Original Research published: 21 March 2017 doi: 10.3389/fenrg.2017.00003
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Herek L. Clack* Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI, USA
Edited by: Kalpit V. Shah, RMIT University, Australia Reviewed by: Milinkumar Shah, Curtin University, Australia Laltu Chandra, Indian Institute of Technology Jodhpur, India *Correspondence: Herek L. Clack
[email protected] Specialty section: This article was submitted to Advanced Fossil Fuel Technologies, a section of the journal Frontiers in Energy Research Received: 05 September 2016 Accepted: 16 February 2017 Published: 21 March 2017 Citation: Clack HL (2017) Numerical Simulation of Simultaneous Electrostatic Precipitation and Trace Gas Adsorption: Electrohydrodynamic Effects. Front. Energy Res. 5:3. doi: 10.3389/fenrg.2017.00003
Electrostatic precipitators (ESPs) are now being tasked with simultaneously removing particulate matter (PM) and trace gas-phase pollutants such as mercury released during coal combustion. This represents a significant expansion of their original operational mission, one which is not captured by decades old quasi-1-D analytical expressions developed from first principles for predicting PM removal alone. At the same time, technological advances in ESP power supplies have led to steady increases over the years in the applied voltage achievable in new or refurbished ESPs. In light of these industry trends, the present study extends our previous study to examine the multiphase flow phenomena that may occur during such ESP operations, specifically the effects of electrohydrodynamic (EHD) fluid flow phenomena that can emerge when electrical current densities are high and/or fluid velocities are low. The results show good agreement at low current densities between the present numerical simulation results and ESP performance predictions obtained from classical analytical expressions, with increasing divergence in predicted performance at higher current densities. Under the influence of EHD phenomena, the acceleration of the fluid by electric body forces effectively increases average fluid velocities through the ESP channel with a commiserate reduction in PM removal efficiency. The impact on trace gas-phase pollutant removal is mixed, with EHD phenomena found to variously promote or inhibit gas-phase pollutant removal. Keywords: electrohydrodynamics, electrostatic precipitator, ESP, particulate matter, mercury, activated carbon
INTRODUCTION Over decades of use in controlling particulate emissions, electrostatic precipitators (ESPs) have been proven to be robust devices with electronic control systems that often operate virtually independently with minimal human intervention. Especially for well-defined processes for which particulate emissions vary little over time, such as stationary power generation, well-designed ESPs have operated reliably for decades after initial installation, typically undergoing periodic repair and maintenance or receiving power supply upgrades. When significant changes in particulate control performance have been needed, often due to changes in particle electrical properties, flue gas conditioning additives such as sulfur trioxide, and less commonly ammonia, have been
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injected upstream of ESPs (Shanthakumar et al., 2008), altering particle electrical properties and improving the particulate matter (PM) removal performance of the device. Operating temperature has long been known to affect ESP performance, and the perceived advantages of high-temperature electrostatic largely drove the installation of a number of so-called hot-side ESPs (Calvert and Englund, 1984), and researchers continue to study how elevated gas temperatures may be used to mitigate ESP performance fluctuations due to variable coal quality (Noda and Makino, 2010). The development of combustion systems capable of postcombustion carbon capture has also driven studies of ESP operation during oxy-fueled combustion (Han et al., 2010; Kim et al., 2014). Increasingly, ESPs have been the focus not for PM emissions but rather for simultaneously controlling the emissions of a second pollutant. Upstream injection of lime, limestone, or trona to neutralize acid gases and powdered mercury sorbents such as powdered activated carbon (PAC) to adsorb toxic trace metals such as mercury has grown in the United States as new and more stringent regulations take effect. Wet and dry ESPs operate on the same basic particle collection principles, but differ in how they handle the bulk collected dust. Dry ESPs periodically mechanically or acoustically dislodge the bulk collected dust (dustcake) from the collection electrodes, causing it to fall by gravity into collection hoppers below. Wet ESPs direct a flow of water, either continuously or intermittently, over the surfaces of the collection electrodes, thereby capturing particles in the falling liquid sheet that are extracted from the liquid in a separate process. Because of the added complexity of the separate liquid handling system and the liquid waste stream produced by wet ESPs, their main use has been in controlling liquid mists or sticky particulates, explosive aerosol suspensions, and as mist recovery processes in sulfuric acid production (Cooper and Alley, 2011; Seetharama et al., 2013). The use of ESPs for two-way (or even three-way) air pollutant emissions control is advancing. Compared to dry ESPs, the lower operating temperatures and elimination of particle resuspension into the flue gas during mechanical rapping offered by wet ESPs increase the removal efficiencies of fine particles (PM2.5) (Cooper and Alley, 2011). Because of their high surface area per unit mass, these smallest particles are also most likely to harbor compounds condensed from the flue gas such as heavy metals (Seames and Wendt, 2000) including mercury (Reynolds, 2004; Seetharama et al., 2013), providing modest, incidental removal of such semivolatile pollutants. In addition to higher PM2.5 removal efficiency, wet ESPs are also superior to dry ESPs at removing liquid particles, droplets, and mists (Cooper and Alley, 2011). Although flue gas from coal combustion nominally contains no condensate, the moist, low-temperature operating environment within wet ESPs promotes condensation of acid gases such as sulfuric acid as temperatures approach the acid dew point of the flue gas. In this way, wet ESPs act to reduce acid gas emissions along with PM emissions. Dry ESPs used for two- or three-way air pollutant emissions control have made advances largely through full-scale field tests and extrapolating from decades of ESP operating experience. Given the dominance of dry ESPs in use at electric utilities
