Measuring the Performance
of Media and Systems Filtering Pollutants
By Robert McIlvaine
The Filter industry is being challenged to provide more efficient media and systems to meet ever more stringent limits on the discharge of heavy metals. The challenge is not only to develop a cost effective filter medium but also to prevent leakage at the seams and to supply total systems, which maintain performance over the extensive campaigns conducted by utility power plants. Possibly the biggest challenges is to measure performance. There is a variety of toxic metals; of which range in toxicities of a thousand times. So in order to truly measure the environmental risk one needs to measure the metals individually.
Recent standards promulgated for the cement industry regulate toxic metals. EPA predicts that as many as 100 cement plants will have to install membrane bags in order to meet the toxic metal limits. Stringent standards already exist for municipal solid waste and hazardous waste incinerators.
European regulations on toxic metals have been in place for years. Metals are divided into categories according to their toxicity. Limits are set on each group. Developing countries are following the western regulations closely and are planning to emulate them in future years. China has a major research effort to determine how to best reduce toxic metal emissions including mercury from power plant stack emissions.
These changes to the rules have placed renewed interest on removing toxic metals emissions from stack gases, but to do so a baseline needs to be established via monitoring. Compliance cannot be met by monitoring every couple of years and extrapolate metals emissions. This is due in part to the new requirements within the regulation, but also the necessity of understanding the process changes from feedstock variations and control strategy efficiencies. By characterizing feedstock's and metals emissions, process efficiencies can be exploited reducing energy consumption, control consumable usage and extended maintenance regimes. Near-real time monitoring gives the information to perform this characterization, adapt to changes and understand where users are in respect to compliance limits simultaneously. Eli Lilly and Company used this type of monitoring and control strategy at both their Hazardous Waste Incinerator and Boiler to reduce control and compliance costs.
The most stringent emission limits have been set for new sources. The just promulgated limits for new sources in MATS are tough but do not equal those limits imposed by states as part of the permit process for new plants. In Table 1 on page 8 are the proposed and promulgated limits for new coal plants in the U.S.
The variation in toxicity and harm from various metals is shown in Table 2. The rankings on a relative bases are derived mostly from EPA data. The conversion to tons of CO2 equivalent was just a mathematical derivation based on an EPA life value of $7 million and a value of $20/ton for CO2. The QELD equivalent is based on a radically new approach to harm assessment. It is based on life quality rather than longevity.
Pall's Xact 640 system uses reel-to-reel (RTR) filter tape sampling and nondestructive X-ray fluorescence (XRF) analysis to monitor stack HAP metal emissions. An isokinetic sub-sample of stack gas is taken from a stilling chamber and drawn through a chemically reactive filter tape. Vapor phase metals, including mercury (Hg), are deposited along with the particulate mass (PM) on the filter tape. The deposit is automatically advanced and analyzed by XRF for selected metals as the next sample is being collected. Sampling and analysis are performed continuously and simultaneously, except during advancement of the tape (~20 sec) and during daily automated quality assurance checks.
The EPA also approved the Xact 640 CEMS as an alternative method for periodic Method 29 testing and feed stream analysis, as well as for monitoring emissions during plant operation. In 2007, through its Clean Air Excellence Award, the EPA recognized the Xact 640 CEMS as an outstanding achievement in innovative clean air technology.