Final Report: Nano-Enhanced Composite Electrodes for Electrostatic PrecipitatorsEPA Contract Number: EPD10053
Title: Nano-Enhanced Composite Electrodes for Electrostatic Precipitators
Investigators: Burton, David
Small Business: Applied Sciences, Inc.
EPA Contact: Manager, SBIR Program
Project Period: May 1, 2010 through April 30, 2012
Project Amount: $224,985
RFA: Small Business Innovation Research (SBIR) - Phase II (2010) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Air Pollution
With the increasing volatility of petroleum and natural gas resources, small coal-fired boilers have become increasingly more attractive for steam and power generation in several industries. This switch to small-scale coal-fired plants is expected to continue and increase the need for clean-coal utilization technology. The United States’ energy policy includes a clean-coal technology initiative focusing on the environmental challenges of using coal.
A key technology for removing particulate matter from flue gases generated by coal-fired power plants is the electrostatic precipitator (ESP). ESPs operate with an efficiency of 98 to 99% for the removal of mercury and fly ash from the flue gas stream. ESPs work by applying an electric voltage to an electrode, which in turn, applies a charge to the particulate matter entrained in the gas flow; the charged particulate matter is then collected on a large plate with the opposite voltage. The collection surfaces and electrodes are made from metal, are extremely heavy with a high installed cost, are easily corroded, and have a limited service life unless very expensive alloys are used.
Under this SBIR Phase II effort, Applied Sciences, Inc. (ASI) and its collaborator, Ohio University (OU), developed a technology for lightweight nano-enhanced electrode composites with more efficient electrical conductivity and superior voltage-current characteristics than the current state-of-the-art ESP components. This technology will enhance the capture efficiency and increase the service life of the ESPs, as well as reduce the complexity and cost of manufacturing and installation of ESPs.
Production and Optimization of Nano-Enhanced Composite Tapes
This task involved a series of steps that were designed not only to make the production of the nanocomposites more efficient, but also to improve the electrical conductivity of the composites produced. By identifying and monitoring the process parameters that are directly responsible for the performance of the composite electrodes, the ASI/OU team could ensure the successful outcome of the proposed effort, and provide a good base of support for the development of efficient composite electrode systems.
Under this task, a new die component with a rectangular section was designed and fabricated at OU. The new die component enabled the continuous production of tapes of nano-enhanced composites that can be readily assembled into the electrodes for ESPs, without any post-processing molding requirements.
Also under this task, the screw configuration was designed to take into account the type of CNF used and the level of electrical conductivity that was targeted for the final compound. Configuring a modular screw presents challenges since there are no clear rules adapted for nanocomposites processing. To date, there are no commercial optimization programs and limited scientific awareness on how to determine the processing conditions and how the modular elements should be combined to fabricate good-quality nanocomposites. Therefore, the assembly of the elements, or screw configuration, was done with the guidance of results obtained from previous studies done on similar composites.
Twin Screw Extrusion Process Simulation
Process simulation is a valuable engineering tool that provides a close look on the evolution of flow conditions in processing equipment used to manufacture the composite tapes. With the aid of a one-dimensional simulation program, the energy level and shear rates imparted on the material by a particular set of conditions and screw profile used was calculated. This represents a value aid for this work, as it simplifies the processing equipment/conditions to simple parameters such as stresses and deformations responsible for mixing the CNF in the polymer matrix, which in turn yields a particular electrical conductivity. Another benefit of the simulation software is the ability to scale up from small laboratory-scale extrusion which produces 5-7 pounds per hour to industrial-scale extruders that can produce hundreds of pounds of compound per hour.
ASI used a commercially available software program, Ludovic, to perform the simulation work. This particular program uses a simple global model to describe the flow along the twin screw extruder, from the hopper to the die. The program computes the evolution of process-related parameters such as shear rates, viscosity, pressure, residence time, temperature, specific mechanical energy input, power consumption, etc., by using a local one-dimensional approach that takes into account the specific geometry of each of the screw elements and dies, as well as the rheological and physical properties of the material.
