Entrained-flow System Research Articles

Co-gasification of coal and empty fruit bunch in an entrained flow gasifier: A process simulation study

Co-gasification of coal and biomass is a proven method to improve gasification performance and a platform towards being independent of fossil fuel in power generation. Thus, this research was conducted to assess the feasibility of coal and EFB as fuels in co-gasification using Aspen HYSYS. A sequential model of an EFG was developed to predict the syngas composition and the optimum operating condition of the gasifier. The process was modelled with a set of five reactors to simulate various reaction zones of EFG in accordance with its hydrodynamics. The model considers devolatilization, char and volatile combustion, char gasification and water-gas shift reactions. The model prediction has exhibited excellent agreement with the experimental results. Three parameters of BR, Top and S/F were considered to account for their impacts on syngas composition in the process. The CE, CGE, PE and HHV were adopted as the indicators of process performance. The optimal values of BR, Top and S/F were 50%, 950°C and 0.75, respectively. While the value of CE reached above 90% and the maximum value of CGE, PE and HHV was obtained. This finding should be helpful in designing, operating, optimizing and controlling any co-gasification process especially in the entrained flow system.

Mercury removal and synergistic capture of SO2/NO by ammonium halides modified rice husk char

Rice husk, a common agricultural waste and renewable biomass, was pyrolyzed and then impregnated with NH4Cl and NH4Br to develop an efficient sorbent for gaseous mercury removal from flue gas. The sorbents’ performance was first investigated in a fixed-bed system with N2 atmosphere at 150°C and then in an entrained-flow system with real coal-fired flue gas at 150°C. For comparison, a commercial coal-based activated carbon (CAC) treated with the same impregnation method was also studied. In the N2 atmosphere, the mercury removal performance of rice husk char (RHC) was significantly enhanced after modification with ammonium halides, which are neither strong oxidants nor strong catalysts. The comparison of the RHC modified with HBr, NH4Br, CaBr2, (NH4)2SO4 and NH4OH with the same amount of bromine or ammonia confirmed the pivotal role of halogen in mercury removal. Although ammonia was proven to have no beneficial impact, the mercury removal capacity varied as the cation changed and increased as the pH of the halide impregnation solutions decreased. When tested in real coal-fired flue gas with a high concentration of SO2 (1177ppmv) and NO (254ppmv), the ammonium bromide impregnated RHC sorbent which had an excellent mercury removal performance in the fixed-bed still reached about 80% efficiency for gaseous mercury removal. Furthermore, a synergistic effect on SO2/NO removal was observed for both ammonium halides modified RHC and CAC sorbents injected into the real coal-fired flue gas; the synergistic effect might be due to the oxidation effect of O2 and H2O in the flue gas and the catalytic effect of the carbonaceous sorbent. Most micropore structure of the RHC was maintained after ammonium halides impregnation; however, the physisorption energy decreased in various degrees. Overall, in spite that the BET surface area and micropore volume of the RHC sorbent were smaller than the CAC, the RHC sorbent still reached a comparable level of multipollutants capture.

Combustion and Pollutant Emission Characteristics of Lignite Dried by Low Temperature Air

Lignite is a kind of coal that has high moisture content and needs to be dried before being utilized. In this article, a Chinese lignite was dried in air at 120–180°C and the changes in its physical and chemical structures after drying were investigated. The results showed that the pore volume and specific surface area of the lignite decreased after drying. Some of the methylene and methyl groups were oxidized by the oxygen in the drying air, resulting in an increase in oxygen functional groups. The combustion characteristics of the dried coals and parent coal (dry basis) were studied via thermogravimetric analysis. The total volatile yields of the dried coals increased compared to the parent coal. The burnout temperatures of the dried coals were higher than the parent coal, whereas the ignition temperatures stayed almost unchanged. An entrained flow system was set up to study the release of nitrogenous gas products during rapid pyrolysis and combustion. The HCN yields of the dried coals during pyrolysis were higher than that of the parent coal, and a similar trend was found for the NO yield during combustion. The mechanism changes of combustion and pollutant emission characteristics were discussed according to the results of the physical and chemical structure analyses.

Release of Nitrogen Species during Rapid Pyrolysis of Model Coals

To gain an in-depth insight into the release of nitrogen species during rapid pyrolysis of coals, two model coals containing pyrrolic or pyridinic nitrogen are synthesized, and rapid pyrolysis of model coals is performed in an entrained flow system between 800 and 1300 °C. The effects of pyrolysis temperature, minerals, particle size, and the interaction between the two forms of nitrogen are discussed. The results show that a moderate amount of HCN and small amounts of NO and NO2 rather than NH3 are formed. As the temperature rises, the HCN release increases first and then decreases, and exhibits very similar trends for the two forms of nitrogen. Because of the better thermal stability of pyridinic nitrogen, the releases of both HCN and NOx are lower for pyridinic nitrogen than for pyrrolic nitrogen. When the two forms of nitrogen are pyrolyzed together, the interaction between the nitrogen species released makes the HCN release decrease and approach the HCN release from pyridinic nitrogen alone. For pyrrolic nitrogen, Fe addition suppresses the HCN release at all temperatures and Na addition promotes the HCN release obviously at 1000 °C or above, whereas Ca addition increases the HCN release with increasing temperature first and then decreases. For pyridinic nitrogen, all the metal additions suppress the HCN release and Fe has the strongest catalytic effect. As the catalyst content increases, the HCN release decreases drastically. The HCN release from pyridinic nitrogen increases slightly with increasing particle size at 800 or 1000 °C. When the temperature achieves 1200 or 1300 °C, the particle size dependence of HCN release is not observed.

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.

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.

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.