial design, the feasibility of the proposed cycle was verified by preliminary experiments and a system model was built with Aspen Plus to simulate the process. The simulation study shows that the main components of syngas (H2+CO) in the new process account for over 85%, reaching up to 93.20%; the ratio of H2/CO is adjustable between 1.0 and 2.0, which meets the requirement of the production for downstream chemical products. In addition, pilot experiments were carried out based on the model. As a result, the simulation results agree with the experimental data under the same operation condition in three different cases.

1. Introduction Nowadays, the conventional processes to prepare syngas include mainly fixed bed, fluidized and entrained bed coal gasification process1. The H2/CO ratio among the main components of syngas in fixed-bed varies from 1.0 to 2.02. But the content of (H2+CO) is low, normally 65%, which is non-optimal for the production of syngas. The content of main components (H2+CO) of the syngas in fluidized-bed is also not high3, 4, normally 80%. And the ratio of H2/CO is approximately 1.0, which needs adjustments to meet the requirement of downstream chemical products synthesis. As for entrained-flow, the content of effective syngas (H2+CO) is high (normally over 90%)5 with the ratio of H2/CO between 0.5 and 1.06, but CO transformation is still needed since the content of H2 is not high enough. The high-quality chemical syngas produced can be directly used for the synthesis of downstream chemical products such as dimethyl ether7, which avoids CO transformation and pressure swing absorption process to adjust H2/CO ratio in conventional techniques, thereby reducing production cost. In addition, as the content of H2 in the new process is relatively high,


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the production cost is reduced greatly when synthesizing downstream chemical products such as methanol and ammonia because the amount of CO needed to be transformed is reduced. Shell pulverized coal gasification process and Texaco coal-water slurry gasification process8 both fall into the category of pressurized entrained flow gasification process, which adopt the single furnace as the reactor and the pure oxygen as the gasification agent. These processes have a complicated system and high operating cost. In order to reduce the production and operating costs and acquire high quality syngas suitable for the synthesis of downstream chemical products, a new process for syngas production was developed. 2. Process design The purpose of this paper is to acquire high quality syngas suitable for the synthesis of downstream chemical products, of which the content of effective gas (H2+CO) is above 85% and the ratio of H2/CO is between 1.0 and 2.09-12. On this basis, a new process for syngas production consisting of reduction and oxidation zones was developed. This main cycle operates under normal pressure. Steam is injected into the reduction zone, and then react endothermically with coke to produce syngas. The raw material (pulverized coal) are fed to the oxidation zone, and then react with air and release heat. High-temperature circulating coal ash and char as heat carrier provide heat for the reduction zone. This guarantees the continuous operation of the cycle process. Therefore, purer syngas can be acquired without use of pure oxygen since the coke just reacts with the steam in the reduction zone. The experimental set-up consists of the reduction zone, the oxidation zone, cyclone and refeed valve. Fluidized bed is used in the reduction zone and entrained flow bed is used in the oxidation zone. As the reductant, the steam enters into the reduction zone from the bottom of the fluidized


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bed. And oxidant (air or oxygen enrichment) enters into oxidation zone from the bottom of entrained flow bed. Fig. 1. shows the schematic diagram of syngas production cycle.

Fig. 1. Schematic diagram of syngas production cycle 2.1 Reactions in oxidation zone In the oxidation zone, influenced by updraft, pulverized coal is lifted to the upper oxidation zone from the bottom. During this process, pulverized coal undergoes drying, pyrolysis and volatile combustion process13. See the details as follows: (1) First in the drying process (temperature below 200℃), pulverized coal changed physically, presented as follows:

Coal(wet) → Coal(dry) + H2 O(g)

(1)

(2)Then with temperature rising, the quality of water in the coal decreases. When the temperature reaches 300~600℃, the heated pulverized coal experiences pyrolysis process with a series of complicated physical and chemical changes14. The macroscopic consequence of this pyrolysis process is that volatile precipitates from the pulverized coal and the remaining part


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forms char or coke. The chemical species in volatiles mainly contain H2, CO, CO2, CH4, H2O, C2H6 and N2. This process can be expressed as follows:

Coal → Coke(s)+Volatiles(g)（H2、CO、CO2、CH4、H2O、C2H6、N2）

(2)

(3) Among the volatile precipitates from pulverized coal, CO, H2, CH4 and C2H6 are combustible. The combustion reaction between gas phases occurs more easily than coke combustion. The combustion reactions with oxidant (air or oxygen enrichment) are written as follows: C O (g)+

1 O 2 (g)=C O 2 (g) 2

(3)

H 2 (g)+

1 O 2 (g)=H 2 O(g) 2

(4)

