The Environmental Protection Agency (EPA) has been working for many years to develop and execute stricter standards for energy generating units’ (EGUs) pollution mitigation. Standards have finally been established and now EGUs need to find solutions that meet the mitigation standards, as well as make sense for cost effective and efficient running of their plants. Dry sorbent injection (DSI) systems are showing proven ability to meet the new EPA standards and should be considered as a means to come into compliance.
A Summary of Standards
The Clean Air Act, greatly expanded in the early 1990s, gave the EPA greater authority to enact and implement pollution mitigation standards throughout the United States. The EPA works closely with states, Native American tribes, local governments, and businesses to develop standards that are achievable and can be maintained over the long run.
These standards, once referred to as the application of maximum achievable control technology (MACT), are now finalized as Mercury and Air Toxics Standards (MATS). The final rule was published on December 21, 2011. EGUs have three years to comply with the final standards, with a one year extension possible for final installation of the new technology.
MATS applies to EGUs larger than 25 megawatts (MW) that burn coal or oil for the purpose of generating electricity for sale and distribution through the national electric grid to the public. It establishes numerical emission limits for mercury, particulate matter (a surrogate for toxic non-mercury metals), and hydrochloric acid (HCl) (a surrogate for all toxic acid gases). It also creates alternative numeric emission standards, including SO2 as an alternative to HCl.
The focus of this article is HCl and SO2. The specific limit for HCl is 0.0020 lbs/MMBtu. For SO2, the limit is 0.20 lbs/MMBtu. Dry Sorbent Injection (DSI) has been proven to meet these standards in real world tests and permanent installations. Before we examine those test results, we’ll discuss the basics of DSI.
Basics of Dry Sorbent Injection
SI uses a pneumatic conveying system to inject dry sorbent materials into system ductwork in a controlled manner. The pollutants in the plant emissions interact with the sorbent material and become inert or non-polluting. Sorbents include hydrated lime, powdered activated carbon, sodium bicarbonate (SBC) and trona. The main pollutants mitigated with DSI include SO2, SO3, Hg, and HCl emissions.
DSI systems are designed to meet each of the power plants specific needs. The precise injection rate needed can be estimated based on unit specification, coal properties, duct work, and the desired level of removal, in conjunction with field data. Silos are sized to that injection rate and the number of days of storage a customer needs. Mills can be added, to refine the sorbent material to the most effective size to capture the emissions the plant is mitigating. Milling of the sorbent can greatly increase the removal efficiency of the pollution. And that means less sorbent is needed, decreasing costs. For example, if 10,000 lbs./hour of a rough sorbent were typically injected, only 7,000 lbs./hour of that sorbent in a milled state might be needed. It’s important that your DSI system provider looks at these types of unique circumstances to ensure a cost efficient system is designed and installed.
The use of DSI technology as an industry standard for air pollutants began with SO3 systems to eliminate the “blue plume” that EGUs experienced when running selective catalytic reductions (SCRs). Today, DSI is an industry standard for SO3. Now, DSI is being applied to other pollutants such as HCL and SO2.
The type of mill used would depend on the material to be refined and the precision that is required in the final milled sorbent. Pin mills are fairly common and have typically been used for Trona applications. But Nol-Tec Systems is using proven air classify milling (ACM) technology for Trona and SBC. A single ACM unit can typically achieve up to 4 TPH. It utilizes an exhauster fan to pull a vacuum on the mill and move the sorbent material from the mill to a vacuum receiver, where it is then discharged into the convey line, which conveys the sorbent to the system ductwork. Nol-Tec is able to guarantee a d90 < 25 micron size on the SBC. This ensures a more efficient mitigation of the pollutant. Another advantage of the ACM technology is a tighter distribution of particle size, since the material is classified prior to exiting the mill. With a pin mill style technology, the material has a single pass-through, giving a larger distribution of particle size.
Why use DSI for HCl mitigation?
There are a number of reasons an EGU would want to consider utilizing DSI to meet MATS standards. One of the most attractive reasons is that DSI can be a low capital cost solution. It allows EGUs to supplement their current mitigation systems affordably – or to use DSI as an interim solution until larger, more costly solutions can be evaluated and installed, if needed. Additionally, DSI systems can be quickly installed and started up. Often, the time frame from purchase order to commissioning is less than one year. The time frame variability is based largely on the market and amount of projects being executed by utilities. A relatively quick project execution for DSI systems can allow EGUs to quickly meet the EPA standards, even while they are seeking funding or the necessity of some additional type of solution.
DSI has been demonstrated to be effective for HCl removal. As we will see in the example results that follow, Nol-Tec Systems has proven results from real world testing of our own DSI systems.
