Sorbent Tube Method Approved for HCl Measurement on Coal-fired EGUs
The U.S. EPA recently “upgraded” Other Test Method 40 (OTM-40) to an approved alternative method (ALT-129) for coal-fired Electric Generating Units (EGUs) subject to 40 CFR Part 63 Subpart UUUUU, otherwise known as the Mercury and Air Toxics Standards or MATS. The change from OTM to ALT means the method can now be used by coal-fired EGUs without pre-approval . The method is the same, just the broad approval is new.
The approval does come with some restrictions, however. The method may only be used without pre-approval by coal-fired EGUs and only those with low moisture flue gas at temperatures above 212 °F. The flue gas can contain no entrained droplets. Under these conditions, field testing shows OTM-40 performs similarly to Method 26A (M26A, one of the traditional HCl Reference Methods) but could potentially be biased high. A simpler sample train setup, lack of harmful chemical reagents, and quicker sample recovery may contribute to lower testing costs for affected facilities compared to Method 26 or 26A.
OTM-40 uses HCl sorbent traps and the same equipment and procedures as Method 30B (M30B, mercury sorbent trap method). The sample gas flows through paired sorbent tubes which capture HCl as chloride. Sample recovery and analysis typically consist of a water extraction followed by ion chromatography. Like M30B, method performance is evaluated for each test using NIST-traceable spiking in one of the traps. This paired-sample design helps validate test results and assess method precision which is difficult to accomplish with the traditional test methods.
While M26 uses a set of glass impingers filled with reagent to collect HCl outside of the stack, OTM-40 traps capture the HCl sample in-situ. This difference eliminates potential low bias resulting from insufficient heating of the sampling system, since the sample is collected in the traps before the sample gas is transported through the rest of the sampling equipment. However, this approach is susceptible to the same potential high bias as M26 from the presence of elemental chlorine (Cl2) or other chlorinated compounds in the flue gas. Unlike M26 or M26A, the trap method could also have a high bias if there are any chloride compounds contained in the particulate matter collected on the glass wool plug at the front of the trap. The trap method requires that the glass wool be included in the sample analyses, whereas the particulate filters in the conventional methods are not required to be analyzed.
As mentioned above, OTM-40 is currently only approved for coal-fired EGUs with dry stacks. Stacks with wet scrubbers still require isokinetic sampling and nozzles for representative sampling of possible entrained water droplets. OTM-40 does not sample in this fashion. Also, the method has only been validated on coal-fired utility stacks; therefore, OTM-40 has not been approved for other sources although it may still be used with pre-approval.