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Apr-2017

Firing high sulphur fuel

An evaluation of schemes to improve firing efficiency and ensure the reliability of combustion equipment

ADIL REHMAN, C STEVEN LANCASTER, SANDEEPAN GHOSH, OM PRAKASH SAHU and PAWAN KUMAR SHARMA
KBR Technology
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Article Summary
Pressure to reduce the carbon footprint of processing facilities means a reduction in fuel consumption by the energy consuming plant. At the same time, the ever increasing price of fuel means that refiners are compelled to utilise cheaper fuels like fuel oil, refinery gas, or sub-grade fuels with a high content of sulphur and other impurities. On the credit side, there is the possibility of increased profit margins as a result of increasing energy demand worldwide.

Thus refiners and petrochemical companies aim for process revamps in order to improve their profit margins as well as decrease their carbon footprint. Furnaces are major consumers of fuel in a typical refinery or a petrochemical plant, and various process schemes are explored here in order to process dirty fuels without severe corrosion problems  or compromising  the life of the equipment and avoiding any unwanted shutdowns. 

Basis of study
For the sake of the study, hypothetical base case and revamp case models have been developed, and simulations for a hydrotreater fractionator feed furnace have been carried out using typical industrial data and studied using commercial software, FRNC-5PC Version 4.18 Mod 7.6.

Definition of cases
The various process cases considered for the study are as follows:
• Base Case A is the existing base case of a natural draft heater with an absorbed duty of 15 Gcal/h.
• Revamp Case B1 [Option I] is the revamp case of a balance draft heater with an absorbed duty of 
15 Gcal/h with on-board heat recovery in the form of a conventional air preheater with multiple carbon steel tube bundles.
• Revamp Case B2 [Option II] is the revamp case of a balance draft heater with an absorbed duty of 
15 Gcal/h with heat recovery in the form of an on-board glass coated tube type air preheater.
• Revamp Case B3 [Option III] is the revamp case of a balance draft heater with an absorbed duty of 
15 Gcal/h with heat recovery in the form of an off-board ground mounted cast and glass type air preheater.
• Revamp Case B4 [Option IV] is the revamp case of a balance draft heater with an absorbed duty of 
15 Gcal/h with heat recovery in the form of an off-board ground mounted cast type air preheater with an upstream steam air preheater required to maintain the lowest tube metal temperature above acid dew point.
• Revamp Case B5 [Option IV] is the revamp case of a balance draft heater with an absorbed duty of 
15 Gcal/h with heat recovery in the form of an off-board glass coated tube type air preheater.

Comparing the base case scheme with revamp schemes
Basic schematics of the existing base case along with various other process revamp schemes are shown in Figures 1, 3, 4, 5, 6, and 7.

Methodology for study
Typical base case and revamp case process flow rate, duty of the furnace, process inlet and outlet temperature, process inlet and outlet pressure have been considered and a feed property grid has been generated using commercially available process simulator PRO II. Results of the various process schemes were compared to evaluate their relative advantages and disadvantages

The study was carried out with the following assumptions and data for modelling: the base case is considered to be a process scheme with a 15 Gcal/h natural draft heater without any heat recovery, radiant heat loss of 1.5%, excess air of 20%, ambient air datum temperature of 15.6°C and relative humidity of 60%. The revamp case is considered to be a process scheme with a 15 Gcal/h balanced draft heater with heat recovery, radiant heat loss of 2.5%, excess air of 15%, ambient air datum temperature of 15.6°C and relative humidity of 60%.

The typical refinery fuel gas composition in mole% is as follows: hydrogen, 44; methane, 13; ethane, 15; propane, 22; isobutane, 5; hydrogen sulphide, 0.6; and isopentane: 0.4.

A typical algorithm followed for the purposes of the study is shown in Figure 2.

Important parameters and simulation results for the base case and revamp cases are shown in Table 1.

Results and discussions
Revamp Option I

This scheme is identical to the base case scheme except that conventional on-board metallic tube bundles have been used for heat recovery, thereby targeting improvement in fuel efficiency. Overall structural weight would be increased due to the addition of an on-board air preheater, thereby increasing load on the foundation as compared with the existing base case configuration.

Although extra footprint is not envisaged for an on-board air preheater, ducting and footprint would be required for a flue gas fan and combustion air fan to take care of extra pressure drop across the flue gas side and air side. Thus, where there is a plot limitation, for example in a revamp, such a scheme is a feasible option for improving fuel efficiency, provided space is available for fans and ducting.

In the event of acid condensation, the carbon steel tubes of an on-board air preheater are expected to corrode at an estimated rate of approximately 2 mm/y. Thus for low ambient or winter conditions, a steam air preheater is envisaged for avoiding any acid corrosion issues, which will add to the hardware requirement of the Option I configuration. This situation is not envisaged with the base case as there is no heat recovery and no chance of acid corrosion.

Efficiency improvement for Option I is around 9% compared with the base case and thus would bring down fuel consumption substantially. CO2 emissions would fall from 105 to 95 t/d, thereby  saving 3300 t/y of CO2 being emitted to the environment, considering 330 on-stream operation days. However, quantities of other emissions like NOx will go up for the revamp case compared with the base case configuration.

Because this scheme introduces fans, electrical power consumption would be required compared with the base case, which essentially does not require any electric consumption.

Soot deposits on the outside of the tubes, especially in the case of fuel oil firing, is a matter of concern as it reduces the efficiency of the system. In the case of on-board air preheater designs, water washing in general is not recommended for cleaning of soot since water may spill below and may cause unnecessary damage to coils, refractory and so on. Thus steam cleaning can be recommended by installation of soot blowers in the on-board air preheater region. This would require installation of access platforms for the operation and maintenance of these soot blowers.

Carbon steel tube bundles are easy to manufacture and can be fabricated by most construction vendors compared with a cast and glass type air preheater or a glass coated tube air preheater, which are specialised designs available from only a few vendors. This option in general is mechanically more robust compared with a glass type air preheater or glass coated tube air preheater, which are thought of as fragile in nature. Since a carbon steel tube bundle is relatively robust compared with a glass type air preheater or glass coated tube air preheater, there are no transportability issues with this design. Modularisation is also possible with this type of design.

If designed and operated properly – always maintaining the lowest tube metal temperature above acid dew point – this type of design would require minimal maintenance. However, if acid corrosion is encountered, then carbon steel tubes would corrode in a few years and unwanted maintenance shutdowns/turnarounds may be required.
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