Optimising Dry Sorbent Injection
Published by Lucy Stewardson,
Portland cement (PC) production processes often result in emissions of gaseous pollutants, including sulfur dioxide (SO2), hydrochloric acid (HCl), and mercury (Hg), which are released from the heating of raw materials, as well as the firing of solid fuels inside the kiln. Throughout the US and the world, PC production facilities are required to control their acid gas and Hg emissions, according to limits dictated in their operating permits, consent decrees, and/or other regulatory mandates. In the US, the Clean Air Act has previously driven acid gas emission control requirements; however, other regulations and limits have recently been passed, such as the National Ambient Air Quality Standards, which drives increasingly stringent limits on SO2 emissions. Specific to PC manufacture in the US, the National Emission Standards for Hazardous Air Pollutants provides numerical limits for the emissions of particulate matter, hydrocarbons, dioxins/furans, Hg, and HCl. Dry sorbent injection (DSI) offers a cost-effective solution to comply with these regulatory requirements. During the DSI system design phase, careful attention must be paid to sorbent selection, sorbent application location, and sorbent distribution into the gas stream, to maximise DSI system performance. In this article, various critical aspects of system optimisation will be discussed, for the achievement of the lowest overall cost of compliance.
DSI and activated carbon injection (ACI) are two mature and low capital technologies for acid gas control and vapour-phase Hg reduction, respectively. Both Hg and acid gas control sorbents have proven effective in a variety of industrial plants (i.e. utility, biomass, cement, waste incinerators, etc.). They have been used commercially in Europe and the US for over 20 years. Trial results from the previous HCl test campaign with CEMEX and Lhoist North America were detailed in a previous World Cement article.1
DSI and ACI systems usually consist of storage (either silo storage or bulk bag, i.e. ‘super sack’), after which the product is metered into an air stream and conveyed via dilute-phase into the process gas stream, upstream of a particulate collection device. However, while often considered a low capital solution relative to other acid gas scrubbing technologies, the greatest capital associated with DSI and ACI is the initial equipment procurement and installation. For applications where Hg control is either intermittent or low injection rates are needed, a blended hydrated lime (HL) and powdered activated carbon (PAC) sorbent allows for a single feed system to be used. For example, Lhoist North America’s blended HL-PAC product enables concurrent acid gas and Hg control, using a single sorbent injection system (instead of installing and maintaining two nearly identical systems), to inject the sorbents simultaneously as a pre-blended, homogeneous product. Lhoist North America produces customised enhanced hydrated lime blends (branded Sorbacal® SP and SPS) with brominated PAC. These are produced either in bag or bulk, in 5% PAC (weight by weight) blend increments up to 30%.
While a single, blended sorbent for Hg and acid gas can decrease overall system CAPEX by reducing the need to a single system, careful attention should be paid to optimising the quantity of sorbent required to achieve compliance. DSI system design guidelines are discussed in detail elsewhere.2,3 The focus of this article is to provide sorbent selection and sorbent application guidelines to achieve the most operationally cost-effective DSI programme. To this end, before equipment design and selection phases (or after system commissioning, if this was overlooked during design), plants should consider the following:
- Optimal injection location (which depends on target pollutants).
- Sorbent type.
- Sorbent application/distribution within the gas stream.
Sorbent trials with temporary DSI systems are highly recommended before the system design and selection phases. Alternatively, it is possible to evaluate alternative injection locations after a DSI system has been installed. Sorbent trials should include the measurement of dose-response curves (i.e. parametric) at several different locations within the plant, to identify the most efficient injection strategy.
DSI programme design considerations to minimise operating costs
- Sorbent type: standard hydrated lime? Enhanced hydrated lime? Hydrated lime blended with PAC for simultaneous acid gas and Hg abatement?
- Injection location: sorbent injection at kiln inlet? Gas conditioning tower (GCT) inlet? GCT outlet? Baghouse (BH) inlet? Induced draft fan inlet? The abatement of HCl and SO2 often requires different injection locations.
- Injection lance type and configuration: standard pipe lances? Advanced sorbent distribution technologies? Static mixing lance designs? Dynamic mixing lance designs?
Differences in hydrated lime sorbents
Over the past twenty years, calcium-based sorbents have evolved, driven by the need to improve acid gas capture efficiencies. Realisation of the importance of physical properties, such as particle size distribution, pore volume, and surface area, led to the development of enhanced hydrated lime sorbents (EHLSs) by engineering these properties to create more reactive hydrated lime sorbents. Sorbent physical properties directly impact material handling properties and acid gas removal performance, ultimately dictating annual operating expenditures.