Good candidates for DSI mitigation
There are a number of contexts that must be considered, when determining whether DSI will be effective in a power plant’s mitigation process. An EGU with an existing SO2 wet scrubber may find that the scrubber alone is insufficient to meet the 99+% HCl removal requirement. DSI could supplement the scrubber and bring the plant into compliance.
In coal-burning units, the sulfur content of the coal and the boiler size must be considered. The amount of sulfur in the coal sulfur impacts the SO2 levels. And SO2 levels impact HCl removal with sodium sorbents. The combination of sulfur content in the coal and the boiler size (amount of coal being burned) sets the amount of SO2 in the flue gas. These parameters must be considered when looking at DSI for HCl. In the case of hydrated lime, only a small amount of reagent reacts with SO2 so the sulfur content is less of an issue.
Residence time of the emissions within the system must be considered. The longer the residence time, the more likely the air pollutants will react with the sorbent. Residence time is dependent on flow rate of the emissions and the duct configuration of the system.
The particulate control device (PCD) may also impact the decision to use DSI. A pulse jet fabric filter (PJFF) PCD allows for high residence time because of the filter cake of sorbent on the bags. This in turn increases the removal efficiency of the air pollutant. This allows for lesser amounts of sorbent to be used to achieve the desired removal of pollutant. This factor may help promote DSI technology.
Lastly, each EGU is different in how it operates in accordance with “the grid”. Some EGUs operate in a base load manner. These units desire low operating costs so they may be less apt to choose DSI, because of the ongoing cost of the sorbent. If an EGU operates in a peak load fashion, then it suffices to use a low capital cost solution and deal with the sorbent costs. Simply turn on the DSI when HCl levels rise to bring them back into compliance.
With these considerations in mind, there are a number of good candidates for DSI pollutant mitigation. Smaller, coal-fired units, or any unit that uses low sulfur coal, have achieved high success rates with DSI mitigation for HCl and SO2. We have found that all units are good candidates for SO3 mitigation.
Testing DSI in Your System
Smaller, coal-fired units, or any unit that uses low sulfur coal, have achieved high success rates with DSI mitigation for HCl and SO2. We have found that all units are good candidates for SO3 mitigation.
It is important to work with a DSI supplier who can provide portable test systems that can be incorporated into your own EGU system. Such on-site testing can give you confidence that DSI will ensure your system will meet EPA standards in a timely and cost-effective manner.
An Example of Test Results
Nol-Tec Systems has performed a number of on-site tests for a variety of EGUs. In this example, we will examine a bituminous coal-fired unit with a boiler that is smaller than 150 MW, utilizing an economizer, air pre-heater, and electrostatic precipitator (ESP). The sulfur content in the coal is about 1.2%. Residence time of the emissions in the system is under one second, which is less than ideal.
This unit is a difficult application, for several reasons. The sulfur levels in the bituminous coal impact the mitigation. The low residence time and low mix rate the system has also present challenges. And the PCD is an ESP which does not allow further mitigation, unlike the PJFF which utilizes a filter cake for further mitigation after it reaches the PCD.
The primary objective of this test was to determine whether the new MATS HCl limit could be achieved on this boiler. With that objective and the system’s challenges in mind, Nol-Tec engineers designed a test system to meet that goal, using CFD modeling services. They determined the best location for the injections of sorbent to be made (at the economizer outlet at 600-700°F). Sorbent amounts were calculated and two types were tested. SBC and trona injection were each done for two days out of each week.
Trona and SBC proved to mitigate HCl to MATS limit of 0.002 lbs/mmBtu. This chart, showing percent of HCl removed in relation to the amount of sorbent injected, is “Total Nominal Stoichiometric Ratio (NSR)” meaning both SO2 and HCl are taken into account when calculating the NSR value. On the y-axis, we have the percent of HCl removed (please note that the y-axis starts at 86%.) The level we are looking for is greater than 98%, which is shown with the solid red line. At medium load, we achieved the 98% removal more consistently, using lower injection rates. At higher load, it was more difficult. Though there may be several reasons, one may include residence time as the main contributor.
DSI Technology Works
As we have seen with the results above, and with additional testing that Nol-Tec has done, DSI technology can indeed work to bring HCl levels into MATS compliance. It is important to note that many issues factor into a decision about using DSI for mitigation. It is important to select your mitigation partner carefully, to ensure they have the expertise or equipment to fully explore and test all the variables that can impact your decision. With due consideration to all factors, DSI technology can bring EGUs into compliance with MATS regulations in an efficient and cost effective manner.
Article as appeared in APC, August 2012.
Nol-Tec is a supplier of pneumatic conveying systems and dry sorbent injection technology. For more information about Nol-Tec’s testing process, please contact Sean Dooley at SeanDooley@nol-tec.com or 651.780.8600.Download Article (PDF)