Flue Gas Recirculation System Research Articles

Flue Gas Recirculation System for Biomass Heating Boilers—Research and Technical Applications for Reductions in Nitrogen Oxides (NOx) Emissions

The paper discusses the results of investigations of the change in thermal and emission-related parameters of a heating boiler fueled with biomass after a modification with a proprietary flue gas recirculation system made for this type of equipment. The results provide insight into the combustion process with a multistage flue gas recirculation that materially affected the boiler operation: a reduction in the mass concentration of nitrogen oxides (NOx) by reducing the combustion temperature. The authors also observed a reduction in the emission of particulate matters (PM) and carbon monoxide (CO). For the investigations, the authors used a heating boiler fitted with an automatic fuel feed (timber pellets) and a proprietary patented flue gas recirculation system (Polish patent Pat. 243395) for low power solid fuel heating boilers. Aside from the measurement of the mass concentration of the emitted pollutants, the research focused on the measurements of the temperature inside the combustion chamber, the temperature of the flue gas and the level of oxygen in the flue gas. The aim of the research was to confirm the validity of using the flue gas recirculation technique to reduce emissions of harmful substances from biomass heating boilers, as a technique not previously used for this group of devices. Moreover, the aim of the research was to test an original engineering solution, in the form of a flue gas distribution valve, and investigate its effect on reducing NOx emissions and improving other thermal and emission parameters of the boiler. The obtained research results confirm the validity of the chosen actions and provide a positive premise for the practical use of this technology in solid fuel heating boilers.

INCREASING THE EFFICIENCY OF BOILER PLANTS WITH FLUE GAS RECIRCULATION AND DEEP HEAT RECOVERY

The thermal calculation results of a heating boiler plant with recirculation and heat recovery of flue gases are presented. The thermal efficiency of the complex heat recovery system use designed for heating the return heat-network water and blown air is investigated. Various options of flue gas selection for recirculation into the blowing air are considered, namely: after the boiler, after the water-heating heat recovery exchanger and after the air-heating heat recovery exchanger. The changes in the temperature of the exhaust gases regularities of the studied boiler plant various elements in the case of using the considered options for recirculated gas selection under conditions of changing their share in the air-gas mixture from 10 to 20 % and the relative heat load of the boiler from 30 to 100 % were established. Under these conditions, the levels of increase in efficiency in all elements of the boiler installation and the total efficiency with and without the proposed heat recovery system were determined. It is shown that the use of this system provides deep cooling of exhaust gases in the heat recovery process. The lower the temperature of the flue gas admixed in the options considered and the higher the share of gas recirculation, the lower the temperature of the exhaust gases for all elements of the boiler plant. The increase in the boiler plant efficiency due to the use of the proposed heat recovery system varies from 3 to 5 % in the case of a water-heating heat recovery exchanger and from 1 to 3 % in the case of an air-heating heat recovery exchanger, depending on the boiler load, the part of admixed gases and the options of their selection. The resulting increase in the boiler plant efficiency with a heat recovery and flue gas recirculation system is 1.4–4.7 %.

Numerical study of internal flue gas recirculation system applied to methane-hydrogen powered gas microturbine combustor

Sources of renewable energy have been increasingly used all over the world. This kind of energy is highly desirable because of its unlimited availability. Unfortunately, renewable energy production very much depends on weather conditions. Consequently, it is necessary to store the produced excess energy in order to use it when needed. There is a technology able to produce a hydrogen/methane fuel from excess renewable energy, which may be stored. This technology is called the Power-to-Gas technology (P2G). Since the efficiency of this technological process depends on the hydrogen fraction in the renewable energy fuel, there is a need to increase this fraction. Concurrently, the gas microturbine technology is increasingly widely used in various industries (aviation, energy, automotive, military, etc). The P2G technology and the gas microturbine technology are likely to be integrated in the near future and, as mentioned above, the hydrogen fraction in the methane-hydrogen fuel will tend to increase. In order to power a gas microturbine with the methane-hydrogen fuel, it will be necessary to modify the combustor to avoid an excessive temperature increase and flashbacks. In this paper it is proposed to apply an autonomous internal exhaust gas recirculation system to resolve the hydrogen combustion problems indicated above. The operating principle and the proposed design of the recirculation system and the latter’s impact on the combustor’s operating parameters and emissivity (NOx and CO) are presented.

