Use of Petcoke in Atmospheric Fluidised Bed Combustion (AFBC) Boilers...

What is Petcoke ?

Cocking is a thermal cracking process characterized by long residence time and the production of petroleum coke (Petcoke), a heavy carbon residue. Petcoke is produced by thermal cracking of vacuum residue that is produced from crude oil.
In a petroleum refinery, atmospheric and vacuum distillation processes remove from the crude the oil the components that boil up to about 500oC. The distillation process separates the lighter part of the crude oil into fractions which are then processed into useful refined products such as LPG, petrol, kerosene, diesel, etc. The part of the crude oil which does not boil off during the distillation process even under vacuum is known as the vacuum reside. The vacuum residue consists of heavy, viscous, complex hydrocarbon molecules.

The coking process converts the vacuum residue into gas, liquid and solid products. In delayed coking, the heavy feedstock is heated in a tube still and pumped as rapidly as possible to insulated drums where it the large molecules are cracked into smaller molecules and residue, producing gas, liquid products and coke. The term 'delayed' comes from the delay between heating and coking. As coke buiilds up in the drum, the lighter products of cracking are recovered as overhead vapours and are taken to a fractionator for separation into various products.

Petcoke yield is mainly dependent on the carbon content of the feedstock, while the structural quality of the Petcoke is influenced chiefly by the type of feed, i.e., virgin crude or cracked products. Fuel grade Petcoke is produced from vacuum residues that have high sulfur contents. High sulphur Petcoke is used as fuel whereas low sulphur Petocoke is used in producing anodes.

Fuel grade Petcoke generally replaces iignite and coal.

How does Petcoke compare with Lignite ?

Lignite is a natural product and a fossil fuel Lignite is found at and around Panandro, Rajpardi and Bhavnagar in Gujarat Lignite is the first stage of conversion of wood and biomass product into coal. It has very high moisture content about 30-40%. The volitile conteint is also very high, about 30-40%. Fixed carbon is less unlike Anthracite and Petcoke. Sulphur content is 3.0-5.0%. Calorific vlaue of lignite is very low, about 2800-3800 kcal / kg where as calorific value of Petcoke is 8200-8500 kcal / kg. So, on heat equivalent basis, 2.5 to 3 kg of lignite is replaced by 1 kg of Petcoke. It means that whereas for the same heat equivalent just kg of Petcoke with about 75 gm of Sulphur is injected into the furnace. Hence, the generation of SO2 with lignite is 1.4 times higher as compared to Petcoke. When Petcoke is used as a fuel, limestone is normally injected into the furnace to absorb SO2 from the flue gas as soonas it is genrated. Some users provide wet lime scrubbers to absorb SO2 from the flue gas. Limestone injection is rarely carried out during lignite firing. Thsu the emission of SO2 is much lesse with Petcoke as compared to lignite.

How does Petcoke compare with Coal ?

Coal is also a natural fossil fuel. Domestic coal is mainly in Central and Eastern India and in the North East. The ash content in domestic coal is very high- it can go up to even 45%. Coal is also imported into India mainly from Indonesia and South Africa. The ash content in imported coal is generally about 2 -15%.
One of the main problems with the use of solid fuels is the emission of suspended particulate matter (SPM) into the atmosphere along with the flue gasses. Against an ash content of about 15% in imported coal and about 35-45% in domestic coal, the ash content in Petcoke is less than 1% - typically about 0.6-0.7%. Thus, the main problem in furnaces and boilers i.e. emission of suspended particulate matter, is non-existent with Petcoke, as compared to coal.

Specifications of Petroleum Coke (Petcoke)

RIL is manufacturing three grades of Petcoke, Grade A, Grade B and Grade C statistics