|Approximately 1,400 units at 600 U.S. power plants are affected by the EPA’s Mercury and Air Toxics Standards. They have until April 2015 to comply. Most of these units will require some form of dry sorbent injection (DSI) and/or activated carbon injection (ACI) technology for mercury and/or hydrogen chloride removal. Photo courtesy of United Conveyor Corp.|
By Jon Norman, PE, United Conveyor Corp.
In 2011, the Environmental Protection Agency (EPA) published proposed emission limits for emissions of Hazardous Air Pollutants (HAPs) from coal and oil-fired power plants. Finalized in 2013, the Mercury and Air Toxics Standards (MATS) impact electric generating units (EGUs) burning coal or oil for the purpose of generating electricity for sale or distribution through the national electric grid to the public. Technologies for HAPs reduction and compliance with MATS for coal-fired EGUs are discussed in this article..
Emission Limits Under MATS Regulations
For existing and new coal-fired EGUs, the MATS regulations establish numeric emission limits for filterable particulate matter (PM), mercury (Hg), and acid gases including hydrogen chloride (HCl). Quantifying emissions relative to the MATS requirements serves to inform decisions when it comes to specifying the most efficient and cost effective technologies.
Under the new MATS regulations, emission rates standards for most coal-fired EGUs for the listed pollutants are as follows: Hydrochloric acid (HCl), 0.002 lb/mmBtu; Mercury (Hg), 1.2 lb/TBtu; Particulate matter (PM), 0.03 lb/mmBtu.
Approximately 1,400 EGUs at 600 U.S. power plants are affected by MATS. The EPA estimates dozens of coal-fired plants may already meet at least some of the new standards, but more than 40 percent lack advanced pollution control equipment. The EPA gave utilities three years to comply and up to four years with an extension. Most plants have completed emission testing and many have completed DSI/ACI demonstration testing. Many plants are also well along in the control technology purchasing process and are planning system installation by year-end 2014 to allow sufficient time to ensure compliance with MATS by April 2015. In addition, a large percentage of plants have applied for one-year compliance extensions and the majority have received, or are expected to receive, the extensions.
|United Conveyor’s patented VIPER® Mill reduces operating costs and improves emissions control of dry sorbent injection. Photo courtesy of United Conveyor Corp.|
Utilities are actively evaluating technologies and strategies for compliance. Removal efficiency, capital and operating cost, plant life, outage requirements, scalability, and impact on existing air pollution control (APC) equipment are all considerations and trade-offs when assessing viable technologies for MATS compliance. Also, existing control equipment technologies are already being leveraged for many units.
Technical changes to the initial MATS regulations, including relaxing requirements during start-up and shutdown when emissions are often higher, in addition to only requiring filterable PM limits (versus total PM), have eased somewhat the compliance burden and will allow more EGUs to achieve the limits with less expensive upgrades. For example, the majority of units will now be able to meet the particulate matter limits without replacing ESPs with new fabric filters.
Multi-Pollutant Controls Needed
There is no one-size-fits-all solution for the multi-pollutant control that is required to meet MATS. Industry testing has shown that some PRB coal units already meet the MATS HCl limit without additional controls. Approximately one third are projected to use ACI to achieve Hg removal requirements. Approximately 20 percent will upgrade existing scrubbers; approximately 20 percent will install dry scrubbers; and, an estimated 40 percent will install DSI systems.
Several variations of wet and dry scrubber technologies provide high levels of HCl removal. Units with existing scrubbers will usually meet MATS HCl requirements. Capital cost will likely deter new scrubber purchases, however, except when an EGU requires simultaneous high level SO2 removal to meet other regulatory requirements. Therefore, the majority of units without existing scrubbers will likely install lower cost DSI technology to accomplish any needed HCl removal. Economical and flexible DSI systems can achieve the low HCl removal rates typically needed for PRB coals and the high HCl removal needed for eastern bituminous coals.
