Low Rank Coal Pyrolysis
Low Rank Coal Pyrolysis, Low Rank Coal Upgrading
Low rank coal upgrading technology refers to a class of technologies developed to remove moisture, volatiles and certain pollutants from low rank coals such as sub-Bituminous coal and lignite (brown coal) and raise their calorific values. Coal upgrading through thermal treatment or low rank coal pyrolysis technology produce chars as primary product, and gases, tar as byproducts.
Low Rank Coal Pyrolysis, an upgrading technology
A number of approaches have been investigated in an effort to reduce the moisture content and certain pollutants of low-rank coals. These include simple drying followed by coating the solid with oil, drying in oil, drying with steam, low-temperature pyrolysis, and pelletizing followed by drying. Low-temperature pyrolysis is being paid more attention and studied in for Low Rank Coal Pyrolysis projects, it is the most developed below.
1) Direct contact heating to upgrading the coal
Three kinds of heat carriers have been used to pyrolyze coal. They are hot gas, steam and solid.
Coal is heated by contacting heat carrier directly to produce char, tar and gas. Fluidized bed, tubular bed, rotary bed and moving bed have been used in this process.
2) Indirect contact heating to upgrading the coal
Coal and heat carrier are into two parts of the reactor separately. The heat from heat carrier is transferred to coal by reactor/heat exchanger wall which is heated by heat carrier. Tubular/channeling bed is widely used in this technology.
Tar as byproduct of low rank coal upgrading is the third option to produce liquid fuel from coal, together with DCL (Direct coal liquefaction) and ICL (Indirect coal liquefaction) processes described at Coal to Liquids. Compared to DCL and ICL, coal pyrolysis process only needs 1/3 to 1/5 of the overall capital investments to process the same quantity of coal. Tar contains sulfur, nitrogen, oxygen and solids contaminants. It is deficient in hydrogen and therefore must be further refined. Hydro-processing of tar is now widely pursued not only because it is commercially attractive and environmentally benign.
However, tar is a challenging feedstock for hydro-processing. As compared to petroleum-based residue oil, tar contains more heteroatoms (O, N), asphaltene, resin and ash. It is unstable due to high concentration of active species. Asphaltene, resin, ash content and active species are prone to re-condense or solidify in pipes and valves, significantly increasing process maintenance costs and reducing the operation turnaround cycle.
In terms of economics, the relatively low yield of high-valued products affects revenues compared to DCL or ICL.
Whole Fraction Tar Upgrading Pilot Plant at National Institute of Clean and Low Carbon Energy (NICE), China
600 kt Coal Upgrading Plant in Yulin, Shaanxi, China
This article has benefited the major contribution of National Institute of Clean and Low Carbon Energy (NICE), China
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