In the oil refining industry, low-grade heavy oil generally refers to a heavy oil fraction with high boiling point, high carbon residue, high metal content and high asphaltene content, the types of low-grade heavy oil generally include vacuum residue, heavy oil residue from solvent separation process, de-oiled asphalt and oil sand bitumen, etc.  Low-grade heavy oil has lower commercial value, because on one hand it cannot be used directly as fuel oil under the limitation of environmental regulations due to its own nature, and on the other hand it can easily result in permanent deactivation of catalyst and coking fault of the device due to high asphaltene content, heavy metal content and carbon residue value and, thus, cannot be used as feedstock for conventional catalytic cracking, hydrotreating process and delayed coking process.  For example, fixed bed hydrotreating process generally cannot handle the heavy oil having a total content of heavy metals Ni and V of greater than 150 μg·g−1 and a carbon residue value of greater than 15 wt %, while most low-grade heavy oil have much higher heavy metal content and carbon residue value (which can be considered to be a lower-grade ultra heavy oil having index much higher than that of a low-grade heavy oil and have much deteriorated nature); in delayed coking process, the use of the low-grade heavy oil will cause the radiant tubes of heating furnace easier to coke, which may cause the device not to operate normally. 
Therefore, in oil refining industry, there is a need to provide a method for processing low-grade heavy oil, especially for processing a lower-grade ultra heavy oil, to effectively promote removals of carbon residue, heavy metal and asphaltene from the low-grade heavy oil as well as maintaining high liquid product yield, lower gas yield and coke yield, thereby providing more valuable and relatively cleaner feedstock with high hydrogen content for downstream processes. 
For the processing of low-grade heavy oil, there have been disclosed some improved technical solutions.  For example, U.S. Pat.  No.  3,144,400A discloses a fluidized coking process for continuous production of low-grade heavy oil.  In this process, a preheated low-grade heavy oil is injected into a reactor via a nozzle, a fluidized bed formed by hot coke powder particles is provided in the reactor, the low-grade heavy oil forms a thin layer on the surface of the coke powder particles after being injected into the reactor, which is heated for coking reaction.  In the reactor, the temperature is controlled in a range of 480-560° C. with the pressure slightly higher than the atmospheric pressure.  The coke powder particles are fluidized by means o oil-gas and water vapor entered from the bottom of the reactor.  The oil-gas generated is outputted from the top of the reactor into the scrubber and the fractionation column after separating coke powder particles via a cyclone separator.  In the scrubber, low-grade heavy oil is used to elute the coke powder particles carried by the oil-gas.  The slurry-like liquid is returned to the fluidized reactor as circulating oil, and part of coke, after stripping out oil-gas carried therein by water vapor, enters into a coke-burning device for regenerating.  The coke-burning device is essentially a fluidized bed combustion reactor, from the bottom of which, air is introduced to partially burn the coke particles, thereby maintaining the fluidized bed at a temperature of 590-650° C. Regenerated high-temperature coke particles are recycled into the reactor, acting as a heat carrier for preheating raw oil and supplying required reaction heat. 
In the above fluidized coking process, since coking reaction of the heavy oil will produce coke and diameters of the coke particles already existing in the reactor will increase with the progress of the reaction, large-size coke particles that are not suitable for fluidizing need to be removed in time to maintain the reaction environment, which naturally increases the difficulty of the process; in addition, as fluidized media, coke particles have low strength and are easy to be crushed, which will affect effects of the fluid coking reaction, and in this case, they are also used as catalyst, which will make poor effects of removing residual carbon, asphaltene and metal impurities, obtaining oil with poor quality.  For example, middle distillate oil obtained by fluidized coking process of low-grade heavy oils has high basic nitrogen compound content, unfavourable to further catalytic process and use; the obtained coking gasoline has serious problems of low octane value and high sulfur and nitrogen contents, bringing a lot of obstacles for subsequent modification and refinement. 
Referencing to fluidized coking process technology, an upgrading technique of using cheaper, inert, solid catalytic microsphere particles instead of coke particles for removing carbon, heavy metal and asphaltene from heavy oil has been proposed.  For example, U.S. Pat.  No.  4,243,514 discloses a heavy oil upgrading process, which is called ART process, in which the heavy oil is in short-time contact with a fluidized high-temperature inert catalyst in a riser after being preheated, gasifying light components in the heavy oil, and macromolecular compounds containing heteroatoms of metal, sulfur and nitrogen, such as asphaltene, are deposited on the contacted particles, producing vaporizable small molecules and coke via cracking and condensation reactions.  The oil-gas is rapidly cooled at the outlet, and the solid contacting catalyst for depositing coke is transferred to a regenerator for regenerating.  The process and apparatus for this process are similar to those of FCC process, except for the raw material to be processed and the contacting catalyst.  The actual application results show that the process has certain effect for upgrading heavy oil having relatively low residual carbon and heavy metal content, but fails to achieve the desired effect for the low-grade heavy oil. 
CN 200310110205.7 discloses a combined process for processing heavy oil, in which the hydrogenation and decarbonization technologies are integrated by a combination of ROP, RHT and RFCC processes to treat low-grade residual oil.  In this process, fluidization-to-decarbonization process for treating residual oil uses an inert porous microspherical heat carrier as a catalyst, to contact and react with the residual oil in a riser reactor, the components containing relatively more hydrogen being rapidly gasified after contacting with the heat carrier, while high-boiling components containing carbon residue are not easy to be gasified, so they are cracked, the coke thus obtained by condensation is deposited on the contacting catalyst, as well as the metal impurities and some sulfur and nitrogen elements in the residual oil, separate the contacting catalyst from the reacted oil-gas and strip the catalyst, the stripped contacting catalyst is transferred to a regenerator to regenerate for recycling.  The inert porous microspherical heat carrier used in the process can remove carbon residue, asphaltene and metal impurities and can process low-grade heavy oil having high residual carbon content and high density (such as Iran vacuum residual oil), but the product obtained has poor distribution and low yields of gasoline, diesel oil and total liquid products.  Thus, follow-up process such as residue hydrotreating (RHT) and residue fluidized catalytic cracking (RFCC) is needed in order to meet the processing requirements for low-grade residual oil
In summary, there is a need to provide a method which can process low-grade heavy oils in a convenient and effective manner, obtain liquid products with higher yields, and provide more and lighter hydrocarbon materials for downstream processes.