On April 21, 2009, the Environmental Protection Agency (EPA) proposed to amend its national emissions standards for Portland cement manufacturing in an effort to reduce emissions of Mercury, total hydrocarbons, hydrochloric acid and particulate matter from both new and existing cement kilns. According to a Fact Sheet published by the EPA, in 2005, there were 186 Portland cement kilns currently operating at more than 100 facilities in the United States. Today’s proposed rule would apply to 163 of those kilns. The remaining kilns are subject to a separate regulation as they burn hazardous waste. Kilns will have to comply with the new limits three years after the final rule is published in the Federal Register. This looming deadline is likely to occur in mid 2013. This article focuses on Mercury monitoring challenges specifically for cement kilns and recent innovative technologies that enable successful monitoring and reporting of Mercury emissions.
Complex Raw Mill effect
Throughout the cement manufacturing process Mercury exists in three different forms namely Elemental Mercury as Hg (0), Ionic Mercury as Hg (2) and Particle bound Mercury as Hg (p). Modern Portland cement plants have a Precalciner with Inline Raw Mills that complicate Mercury mitigation within the process and makes it more difficult to adapt consistent removal and monitoring technologies. While analyzing the feed rate of Mercury to the kiln itself during the long term operation of Precalciner kilns with in-line raw mills, it is typically the case that all of the dust captured in the main Air Pollution Control Device (APCD) is returned to the kiln. However these systems periodically go through a maintenance cycle where the raw mill is shut down and the exhaust gases from the kiln bypass directly to the APCD device. Thus, it is important to note that the Mercury emissions never come to short term equilibrium and can typically take weeks to reach long term equilibrium.
There have been studies to show that when the Raw Mill is on, the Hg(0) is at 54%, Hg(2) is at 37% and Hg(p) is at 9% whereas when the Raw Mill is off, the Hg(0) is at 16%, Hg(2) is at 76% and Hg(p) is at 8%. Added to this, because typical plant returns the captured dust from APCD into Kiln and uses exhaust gases for drying in the Raw Mill, there is a complex Mercury mitigation cycle within the process which makes it difficult to achieve an equilibrium state due to the periodical shut down of Raw Mills for maintenance. Sometime the equilibrium cycle may take weeks. The highly volatile nature of the equilibrium cycle renders periodical stack measurements as an inadequate method in garnering data to make decisions of appropriate Mercury control strategies. A Continuous Emissions Monitoring System (CEMS) is a more appropriate solution for determining this strategy.
Innovative & Emerging Monitoring Technologies
Mercury CEMS are undergoing constant development as new challenges emerge regularly across the wide industrial spectrum and as new environmental regulations are being implemented. There are two broad categories of monitoring as recommended by the Clean Air Mercury Rule (CAMR) and Part 75 for Utility Boilers namely Mercury CEMS and Sorbent Trap based periodical monitoring under Appendix K of Part 75. However for Cement plants the EPA has mandated Mercury CEMS as Sorbent systems which are not considered continuous systems and thus need regular attendance and maintenance. Sorbent traps frequently suffer from clogging due to dust in the flue gas and high concentrations of Mercury when the Raw Mill is off. The traps also need to be pulled out from the stack and analyzed in a Laboratory in a very short time which is usually a difficult task. Compounding the problem is that this can be a costly endeavour due high labor costs involved. Some Mercury CEMS currently available, have been developed to perform with minimal maintenance for CAMR Utility market applications and can provide continuous reporting of Mercury in all three phases namely Hg(0), Hg(2) and Hg(p). Out of the two types of Mercury analyzers the AAS (Atomic Absorption Spectroscopy) and AFS (Atomic Fluorescence Spectroscopy), the AAS Technology is more suitable for wider range detection as it does not suffer from the ‘Quenching effect’ while AFS technologies are limited to a narrower range of detection limits.
A new ‘Adjustable Sample Volume’ (ASV) method has been developed and is now commercially available to specifically meet the demands of complex Mercury monitoring challenges in the cement industry. The ASV feature in the AAS Mercury analyzer automatically detects whether the Raw Mill condition is either on or off, and adjusts the sample volume in the sample cell automatically to provide accurate measurements in both extremely high and low ranges of Mercury variations during this process. By implementing this feature, the inability to measure due to fluctuating Mercury level conditions in the stack is avoided.
Multiplexing several kiln stacks using one Mercury CEMS has also been actively pursued and many tests have been conducted at cement kilns to demonstrate a cost-effective Mercury CEMS package where multiple kiln stacks have been monitored and reported. By employing advanced multiplexing sample extraction technologies, cycle time between stacks has been reduced to a few minutes and provides virtually continuous measurements.
The Needs of the Cement Industry
Although the cement plant operators have to comply within three years from the promulgation of the Rule likely to be in effect by middle of 2010, most of the plant owners and operators are getting familiarized with the monitoring technologies, how they can be adapted to their specific plant environment, operational and maintenance experience, and have sufficient time to take alternate actions to be ready well ahead of the pending compliance deadline.
Unlike the Utility Mercury Rule, Cement Kilns’ complex process involves Mercury mitigation and cross-transfer issues between fuel and raw material. A thorough study would have to be made on related Mercury emissions characteristics before any meaningful control strategy can be adapted as there is a wide variation in process and emissions from plant to plant, as the fuel and raw material matrices vary a lot. Cement plant operators are actively conducting emissions test programs to collect data on speciated Mercury under various plant operating conditions to evaluate proper control strategy decisions.
Mercury CEMS for Cement Kilns must be capable of handling a wide range of speciated Mercury concentrations by using an adjustable sample volume technique in view of Raw mill operations, performing consistently in a dusty environment, and should use a fast loop sample extraction to minimize data update time if stacks are multiplexed. A unique alternative that some cement plants are using are transportable Mercury CEMS that can be easily moved from stack to stack for initial studies or to compare with stack test results apart from performance evaluations of Mercury removal equipments.
Unlike the CAMR and Part 75 for Utility market where particle bound Mercury was excluded from reporting, it is likely the EPA will require particle bound Mercury to be reported from Cement kilns as well. Therefore Cement plant operators will have to ensure that the Mercury CEMS can handle, speciate, and report particle bound Mercury without losing the ability for quantitative analysis of Mercury in the sample treatment and conditioning phase.
Like any other emissions monitoring challenge, the new Cement Mercury Rule is posing a stringent measurement and reporting task ahead for the industry. With emerging technologies and an increasing number of Mercury monitoring studies taking place in since the last several decades’ new solutions are being developed and offered to meet those challenges. The important key aspect is to plan monitoring and control strategies well ahead of time so that compliance can be achieved successfully.
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