Weighing, Conveying, and Injecting Sorbents for Mercury Control
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An Evolving Market
The Environmental Protection Agency (EPA) standards for electrical generating units (EGUs) and industrial boilers (IBs) pollution mitigation are continuing to move forward, now also covering mercury and various acid gas pollutants. Dry sorbent injection (DSI) has worked to bring acid gas pollutants into compliance. But with the changing regulations, attention has focused on the sorbents to capture mercury. Due to the stringent regulation on mercury and the many variables surrounding in-duct capture technology, the mercury and PAC market is evolving. It is not what it used to be. But activated carbon injection (ACI) continues to show proven results in meeting the increasing standards for mercury (Hg) compliance. Years ago, the solution was first generation PACs, short convey distances, low injection rates, and temperamental yet cheap eductor equipment. And now, the stakes are higher – MATS compliance.
The market for Hg mitigation is changing rapidly. The sorbents are not just first generation PACs. New generation PAC products, non-Carbon sorbents (e.g. amended silicates), and a combination of the two are being developed in finer particle sizes with uniquely designed features so as to increase the mercury removal rates to meet compliance. The finer sizes and the unique features impact the pneumatic conveying properties of the material. Injection rates are higher than previous years in order to meet the newest standards. There is also an increasing demand for longer conveying distances, so as to place multiple silos in one offloading location. These various aspects increase the convey line back pressure above the eductor limit and require more energy to convey. Therefore, the injection system must be designed properly to convey various sorbents, at any injection rate wanted, for whatever distance is needed.
Balancing the competing demands to determine the best sorbent, and deciding the best place to inject the sorbent can be overwhelming. Ideally, EGUs and IBs need a system that allows them to inject any sorbent they want, at whatever rate they want, through a convey routing system that they choose. A flexible convey system that is also reliable is critical to the success of meeting MATS.
Eductor technology is limited and can’t meet these needs for the Hg mitigation market. With a 3 PSI maximum convey pressure, an eductor-fed system will not allow for the increased material amounts and distances required for appropriate residence times to meet regulations.
Some systems have moved to using a zero clearance airlock. This technology doesn’t greatly increase the energy in the system, though it will produce up to 6 PSI of convey energy. However, this level is not entirely reliable at the top end of its range. Also, because of the size of PAC particles, for example, airlock leakage is still a substantial problem with a rotary valve. Effectiveness of the sorbent and system is negatively impacted.
By listening to customer needs, Nol-Tec Systems has designed and innovated a convey system able to meet the need to convey higher rates, longer distances, and different sorbent types. This is a continuous transport system for the conveying of Hg sorbents. The system features dual hoppers, no pressure differential across the rotary valve between the weigh hoppers and convey line, and rotary valves that function as metering devices, rather than an airlock feeding material into a pressure zone. This design provides the capability to meter material into pressure zones up to 12 PSI and addresses the need for longer distance conveying of complex materials. With this new approach, Nol-Tec started implementing this proven technology during on-site demonstrations by conveying sorbents for mercury mitigation.
2013 – A Year of Testing
Nol-Tec invested a lot of time in MATS testing in 2013. Utilizing a large fleet of portable, self-contained testing equipment, this testing took place on site of both power plants and industrial boilers, in real-world conditions. This equipment included silos, bulk-bag unloaders, pin mill and air classifying mills, and eductor and non-eductor systems for conveying.
Most tests were for mercury compliance with MATS standards. However, EGUs and IBs also wanted to control SO3 emissions that are not directly included in MATS, but necessary nonetheless for enhanced Hg removal. Many tests included injecting both alkaline products and Hg sorbents.
An early spring 2013 trial looked at enhancing PAC performance in conjunction with SO3 control. The two test units were 200-300 MW each. HCl control was done via an existing wet FGD. (If that hadn’t already been in place, the test would have included a DSI process for HCl mitigation.) The goal was to reach 1.2 lb/Tbtu Hg emission, then optimize from there. The challenges were a short residence time between lime and PAC (less than 1 second) and between ACI and ESP (also less than 1 second).
Figure 1 – SO3 control to enhance PAC performance
Nol-Tec injected PAC at a consistent rate, while increasing the hydrated lime for SO3 removal. Testing showed that increasing the hydrated lime rate allowed PAC to remove Hg to below MATS limits, without increasing PAC usage. Many of the tests utilized our proprietary Gen 3 non-eductor system.
On one initial test site, Nol-Tec injected PAC at higher rates than standard eductor technology allowed for. As shown in Figure 2, non-eductor technology allowed for conveying pressures that are beyond an eductor’s capability.
Figure 2 – Test site A for Gen 3 technology
The increased injection rate required more energy to convey the material into the duct, as was expected. The non-eductor system handled that increased rate without failure, which an eductor could not.
One reality of conveying systems is that back pressures will vary when a run produces fluctuating pressures. When an eductor runs at the borderline of its capabilities at higher pressures, it will be impacted by these upset conditions. Another test site shows this. Using our non-eductor technology, conveying was not affected by the upset conditions.
