A new look at SO2 removal

By Mark A. Shand Mark A. Shand, Director Technical Services, Premier Chemicals, LLC

BRUSSELS, April 28, 2009 (Viewpoint) - Flue gas desulfurization (FGD) in power and steam generation systems has traditionally been accomplished using caustic soda, lime or limestone. The use of magnesium hydroxide (Mg(OH)2) slurry for FGD is fairly uncommon in the United States, although its use in Japan is much more widespread. In the US, Exelon Power uses magnesium hydroxide in its FGD system at two coal burning power generation facilities, and has done so successfully for a number of years. The development of the Magnesia FGD scrubbing process in the US was undertaken primarily by Chemico-Basic along with Babcock and Wilcox in the late 1960s, with some supplemental research being carried out by the Tennessee Valley Authority.

The recent steep price increases for caustic soda has resulted in plant engineers searching for viable alternative scrubbing chemicals for their FGD systems. Several pulp mills have already successfully made the transition to magnesium hydroxide slurry. The transition has resulted in maintaining SO2 scrubber removal efficiencies seen while using caustic soda. The net consumption of scrubber chemical is about 30% less (dry basis) when using magnesium hydroxide. Cost savings based on $1,000/dry ton for caustic soda are estimated at $500/ton of caustic soda used when magnesium hydroxide slurry is substituted for caustic soda.

In many cases magnesium hydroxide can be directly substituted for caustic with little modification to the existing FGD system. The only modifications that are needed are to the storage system, which consist of mechanical agitation for storage tank and positive displacement feed pump. Since magnesium hydroxide is an aqueous suspension, its freezing point is the same as water: 32oF (0°C). Also, magnesium hydroxide is classified as a non-hazardous chemical and is a much safer alkali than caustic since it will not cause chemical burns if it comes into contact with skin or mucous membranes. Magnesium hydroxide slurry is supplied in bulk with solids content ranging from 56-63 wt%.

Figure 1 - Boiler flue gas desulfurization unit

Scrubbing chemistry

The chemistry of scrubbing SO2 with Mg(OH)2 is well understood and documented, the principal chemical reactions being as follows.

Sulfur dioxide absorption: This is achieved by spraying dilute (10 wt% solids) slurry into a Venturi, mobile bed or spray type scrubber, where the magnesium hydroxide reacts with sulfur dioxide to form magnesium sulfite hexahydrate (see reaction 1). The scrubber slurry is maintained at a pH of about 6.5, with an operating range of 5.0-7.5, by adding magnesium hydroxide slurry to the recycled scrubber liquid.

Mg(OH)2 + 5H2O + SO2 → MgSO3.6H2O (1)

SO2 + MgSO3.6H2O → Mg(HSO3)2 + 5H2O (2)

Magnesium sulfate hexahydrate can further react with SO2 to form soluble magnesium bisulfate (see reaction 2). These reactions together are capable of removing 90% or better of the scrubber inlet sulfur dioxide.

Neutralization of magnesium bisulfate is achieved by further reaction with magnesium hydroxide, which forms two molecules of magnesium sulfite hexahydrate (see reaction 3).

Mg(HSO3)2 + MgO + 11H2O →2MgSO3.6H2O↓ (3)

Mill experiences

A pulp and paper mill located in the Florida panhandle has recently completed conversion to magnesium hydroxide slurry from 50% caustic soda. The power boiler is permitted to burn coal, wood waste, natural gas or oil and the maximum allowed emissions of SO2 is 1.3 lb/MMBtu. A basic schematic diagram of the FGD unit is shown in Figure 1. The scrubber is operated at a pH of about 5.5. Magnesium hydroxide slurry, normally delivered with a solids content of about 56%, is first diluted to about 10% solids before entering the scrubber system. The scrubber slurry is then injected into a Venturi type scrubber. Spent scrubber liquor is collected in the recycle tank after exiting the scrubber separator. Scrubber liquor is bled-off from the recycle tank some being pumped to the waste treatment system. There is also a recycle loop back to the recycle collection tank from the bleed flow where scrubber solution pH is monitored. Scrubber liquor from the recycle tank is pumped back to the Venturi unit with make-up magnesium hydroxide slurry being added to this line in order to maintain a scrubber solution pH of 5.5. The plant has reported no significant operating issues after switching to magnesium hydroxide and is currently operating at about 60% SO2 removal efficiency. Consumption of scrubber neutralization chemical is about 30% less with magnesium hydroxide when compared on a 100% basis.

Other uses inside a mill

Magnesium hydroxide slurry has been successfully tested at a number of mills as a lower cost replacement for caustic soda in Eop stage bleaching with no negative effect on kappa number reduction, pulp viscosity, or brightness. Magnesium hydroxide has eliminated the need to use magnesium sulfate and has also reduced caustic soda requirements, and in some instances it has also reduce the consumption of hydrogen peroxide. Magnesium hydroxide bleaching generates lower effluent BOD/COD, higher pulp yield lower anionic trash on the paper machines and can reduce oxalate scaling.

Mark A. Shand is Director Technical Services, Premier Chemicals, LLC, Findlay, OH: mshand@premierchemicals.com

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