CH 4 (g)+2O 2 (g)=CO 2 (g)+2H 2 O(g) C 2 H 6 (g)+

7 O 2 (g)=2C O 2 (g)+3H 2 O (g) 2

(5) (6)

(4) As the quality of the oxidant is not sufficient to complete the coke combustion reaction, only part of the coke reacts with oxidant (negative sign indicates that these reactions are exothermic, kJ/mol), as shown below: 1 C(s)+ O 2 (g)=CO(g) -110.4kJ/mol 2

(7)

C(s)+O 2 (g)=CO 2 (g) -394.1kJ/mol

(8)

The coal ash and coke in the oxidation zone are heated by the heat release and continue to be lifted to the upper section of the oxidation zone. After cyclone seperator, flue gas is discharged from the upper section of the oxidation zone to waste heat boiler, while circulating coal ash and


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coke enter the reduction zone through refeed valve. The steam produced in waste heat boiler as the reductant enters reduction zone. 2.2. Reactions in the reduction zone In the reduction zone, since the coke reacts with the steam, the gas composition mainly contains H2 and CO. The reduction reactions are shown as below (positive sign indicates that these reactions are endothermic, kJ/mol): C(s)+H 2 O( g ) → CO(g)+H 2 (g) +135.0kJ/mol

(9)

CO(g)+H 2 O(g) → CO 2 (g)+H 2 (g) -38.4kJ/mol

(10)

Compared with the conventional process to produce syngas, the proposed cycle has the following advantages: making lower costs of raw materials by using low rank pulverized coal; improving the reliability of the system by operating at atmospheric pressure; decreasing the operating costs by using air or oxygen enrichment instead of pure oxygen. Meanwhile, volatile burns in the oxidation zone to provide heat for reduction zone, and the coke just reacts with the steam in the reduction zone. Therefore, it will not contain volatile and nitrogen in the syngas and high quality syngas can be produced in reduction zone continuously. The content of main compositions in syngas (CO+H2) can be maintained at round 90% and the ratio of H2/CO is between 1.0 and 2.0, which is applicable for producing methanol, dimethyl ether, ammonia and other downstream chemical products. A series of preliminary experiments have been conducted in the lab. China Shenmu bituminous coal was used as a raw material. The particle size varies between 0 and 8 mm. The schematic diagram of syngas production cycle is similar to Fig. 1. But heat recovery system was not took into account in preliminary experiments in order to simplify the experiments. Instead, electric boiler and steam generator was used to produce steam. According to the experiments, we found that many parameters affect


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compositions and productivity of syngas, like the steam-to-coal ratio (Rs) (define as the ratio of the steam mass to the pulverized coal mass), operating temperature in oxidation zones, and preheating temperature of steam and air. The preliminary experiments were conducted steady and no operation glitches appeared in the experimental processes. 3. Simulation and optimization of operation parameters 3.1 Materials and models used In order to get operation parameters for the cycle and provide a design basis for a serial industrial production, Aspen plus was used to model the cycle. China Shenmu bituminous coal was used as the raw material. The results of proximate and ultimate analysis of this coal are shown in Table 1 which was obtained by experiment in the lab. Table 1 Proximate and ultimate analysis of Shenmu bituminous coal Proximate analysis

(wt%)

Moisture

8.67

Volatile matter

30.55

Fixed carbon

50.30

Ash

10.48

Ultimate analysis (dry basis)

(wt%)

Carbon

68.86

Hydrogen

3.99


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Nitrogen

1.23

Oxygen

13.89

Sulfur

0.56

Ash

11.47

Due to the complication of multiphase thermal chemical reaction under high temperature, the following assumptions are considered in this process. (a) The reduction and oxidation processes operate at a steady state and the reactions reach thermodynamic equilibrium under operation conditions; (b) the circulating coal ash is inert and does not participate in chemical reactions.15 ,16 In the syngas production process of simulation, the dynamic model17, 18 and thermal balance model19-21 are considered. Dynamic model is based on the gasification system reaction kinetics, it can truly reflect the gasification process, commonly used in CFD simulation. Thermal balance model is based on the reaction thermodynamics which can simulate coal gas compositions precisely, so the proposed cycle model is built using the thermal balance model based on Gibbs free energy minimization22 and according to equations (13)-(15). The specific model types for different units are shown in Table 2. Table 2. The specific model types for different units Unit operation