Lhoist’s EHLS products are branded Sorbacal®. The second generation product is Sorbacal® SP and third generation is Sorbacal® SPS. Sorbacal® SPS is a chemically-activated formulation of Sorbacal® SP, specifically designed to provide best-in-class acid gas capture performance. Surface area and pore volume are the key performance drivers for acid gas capture. Sorbent particle size dictates material handling properties and removal efficiencies in electrostatic precipitators (ESPs) and BH filters. Empirical data from the field, as well as laboratory flow testing, has demonstrated that larger median particle diameters (i.e. D50) are recommended for optimum handling.2,6 Specifically, a 32% improvement in flow properties was demonstrated between particles with D50 = 2 µm and particles with D50 = 11 µm.7,8 This is likely due to small particle-sized hydrated lime sorbents being more cohesive than larger particles; small particles can facilitate pluggage in the conveying system.7 Additionally, fine particle-sized hydrated lime can become irreversibly lodged in BH filter bags and bin vents (this is called ‘blinding’), and can result in premature wear and poor bag cleaning efficiencies. Users should refer to their manufacturer’s design information regarding particle size and carefully weigh the impacts of introducing particles outside of the design range. Likewise, ESP particulate capture efficiencies decrease below approximately 6 µm and can result in increased particulate emissions.7,8
The key parameters to consider when choosing sorbents are surface area, pore volume, and median particle size (D50). Surface area and pore volume are the most critical performance drivers for acid gas capture. Larger median particle sizes (≥ 6 µm) have been found to offer the best handling7 and particle capture results.7,8 It is noteworthy that ‘available Ca(OH)2’ impacts acid gas removal performance to a much lesser extent than surface area and pore volume. This is because sorbent utilisation rates (i.e. the fraction of calcium ions consumed in the reaction) are seldom in excess of 30%.
EHLSs provide the following benefits compared to standard hydrated lime:
- Operating cost savings: EHLSs typically reduce sorbent usage by ≥ 30% over standard hydrated lime sorbents. This results in a lower annual spend on sorbents.
- Less impact on the ESP/BH filter: lower sorbent dosage rates will result in less dust loading to particulate capture equipment. Less dust to an ESP may directly impact particulate collection efficiency, and for a BH filter this could impact bag cleaning cycle frequency. Particle sizes play a critical role in ESP/BH operational efficiencies. Respective equipment manufacturers should be consulted on particle size guidelines.
- Fuel and raw material flexibility: if a lower cost fuel or raw material becomes available but results in increased acid gas emissions, then an EHLS can provide additional flexibility. This is because it has the ability to achieve higher acid gas removal efficiencies than standard hydrated limes, without having to modify the existing DSI system.
- Increased storage silo capacity: lower sorbent consumption using EHLSs results in more days of available storage in a fixed silo volume. Hence, reducing sorbent consumption by 50% equates to doubling the silo storage capacity.
- One DSI system for acid gas and Hg control: EHLS blended with PAC is available and precludes the need for two separate systems.
These benefits are a result of the EHLS’s engineered and improved physical properties, which are designed to enhance acid gas reactivity.
Choosing the most cost-effective sorbent
The two most critical components when implementing a successful compliance strategy are the following:
- Proper sorbent selection.
- Sorbent distribution in the gas stream.
Assuming that the DSI system is properly designed, installed, and operated,2,6 choosing the most effective sorbent, injection location, and injection grid design are the next critical steps to optimising system cost effectiveness. Although EHLSs are typically more costly than standard sorbents on a delivered basis (i.e. US$/t), higher sorbent efficiencies often result in an overall lower total cost of ownership. For example, an EHLS may cost 30% more than standard hydrated lime; however, EHLS usage rates are often between 30% and 50% lower than those of standard hydrated lime, resulting in net cost savings.
Requesting proposals from sorbent suppliers
Care should be taken when preparing sorbent requests for proposals (RFPs), since the quality specification outlined in the RFP could inadvertently result in the selection of an unsuitable or single-source supplier. Potential sorbent suppliers should also be identified and communicated with, to better understand the most critical sorbent attributes, as well as the chemical and physical properties of the sorbents they offer. For example, not understanding that sorbent purity (i.e. ‘available Ca(OH)2’) is less critical than surface area and pore volume, or that large particles are superior to smaller particles may result to choosing a single supplier, which may not be the most cost-effective choice.
Sorbents blended with PAC
For simultaneous Hg and acid gas abatement, Lhoist’s Sorbacal® acid gas sorbents can be blended with PAC. The simultaneous capture of Hg and acid gases offers the advantage of requiring only one feed system for installation and operation. For applications in which Hg control is either intermittent (e.g. when using certain raw materials) or is only needed for low injection rates, a blended product can be advantageous. Lhoist’s blended product enables concurrent acid gas and Hg control using a single sorbent injection system (instead of installing and maintaining two nearly identical pieces of equipment), injecting the sorbents simultaneously as a pre-blended homogeneous product. The relative quantity of PAC blended with Sorbacal® can be custom-tailored between 5% and 30% to meet specific needs.
Injection location and lance configuration
Another critical aspect of the DSI process is choosing the best injection location and the specific design of the injection grid. Both the injection location and the grid design directly impact how the sorbent is introduced into the gas stream. Sorbent distribution and coverage in the gas stream dictate pollutant removal efficiencies and resulting operating costs.
