SO2 emission control for resid combustion
Regenerable and non-regenerable SO2 scrubbing systems for high-sulphur residuum combustion are compared
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Various environmental and market pressures may provide the refiner with an incentive to consume modest quantities of high-sulphur residuum on site, to generate power or steam that is consumed locally.
Resids contain elevated amounts of sulphur compared to sweet liquid fuels or natural gas. SO2 scrubbing is likely to be required to support a refinery’s co-generation project that uses a heavy sour liquid fuel stream. Designers need to consider whether to install a non-regenerable SO2 scrubbing system that uses an alkaline agent such as caustic soda (NaOH), lime or limestone, or whether a regenerable system will be used that generates a pure stream of SO2 that can readily be converted to elemental sulphur or 98% sulphuric acid. Depending on the sulphur content of the fuel in question, one or the other type of system is preferred. This article describes the regenerable Cansolv SO2 system and compares it to non-regenerable systems that employ NaOH, lime or limestone as the sorbent.
Resid combustion and cogeneration
The refining industry continues to be pressured to process more difficult crudes and, at the same time, respond to ever-tighter limitations on product sulphur content. Investment in bottom-of-the-barrel hydrocracking or coking systems is economical for large volumes of resid, but options are limited when smaller volumes of high-sulphur products must be accommodated. Co-generation projects may be able to serve as outlets for future quantities of high-sulphur refinery streams. These projects will require SO2 scrubbing to prevent the sulphur in the fuel mix from escaping to the atmosphere.
A refiner may wish to consider a co-generation project for three reasons. First, co-generation projects can be designed to accommodate a wide range of fuels, from cycle oils to asphalt and coke. Second, on-site generation of power and steam reduces the refiner’s reliance on external, purchased balancing fuels. Finally, the versatile nature of the co-generation system to accept variable fuel qualities increases the refinery planner’s ability to fill the refinery crude slate with a wider range of crudes and to maximise margins.
SO2 scrubbing can be effected by non-regenerable and regenerable means. Non-regenerable systems consume an alkaline agent such as sodium hydroxide, limestone or dry lime and generate a waste stream of sodium sulphate or gypsum (calcium sulphate). Sodium sulphate is disposed of in wastewater treatment systems, while gypsum is most often disposed of in a landfill site.
Regenerable systems use an alkaline agent, such as sodium sulphite or Cansolv DS, to capture SO2 and release it in pure form from a regenerator that is designed to split the reagent from the SO2. In the refinery, SO2 is converted to sulphur in the refinery’s sulphur recovery unit (SRU).
Non-regenerable systems capture one tonne of SO2 from flue gas and generate between two and three tonnes of dry equivalent waste, which is discharged as a dilute stream of liquid (sodium sulphate) or as a wet, hydrated solid (CaSO4). Regenerable systems, which convert SO2 to elemental sulphur in the refinery, remove a tonne of SO2 from flue gas and generate only half a tonne of high-value, marketable product.
When external SO2 from a regenerable SO2 scrubbing system is fed to the refinery SRU, it displaces combustion air and can be used alone or in combination with other strategies, to debottleneck or reduce the cost of the SRU, which is often under pressure to process greater quantities of H2S from elsewhere in the refinery.
SO2 scrubbing systems are composed of up to six process blocks (see Figure 1). The lime and limestone non-regenerable systems include a reagent preparation area (block 2) and a gypsum filtration area (required for blocks 5 and 6). Caustic requires no reagent preparation or byproduct management block, since NaOH is sourced as a bulk liquid and sodium sulphate is discharged to wastewater-treating systems as a dilute solution of sodium sulphate. The non- regenerable systems, by definition, do not have a regeneration block.
Regenerable systems have little or no requirement for a reagent preparation area, but dedicate significant resources and space to solvent regeneration. While both regenerable and non- regenerable systems can produce a marketable byproduct, it is more common for the caustic, lime and limestone non-regenerable systems to direct their byproducts to waste, as markets for sodium sulphate or gypsum are limited. This contrasts with the regenerable systems that direct SO2 into sulphuric acid or elemental sulphur markets.
Table 1 compares the process areas required for several common non-regenerable systems and the Cansolv SO2 scrubbing system.
Caustic systems have lower capital costs than other non-regenerable systems. Reagent is purchased as a concentrated liquid or dry solid and wastes are directed to the refinery wastewater treatment system. NaOH prices are quite volatile, because it is a co-product in the manufacture of chlorine. When chlorine is in high demand, NaOH pricing tends to drop, whereas when chlorine is in low demand NaOH prices tend to rise. Chlorine markets are sensitive to world demand for chemical products such as vinyl chloride monomer, used to manufacture polyvinyl chloride plastics.
Sodium carbonate and sodium bicarbonate are less expensive alternatives to caustic and can be substituted for caustic, but their cost is higher than for limestone or lime, and they require investment in reagent preparation and management systems.
Capital costs for limestone- and lime-based systems show an advantage over regenerable systems. Although investment is required for reagent preparation and byproduct or waste management, by definition, no investment is required for solvent regeneration systems.
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