Figure 3 – Test site B for Gen 3 non-eductor technology
With a non-eductor system, process failures due to upset pressure conditions were minimal. Nol-Tec’s proprietary technology provided the rates and results needed, despite the upset pressure conditions.
At another site, Nol-Tec was able to preview how differing PAC formulations impacted the pneumatic conveying system by monitoring the back pressure in the convey line. In figure 4 for test site C, the bottom data set circled displays how differing PACs convey differently. The top data set circled displays how increasing the convey distance greatly impacts the back pressure. This may be obvious, but designing a system to handle this back pressure is not. Nol-Tec’s non-eductor technology can handle the longer distances.
Figure 4 – Test site C for Gen-3 non-eductor technology
Note for Figure 4: The top data set is conveying PAC to a further location than the bottom data set.
On-site testing can be invaluable for an EGU or IB who is making decisions about most effective sorbents and which has the highest value. In this case, Nol-Tec was able to run all the tests without having to move equipment, which would have introduced costly delays to the process.
One customer had previously tested with eductor technology. They needed to convey sorbent into three ducts. They learned quickly that their convey route and distance was too much for an eductor system. The result: the eductor system could not distribute sorbent into all three ducts evenly. The sorbent was all being conveyed into a single duct, as the eductor system didn’t have the energy to equally distribute materials through the three ducts available to them. They requested non-eductor testing, to find whether these problems could be fully addressed with this new technology.
The test was a success. Non-eductor conveying allowed for accurate conveyance to the three ducts and provided enough energy to allow for resistive splitting and thus equal distribution into those ducts. Equal distribution and dispersion was achieved through each duct. The end result of the testing was that the boiler unit was brought into compliance.
The Use of DSI Systems for Mercury Mitigation
Dry sorbent injection (DSI) has historically been used for SO3 removal for the infamous “blue plume”. As of late, DSI has been utilized for HCl removal for MATS compliance and also SO2 removal in specialized cases. This DSI technology utilizes a pneumatic conveying system to inject alkaline sorbent materials into the system ductwork of an EGU in a controlled manner. The pollutants in the EGU’s emissions interact with the sorbent material and become inert or non-polluting. DSI sorbents typically include hydrated lime, sodium bicarbonate (SBC), and trona.
These EGUs can be as large as a major electrical provider and as small as an industrial boiler operation for a local manufacturing plant. DSI can be designed to meet the specific needs of each type of EGU, taking into consideration unit specifications, coal properties, injection rates, and pollutant removal levels, to name a few of the variables.
Now, DSI is being used to remove SO3 from the flue gas stream prior to activated carbon injection so as to improve Hg removal. Removing SO3 allows Hg sorbents to capture the mercury pollutant by freeing the carbon to capture the mercury. Hg sorbent designers are creating materials to capture mercury even in the presence of SO3. Again, new generation sorbents are more effective than previous generation sorbents and they will require the right conveying system to inject them into the duct.
Non-Eductor Systems Work for MATS Compliance
Nol-Tec testing throughout 2013 has shown that ACI systems can work – and work well – for MATS compliance. It’s important, however, that the system be properly designed for an EGU or IB’s specific needs. Proper material distribution within the ductwork is an essential aspect of ensuring ACI meets the mercury mitigation standards. Flexible injection rates will help ensure successful mitigation, and flexibility in sorbent types will allow the ACI system to work for more than Hg mitigation.
With a variety of delivery methods – eductor, zero clearance airlocks, and continuous transport – ACI systems offer great flexibility to meet MATS requirements. However, eductor and airlock systems can work, but are limited by the relatively low convey pressure each can muster. Short convey distances, low material injection rates, simple Gen 1 PACs, and single duct dispersal are the rigid parameters of these types of systems.
Non-eductor technology is proving to provide striking benefits for MATS compliance. With convey pressure potential of up to 12 PSI, convey distance is no longer a limiting factor. This allows for the needed residence times, as well as greater flexibility in plant layout. This higher pressure also enables the use of resistive splitter technology. Reliable material dispersal is ensured, as the system can be designed with more ducts that can be consistently filled with sorbent.
Non-eductor design provides great flexibility, in injection rates and in sorbent usage. Non-eductors can provide rates as low as 10 pph and as high 5,000+ pph. With a broad range of injection rates, an EGU or IB can select less expensive sorbents or more effective sorbents that may require more material, depending on their operational budget. As regulations continue to change, flexibility in the amount and types of sorbents that can be used can have a huge impact on capital and maintenance costs.
As has been found with DSI in the past, it is important to note that many issues factor into a decision about using ACI 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, Hg sorbent injection technology can bring EGUs into compliance with MATS regulations in an efficient and cost effective manner.
Michael Thiel is the Technical Services Manager of Nol-Tec Systems, Inc. in Lino Lakes, MN. Nol-Tec is a supplier of pneumatic conveying systems and dry sorbent injection technology known as Sorb-N-Ject®. For more information about Nol-Tec’s testing process, please contact Michael via the Contact Us form.