Performance research and application of the vapor pump boiler equipped with flue gas recirculation system

To improve the boiler thermal efficiency and reduce oxynitride emissions, a novel flue gas waste heat recovery system is proposed. The vapor pump boiler equipped with a flue gas recirculation system is first applied for practical engineering. The oxidizing air is preheated and humidified before the combustion process, which causes the dewpoint temperature of the flue gas to increase. Thus, more waste heat in the flue gas can be recovered by the boiler feedback water. Heat recovery efficiency of the system can reach 82.6%, resulting in a 7–8% increase in the boiler efficiency. In addition, the increases in oxidizing air humidity and flue gas recirculation both reduce oxynitride production. As a result, oxynitride emission decreases from 129 mg/m3 to 23 mg/m3, resulting in a reduction of 82.3%. The gas-fired boiler realizes ultra-low oxynitride emission. For a 1.8 MW boiler, annual natural gas consumption can be saved by 7–8%. Given current energy prices in Beijing, the proposed system is economical with a static payback period of 1.2 years. An overall system performance is provided as evidence of the potential benefits. The proposed system is proved to have advantages in energy savings, pollutant reduction, and economic benefits.

Numerical investigation of the internal flue gas recirculation system applied to methane powered gas microturbine combustor

The gas microturbines gain significance in various industry sectors. One of their most crucial advantages is the capability of utilizing variety of fuels. At the same time, the emissions regulations become increasingly strict. This is why there is a need to look for a new technological solution to limit the emissions of selected substances, like carbon monoxide (CO) and nitrogen oxides (NOx). The internal recirculation of the flue gases is well known to limit the temperature peak and for the homogenization of the temperature field gradient in different combustion chambers. This paper presents a numerical investigation of a novel internal flue gas recirculation system applied to gas microturbine combustors. The ability to perform an internal exhaust gases recirculation by adding a combustor internal pipe system was verified numerically. This paper exposed the numerical investigation methods and obtained results. The study presents the concept and results performed on three cases of internal exhaust gases recirculation systems applied to a reference combustor. The work permitted to demonstrate numerically that it is possible to perform an autonomous exhaust gases recirculation inside gas microturbine combustor at a maximum global rate of 0.51%, and that the recirculation system has an impact on the combustion processes without specially modifying the combustor work parameters.

Effect of Flue Gas Recirculation on Nitrogen Oxide Emission of Coal-Fired Unit Boiler

Taking the flue gas recirculation system of 220MW coal-fired unit as the test object, effects of the operation mode of the flue gas recirculation system on the operating characteristics of the boiler under different working conditions was studied.The results show: the operation of flue gas recirculation system will suppress the amount of NOx generated in the furnace, and the best way to restrain the formation of NOx is to send the recycled flue gas into the furnace through primary and secondary air simultaneously. The flue gas recirculation system can effectively improve the inlet flue gas temperature of SCR, especially when the secondary flue gas recirculation system is put into operation, and the effect of increasing the inlet flue gas temperature of SCR is obviously better than that of the other two operation modes. The operation of flue gas recirculation system can effectively save net coal consumption.

Effect of Flue Gas Recirculation on Operating Performance of Coal-Fired Unit Boiler

Taking the flue gas recirculation system of 220MW coal-fired unit as the test object, effects of the operation mode of the flue gas recirculation system on the operating characteristics of the boiler under different working conditions was studied. The results show: To a certain extent, the operation of flue gas recirculation system will reduce boiler efficiency, which is more obvious under low load conditions. However, The operation of the flue gas recirculation system can effectively increase temperatures of reheat steam and superheated steam.Comparing the economics of the front and rear units of the flue gas recycling system, it can be find that net coal consumption of the unit under the load of 200MW and 140MW can be reduced by up to 1.51g/kW·h and 1.86g/kW·h respectively.

Nitrogenous Gas Emissions from Coal/Biomass Co-combustion under a High Oxygen Concentration in a Circulating Fluidized Bed

Oxy-fuel combustion of coal/biomass is able to realize negative CO2 emission. Increasing the total oxygen concentration in oxy-fuel combustion surely reduces the scale and cost of the flue gas recirculation system. The increase of the total oxygen concentration leads to considerable changes in the emission characteristics of nitrogenous gases (nitrogen oxides and their precursors). Here, coal/biomass co-combustion tests were conducted in a 0.1 MW oxy-fuel circulating fluidized bed combustion apparatus to investigate nitrogenous gas emissions from oxy-fuel combustion of coal/biomass under a high total oxygen concentration (50%). HCN was also detected in the flue gas, besides NO and N2O, especially in the mixed fuel tests. A lower excess oxygen ratio led to less NO and N2O emissions, the same as conventional air combustion. The secondary flow ratio affected emissions of nitrogenous gases depending upon fuel types and atmospheres. Under the same total oxygen concentration, a higher oxygen concentration in th...

Suppression of dioxins by S-N inhibitors in pilot-scale experiments.