Model type

DECOMP

Ryield

RED-ZONE

RGibbs


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SEP

Sep

OX-ZONE

RStoic

FLUE-CYC and SYN-CYC

Cyclone

BOILER and AIR-HEAT

MheatX

In the decomposition unit, the coal is converted into its constituting components such as carbon, oxygen, hydrogen, sulfur, nitrogen, ash and water. Ryield reactor is used to model this process by specifying the yield distribution vector according to the coal ultimate analysis. The enthalpy balance equation in this unit is show as follows: NP

NP

mcoal H 0f ,coal ,298 + mcoal H coal (Tfeed ) = ∑ ni H 0f , prod ,298,i + ∑ ni H i (Tprod ) + Q p i =1

(11)

i =1

Where mcoal is the quality of coal, kg; H 0f ,coal ,298 is the formation enthalpy of coal at 298K, kJ/kg, the expression is show as equation (12); Hcoal is the enthalpy value of coal, kJ / ( kg ⋅ K ) ; T feed is the temperature at the entrance of coal, K; NP is the number of simple molecules by coal

pyrolysis; n is the mole number of components;

H 0f , prod ,298,i is the formation enthalpy of

pyrolysis product, kJ/kg; Tprod is he temperature of pyrolysis product, K; Q p is decomposition heat of coal, kJ.

H 0f ,coal ,298 =Q − (327.63wC +1417.92wH + 92.57wS + 158.67wM )

(12)

Where Q is high heating value of coal, kJ/kg; wC is mass fraction of element C; wH is mass fraction of element H; wS is mass fraction of element S; wM is mass fraction of water;


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S

c

min G; G = ∑ G 0j n j + j =1

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p

∑ ∑G n jl

(13)

jl

j = s +1 l =1

Where G is Gibbs free energy, J; S is isolated phase; C is number of component; P is number of phase in the system. S

c

bk = ∑ m jk n j + j =1

c

c

∑n

i

i =1

p

∑ ∑G

jk

n jl k = 1, 2 ⋅⋅⋅, E

(14)

j = s +1 l =1

c

c

H 0f ,coal ,298,i + ∑ni H feed ,i (Tfeed ,i ) + Qp = ∑ni H 0f , prod ,298,i + ∑nprod ,i Hi (Tprod ,i ) + QL i =1

i =1

i =1

(15)

ni ≥ 0 Where bk is mole number of element, mol; m jk is atom matrix of components, n is the mole number of components; E is number of element;

H 0f , feed ,298,i is the formation enthalpy of

imported component, kJ/kg; H feed ,i is the enthalpy value of imported component, kJ / ( kg ⋅ K ) ; T feed ,i is the temperature of imported component, K; QL is heat loss of system.

The flow diagram of the process used for simulation applying Aspen Plus software is shown in Fig. 2.


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Fig. 2. Process flow diagram for simulation of the cycle 3.2 Simulation results and optimization of operating parameters A detailed model of the syngas production cycle has been built to achieve an accurate process description. The syngas can be influenced by various factors such as steam-to-coal ratio ( R s ), volume fraction of oxidant, oxidation zone temperature (Tox-zone), preheating temperature of steam (Tsteam) and air (Tair). The composition of syngas (CO+H2), H2/CO ratio and syngas yield are analyzed when the operating parameters change. Then the new process is optimized. 3.2.1 Impact of steam-to-coal ratio (Rs) The impact of Rs on syngas compositions is illustrated in Fig. 3. The contents of H2, CO and (H2+CO) are plotted on a dry basis against Rs. H2 increases with the rise of Rs from 0.8 to 1.2 while the contents of CO and (H2+CO) reveal an opposite trend that (H2+CO) decreases from 93.20% to 85.77% and the ratio H2/CO increases from 1.3 to 1.9. These trends may be explained as follows: With the increase of Rs, the reactions (9) and (10) react forwards, which indicates the increasing of H2 and decreasing of CO in the syngas.


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Fig. 3. The effect of steam-to-coal ratio (Rs) on syngas compositions. The effects of Rs and O2 volume fraction of the oxidant on syngas yield are shown in Fig. 4. Syngas yield increases by 7.6% with the increasing of Rs from 0.8 to 1.2, since the reactions (9) and (10) react forward, which symbolizes the growing composition of syngas (CO+H2). Besides, when O2 volume fraction of the oxidant increases, syngas yield increases. Oxygen-enriched air is an effective way to increase the thermal efficiency of the system and more syngas can thus be produced. When Rs increases from 0.8 to 1.2 and O2 volume fraction of the oxidant from 21% to 60%, syngas yield is increased by 7.6% and 6.1% respectively.


