Pyrolysis Of Small-size Low-rank Coal In Indirectly Heated Moving Bed With Internals

Of China's total coal reserve more than 50% is low-rank coal. Low rank coal has high content of volatile matters that can be extracted as tar and gas. The direct combustion and gasification technologies waste in fact such inherent oil and gas resource containing in the low-rank coal. Pyrolysis can extract high-value coal tar and gas under relative mild conditions, while its solid char can be fuel for combustion or gasification. Thus, coal pyrolysis is the core technology for high-value utilization of low-rank coal. However, there is not commercial technology for pyrolyzing low-rank coal, especially small-size coal. Aims at solving the problems for low-rank fine coal pyrolysis including low tar yield, high content of heavy species and high dust in tar, the study proposes a new pyrolysis technology using the so-called moving bed pyrolyzer enhanced with internals. This thesis work first tested the pyrolysis characteristics of Yilan coal, one kind of subbituminous coal, under various conditions to get the fundamental data supporting reactor design and operating conditions definition. Then, the batchwise pyrolysis at capacities of 1.0 kg and 100 kg per test were conducted, and finally a 1000 t/a pilot plant was designed, constructed and commissioned into successful running. The result showed well the good prospective of newly proposed reactor and pyrolysis process.1. Fundamental experiments. The pyrolysis of Yilan coal was conducted to study the effect of heating rate, particle size, accumulation mode and coal bed thickness on tar yield and quality. The results show that tar is generated in 350-500 ? for Yilan coal. Increasing the heating rate and decreasing the coal particle size increased the yield of tar but increased the content of heavy species. By passing through the thicker coal bed it slightly decreased the tar yield but the content of light species increased.2. Pyrolysis tests at capacity of 1.0 kg. A series of bathwise coal pyrolysis tests were conducted in a 1.0-kg laboratory moving bed pyrolyzer with internals, in comparison with that in a conventional moving bed without internals. The results clarified the effects on pyrolysis of internals consisting of heating transfer plates and a gas collecting pipe. The reaction time was shortened by almost 50% in the pyrolyzer with internals in comparison with that in the conventional moving bed at a furnace heating temperature of 700 "C. The tar yield was 9.82 wt.% and 7.67 wt.% for such two reactors, while the light tar content was also slightly higher for bed with internals. When raising the heating furnace temperature from 600 to 1000 ?, the tar yield increased from 8.50 to 10.64 wt.% for the reactor with internals, whereas the tar yield decreased remarkably from 7.98 to 4.77 wt.% in the conventional bed without any internals. The use of internals greatly changed the variation tendency of tar yield with heating temperature. It was shown also that the internals of heating transfer plates mainly worked to enhance the heat transfer from the reactor wall (heated) to coal, thus increasing the heating rate of coal. The internals of gas collecting pipe also facilitated the heat transfer to some extent but mainly worked to direct the flow of gaseous pyrolysis products from high-temperature wall zone to low-temperature core zone to realize the selective secondary cracking and much improve the tar yield and quality. The heavy species in the product can be captured in the low-temperature coal bed and in turn cracked there when the temperature becomes higher.3. Pyrolysis tests at capacity of 100 kg. A series of experiments were further conducted in the enlarged reactors with a coal load of about 100 kg for each test. At this capacity the effects of the coal particle size, heating furnace temperature and coal moisture on pyrolysis were also studied in the pyrolyzer with internals. Furthermore, a large cold model moving bed with internals was constructed to show the possibility of continuous granular flow. In the enlarged reactor, the use of internals increased the heating rate of coal bed, and the tar yield was 6.11 wt.%and reached 87.0%of the Gray-King (G-K) tar yield. The tar yield is obviously higher than 3.29 wt.% realized in the reactor without internals. The content of light tar (boiling point below 360 ?) was 71 wt.% for the reactor with internals, higher than 67 wt.% for the conventional reactor. All of these demonstrate that the internals improved the tar yield and quality in the enlarged reactor. For the lump coal in 10-50 mm the realized heating rate is low due to low heat transfer in large-particle bed. The tar yield and light tar content are both low, possibly because of the secondary pyrolysis on the high-temperature surface of the lump coal. For the reactor with internals, raising the furnace temperature from 900 to 1100 ? increased the tar yield from 5.65 wt.%to 6.34 wt.% and decreased the light tar content from 74 wt.% to 69 wt.%. The coal with higher moisture content caused the lower heating rate and longer reaction time to lower thus the tar yield, but the light tar content was higher. In the cold model moving bed in size of 0.6 m × 0.35 m × 6 m (length× width × height) and 0.2 m × 0.175 m × 6 m (length × width × height), both coal and char in moisture contents below 10 wt.% continuously discharged without any blockage by the internals, showing the possibility of scale-up of the moving bed reactor with internals for coal pyrolysis.4. Pyrolysis tests via 1000 t/a pilot plant. A 1000 t/a pilot plant using the moving bed with internals as the coal pyrolyzer was designed, constructed and commissioned into successful running for a series of tests. Two kinds of fine coal (Yilan coal and Shenmu coal) and two kinds of fine oil shale (Longkou oil shale and Beipiao oil shale) were tested in the pilot plant to show the continuous operation possibility and pyrolysis performance of the technology based on the new reactor. Fine coal and oil shale in size of 0-10 mm were processed in the pilot plant, showing stable continuous production of tar and gas in high quality. The coal bed temperature was controlled to achieve the optimal conditions by setting the heating furnace temperature and adjusting the coal feeding and char discharging rates. The tar yield was higher 80% of the Gray-King tar yield at the furnace temperature of 1000 ?, dust content in tar was about 0.1 wt.%, and light tar content was above 70 wt.%. All of these proved the technology feasibility and also superiority based on the moving bed reactor with internals for coal pyrolysis producing tar and gas in high yield and high quality.Consequently, the thesis work demonstrated that the pyrolysis technology using moving bed reactor with internals is highly suitable for processing non-caking fine coal and oil shale in sizes of 0-10 mm. The technology has the advantages of stable running, easy controlling, low dust and high light oil contents in tar or oil product, and simultaneous production of high-quality pyrolysis gas. These show thus the good prospective of the new technology for industrial application.