Biological-based emissions control has been demonstrated to be an efficient and cost-effective alternative to thermal-oxidation technology or flaring for volatile organic compounds (VOCs) from the forest-products and paint and coatings industries. This type of technology application has promising advantages such as the potential for a low carbon footprint, low secondary pollutants such as NOx and SOx, lower energy demands, and lower cost. The objective of this project was to design and implement a sequential field-scale biotrickling/biofilter treatment unit to remove VOCs and hazardous air pollutants (HAPs) emissions at the Apache TAMU#2 well-storage-tank battery in Snook, Texas.
The field-scale biotreatment system included a biotrickling filter followed by a biofilter with the total treatment volume of 100 ft3, skid-mounted on a 22-ft trailer. The biotrickling filter was packed with structured cross-flow media with large surface area and high void fraction designed to remove the more-water-soluble compounds and control the humidity and temperature variations of the inlet gas stream. The biofilter unit was loaded with plastic spheres packed with compost, which is referred to as the engineered media. Each of the bio-oxidation units was operated at the air-flow rate of 25 ft3/min and empty-bed-residence time of 2 minutes. The system was inoculated with local storm water and waste water from a sedimentation basin clarifier of a local refinery to provide a mixed culture of microorganisms for degradation of the VOC emissions.
VOC emissions were collected from the headspace of a storage-tank battery leading into a pressure-relief-vent system. On the basis of photo-ionization-detector measurements at the inlet of the bio-oxidation unit, VOC concentration loading was cyclic and appeared to be correlated to the gas-lift cycle of liquid loading to the crude oil storage tank.
During the evaluation period, the biotrickling unit demonstrated a surprisingly higher removal efficiency compared with the biofilter. This may be related to the more-stable and higher-density biomass growth observed on the surface of the cross-flow media. The lower removal efficiency in the biofilter unit could be from the lack of uniform moisture and nutrients in the second vessel as a result of spray-nozzle inefficiency. This aspect of operation can be optimized further by changing the nozzle and the frequency of watering/spraying of the compost media. A removal efficiency of 50–60% for the total VOCs, across the complete unit, was achieved during the 3-month evaluation period while the unit was operated at an average inlet VOC concentration of 400 ppm.
The relatively high concentration of alkenes and alkanes (compared with aromatics and water-soluble organics in the crude-oil vapor), may have decreased the degradation of the total VOCs in the bio-oxidation unit because these long-chain compounds are more difficult to biodegrade by bacterial biofilms in an aerobic environment.
The results suggest biological emission treatment systems may be cost-effective when compared with thermal oxidizers and flares and should be evaluated as a maximum achievable control technology to mitigate HAPs (and VOCs) from some oil and gas operations.
This innovative biological-emissions-control technology effectively controlled the cyclic emissions produced at the remote site. The strong increase in removal of VOCs after the oil refinery wastewater inoculation suggests an important optimization parameter for more-rapid acclimation and increased efficiency for the system in future applications.
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