Decarbonisation Technology May 2022 Issue

Plant

Power

Year Lo-Cat

Currently operating

Plant name

Raw gas flow rate (LGFR)

Inlet H 2 S LTPD recovered

name generated (MW)

installed

(vol%)

sulphur

Navy 1 Navy 2 Navy 210

75 90 90

1993 1993 1996

24/7 On standby 24/7

Navy 1 Navy 2 Navy 210

3688 6774 6228

1.18 1.55 3.00

2.4 5.8 10.0

Table 1 Year built and current status

Table 2 Unit design parameters

Lo-Cat process The Lo-Cat process converts H₂S contained in the following equation:

to a filter that separates the sulphur from the catalyst solution and washes the filter cake. The sulphur is discharged into a sulphur bin while the clarified solution (i.e., filtrate) is returned to the auto-circulation vessel. Even with water washing of the sulphur filter cake, some catalyst solution exits with the solid sulphur. Makeup catalyst is added to maintain the solution at optimum concentrations. A surfactant is also added to help prevent foam and floating sulphur. Potassium hydroxide (KOH) is added for pH control. Operations review Two key parameters ensure consistent Lo-Cat operations as follows: (1) Prevent sulphur from settling in undesired locations, and (2) Maintain proper solution chemistry. Catalyst makeup and chemical addition rates are discussed later. The main method to prevent undesired sulphur settling is to use ‘air blasts’ placed strategically throughout the unit in regions of low flow. Nozzles send bursts of air into these areas within the auto-circulation and settler vessels, preventing sulphur build-up. When feed gas flows through the unit at the process design rate, undesired sulphur settling is less likely to occur.

H₂S (g) + 1/2 O₂ (g)  H₂O + S0

Before entering the Lo-Cat unit, raw feed gas passes through an activated carbon bed to absorb mercury and other heavy metals. The raw gas then enters the auto-circulation vessel, where the H₂S is absorbed into a proprietary Lo- Cat catalyst solution. The catalyst is deactivated in the absorber section, where H₂S is converted to elemental sulphur. Subsequently, the catalyst is regenerated in the oxidiser section of the same auto-circulation vessel. Regeneration is achieved by contacting the catalyst solution with oxygen contained in air. The air and sweetened gas exit into the atmosphere as vent gas. The solution is circulated between the absorber and oxidiser sections via a system of baffles and weirs with density difference as the driving force. Elemental sulphur formed via the reaction becomes suspended in the catalyst solution. A circulation pump sends a slipstream of solution to a settler vessel to remove the elemental sulphur from the process, which allows the sulphur to concentrate and form a slurry The slurry is routed

Vent gas

Catalyst

Settler vessel

Proprietary Sulphur lter package

Autocirculation vessel (Absorber / Oxidiser)

Raw gas

Sulphur cake

Solution (Circulation) pump

Air

Air blowers

Filtrate return line

Figure 1 Simplified flow diagram of the Lo-Cat unit