Operation and Performance of a Double-Alkali Scrubber
Authors: Sachtschale James R. , Dydo John F. 
Published: December 2007

Summary
Santa Fe Energy Co. (SFEC) installed a double-alkali sulfur dioxide absorber at its Kern River field near Bakersfield, CA. The flue gas desulfurization (FGD) system is designed to remove 95% of the sulfur dioxide from the exhaust of eight oil-fired steam generators when 1.5 wt% sulfur fuel oil is burned. The chemistry of the double-alkali process and the results of compliance and performance emissions testing are presented. Sulfur dioxide emission results are analyzed. Performance tests showed that total outlet sulfur dioxide emissions were 28.7 lbm/hr (0.00363 kg/s). If offset emissions are included, the net sulfur dioxide emissions were 0.22 lbm/hr (0.00003 kg/s).

Introduction
In Dec. 1977, SFEC contracted with FMC Corp. to install a double-alkali sulfur dioxide absorber serving a group of eight generators consisting of five 62.5-MMBtu/hr (18.44-MW) new steam generators, one 25-MMBtu/hr (7.33-MW) new steam generator, and two 25-MMBtu/hr (7.33-MW) existing steam generators. The absorber was guaranteed to reduce sulfur dioxide emissions to 28.1 lbm/hr (0.00355 kg/s), which wouldcomply with U.S. Environmental Protection Agency (EPA) permitting requirements of 28.14 lbm/hr (0.00355 kg/s) for sulfur dioxide emissions, andcomply with the Kern County Air Pollution Control Dist.
permitting requirements of 15 lbm/hr (0.00190 kg/s) when sulfur dioxide emission offsets were included. This paper discusses the double-alkali process, its chemistry, and the results of the performance and compliance emission tests that were conducted in May 1980.

Process Chemistry and Equipment Description
SFEC, at its Kern River oil field steam drive project, installed eight steam generators with a total maximum rated heat output of 310 MMBtu/hr (90.85 MW). In partial fulfillment of Authority to Construct Air Permits, SFEC contracted with FMC to design and install a double-alkali sulfur dioxide absorber. A description of the double-alkali process and the equipment installed follows.

Process Overview
The overall process schematic of the double-alkali process is shown in Fig. 1. Flue gas from the steam generators passes through a disk and doughnut-type absorber. Sulfur dioxide is absorbed by a sodium sulfite solution in the absorber. The sodium sulfite is converted to sodium bisulfite. A portion of the solution then is bled from the absorber to a lime reactor tank. Hydrated lime, calcium hydroxide, reacts with sodium bisulfite in the stream from the absorber, forming calcium sulfite and sodium sulfite. Sodium sulfite and calcium sulfite then flow into a thickener tank, where calcium sulfite precipitates. Sodium sulfite solution overflows into a surge tank and then is returned to the sulfur dioxide absorber vessel. The calcium sulfite precipitate is pumped to a rotary vacuum filter where excess moisture is removed, producing a semidry solid filter cake. This semidry filter cake then is disposed in a solid waste disposal site. The two additional vessels shown in the schematic are for chemical storage. One vessel is a pebble lime (CaO) storage silo. Pebble lime is air-blown to a lime slaker where calcium hydroxide is formed. Calcium hydroxide flows into the lime reactor tank. The other vessel shown is a sodium carbonate (Na2CO3) storage tank.
JPT
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