In February of this year, we completed a ten day test of MSA injection at a 2,000MW+ power plant. The results of that test have suggested a few things about our chemical MSA, used for the simultaneous control of Arsenic, Selenium, and Mercury by addition into the WFGD Absorber Recirculation Tank.
1 – MSA certainly does precipitate selenium from the slurry liquid aqueous phase, within the solids-burdened, sulfate-laden solution, and;
2 – Sufficient molar excess of chemical is required to overcome the additional selenium and mercury that appears to be absorbed as a result of the MSA addition.
Another test is currently scheduled for early 2022 to validate molar ratios needed to ensure continued sub-saturation throughout the selenium and mercury capture scenarios.
In November of 2019, the EPA issued newly proposed Effluent Limitation Guidelines for Steam Electric Power Generation Units applicable to flue gas desulfurization wastewater. A key component of the newly proposed rules deal with the limits for selenium. While the newly proposed limit for mercury was revised downward, the newly proposed monthly average for Se is now 31 ug/L (ppb), still under the US drinking water limit, but higher than before.
In July of 2018, we filed a provisional patent for the use of a unique inorganic sulfide salt for the simultaneous capture of arsenic, mercury and selenium in Wet Flue Gas Desulfurization WFGD systems, wastewater and leachate. We refer to the chemical as MercSelAs, or MSA for short. It was felt this unique chemical intermediary might not only precipitate mercury from the slurry liquid aqueous phase within the WFGD, allowing stack mercury limits to be achieved, but provide phase-partitioning of the metalloid arsenic, and the non-metal selenium, into the slurry solid (gypsum) phase.
In October of 2018, beaker tests carried out on slurry from a full scale operating unit confirmed we achieved significant phase-partitioning of total selenium from the slurry liquid aqueous phase to slurry solid phase.
In December of 2019, a five day, full scale test was planned at a plant operating two absorbers at a total gas flow equivalent of 1,000MW for the test. Unfortunately, the unit(s) tripped before the end of the test, which limited the test to forty-four (44) hours of duration.
Below is a graphical representation of the selenium results for the full scale test:
It should be added the stack mercury concentration began trending down within five minutes of MSA introduction, similar to many other sulfide chemicals that are a standard in the industry.
What is so very interesting about this unique chemical is not only its simplicity, but in an Absorber Recirculation Tank with almost 7 million pounds of water, and 200,000 pounds of gypsum, this chemical seeks and finds the roughly 2 pounds of selenium that exists in the slurry liquid aqueous phase. In another somewhat remarkable result, after 44 hours of MSA injection, each absorber achieved ~55% reduction in aqueous selenium, even though their baseline selenium concentrations were different.
In our longer test scheduled for this summer we hope to confirm additional injection time will lead to further selenium incorporation into the gypsum crystal via aggregation, and potentially provide an even greater selenium reduction in the slurry liquid aqueous phase.
Should the longer test confirm even greater selenium aggregation into the gypsum crystal, this may provide significant savings for those owners/operators who are currently looking to bioreactors to achieve their 31 ppb limit for selenium.
Please contact us to purchase our services for the introduction of MercSelAs™ Solution, MSA™, into your WFGD system and see how our unique sulfide salt can provide a chemically-coupled approach to 1 – achieving MATS limits for mercury, and 2 – reducing slurry liquid aqueous phase selenium concentrations.
