Lighter tars are largely useful in chemical industries but their quantity is quite little. Catalytic cracking is applied to improve the yield of light tars during pyrolysis. Consequently, in situ upgrading technique through a MoS2 catalyst has been explored in this research work. MoS2 catalyst is useful for the conversion of high energy cost into low energy cost. The variations in coal pyrolysis tar without and with catalyst were determined. Meanwhile, the obtained tar was analyzed using simulated distillation gas chromatograph and Elemental Analyzer. Consequently, the catalyst reduced the pitch contents and increased the fraction of light tar from 50 to 60 wt.% in coal pyrolysis tar. MoS2 catalyst increased the liquid yield from 18 to 33 (wt.%, db) and decreased gas yield from 27 to 12 (wt.%, db) compared to coal without catalyst. Moreover, it increased H content and hydrogen-to-carbon ratio by 7.9 and 3.3%, respectively, and reduced the contents of nitrogen, sulphur, and oxygen elements by 8.1%, 15.2%, and 23.9%, respectively, in their produced tars compared to coal without catalyst.
Coal is heterogeneously complex structure having organic and inorganic macromolecules. Low-rank coals have great importance in utilization but have some problems due to low efficiency and higher CO2 emissions compared to high-rank coals. These coals are difficult for storage and transportation and are not suitable for direct combustion [
To improve the tar yield and fraction of light components in coal tar, catalytic coal pyrolysis experiments have been performed through mixing or without mixing of coal with catalysts in previous study [
In foregoing work, mechanical mixing and impregnation of MoS2 catalyst with lignite coals were applied on the basis of research aims during catalytic hydropyrolysis [
Low-rank Shengli coal was grounded to the required size (20–40 mesh) for testing of experiments. Table
Results of Shengli raw coal.
Proximate (air-dried base, wt.%) | Ultimate (dry-free base, wt.%) | |||||||
---|---|---|---|---|---|---|---|---|
M | A | Vol.Mat. | Fix.C. | C | H | N | S |
|
15.0 | 4.2 | 32.04 | 48.76 | 74.55 | 4.38 | 1.03 | 0.35 | 19.69 |
M: moisture; A: ash; Vol.Mat.: volatiles matter; Fix.C: fixed carbon.
The testing section consisted of gas supply, a dual-stage fixed bed reactor, electric ring furnace, and tar and gas collection system as shown in Figure
A schematic figure of the experimental apparatus. (
High purity N2 carrier gas (Beiwen, Beijing, China, 99.99%) was controlled with a mass flow meter. It gave pyrolysis initial reaction in the dual-bed reactor. A 5 g coal was kept in the upper part of the pyrolysis and 2.5 g catalyst was kept in lower part of the catalytic upgrading of coal. The pyrolysis products from the upper section were transformed and go through the catalyst bed in lower part. Finally, obtained product was passed through condenser; it was cleaned with acetone thoroughly to get coal pyrolysis tar. Noncondensable pyrolysis gases were dewatered through calcium chloride block and received in gas bags. All pipelines were cleaned using acetone and then the liquid was obtained. The obtained liquid (acetone plus water) including tar was first dehydrated with MgSO4. It was kept on filter paper for filtration and the acetone was removed at 30°C in a rotary machine to get coal pyrolysis tar.
The experiments represented that reaction can finish within about 40 min during pyrolysis. The testing methods for coal with and without catalyst were as follows. A 5 g coal was pyrolyzed without catalyst in the upper part through electric furnace. It was performed for heating the coal through a temperature controller with 100°C per minute to 600°C temperature. Coal (5 g) was pyrolyzed and catalyst (2.5 g) was also put in lower part during upgrading of coal pyrolysis tar. Both parts were heated with a rate of 100°C per minute at 50 mL per min of gas flow from ambient to 600°C temperature. Both pyrolysis and cracking temperatures were kept within 30 min at 600°C. All performed tests were repeated three times and average was calculated as the final reading.
Noncondensable and hydrocarbon gases were measured through a gas chromatograph as described in our last work [
The catalyst characterization was analyzed through X-ray pattern machine and micropore analyzer as described in our last article [
Coal tar yield (
Tar yield is given as follows:
Light tar fraction is given as follows:
Light tar yield is given as follows:
Pyrolysis gas yield is given as follows:
Gas yield is given as follows:
Char yield is given as follows:
Liquid yield is given as follows:
Coal having tar yield from 300 to 700°C is shown in Figure
Tar yield based on temperatures without catalyst.
Pyrolysis product yields based on temperatures without catalyst.
The catalyst having influence on yield of pyrolysis products during coal pyrolysis is described in Figure
Pyrolysis product yields without and with catalyst.
Pyrolysis gas components and hydrocarbon gases (
Gas components without and with catalyst.
Tar fraction as a function of simulated distillation temperature without and with catalyst is shown in Figure
Tar fraction as a function of simulated distillation temperature without and with catalyst.
Tar composition of coal without and with catalyst.
The tar yield, light tar fraction, and yield are represented in Figure
Analysis of produced coal pyrolysis tars before and after reaction.
Catalyst | Elemental analysis of coal pyrolysis tar (wt.%) | |||||
---|---|---|---|---|---|---|
C | H | N | S |
|
H/C | |
Without | 68.92 | 10.56 | 5.58 | 1.12 | 13.82 | 1.84 |
MoS2 | 72.0 | 11.40 | 5.13 | 0.95 | 10.52 | 1.90 |
Yields and fraction of coal pyrolysis tar without and with catalyst.
The XRD pattern of MoS2 catalyst is shown in Figure
XRD pattern of MoS2 catalyst.
The porosity of fresh and spent catalysts was calculated from nitrogen adsorption/desorption isotherms. The BET surface areas of the fresh and spent catalysts are 3 and 5 m2g−1, respectively, and there was no significant difference between fresh and spent catalysts. The volume of pore in spent catalyst increased from 0.015 to 0.025 cm3/g, while pore diameter was reduced from 23.65 to 16.66 nm. It is realized that BET surface area and volume of pore increment might be attributed to the breakdown of pore which was suffered from high volatiles during pyrolysis and the cracking of coal pyrolysis tar.
The surface structure of fresh and spent catalysts with high magnification is shown in Figures
SEM diagrams of fresh MoS2 catalysts (a) and spent MoS2 catalyst (b).
TEM diagrams of fresh MoS2 catalysts (a) and spent MoS2 catalyst (b).
It was testified that MoS2 catalyst was able to increase liquid product yield and produced the highest liquid yield. Moreover, it is found that the catalyst decreased the pitch in the tar and showed the increment in the lighter weight tars fraction. Therefore, the catalyst was also active for the increment in hydrogen-to-carbon ratio and reduction in nitrogen, sulphur, and oxygen elements in coal pyrolysis tar. The catalyst in this study was more effective to upgrade the coal pyrolysis tar to decompose the coal catalytically, although MoS2 catalyst can promote the energy efficiency during the operation of industrial equipment.
The authors declare that they have no competing interests.
In this research study, authors acknowledge gratefully the funding support of Beijing Natural Science Foundation (no. 3142020), Strategic Priority Research Program of CAS (no. XDA07010300), CAS Province Cooperation Program (no. 2014JZ0012), and National Major Instrumentation Development (no. 2011YQ12003907).