| The ever increasing demand of natural gas in China as well as the limited domestic supply provides a strong incentive to produce synthetic natural gas (SNG) from coal, especially low-rank coal. A high initial yield of CH4 produced from the gasifier is highly expected for the construction of high-effective and low-cost coal to SNG project since the reaction of H2 with CO to form CH4 (methanation) involves approximately 20% heat production. Now, Lurgi moving bed gasifier has been widely used for SNG production by result of its producer gas with a high content of CH4. This gasifier also produces a considerable amount of coal tar which is highly demanded by chemical industry due to the limited availability of crude oil in our country. But Lurgi gasifier can only adopt lump coal above 6 mm, thus there is a great need to treat more abundant powder coal, such as below 10 mm, via Lurgi-type gasifier. Accordingly, an integrated fluidized bed (IFB) coupling an upper transport bed pyrolysis with a bottom fluidized bed gasification was proposed to process low-rank powder coal and co-produce coal tar and CH4-rich syngas. In this study, bottom fluidized bed gasification and upper transport bed pyrolysis were separatedly carried out to obtain the methods of increasing CH4 content and the parameters of improving tar production. Finally, the CH4-rich syngas and coal tar were successfully prepared by coupling coal pyrolysis and gasification in an integrated fluidized bed.The main results from this study are summarized as follows:1. Coal gasification in a fluidized bed. (1) The gasification characteristic of a subbituminous coal and two lignites with sizes of 0.5-1.0 mm was studied in a fluidized bed. Increasing excess oxygen ratio (ER) and steam to carbon mole ratio (S/C), and raising gasification temperature and residence time were beneficial to the conversion of carbon to gas. At atmosphere pressure, CH4 was mainly derived from coal pyrolysis and raising carbon conversion thus led to the reduction of CH4 content. Increasing ER, gasification temperature and residence time, and adding Ca(OH)2 catalyst resulted in the decline of H2/CO mole ratio while increasing S/C was help to increase H2/CO mole ratio. The presence of calcium in lignite promoted the conversion of carbon to gas and adding Ca(OH)2 into subbituminous coal markedly improved the gasification reactivity. (2) The catalytic gasification characteristic of a lignite with sizes of 1-1.6 mm after impregnating 10 wt.% Ca(OH)2 was investigated in the same fluidized bed. At atmosphere pressure, adding catalyst increased carbon conversion and gas yield while CH4 yield kept no change. Increasing ER, S/C and gasification temperature could significantly raise carbon conversion but went against the production of CH4. Under conditions of appropriate temperature and S/C, H2/CO ratio in syngas was about 3. Elevating gasification pressure could enhance carbon conversion and CH4 production. At 1.5 MPa, under conditions of ER = 0.05, S/C= 1 and gasification temperature of 800℃, catalytic gasification reached good results with carbon conversion of about 70% and CH4 yield of 0.142 Nm3/kg-coal. And its producer gas, with the best H2/CO ratio of about 3, was highly rich in CH4 with its content of 10.1 vol.% that is well-matched in terms of SNG production.2. Coal rapid pyrolysis in a transport bed. An integrated fluidized bed consisting of an upper transport bed section and a bottom fluidized bed section was adopted to investigate the transport bed pyrolysis using a kind of subbituminous coal with sizes of 0.3-0.4 mm by varying its reaction temperature and reaction atmosphere adjusted to simulate steam-containing syngas produced by the bottom fluidized bed char gasification. Steam and syngas, in comparison with N2, as the reaction atmosphere little affected the tar yield below 600℃ but significantly decreased it for the former and increased it for the latter at rather higher temperatures. The presence of H2 in the syngas increased the tar yield significantly because it could suppress the polymerization and condensation reactions through providing H as radical stabilizer and hydrogenation agent. In the steam-containing syngas atmosphere, the tar yield obtained from transport bed rapid pyrolysis increased rapidly with raising temperature to a peak value of 10.5 wt.%(daf) at 600℃, about 1.1 wt.% higher than the Gray-King assay yield, and then decreased due to the excessive secondary reactions. Analyzing tar composition further showed that steam-containing syngas combined their respective advantages that syngas improved the yields of both light and heavy tars while steam reduced the heavy tar yield, especially at temperatures above 600 ℃. The steam-containing syngas atmosphere promoted CH4 production in comparison with the case in N2 atmosphere.3. Coupling coal pyrolysis with gasification in an integrated fluidized bed. (1) This part investigated the effects of major operating parameters on CH4 content in the syngas produced from processing a kind of subbituminous coal with sizes of 0.2-0.5 mm for upper pyrolysis and 1.0-2.0 mm for bottom gasification in an integrated fluidized bed. It was found that the formation of CH4 was facilitated under the conditions of lower overall excessive oxygen ratio (ER), lower temperature of gasification (830-970℃), higher temperature of pyrolysis (500-700℃) and higher holdup of coal in the transport bed. The CH4 content in the produced syngas increased with elevating operating pressure but decreased with increasing the steam to carbon mass ratio (S/C) for gasification. Coupling transport bed pyrolysis with fluidized bed gasification at atmospheric pressure caused the CH4 content in the syngas to be 7.1 vol.% under the conditions of ER= 0.1, S/C= 0.1, gasification temperature of 900 ℃ and pyrolysis temperature of 700 ℃, which was higher than 5.1 vol.% for the case without coupling coal pyrolysis. At the operating pressure of 1.4 MPa, the CH4 content reached about 11.2 vol.%, about six times higher than 2.0 vol.% for the usual fluidized bed gasification and close to 12.0 vol.% for the Lurgi gasifier. (2) A kind of lignite before and after impregnating 10 wt.% Ca(OH)2 with sizes of 0.2-0.3 mm for upper pyrolysis and 1.0-1.6 mm for bottom gasification was investigated respectively in an integrated fluidized bed. Catalytic coal gasification could significantly enhance carbon conversion. For example, at 1.0 MPa catalytic gasification resulted in a higher carbon conversion of 78.9% compared to 57.4% without catalyst in lignite. At atmospheric pressure, CH4 yield with about 0.045 Nm3/kg-coal kept no change after using catalyst. Under elevated pressure of 1 MPa, CH4 yield increased to 0.105 Nm3/kg-coal and 0.167 Nm3/kg-coal for both without and with catalyst, respectively. It is obvious that catalytic gasification gave rise to a higher CH4 yield. After coupling with transport bed pyrolysis, the content of CH4 in the syngas was raised to one degree or another. For example, at 1.0 MPa, CH4 content increased from 9.1 vol.% and 9.3 vol.% to 11.3 vol.% and 10.8 vol.% before and after adding catalyst, respectively. Coupling coal pyrolysis led to a slight decline of H2/CO ratio, while its value was still about 2. After adding transport bed pyrolysis, a considerable amount of tar was produced. Adding catalyst and elevating pressure decreased the tar yield while improved its quality. The integrated fluidized bed catalytic gasification using inexpensive Ca(OH)2 resulted in CH4-riched syngas with a better H2/CO and a high quality tar at a higher carbon conversion of 78.9%, which is highly anticipated in the SNG plant. |
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