DARCO Hg-LH Research Articles

Preparation and characterization of high efficiency modified activated carbon for the capture of mercury from flue gas in coal-fired power plants

On a global scale, coal-fired power plants supply approximately 25 percent of current energy needs. Mercury, a highly toxic metal, is commonly found as a ubiquitous trace element in coal. Flue gases produced from the combustion of coal contain both mercury and mercury ions. Currently, halogenated activated carbon (e.g. DARCO Hg-LH@) is considered to be the most effective commercially available mercury sorbent. We report the preparation and characteristics of an activated-carbon-based, polyethyleneimine-modified material, which was subsequently cross-linked using elemental sulfur and diethyldithiocarbamate as an accelerator. The synthesized material exhibits a three-fold increase in the removal of elemental mercury compared to DARCO Hg-LH. In contrast to activated carbon, which adsorbs mercury, the material developed in our laboratory reacts with mercury to form metacinnabar (HgS), which can be safely processed or disposed of. Moreover, the prepared material can adsorb up to 99 percent of ionic Mercury (II) species in solution, compared to 78 percent absorption of Mercury (II) by DARCO Hg-LH. These comparative bench-scale results were obtained under identical experimental conditions. In addition, we report data obtained from a simulated bench-scale pilot plant study, supporting the superior performance of the polymer modified material.

First full-scale demonstration of mercury control in Alberta

Alberta electricity companies, TransAlta, ATCO, and EPCOR, teamed with GE Energy to conduct full-scale evaluation of sorbent injection in Sundance Unit 5 operated by TransAlta. Sundance Unit 5 fires a Western Canadian sub-bituminous coal and is equipped with cold-side ESP for PM control. Goals of the program were to evaluate: (1) the ability of achieving 70% or greater mercury reduction using activated carbon injection in long-term tests (30 days), (2) the effect of sorbent injection on ESP performance and opacity in long-term testing, and (3) the effects of combustion conditions on “natural” mercury removal in fly ash. DARCO Hg-LH was injected upstream of ESP at average injection rate of 2.1 lb/MMacf and achieved an average mercury removal of 80%. During the test, the sorbent injection rate was varied from 0.55 lb/MMacf to 8 lb/MMacf with mercury removals from 65% to > 95%. The continuous 30-day DARCO Hg-LH injection testing demonstrated that 70% mercury removal could be achieved at DARCO Hg-LH injection rate of 1.2 lb/MMacf. Tests were conducted to optimize combustion conditions to improve “native” mercury capture in the fly ash. Testing demonstrated that combustion conditions that resulted in reduction of NO x emissions also corresponded to reduced mercury emission. Mercury emissions were reduced by up to 50% and NO x emissions by up to 35% from baseline levels as a result of changes in the way Unit 5 operated. Integration of sorbent injection with combustion conditions reduced requirements for sorbent injection by 20–30%. Testing has demonstrated that sorbent injection did not have an effect on opacity and ESP performance.

Mercury oxidation and adsorption characteristics of chemically promoted activated carbon sorbents

Previous entrained-flow tests conducted under elemental mercury (Hg 0)-laden air found that significant amounts of oxidized mercury (Hg 2+) are not adsorbed onto cupric chloride-impregnated carbon (CuCl 2-AC) and brominated activated carbon (DARCO Hg-LH), but entrained to the gas phase. In this study, these sorbents were tested in a fixed-bed system and a filter-added entrained-flow system to further investigate Hg 0 oxidation and adsorption characteristics of CuCl 2-AC and DARCO Hg-LH. These test results suggested that CuCl 2-AC has different sites available for Hg 0 oxidation and Hg adsorption, and the resultant oxidized mercury generated from the reaction between Hg 0 and CuCl 2 is re-adsorbed at the site of CuCl 2-AC available for adsorption. The resultant oxidized mercury was also found to be easily re-adsorbed onto CuCl 2-AC and DARCO Hg-LH in the filter connected to the entrained-flow reactor.

Bench-Scale Studies of In-Duct Mercury Capture Using Cupric Chloride-Impregnated Carbons

A brominated activated carbon (Darco Hg-LH) and cupric chloride-impregnated activated carbon (CuCl2-ACs) sorbent have been tested in a bench-scale entrained-flow reactor system which was developed for simulating in-flight mercury capture in ducts upstream of particulate matter control devices. The bench-scale experimental system has been operated with the conditions of a residence time of 0.75 s and a gas temperature of 140 degrees C to simulate typical conditions in the duct of coal-fired exhaust gas. In addition, sorbent deposition on walls which can occur in a laboratory-scale system more than in a full-scale system was significantly reduced so that additional mercury capture by the deposited sorbent was minimized. In the entrained-flow system, CuCl2-ACs demonstrated similar performance in Hg adsorption and better performance in Hg0 oxidation than Darco Hg-LH. In addition, the carbon content of those sorbents was found to determine their Hg adsorption capability in the entrained-flow system. The bench-scale entrained-flow system was able to demonstrate the important Hg adsorption and oxidation characteristics of the tested sorbents.