Depending on mercury levels in the coal, units may need ACI systems or other Hg removal technologies, and some may need ESP upgrades or the addition of a fabric filter. While some units may use novel technologies such as Hg removal filters, powdered activated carbon (PAC) injection (or possibly low-carbon/non-carbon sorbent injection) in some form has become the industry standard.
Given the above, the use of DSI and ACI for MATS compliance with HCl and Hg limits will be the focus of the remainder of this article.
Full-Scale DSI/ACI Demonstration Testing
Testing is the first step to determine emission rates and ascertain if existing APC equipment can achieve compliance on their own. Attaining an accurate measure of emissions will identify gaps and point to viable control solutions. Each plant has a unique footprint in terms of size, life expectancy, efficiency target and financial position and should consider all variables in their specific evaluation. A majority of EGUs affected by MATS will require some form of DSI and/or ACI technology for Hg and/or HCl removal, and these units must simultaneously meet the emission limits for both, while not increasing PM emissions. For this reason, considerable value can be derived from DSI and ACI demonstration testing, including adequate data to support guarantees for Hg, HCl and PM emissions that provide a sufficient and predictable performance margin for MATS compliance.
A confluence of factors—coal type, combustion conditions, ductwork, temperature and other APC equipment—impact emission levels. DSI system design and sorbent selection requires comprehensive experience in complex chemistry, fluid dynamics, injection location and interaction with existing APC equipment. DSI demonstration testing can not only validate a plant’s compliance strategy, but optimize total system performance. With over 25,000 hours of data from 85 tests, United Conveyor Corporation (UCC) has conducted the industry’s largest number of on-site DSI demonstrations. Plants tested have spanned the range from large, PRB-fired units, to smaller, E. bituminous-fired units. This testing forms the basis for developing strategies for MATS compliance for different coal types and unit configurations.
HCl Removal with DSI
Hydrated lime, trona, and sodium bicarbonate are all effective sorbents than can be injected to achieve HCl removal. Although there can be exceptions, generally the most economical way to reduce HCl emissions is with hydrated lime, unless SO2 removal is also desired. The reason is that less hydrated lime will be used since it is not as reactive with SO2 as trona or sodium bicarbonate, and will therefore not be consumed through reaction with SO2.
If trona is used for HCl removal, performance is better with injection at the air heater inlet as is also shown in Figure 1. In line milling will also significantly reduce trona use as can be seen in Figure 1, which shows that trona use can be reduced by up to 50 percent when milled in-line with UCC’s VIPER® mill.
All three sorbents have been demonstrated to achieve high removals of HCl, if needed, up to 99 percent. However, for units burning eastern bituminous coals that often require very high HCl removals, the corresponding high injection rates can be a concern. This is particularly true for units with ESPs that can sometimes not accommodate high rates of hydrated lime without an opacity or PM emissions increase.
Hg Reduction with ACI
A variety of PAC and other mercury removal sorbents have been used on coal-fired units with success. These can be grouped into the following broad categories, along with their common uses:
- Non-halogenated PAC
- – High Cl E. Bituminous coals when a high percentage of the Hg is oxidized
- – Used in combination with CaBr2 fuel additive for PRB coals; this often is the best performing strategy for PRB coals
- – Some products have other additives to enhance performance
- Non-Carbon/Low Carbon Sorbents
- – Used when want to retain ash sales
- – Use when ESP cannot accommodate carbon
- Brominated PAC
- – Use for both PRB and bituminous coals
- – Use when want to inject alkali sorbent at the air heater inlet and therefore can’t use CaBr2 fuel additive (which will react with the alkali sorbent)
- SO3 Tolerant PAC
- – Use with higher sulfur eastern bituminous coals with moderate SO3 levels (< 10 to 15 ppm) in the flue gas
Numerous ACI tests have shown that 90 percent to 95 percent or more mercury removal is readily achievable when the correct sorbent is injected in the proper location on a given unit. Although a customized strategy must be used for each unit, taking into account a number of factors, high performance can often be grouped into the following for PRB and eastern bituminous coals.