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Fig. 4. The effects of steam-to-coal ratio (Rs) with different O2 volume fractions of the oxidant (21%, 40% and 60%) on syngas yield 3.2.2 Impact of the oxidation zone temperature (Tox-zone) The effect of the oxidation zone temperature (Tox-zone) on syngas compositions on a dry basis is shown in Fig. 5. The compositions of H2, CO and (H2+CO) almost remain the same when Tox-zone increases from 950℃ to 1100℃. Increasing Tox-zone can effectively accelerate reaction speed but has little effect on syngas compositions. The component of H2+CO is above 92.5%, and the ratio of H2/CO maintains at 1.3 which belongs to high quality syngas.

Fig. 5. The effect of the oxidation zone temperature (Tox-zone) on syngas compositions. The effects of the oxidation zone temperature (Tox-zone) and O2 volume fraction of the oxidant on syngas yield on a dry basis are shown in Fig. 6. Syngas yield decreases with increase of Toxzone

cause more heat is needed to maintain the high Tox-zone that makes thermal efficiency

decreases. The O2 volume fraction of the oxidant and syngas yield have an inverse proportion relation cause Oxygen-enriched combustion will accelerate reaction rate, increasing the released heat and improving temperature in oxidation zone. As a result, after the circulating ash gets


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heated in the oxidation zone and enter the reduction zone, the temperature of reduction zone will be improved, which accelerates the reaction rate and makes syngas yield increased. When Tox-zone increases from 950℃ to 1100℃ and O2 volume fraction of the oxidant increased from 21% to 60%, syngas yield decreased by 4.4% and increased by 7.4% respectively.

Fig. 6. The effect of the oxidation zone temperature (Tox-zone) on syngas yield. 3.3.3 Impact of preheating temperature of steam (Tsteam) and air (Tox) In the reduction zone, steam is used as the reductant. The effect of the steam temperature (Tsteam) on syngas compositions is shown in Fig. 7. The volume fractions of H2, CO and (H2+CO) are plotted on a dry basis and almost remain the same when Tsteam increases from 200℃ to 500℃ . Increasing Tsteam can effectively accelerate the reaction speed but have little effect on syngas compositions. The component of H2+CO is above 92.5%, and the ratio of H2/CO maintains at 1.3 which belong to high quality syngas.


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Fig. 7. The effect of steam temperature (Tsteam) on syngas compositions: air preheated to 500℃ The effects of steam temperature (Tsteam) on syngas yield under different air temperature (300 ℃, 400℃ and 500℃) are shown in Fig. 8. Both increasing Tsteam and Tair can promote syngas yield. Air and steam preheating is a means of accelerating reaction rate as well as thermal efficiency of system and thus more syngas can be produced. When Tsteam increases from 200℃ to 500℃ and Tair from 300℃ to 500℃, syngas yield is increased by 2.8% and 5.3% respectively.

Fig. 8. The effect of steam temperature (Tsteam) on syngas yield for different air temperature (300 ℃, 400℃ and 500℃).


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Based on the above analyses, it can be concluded that Rs mainly affects the compositions of the syngas: the composition volume of (H2+CO) decreased and the ratio of H2/CO increases with the rise of Rs. The operating parameters Tox-zone, Tsteam and Tair mainly affect syngas yield: syngas yield increases with the decreasing of Tox-zone and increasing of Tsteam and Tair.

Table 3 the reasonable operating parameters steam-to-coal ratio Rs

0.8~1.0

oxidation zone temperature Tox-zone (℃)

950

preheating temperature of steam and air 500 Tsteam=Tair (℃) In order to acquire high quality syngas suitable for the synthesis of downstream chemical products, the reasonable operating parameters for this preparation technology of high-quality chemical syngas production are shown in Table 3. Under these conditions, high quality syngas with the composition of syngas (H2+CO) over 90%, reaching up to 93.20% can be acquired, the ratio of H2/CO is adjustable between 1.3 and 1.6, and the syngas yield is adjustable from 1.61 Nm3/kg to 1.68Nm3/kg. 4. Pilot experiment and application In order to validate the simulation results, a pilot experiment of 450 kg coal/h was set up. China Shenmu bituminous coal was used as the raw material in experiments, of which the


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properties were shown in Table 1. Coal, with particle size varing between 0 and 8 mm, is fed into the gasifier by spiral feeder. Three main gases (i.e. H2, CO, CO2) were considered to study gas production. The experimental data and simulation results were listed in Table 4.