The effect of methods of preparation on the performance of cupric chloride-impregnated sorbents for the removal of mercury from flue gases

Cupric chloride-impregnated carbon sorbents (CuCl 2-ACs) showed comparable performance in elemental mercury (Hg 0) oxidation and adsorption to a brominated activated carbon sorbent (Darco Hg-LH) in our previously reported experimental studies using fixed-bed and entrained-flow systems. This study tested several CuCl 2-ACs prepared in different solvents to investigate the effects of solvents on the Hg 0 removal capability of CuCl 2-ACs and find the most efficient and economical method of sorbent preparation. The performance of CuCl 2-ACs prepared in different solvents was also examined with respect to surface tension and pH of solvents. As a result, the sorbents prepared in isopropyl alcohol and acetone showed the best performance in these tests, but low surface tension and pH of isopropyl alcohol and acetone were not found to be critical factors.

Performance of Copper Chloride-Impregnated Sorbents on Mercury Vapor Control in an Entrained-Flow Reactor System

An entrained-flow system has been designed and constructed to simulate in-flight mercury (Hg) capture by sorbent injection in ducts of coal-fired utility plants. The test conditions of 1.2-sec residence time, 140 °C gas temperature, 6.7 m/sec (22 ft/sec) gas velocity, and 0–0.24 g/m3 (0–15 lbs of sorbent per 1 million actual cubic feet of flue gas [lb/MMacf]) sorbent injection rates were chosen to simulate conditions in the ducts. Four kinds of sorbents were used in this study. Darco Hg-LH served as a benchmark sorbent with which Hg control capability of other sorbents could be compared. Also, Darco-FGD was used as a representative raw activated carbon sorbent. Two different copper chloride-impregnated sorbents were developed in our laboratory and tested in the entrained-flow system to examine the possibility of using these sorbents at coal-fired power plants. The test results showed that one of the copper chloride sorbents has remarkable elemental mercury (Hg0) oxidation capability, and the other sorbent demonstrated a better performance in Hg removal than Darco Hg-LH.

Copper-Doped Silica Materials Silanized With Bis-(Triethoxy Silyl Propyl)-Tetra Sulfide for Mercury Vapor Capture

The use of Cu−S sites for Hg capture from the gas phase has been successfully applied to a silica-based platform using an S4 organic polysulfane and copper sulfate. The maximum fixed-bed equilibrium capacity achieved using these materials was 19 789 μg Hg·g −1 sorbent for a material with 2.5 wt % Cu and 6 wt % S. An optimal S level was determined to be around 3 wt % because enhancement of capacity was only 18% when increasing from this 3 to 6 wt %. The rate of adsorption in pure beds ranged from 0.6 to 1.6 μg Hg·min −1 depending on the inlet concentration. Differences in breakthrough times suggest that material deposition is not uniform. When compared to two other platforms, commercially available Darco HG-LH and previously tested Fe−Cu−S4 nanoaggregates, the Si-1 material performed the best in fixed-bed testing. During entrained-flow testing, a steady-state Hg removal of 82% was achieved using Si-1 at injection rates of both 6 × 10 −5 and 1.2 × 10 −4 g·L −1·h −1. The lack of increase in Hg removal when the injection rate is doubled suggests that pore accessibility is the rate-controlling step during dynamic Hg capture. A calculation of the approximate pore usage based on injection testing helped confirm this observation. During injection testing, the performance of Si-1 was only diminished 10% when exposed to 20 ppm SO 3. This is an encouraging result for flue-gas applications where SO 3 levels range from 1 to 40 ppm. Testing demonstrated that Si-1 is stable when exposed to leaching conditions after concrete blending and cement impregnation. This is an important aspect to consider for injection because the sale of fly ash for concrete is a key cost-recovery tool for power plants.

Novel sorbents for mercury emissions control from coal-fired power plants

The authors have successfully developed novel efficient and cost-effective sorbents for mercury removal from coal combustion flue gases. These sorbents were evaluated in a fixed-bed system with a typical PRB subbituminous/lignite simulated flue gas, and in an entrained-flow system with air simulating in-flight mercury capture by sorbent injection in the ductwork of coal-fired utility plants. In both systems, one of the novel sorbents showed promising results for Hg 0 removal. In particular, this sorbent demonstrated slightly higher efficiencies in Hg 0 removal than Darco Hg-LH (commercially available brominated activated carbon) at the similar injection rates in the entrained-flow system. The other novel sorbent showed excellent Hg 0 oxidation capability, and may enable coal-fired power plants equipped with wet scrubbers to simultaneously control their mercury and sulfur oxides emissions. In addition, fixed-bed results for this sorbent showed that co-injection of a very small amount (∼10%) of raw activated carbon could eliminate almost all of the mercury generated by reactions of Hg 0 with the sorbent.

Elemental Mercury Control by Novel Oxidant and Sorbent in an Entrained-Flow System

An entrained-flow system has been designed and constructed to simulate in-flight mercury capture by sorbents in ducts of coal-fired utility plants. The test conditions of 1.5 s residence time, 140°C temperature, 4.5 ppbv inlet Hg0 concentration, and 0–20 lb/MMacf sorbent injection rates were chosen to simulate conditions in the ducts. Novel oxidants developed in previous fixed-bed tests and novel sorbents derived from the novel oxidants were tested for their Hg0 capture in the entrained-flow system to examine the possibility of using those sorbents in a full-scale system. Darco-FGD and Darco Hg-LH served as benchmark sorbents with which mercury control capability of the novel oxidants and novel sorbents could be compared. The test results showed that the novel oxidants have remarkable Hg0 oxidation capability, and the novel sorbents showed a better performance in Hg0 removal than Darco Hg-LH.