Dispersion Analysis of the Nano Reinforced Phase in the Composite Tapes
Understanding the relationships between processing and a given set of functional properties in a composite requires a direct evaluation of the CNF dispersion. Since dispersion of carbon nano materials is a complex process with multiple dependencies, and because different functionalities require different levels of dispersion, qualifying the dispersion by non-quantitative methods is not sufficient. By using measurements of absolute values, one can discuss the developed dispersion quantitatively rather than qualitatively, specifically, whether or not the production method was efficient in dispersing the CNFs. Under this task, microtomed sections of the manufactured tapes were analyzed by transmission optical microscopy, and the quality of the dispersion of the CNF was analyzed by the Multiscale Image Analysis Technique.
The dispersion level of the fabricated compounds will be characterized by multi-scale image analysis (MSIA), a method originally developed by Spowart et al. to quantify dispersion in metal matrix composites, but which was later adapted to nanocomposites by Van Hattum and Leer. This method can be used to make quantifiable relationships between the carbon nanomaterials used, the resulting dispersion and the measured physical properties of the manufactured composites tapes.
Production of Conductive Tapes
Based on the results obtained from the optimization task, new composite tapes were manufactured at ASI on a 20 mm Theyson intermeshing twin screw extruder. The base polymeric material in these tapes was an ESD (electrostatic discharge) electrical conductive (109 – 106 Ohm/sq) polypropylene with 9 wt % short glass fibers and carbon black from RTP. This material was selected due to its low cost and availability in large volumes. However, the resistivity of the base material is three orders of magnitude higher than required by the program. To achieve a lower surface resistivity value, 3 wt % carbon nanofibers were added to the composition in a one-step extrusion process. The temperature of the barrel and feeding zones were kept constant at 392o F and the total throughput was held at 0.7 Kg/h. The screw speed was kept constant at 200 rpm, and melt pressure also was kept constant at 270 bar. The fabricated tapes showed good dimensional stability.
Electrical Characterization of the Composite Tapes
The surface electrical resistivity was measured at ASI. All measurements were done at room temperature in direct current (DC) by using the two-probe method.
Assembly of Electrostatic Precipitator Electrode
The composite tapes manufactured by ASI were assembled for testing at OU in a short ESP vertical chamber. For that effect, a 3-feet glass fiber tube was used as the support rod and was fitted with the manufactured tapes. The composite tapes were attached to the supporting rod along its length. This new design is easier to fabricate and more stable than the previous approach, the helical configuration.
Performance Evaluation of Composite Tapes in Electrostatic Precipitator
To evaluate and compare the performance of composite discharge electrodes made of a combination of polymer, carbon fiber and nanofiber. The electrodes were compared on the basis of their current discharge as a function of voltage (V-I curve). The following materials were tested as the source the discharge current from the electrode:
- Continuous carbon fiber in the form of a roving (upper limit);
- Tape of continuous carbon fiber in a polymer matrix (made by Ticona);
- Tape of carbon nanofiber in a polymer matrix (produced by ASI); and
- A steel strip with the dimensions of the ASI tape (baseline).
The conductive composite tapes produced by ASI and tested at OU showed significant advantages over the current material in use for ESP. Through the use of a systematic approach in the first task, the right processing conditions were identified and the process optimized. This led to the production of electrically conductive tapes, with good a dimensional stability and current discharge, making these materials good candidates for use in electrostatic precipitators.
Discussions were held between OU, ASI and Southern Environmental, Inc. (SEI) (an original equipment manufacturer for ESP) to determine the ideal test conditions for the composite electrodes tapes produced by ASI, and to discuss future commercialization aspects. SEI proposed that accelerated tests should be performed on the composite tapes, so that their life span can be assessed. The typical life cycle of a traditional ESP (metal alloy) is 20 years. Thus, for the composite tapes to be commercially competitive and attractive, they should meet the same requirements. Future testing involves the use of the 10-ft long 12” x 12” ESP with high spark conditions, which consists of 100 sparks/minute at a high voltage range between 50-60 kV. The current trend in the ESP industry is to design new precipitators to operate at higher voltage conditions in order to reduce the size of the ESP box. Ideally, the new generation of composite electrodes would withstand 80 kV.
Finally, ASI developed a cost model for the composite tapes. ASI cost projections point out that the manufactured tapes will be commercialized at $0.10/linear foot. SEI’s marketing director showed renewed interest in the material, stating that at this cost, the electrostatic precipitators (ESP) will be very competitive with the current material currently used for wet ESP.