Table 4 Experimental data and simulation results Syngas composition (dry) (V%) Case CO2

CO

H2

H2+CO

H2/CO

7.15

39.25

50.77

90.02

1.29

simulation

6.25

40.46

52.74

93.20

1.30

experiment

9.60

35.90

52.10

88.00

1.45

simulation

8.49

37.04

53.90

90.94

1.46

experiment

11.53

32.93

53.21

86.14

1.61

10.42

34.01

54.85

88.86

1.61

experiment 1

Rs=0.8

2

Rs=0.9

3

Rs=1.0 simulation

Table 4 shows the experimental data of three cases as well as the comparison with simulation results. In the reduction zone, the temperature ranges from 1138 to 1158 K; steam-to-coal ratio Rs is within the range of 0.8 to 1.2. In the oxidation zone, the temperature ranges from 1225 to


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1245K. The syngas compositions in the experiment have volume fractions of CO2 which are little higher than the simulation results 9%-10%, while the volume fractions of H2 and CO are slightly lower by 3%-4%. The experimental result shows that the process proposed can realize high proportions of H2/CO 1.3-1.6 and the composition of syngas (H2+CO) is maintained at round 90%. Besides, when the steam-to-coal ratio Rs is 1.2, the simulation result shows that the process proposed can also acquire high proportions of H2/CO, reaching up to 1.9 while the composition of syngas (H2+CO) is at round 85%. In the subsequent experiments, low rank coal (Wenshan lignite) was used as the raw material in experiments, and ideal results were also acquired: the proportions of H2/CO 1.0-2.0 and the composition of syngas (H2+CO) is above 85%, but the syngas yield decreased by 40%-45% than Shenmu bituminous coal’s result. Overall, simulation results and experimental data match well. The proposed process can be used for the synthesis of downstream chemical products such as dimethyl ether, methanol, ammonia and other downstream chemical products. In dimethyl ether synthesis process, the main reactions can be expressed as follows:

CO(g)+2H2 (g)=CH3 OH(g)

(16)

2CH3OH(g)=CH3 OCH3 (g) + H2O(g)

(17)

CO(g)+H2O(g)=CO2 (g) + H2 (g)

(18)

Simultaneous equations: 3CO(g)+3H 2 (g)=CH 3 OCH 3 (g) + H 2 O(g)

(19)

In the process of dimethyl ether synthesis, the stoichiometric ratio of H2/CO is 1.0. But in industrial production, due to the influence of adsorption of catalyst surface the ratio of H2/CO is


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controlled between 1.1 and 1.3. So the chemical syngas produced by the process can be directly used in downstream production of dimethyl ether. In methanol synthesis process, the main reaction is shown as follows:

CO(g) + 2H 2 (g) → CH 3 OH(g)

(20)

In ammonia synthesis process, the main reaction is shown as follows:

N2 (g)+3H2 (g) → 2NH3 (g)

(21)

In the synthesis of methanol (optimal ratio of H2/CO≈2.05~2.15 in the actual production) and ammonia synthesis process, CO needs to be shifted when synthesizing the downstream chemical products so that the content of H2 meets the requirement of the synthesis of chemical products. Compared with the conventional process, the content of H2 in the new process is relatively high, so the transformation cost is reduced since less amount of CO is needed. 5. Conclusions A new preparation technology of high-quality chemical syngas through alternative oxidationreduction process of pulverized coals is proposed. Compared with the conventional syngas production process using single furnace, the proposed process has the following advantages, using low rank pulverized coal contributes to lower cost of raw materials; operating at atmospheric pressure improves the reliability of the system; replacing pure oxygen with air or oxygen enrichment decreases the costs. Purer syngas can be acquired in this new process. The Aspen stimulation study shows that main components (H2+CO) in the new process account for over 85%, reaching up to 93.20%; the ratio of H2/CO is adjustable between 1.0 and 2.0, which meets the requirement of the production of downstream chemical products such as methanol, dimethyl ether, ammonia and other downstream chemical products.


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The process has been optimized and the optimal parameters were listed below: steam-to-coal ratio Rs=0.8~1.0, oxidation zone temperature Tox-zone=950℃, preheating temperature of steam and air Tsteam=Tair=500℃ .In this condition, high quality syngas with the content of syngas (H2+CO) is over 90%, reaching up to 93.20%, the ratio of H2/CO is adjustable between 1.3 and 1.6 and the syngas yield is adjustable from 1.61 Nm3/kg to 1.68Nm3/kg. Corresponding Author Dehong Xia* E-mail: [email protected] Present Addresses School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Dehong Xia, † Qiang Zhang, ‡ Hongbin Wu, § Weiwei Xuan.⊥ These authors contributed equally. ACKNOWLEDGMENT The article was supported by the Fundamental Research Funds for the Central Universities (No. FRF-SD-12-007B). We would like to thank Dr. Dadyburjor and three reviewers for their helpful comments of this paper. Reference


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New Process of High-Quality Syngas Production through Sequential

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