Process for the thermal conversion of biomass to liquids

Details Process for the thermal conversion of biomass to liquids Process for the thermal conversion of biomass to liquids

1995-07-31

1997-02-25

Medley, Margaret

Fuel and related compositions

Liquid fuels

Plant or animal extract mixtures or extracts of...

44606, 44628, 48197R, 201 25, 201 28, 201 29, 201 36, 201 37, C10L 102, C10G 102

Patent

active

056055510

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION

This invention discloses an improvement on present methods of thermal conversion of biomass to liquids which eliminates safety problems present in the current practice due to high concentrations of carbon monoxide and the handling of large volumes of explosive gases at high temperatures. At the same time, potential savings in energy efficiency are made possible.


BACKGROUND TO THE INVENTION

Current practice in obtaining high liquid yields from the thermal decomposition of biomass at short reaction times (sometimes called "fast", "flash" or "rapid" pyrolysis) makes use of reactor types capable of high heat transfer rates to small biomass particles, in order to achieve the rapid heat-up rates necessary. Three of the most commonly used types are the fluidized bed, the circulating fluidized bed or the transport reactor. In the first two of these, hot gases and solids, normally inert, are brought into intimate contact with the biomass particles. In transport reactors, either hot gas alone or a mixture of hot gas and solids may be used. All of these reactors have in common a requirement for a significant gas flow, usually from 1 to 10 times the weight of biomass being processed. If pyrolysis is carried out in the absence of oxygen, then the non-condensable gases formed will have significant contents of carbon monoxide, hydrogen, methane, and other light hydrocarbons or organics, and are of medium calorific value and can be readily burned in air.
In order to preserve the high calorific value of these non-condensable gases, and to prevent the loss of organic liquid yield due to uncontrolled oxidation reactions if air is present, it is current practice to use these gases as a recycle stream to supply the necessary fluidizing or conveying gas for reactor operation. It is also current practice to heat this recycle stream indirectly in order to supply part or all of the heat necessary for the pyrolysis reaction.
The operation of a fluidized bed process has been described by Scott and Piskorz (1) (2). When poplar wood was used as feed in a fluidized bed of sand at a temperature of 500.degree. C. and a gas apparent residence time of 0.48 seconds at a gas to-feed weight ratio of 3:1, the recycle gas composition, on a moisture-free basis, was:


______________________________________ Hydrogen 1.49% by volume Carbon monoxide 47.83% Carbon dioxide 39.40% Methane 6.97% Ethylene, ethane, etc. 4.31% ______________________________________ expressed as weight % of the moisture-free feed. The balance of 10.8% was water formed in the pyrolysis reactions.
It is apparent that a gas with such a high concentration of carbon monoxide would be extremely toxic, and even small leakages or emissions would pose a severe hazard to life. In addition, the gas can readily form explosive mixtures with air due not only to the carbon monoxide content but also due to the content of other inflammable hydrocarbons and hydrogen. However, in the prior art as described in these publications and also in our earlier Canadian Patent No. 1,241,541 (September, 1988), it is specified that the gas used must be oxygen-free.
Pyrolysis with a transport reactor in which both gas and hot solids were mixed and transported with the biomass is described by Graham et al (3). Poplar wood was pyrolysed at 650.degree. C. and 0.524 seconds apparent residence time. Although gas recycle was not used in the reported experiments (nitrogen was used), if gas recycle had been practised as would be expected in a larger scale unit, then, on an inerts-free basis, this reactor would give a gas for the above conditions having the following analysis:


______________________________________ Hydrogen 4.18% by volume Carbon monoxide 63.10% Carbon dioxide 11.80% Methane 12.55% Ethylene, ethane etc. 8.37% ______________________________________ both highly toxic and readily forming explosive mixtures with air. In this work, the carrier gas used was inert. In subsequent disclosures by Underwood and Graham (U.S. Pat. No. 4,876,108, Oct

REFERENCES:
patent: 1395867 (1921-11-01), Pearce
patent: 3852048 (1974-12-01), Pyle
patent: 4329202 (1982-05-01), White et al.
patent: 4344770 (1982-08-01), Capener et al.
patent: 4530700 (1985-07-01), Sawyer et al.
patent: 4564368 (1986-01-01), Sawyer et al.
patent: 4876108 (1989-10-01), Underwood et al.
patent: 4994297 (1991-02-01), Underwood et al.

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