Working Mechanism Of Internals For Oil Shale Pyrolysis In Fixed Bed Reactor Externally Heated

Oil shale is an unconventional fossil energy resource with large reserves,which has ash content of 50-85 wt.%,high H/C atomic ratio of about 1.2.Pyrolysis or retorting is the major technical way to utilize oil shale.Its main product is shale oil which has similar composition with crude petroleum oil.With gradually higher demand of oil fuels in many countries,oil shale is becoming an important supplementary unconventional oil resource.In recent 5 years,the oil production from oil shale retorting was about 750 kt/a in China.However,Fushun retorting furnace is the major oil shale retorting technology used,and it is limited to use only large particles,for example,over 12 mm.In order to use fine oil shale,it is in pressing need to develop a new technology(mainly reactor)to pyrolyze fine oil shale particles.On the other hand,the reactor indirectly heated has the superiority that it can avoid violent movement of particles to essentially lower the intake of dust in produced shale oil.Whereas,low oil yield and heat efficiency of the indirectly heated method enormously hinder its industrial development and application.This study intends to investigate the pyrolysis behavior,product flow and temperature distribution inside the indirectly heated fixed bed reactor,and thereby to clarify the mechanism and key points for possible technical improvement.Particularly designed internals are employed to optimally match the secondary pyrolysis or volatile reactions with the fields of product flow and temperature inside the particle bed to achieve high yield and high quality of shale oil.The main contributions of this dissertation are summarized as follows:(1)Fundamental research for oil shale pyrolysis.Thermogravimetric analysis of different oil shales was conducted to identify their oil-production temperature ranges,which are 380-550 ? for Huadian oil shale,370-500 ? for Longkou oil shale and 350-520 ? for Yilan oil shale.Oil shale and shale char samples at different heating temperatures were prepared for BET analysis.A minimum of surface area and pore volume of oil shale or shale char was found to occur at 350 ? before substantial pyrolysis,implying the diffusion of intermediates named asphaltene inside pores to vary the surface area and pore volume of oil shale or shale char.Pyrolyzing oil shale of different particle sizes(0-5 mm)in Fischer Assay reactor found little difference in product distribution.(2)Regulation of pyrolysis product flow in indirectly heated fixed bed reactor.Measuring the pressure drop of oil shale and in-situ shale char bed demonstrated that shale char was more porous to generate lower resistance to gas flow.It indicates that in an indirectly heated fixed bed reactor the pyrolysis product would select an exhausting path in high-temperature char bed near the reactor wall and have to encounter serious secondary reactions.A central gas collection pipe was employed to matching the volatile product flow direction and temperature gradient inside the particle bed.Oil shale pyrolysis in reactors without and with a central gas collection pipe was conducted to verify the effect.The regulation via internals obviously decreased secondary reactions in particle bed near the reactor wall and improved the oil yield and also quality.At the heating temperature of 1000 ? the shale oil yield was doubled in the reactor with a central gas collection pipe in comparison with that in the reactor without the gas collection pipe.The working performance of a central gas collection pipe also depended on temperature distribution and thickness of the particle bed.(3)Heating enhancement in indirectly heated fixed bed pyrolysis reactor.Metallic heating plates were vertically welded on the reactor wall as heating enhancement internals to provide extra heating surfaces inside the oil shale bed.Adding 210%extra heating surface area,against the surface area of heated reactor wall,doubled the heat efficiency and shortened 50%pyrolysis time.Its oil yield reached 91%of the Fischer Assay oil yield and the content of gasoline and diesel reached 66 wt.%when heating surface area was increased by 70%.With an increase of heating surface area above 110%,the shale oil yield had conversely a slight decrease.Characterizing the char samples at different lateral positions of the oil shale bed justified that the flow of gaseous pyrolysis product in the reactor was guided to the vicinity of heat transfer plates to speed up gas exhaust and facilitate secondary cracking.(4)Distinctive pyrolysis behavior in indirectly heated fixed bed with internals.Pyrolyzing oil shale respectively in four reactors respectively without internals,with a central gas collection pipe,with heat transfer plates and with both internals(named reactor with internals herein)demonstrated that adopting the particularly designed internals achieved perfect pyrolysis performance,including heating efficiency and pyrolysis product.Raising heating temperature(to reactor)from 600 ? to 1000 ? increased the shale oil yield from 9.5 wt.%to 10.60 wt.%in the bed with internals.Furthermore,the shale oil had its relatively stable aliphatic hydrocarbon content.In the bed without any internals such a temperature rise sharply decreased the shale oil yield from 9.53 wt.%to 4.60 wt.%and seriously cracked aliphatic components.Comparing particle characteristics at different radial positions of the reactors without and with internals demonstrated that the product flow inside the particle bed with internals is regulated to be from high-temperature zone(outboard)to low-temperature zone(center),which reduces secondary reactions toward the released volatiles.Terminating oil shale pyrolysis at central particle bed temperatures of 150,300,450 and 550 ? found that the content of VGO(vacuum gas oil)and heavy oil in produced shale oil increased from 9.63 wt.%to 53.29 wt.%.The liquids adsorbed or condensed on the surface of particles gradually increased f-rom the outboard to central region of the reactor.Sustaining heating and pyrolysis increased contents of alkane and heteroatom-compounds in shale oil,while it decreased the contents of alkene,aromatics and cycloalkane.All these show in fact that adopting internals in indirectly heated fixed bed oil shale pyrolysis optimized product flow direction to match temperature gradient and volatile reactions secondary reactions to avoid serious secondary cracking.