What is Petcoke?

Coking 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 500o C. 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 builds 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 Petcoke is used in producing anodes.

Fuel grade Petcoke generally replaces lignite 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 volatile content is also very high, about 30 – 40%. Fixed carbon is less unlike Anthracite and Petcoke. Sulphur content is 3.0 –5.0%. Calorific value 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 with average 4% Sulphur, 3 kg of lignite will inject 120 gm of Sulphur to the furnace whereas for the same heat equivalent just 1 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 soon as it is generated. Some users provide wet lime scrubbers to absorb SO2 from the flue gas. Limestone injection is rarely carried out during lignite firing. Thus the emission of SO2 is much lesser 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 12 - 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 gases. 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-existant 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 whose specifications are given below:

Characteristics of Lignite (Panandro, Kutch, Gujarat)

Typical Characteristics of Coal

Emission Control Mechanism

SO2 Emission Control

Fluidised bed boilers are able to remove sulphur dioxide directly in the combustor. This is accomplished by using limestone in the fluidised bed. Limestone consists mainly of CaCO3. When heated, limestone calcines to form calcium oxide (CaO) which reacts with SO2 to form calcium sulphate as follows:

CaCO3 ® CaO + CO2

CaO + SO2 + ½ O2 ® CaSO4

Bed temp of 850-870 °C and Ca / S ratio of 2.5 are found to be the optimum conditions for sulphur capture in AFBC boilers.

Some of the major points to be considered for use in AFBC boilers are as under:

• Volatile matter content: Petcoke contains 9%-11% of VM and lignite / domestic coal contains 20%-25%. For AFBC Boilers volatile content is not a matter of concern. The fluidised bed gives sufficient residence time for combustion of the fixed carbon. If cinder recovery and reinjection system is provided, then the residence time can be increased further and this would ensure complete combustion of the Petcoke. With a Petcoke blend of 20%-50% the average volatile matter content of the blended fuel would be 15%-20%, which would be high enough to initiate combustion of Petcoke. Since the furnace is at 850oC – 900oC, the 9% - 11% VM in Petcoke is sufficient to initiate combustion.

• Fuel preparation: It may be noticed from the Petcoke particle size distribution that typically 63% of the material under 6.3 mm in size and 11% is between 6.3 – 12.5 mm, thus around 30% of the material is to be crushed to get the product for AFBC use.

• Sulphur: As explained above, sulphur in the fuel can be captured by addition of limestone in the furnace. Limestone (CaCO3) gets heated in the furnace and forms CaO. CaO reacts with SO2 and forms CaSO4. It is possible to capture 80% - 90% of SOx in the furnace depending on the reactivity of limestone. Thus SOx emissions can be restricted to the desired level. Also, the flue gas temperature at the air pre heater outlet remains unchanged at 130oC–140oC with Petcoke. It is mandatory to use limestone when Petcoke is used as a fuel (100% Petcoke or as a blend with Coal / lignite).

Limestone purity – Limestone having 75 to 85% of CaCO3 content

Particle size – 1 to 2 mm

• Furnace temperature: The bed temperature would have to be monitored and maintained at 850 - 900°C for Petcoke. It is likely that when Petcoke is used, the bed temperature may increase since the quantity of ash in Petcoke is much lower than that in coal. As Petcoke has practically no ash, the possibility of slagging is lower with Petcoke as compared to coal. Reduced slagging results in higher radiant heat transfer.

• Bed temperature: Since Petcoke has less moisture as compared to coal, the bed temperature is expected to be high on account of this as the loss due to moisture is reduced. Also, as the ash content is less, the bed temperature will increase since the heat carried away by ash is also reduced. The bed temperature may be maintained at the desired level by dosing bed material over and above the limestone required for Sulphur control.

• Stockpiling: As the volatile matter content in Petcoke is low chances of spontaneous combustion are remote. Storage of Petcoke is safer than storage of coal or lignite.

• Scheme for Blending: Blending of Petcoke and coal / lignite should be homogeneous to get the predicted results.

Conclusion:

Petcoke can be used as a replacement of coal / lignite. It can be used in blend form along with coal / lignite or 100% in AFBC / CFBC boilers.

AFBC Boilers are capable of absorbing SOx and restrict SOx emissions to the desired levels by limestone injection system.

Ash content in Petcoke is much lower than coal / lignite. Hence particulate emissions are always lower as compared to coal.