S-N inhibitors like thiourea and sewage sludge decomposition gases (SDG) are relatively novel dioxins suppressants and their efficiencies are proven in numerous lab-scale experiments. In this study, the suppression effects of both thiourea and SDG on the formation of dioxins are systematically tested in a pilot-scale system, situated at the bypass of a hazardous waste incinerator (HWI). Moreover, a flue gas recirculation system is used to get high dioxin suppression efficiencies. Operating experience shows that this system is capable of stable operation and to keep gaseous suppressant compounds at a high and desirable molar ratio (S + N)/Cl level in the flue gas. The suppression efficiencies of dioxins are investigated in flue gas both without and with addition of S-N inhibitors. A dioxin reduction of more than 80% is already achieved when the (S + N)/Cl molar ratio is increased to ca. 2.20. When this (S + N)/Cl molar ratio has augmented to 4.18 by applying suppressant recirculation, the residual PCDD/Fs concentration in the flue gas shrank from 1.22 to 0.08ng I-TEQ/Nm(3). Furthermore, the congener distribution of dioxins is analysed to find some possible explanation or suppression mechanism. In addition, a correlation analysis between (S + N)/Cl molar ratios and PCDD/Fs is also conducted to investigate the chief functional compounds for dioxin suppression.

Effects of Flue Gas Addition on the Premixed Oxy-methane Flames in Atmospheric Condition

This numerical study investigates the flame characteristics of premixed methane with various inert gas dilutions in order to simulate oxy-combustion of hydrocarbon fuels with flue gas recirculation system. In general, a flue gas consists of high concentration CO2/H2O and high gas temperature. The diluent gas in oxy-combustion has been changed compared with air-combustion, so that the flame behavior and combustion characteristics of oxy-fuel must be influenced. By observing the resultant flame temperature and species concentration profiles can identify that the flame location shifts, and the concentration profile of major chemical reaction radicals varies, indicating the change of flame structure and flame chemical reaction paths. The dominant initial consumption reaction step of methane shifts from R53 (H+CH4=CH3+H2) to R98 (OH+CH4=CH3+H2O) when nitrogen is replaced by the recirculated gases. It is because that the chemical effects of the recirculated gases changes the flame reaction pathway, and further affect reaction rate, species and radical concentrations.

Effects of flue gas recirculation on the premixed oxy-methane flames in atmospheric condition

This numerical study investigates the flame characteristics of premixed methane with various dilutions in order to simulate oxy-combustion of hydrocarbon fuels with flue gas recirculation system. In general, a recirculated flue gas consists of high concentration CO2/H2O and high gas temperature. The effect of various diluent gases on laminar burning velocity and adiabatic flame temperature is discussed via using Chemkin-pro simulation. By observing the resultant flame temperature and species concentration profiles one can identify that the flame front shifts, and the concentration profiles of major chemical reaction radicals varies, indicating the change of flame structure and flame chemical reaction paths. The dominant initial consumption reaction step of methane shifts from R53 (H + CH4CH3 + H2) to R98 (OH + CH4CH3 + H2O) when nitrogen is replaced by the recirculated gases. H and OH radical concentration would be influenced in various diluent gas cases, so that it leads to affect hydrogen formation and methane consumption. It is because that the chemical effects of the recirculated gases change the chemical reaction of the oxy-fuel combustion, and further affect reaction rate, species and radical concentrations.

Sensitivity Analysis and Optimization of a Coal-fired Power Plant in Different Modes of Flue Gas Recirculation

In a coal-fired power plant with flue gas recirculation, recirculation rate and coal input have a great effect on the performance of the power plant. In this paper, a 600 MW coal-fired boiler is taken as base case, the main parameters of the boiler are calculated at different recirculation rates and coal input conditions, an optimization is carried out and the optimum recirculation rate and coal input are reported. The results show that under optimum recirculation rate and coal input conditions, the net coal consumption rate can be reduced by 3.5g/(kW·h) at 575MW load; while it is 4.36g/(kW·h) and 5.11g/(kW·h), respectively, at 450MW load and 300MW load. Compared to the conventional flue gas recirculation system, the net coal consumption rate can be reduced by 2.31 g/(kW·h), 2.42 g/(kW·h) and 2.41 g/(kW·h), respectively, at 575MW, 450MW and 300MW load.