PRB – a combination of a CaBr2 fuel additive and non-halogenated PAC, or a brominated PAC, or a non-carbon/low carbon product injected at the air heater inlet has been shown to give the highest performance.
Eastern bituminous – non-halogenated PAC, or brominated PAC, or a SO3 tolerant PAC injected at the air heater outlet can be used for the best performance. If there are significant levels of SO3 present in the flue gas (> 5 ppm), then either hydrated lime or trona is often injected at the air heater inlet to lower the SO3 levels that interfere with Hg adsorption.
Note that mercury removal strategies for lignite coals will depend on the specific coal and its sulfur level, but will often be more similar to eastern bituminous coals.
Most units are able to meet the MATS limit of 0.03 lb/MMBtu filterable particulate, or the individual metals limits, with their existing PM control devices. However, in some cases it is necessary to either upgrade an ESP or to install a fabric filter. If a fabric filter is installed, this will substantially increase the effectiveness of both DSI and/or ACI needed for HCl/Hg removal. For example, data has shown that hydrated lime injection rates can be as much as 5 times less for a similar-sized unit/similar coal when a fabric filter is used versus an ESP.
Even when relatively high injection rates for DSI/ACI are found to be needed for HCl/Hg reduction to meet MATS limits, experience has shown that this does not result in a PM emissions increase for most units equipped with ESPs. Sodium sorbents usually have neutral to even positive effects on ESP efficiencies as can be seen below in Table 2. PM emissions are often stable or even lower with hydrated lime injection as well, although testing has shown that above a given injection rate, PM emissions can increase. That threshold hydrated lime injection rate, however, can vary greatly between ESPs. UCC has test results showing that hydrated lime injection rates as high as 10,000 lb/hr or more can be used without deteriorating ESP performance in some cases. For other ESPs, the threshold injection rate can be much less.
Interactions Between DSI and ACI
UCC has conducted dozens of combination DSI/ACI tests for MATS compliance and has found that the interactions between the two technologies can be quite complex.
|Dry sorbent injection silos. Photo courtesy of United Conveyor Corp.|
Hydrated lime or sodium injection can have substantial effects on Hg removal and corresponding ACI injection rates, and these effects must be understood before choosing a final MATS compliance strategy. For any given unit and fuel, it is best to conduct testing in order to quantify the exact injection rates needed for both DSI and ACI, but it is still helpful to understand the interactions between DSI and ACI before even designing the test plan. If testing cannot be conducted, understanding the interactions is critical to avoid surprises that could lead to undersized or underperforming DSI/ACI systems.
Although the interactions are complex and unit/fuel specific, common observations from testing are summarized in Table 3 in an attempt to give simplified guidelines.
Each plant will usually specify emission levels that must be met to ensure MATS compliance, and these levels generally include an appropriate margin to ensure performance below the regulatory limits.
Typical guarantees are shown below. In addition, other parameters commonly guaranteed include sorbent consumption rates, utilities (power, compressed air) consumption values, noise levels, and system availability.
Common Emission Guarantees:
|HCL – 0.0018 lb/MMBtu
Hg – 1.0 lb/TBtu
PM – no increase
DSI/ACI Costs for MATS Compliance
As expected, costs for DSI and ACI systems for MATS compliance can vary significantly because sorbent injection rates are specific to unit size and fuel characteristics. However, approximate ranges for capital and operating costs are given in the table below to help give indicative costs only.
Since operating costs are a significant factor for any DSI or ACI system, it is critical that the systems be designed to optimize sorbent use.
Due to the multitude of variables and unique characteristics at each plant, there is no single solution that can be applied to all units to achieve MATS compliance. Multiple factors must be considered when selecting control technologies, as well as the interactions between the technologies. ACI or DSI, or combinations of the two technologies will enable a majority of units in operation to achieve cost-effective MATS compliance.
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