A key question is where to locate the injector(s). The target pollutant(s) typically guide where to locate injection lances; however, it is recommended that each facility performs site-specific testing, especially for cement applications. It is recommended that several injection locations are evaluated during a trial with a temporary DSI system. For example, SO2 capture by hydrated lime is typically favoured with injection at higher temperatures, whereas HCl capture tends to be favoured at cooler temperatures. Without a trial to determine the best injection location, incorrect injector location selection can result in higher usage rates and annual costs.
Once the injection location is determined, the injection grid design is the next key performance driver. Injection grid designs can be as simple as a single injection lance or as complicated as a multi-lance design with various penetration depths. Over the past few years, new injection technologies have emerged, significantly improving sorbent distribution within the gas stream and reducing sorbent consumption. These systems can result in operating cost savings with a relatively quick return on investment. Computational fluid dynamics modelling is a beneficial tool that can be used to guide the injection grid design, in order to optimise sorbent distribution. In-duct cameras can also be employed to visually inspect sorbent distribution following system installation, and to corroborate good distribution by identifying distribution inefficiencies.
Sorbent selection, proper location of injectors, and injector grid/lance design are the most critical parameters that determine overall DSI system efficiency. Over the past two decades, EHLSs have been specifically optimised for acid gas abatement applications. In the past, sorbent selection was driven by geologically-dictated hydrated lime purity (i.e. available Ca(OH)2). Today, sorbent purity has little impact on performance, and sorbent performance is primarily driven by porosity (i.e. surface area and pore volume). Additionally, blended sorbents (PAC and EHLS in one sorbent) can reduce system costs because only one injection system is needed. EHLS particle sizes have been optimised for superior material handling and particulate capture by BH filters and/or precipitators (i.e. larger particles are better). Locating injectors in a cement plant should be driven by data from trials with temporary DSI systems. Once injectors are located, to maximise sorbent efficiency injection grid design should be guided by computational fluid dynamics modelling. Following system installation, in-duct cameras can be used to evaluate and tune sorbent injection grids to ensure proper distribution and coverage. Many of these critical parameters are easily evaluated during a short product trial and can result in significant operating cost savings in the long run.
- CIRO, W. and SEWELL, M., ‘HCl Control for MACT Compliance’, World Cement, April 2014, pp. 50 – 53.
- HUNT, G. and SEWELL, M., ‘Optimising Dry Sorbent Injection Technology’, World Cement, April 2015.
- FILIPPELLI, G., ‘Living with Your DSI System’, CIBO Boiler Operations, Maintenance and Performance Conference, May 2016.
- FOO, R., DICKERMAN, J., HUNT, J., JOHNSON, L., and HEISZWOLF, J.J., ‘ESP Compatible Calcium Sorbent for SO2 Capture at Great River Energy’s Stanton Station’, MEGA Symposium Conference Proceedings, August 2016.
- HEISZWOLF, J.J., HUNT, G., and SEWELL, M. ‘Enhanced Hydrated Lime – A Simple Solution for Acid Gas Compliance’, IEEE-IAS/PCA Cement Industry Conference, May 2017.
- ‘Dry Sorbent Injection for Acid Gas Control: Process Chemistry, Waste Disposal and Plant Operational Impacts’, Institute of Clean Air Companies (July 2016).
- WOLF, D., ‘Results of Hydrated Lime DSI Field Trial Tests for HCl Removal from Industrial Coal Fired Boilers’, CIBO Industrial Emissions Control Technology XII Conference, August 2014.
- GUOQUAN, Z. and ZHIBIN, Z., ‘Investigations of the Collection Efficiency of an Electrostatic Precipitator with Turbulent Effects’, Aerosol Science and Technology, Vol. 20, No. 2 (1994), pp. 169 – 176.
- CHEN, T. M., LIN, G. Y., and TSAI, C. J. A., ‘Modified Deutsch-Anderson Equation for Predicting the Nanoparticle Collection Efficiency of Electrostatic Precipitators’, Aerosol Air Quality Research, Vol. 12, No. 5 (2012), pp. 697 – 706.
About the authors
Dr Ian Saratovsky is Director of Lhoist North America’s Flue Gas Treatment group. He holds a PhD in inorganic chemistry and environmental engineering from Northwestern University and was a Fellow at the University of Oxford before entering the air pollution control industry. Dr Saratovsky has 12 years of experience in air pollution control, wastewater treatment, and industrial process optimisation.
Manager of Flue Gas Treatment Applications with Lhoist North America, Martin Dillon holds a masters degree in engineering from Old Dominion University and is a registered Professional Engineer in Colorado. He has over 11 years of experience in the air pollution control industry and has worked on numerous multi-pollutant control demonstration projects.
Gerald Hunt holds bachelors and masters degrees in chemical engineering from the State University of New York at Buffalo. He is currently a Manager of Flue Gas Treatment Applications with Lhoist North America. He has over a decade of experience in the air pollution control industry, including performing field trials, proposal management, and process engineering in dry sorbent injection and wet flue gas desulfurisation technologies.
Read the article online at: https://www.worldcement.com/special-reports/06032019/optimising-dry-sorbent-injection/
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