Development of gas phase bioreactors for the removal of nitrogen oxides from synthetic flue gas streams☆
Introduction
Nitrogen oxides (NOx) are hazardous air pollutants that lead to the formation of acid rain and tropospheric ozone. Approximately 24 million tons of NOx were released to the atmosphere from US sources during 1998 [1]. Titles I and IV of the 1990 Clean Air Act Amendments regulate NOx emissions from major stationary sources [2]. The overall goal of these programs was to achieve NOx reductions of 2 million tons per year below 1980 levels by the year 2000.
Concern for environmental and health issues coupled with stringent NOx emission standards indicate a need for the development of efficient low-cost NOx reduction technologies. Biological removal of NOx from contaminated gas streams offers promise as a novel alternative treatment method. This approach relies on the activity of denitrifying bacteria indigenous to composts and soils. Microbial denitrification is a dissimilatory reductive process in which nitrogen oxides are sequentially reduced to nitrogen gas. By this process, toxic nitrogen oxides are converted to environmentally benign nitrogen gas [3], [4], [5].
Contaminated gas streams can be biologically treated in a variety of gas phase bioreactor configurations including biofilters and biotrickling filters [6]. These treatment systems are effective for contaminants that display high water solubility and high biodegradability. Biofilters utilize microorganisms immobilized as biofilms on packing materials such as compost, peat or soil [7]. Contaminants in the vapor phase are absorbed by the bed and biologically converted into benign end products, such as water and carbon dioxide. Biotrickling filters [8], [9] are similar to biofilters, but contain an inert packing material (such as ceramic or plastic Raschig rings) that provides surface area for gas/liquid mass transfer and biofilm growth. A recirculating liquid phase flows through the reactor in order to keep the packing wetted. Both laboratory and field scale gas phase bioreactors have been shown to be effective for the removal of petroleum hydrocarbons including gasoline vapors [10], the benzene, toluene, ethylbenzene and toluene (BTEX) class of compounds [11], [12], [13], alcohols such as methanol [14], [15] and ethanol [16], [17], [18]; and reduced sulfur compounds such as hydrogen sulfide [19], [20], [21], [22].
The development of a gas phase reactor for the biological treatment of fuel combustion gases offers a number of unique challenges. Nitric oxide (NO) represents 85–90% of the NOx formed during the combustion of coal [23] and many coal combustion streams are scrubbed with a water/limestone slurry to reduce sulfur dioxide emissions. Gases exiting the scrubbers typically exhibit temperatures between 50 and 60 °C. The bioreactor packing materials should therefore exhibit long-term thermal stability within this temperature range and must contain suitable concentrations of thermophilic denitrifying bacteria. Blower operating costs for coal-fired power plants can be significant, thus the treatment system must operate with minimal back pressure. Finally, many fuel combustion applications generate flue gas streams with very large volumetric flow rates. The bioreactor must therefore operate with a short gas residence time in order to achieve a competitive capital cost and a reasonable footprint area.
The purpose of the present research is to demonstrate the initial phases in the incremental development of gas phase bioreactors for the thermophilic removal of NO from an oxygen-free synthetic flue gas using denitrifying microbial populations. Since flue gas can contain up to 8% oxygen and since oxygen inhibits the removal of NOx compounds by denitrifying bacteria, eventually understanding the effect of oxygen on NO removal in gas phase bioreactors will be important. However, to maximize differences in NO removal under optimum conditions for denitrification to occur oxygen was omitted from the gas stream for the current research. To address factors relating to maximum removal efficiency, short gas residence time and low pressure drop, a number of bioreactor packing materials that exhibit long-term thermal stability and can achieve biological NOx removal with reduced back pressure were considered. Following comparison of compost and inert packings, bed medium consisting of various ratios of compost to inert material (i.e. lava rock and perlite) were also compared in an attempt to combine the elimination capacity demonstrated by compost and the ideal flow characteristics associated with inert packings.
Section snippets
Biofilter packings
Four types of bed packings were used during this study: wood compost with bulking agents, perlite, biofoam and then various ratios of compost and lava rock or perlite. Perlite and biofoam were chosen as potential alternatives to compost since both materials potentially offer a lower resistance to gas flow and greater long-term thermal stability.
Unfinished wood and yard waste compost (courtesy of Nature Gro Compost, Pocatello, ID) were used as the base for the biofilter bed medium during this
Schenectady compost
The Schenectady compost biofilter was operated for 71 days at a gas flow rate of 1.1 l/min (corresponding to an empty bed gas residence time of 71 s). The average NO removal efficiency during the first 14 days of operation was 86%, but began to drop gradually with time to a NO removal efficiency of 70%. The lactate delivery rate was 0.8 times the stoichiometrically required amount during the first 35 days, and from 1.1 to 1.8 times the stoichiometrically required amount thereafter. Increasing the
Conclusions
This study describes steps that were taken in an effort to develop a gas phase bioreactor system suitable for the biological treatment of NOx in gas streams. Experimental data from the study indicate that perlite and biofoam packings offer long-term thermal stability and reduced back pressure compared with compost. Although the NOx removal ability of the alternative packings was comparable to compost at the longer residence time of 71 s; the compost biofilters performed significantly better at
Acknowledgements
The INEEL lead research for the project was supported through the US DOE, Office of Advanced Research and Technology Development, Office of Fossil Energy under DOE Idaho Operations Office Contract DE-AC07-94ID13223.
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