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in the United States and elsewhere, the remaining discussion focuses on this primary population of devices; references to ESPs beyond this point imply dry ESPs. The reliance on ESP PM removal principles to interpret trace gaseous pollutant emissions reductions observed from ESP field tests, however, can lead to erroneous inferences of causal relationships. For example, ESP PM removal efficiency is understood to increase with increasing collection electrode area; however, mass transfer limitations prevent the accumulated dustcake on collection electrodes from contributing significantly to the overall reduction in trace pollutant concentrations across an ESP, thereby greatly weakening the influence of collection electrode area on trace pollutant removal. Suspended adsorbent particles in the flue gas contribute to a greater degree to trace pollutant removal in ESPs of conventional design. Even the implicit assumption that multiple mechanisms of trace pollutant adsorption can be dealt with in an additive fashion is questionable. Results from a recent numerical simulation of trace mercury adsorption by suspended aerosols and the accumulated dustcake on the collection electrodes of an ESP channel (Clack, 2015) indicate that the mercury-lean boundary layers that develop over the adsorbent dustcake suppress adsorption by suspended aerosols as they drift toward the collection electrodes. Because all aerosols must traverse these boundary layers before they are collected on the plate electrodes, this suppression has an outsized impact on in-flight adsorption. In the extreme case of collection of electrodes covered by a highly adsorbent dustcake layer, the deeply mercury-lean concentration boundary layers that are produced actually result in lower mercury removal by the combined in-flight and wall-bounded mechanisms that would be predicted by summing the separate contributions of the two adsorption mechanisms. Nevertheless, anecdotal evidence from full-scale field tests suggests that larger ESPs as measured by space velocity broadly tend to exhibit higher levels of mercury removal. While increased space velocity can reflect greater collection electrode, and thus dustcake, surface area, it can also reflect reduced flue gas velocities. Reduced gas velocity increases the potential for electrohydrodynamic (EHD) phenomena to occur within ESP channels by reducing fluid inertial forces compared to EHD forces, the ratio of which is represented by EHD number or Masuda number and which can be used to predict the onset of prominent EHD phenomena and flow features (IEEE-DEIS-EHD Technical Committee, 2003). Study of EHD phenomena in principle began in the 1980s (Leonard et al., 1983; Yamamoto and Sparks, 1986; Kallio and Stock, 1992) with detailed simulations (Zhao and Adamiak, 2008) and experimental measurements (Zouzou et al., 2011) arising more recently that focus on conventional ESPs and PM control. This study seeks to shed light on how EHD phenomena might affect, either incidentally or intentionally, the performance of ESPs operated as multipollutant collectors that simultaneously remove PM and trace toxic metals from combustion flue gas. Considerations of EHD effects have particular relevance given operating and design conventions that call for increases in ESPspecific collection area (SCA) and/or power supply upgrades to improve PM collection efficiency; both actions have the effect
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constrains Φ(x,y) such that 𝛻qi(x,y) and 𝛻Φ(x,y) satisfy Eqs 2 and 3 is determined through repeated, iterative solutions of the electric field, as described previously (Clack, 2015). The discharge corona constitutes a negligible fraction of the computational domain, and thus, it is not numerically simulated.
of increasing the relative influence of electric body forces and the relative extent in the flow field of EHD effects.
MATERIALS AND METHODS The methodology of the numerical simulation has been presented in detail previously (Clack, 2015) and thus will be summarized here. Solved are the steady conservation equations for the electric and fluid fields (including EHD forces); charged-induced particle motion; and associated convective gas-particle mass transfer rates. There are two 2-D computational domains representing partial segments of a complete ESP channel: a 3-wire segment measuring 2 m × 0.3 m (L × W) or a 9-wire segment measuring 5 m × 0.3 m, each containing 1 mm diameter wire discharge electrodes spaced 0.5 m apart. Figure 1 presents a schematic view of the 3-wire segment; the 9-wire segment is longer but of similar design to the 3-wire segment, both having the same spacing between the wire discharge electrodes and the same distance separating the inlet from the first electrode and the last electrode from the outlet.
Fluid/Continuous Phase
The incompressible Reynolds-averaged Navier–Stokes equations (Eqs 4 and 5) modified to include a term representing the electric body force (qi∇Φ) are as follows: ρ ∇ • U = 0 (4)
(
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ρ U • ∇U = −∇P + µ∇2U + qi∇Φ (5) In Eqs 4 and 5, U is local gas velocity vector [m/s], P is local pressure [Pa], ρ is gas density [kg/m3], μ is the gas dynamic viscosity [Pa − s], qi is local space charge density [C/m3], and Φ is local electric potential [V]. The material property database that is native in COMSOL includes the temperature dependency of fluid properties and the native k − ε turbulence model; its default parameters are used without modification.
Electric Field
The electric field, assumed to depend only on the continuous phase fluid properties, derives from Eq. 1 for the electric potential: −∇Φ = E (1) where Φ is voltage [V] and E is the electric field vector [V/m]. The solution to Eq. 1 must also satisfy Poisson’s equation (Eq. 2) and current continuity (Eq. 3) assuming Fickian ion diffusion is neglected compared to fluid and electric field-driven ion advection: q ∇2Φ = − i (2) ε ∇ • ( ( bi∇Φ + u ) qi + α∇qi ) = 0 (3)
(
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Particle/Dispersed Phase
Particle volume fraction ϕ and number density NDp are treated as scalar quantities, spatially distributed in the fluid flow, i.e., ϕ(x,y) and NDp(x,y). Both because particle charging and particle dynamic motion are highly dependent on particle size, both quantities must be tracked by particle size, i.e., ϕ(x,y,dp) and NDp(x,y,dp). Considered here are two particle size distributions entering each ESP channel segment: a log-normal distribution and an algebraic representation of a measured size distribution reported previously (Clack, 2015). Figure 2 compares the two size distributions and Table 1. The two particle size distributions are discretized into 11 size bins (1.5