Study of Hg and SO 3 behavior in flue gas of oxy-fuel combustion system

Oxy-fuel combustion systems have been under development to reduce CO 2 emissions from coal-fired power plants. In oxy-fuel combustion system, Hg in the flue gas causes corrosion in CO 2 purification and compression units. Also, SO 3 in the flue gas corrodes the equipment and ducts of oxy-fuel combustion system. Therefore, Hg and SO 3 need to be removed. Babcock-Hitachi conducted tests using a 1.5 MWth Combustion & Air Quality Control System (AQCS) test facility which consists of oxygen supply unit, furnace, Selective Catalytic Reduction (SCR) catalyst, Clean Energy Recuperator (CER), Dry Electrostatic Precipitator (DESP), flue gas recirculation system, Wet Flue Gas Desulfurization (WFGD), and CO 2 Compression and Purification Unit (CPU). In both cases of air and oxy-fuel combustion, the Hg removal across the DESP could be improved, and SO 3 concentration at the DESP outlet could be reduced to less than 1 ppm by installing a CER upstream of the DESP and reducing the gas temperature at the DESP inlet. Hg was not dissolved in the drain recovered from CO 2 compressor, and may be adsorbed at an inner part of CO 2 compressor. This indicated that Hg needs to be removed at a location upstream of the CO 2 compressor to prevent corrosion of the compressor.

Operating pressure dependence of the pressurized oxy-fuel combustion power cycle

Oxy-fuel combustion technology is an attractive option for capturing carbon dioxide (CO2) in power generation systems utilizing hydrocarbon fuels. However, conventional atmospheric oxy-fuel combustion systems require substantial parasitic energy in the compression step within the air separation unit (ASU), the flue gas recirculation system and the carbon dioxide purification and compression unit (CPU). Moreover, a large amount of flue gas latent enthalpy, which has high water concentration, is wasted. Both lower the overall cycle efficiency. Pressurized oxy-fuel combustion power cycles have been investigated as alternatives. Our previous study showed the importance of operating pressure for these cycles. In this paper, as the extended work of our previous study, we perform a pressure sensitivity analysis to determine the optimal combustor operating pressure for the pressurized oxy-fuel combustion power cycle. We calculate the energy requirements of the ASU and the CPU, which vary in opposite directions as the combustor operating pressure is increased. We also determine the pressure dependence of the water-condensing thermal energy recovery and its relation to the gross power output. The paper presents a detailed study on the variation of the thermal energy recovery rate, the overall compression power demand, the gross power output and the overall net efficiency.

Clearing the Skies of Texas

A modern technology promises to reduce the emissions from gas-fired power plants. The solution that the John Zink Co.’s Todd Combustion Group of Shelton tested is a combustion technology that is yet to be tried in a utility-scale generation facility. The solution that the Todd engineers wanted to test was a flue gas recirculation system that differs significantly from conventional systems, which bring in the flue gas with the intake air. The new system, called COOLfuel, instead mixes the flue gas with the fuel stream, to create a gas that has a much lower energy density. Through computational fluid dynamics modelling, modifications were made to the gas nozzles and flue gas connections during normal shutdown periods to try to reduce the pressure drop due to the introduction of the flue gas. While it is expected that coal-fired power plants will be the focus of the push to lower nitrogen oxide emissions in Texas, utilities are likely to retrofit their gas-fired plants too.

Carbon dioxide removal via a membrane system in a natural gas combined-cycle plant

Gas separation using polymeric membranes is generally believed to be poorly suited for CO2 capture from gas turbine based power plants, because the high air-to-fuel ratios produce large amounts of exhaust gas containing strongly diluted CO2. The driving force for selective permeation is obtained by compressing the entire exhaust stream, resulting in a significant energy penalty. Additional energy is required for permeate compression, which is impaired by the other gases emitted with CO2. In spite of these difficulties, the present study is concerned with membrane systems, which the authors believe warrant further investigation. The attention was focused on natural gas combined cycles (NGCC), so as to have a starting point with high energy efficiency and low baseline emissions. To reduce the exhaust gases and to concentrate the CO2, the NGCC is fitted with a flue gas recirculation system, which has proven to leave plant performance substantially unaffected. The energy requirements for exhaust gas compression are reduced by introducing a heat exchanger between the compressed and residual exhaust gas and recovering heat from the latter. The membrane separator has been modeled using conservative data reported in the literature. Various configurations, such as multiple compression-expansion and multistage membrane units, are considered. Permeate compression is also modeled, so as to highlight the influence of permeate purity on this process. The results show a rather moderate performance penalty. A specific emission of 100 g CO2/kWh corresponds to 48 percent efficiency. Admittedly a CO2 separator for a power plant would be larger than any other membrane system designed for other purposes. However, the system appears technically feasible and would offer valuable benefits in terms of emissions.

Environmentally Safe Coal Processing

A combined technology for coal and sulfur processing in a flue gas recirculation system is considered. Sulfur oxides and nitrogen oxides are absorbed from the flue gas by the products of sulfuric acid synthesis and are converted into sulfuric and nitric acids; carbon dioxide is liquefied.