Flue Gas Recirculation Research Articles

A Novel Method of Pure Oxy-fuel Circulating Fluidized Bed Combustion with Zero Recirculation Flue Gas – Experimental Validation

Applying oxy-fuel combustion requires more advanced combustion control methods to avoid inadmissible high flam temperature. In fluidized beds and pulverized unites, enhanced heat transfer and recirculation flue gas are used. On other hand, higher oxygen concentration has pluses viz. better heat transfer, higher efficiency, compact setup and lower installation and operating costs. In pulverized power unites, pure oxy-fuel combustion is used with 100% O2 in the oxidant. In contrast, the highest experimental O2 % in oxy-fuel circulating fluidized bed (CFB) combustor is 70%. To the best of authors’ knowledge, there is no single CFB power plant operating under pure oxygen condition. In this work, we are aiming to use pure oxygen for oxy-CFB combustion, with new temperature controlling method for CFBs depending on combustion staging by fuel staging rather than using RFG. Fuel staging allows controlling combustion and varying SR. At the first stage, the used oxidant is 100% O2, and fuel is fed to achieve over SR (λ>1), where the excess oxidant absorbs heat and does not take a part in the reaction. The products of the first stage are reach of O2 and subsequently it is used as an oxidant for the second stage. For validation, a series of experiments are conducted using mini-CFB, and an oxidant of 100% O2 concentration is used with three SR ratios λ=1.25, 2.0, and 3.0. The resulted average temperatures along the riser for biomass are 1031°C, 950°C, and 798°C; and for coal 1129 °C, 1051 °C, and 961 °C respectively. The controlling of AFT with pure oxy-fuel combustion eliminates the recycled flue gas (RFG) in oxy-fuel CFB combustion and flue gas recirculation section; this simplifies the power plants’ design, fabrication and its installing-operating costs. Familiarising this concept can accelerate adapting oxy-fuel combustion in CFB power plant for Carbon Capturing and Sequestration (CCS). This contribution can commence and commercialise the third generation of oxy-fuel CFB combustion with zero recycled flue gas. Finally, the concept of controlling AFT by SR (λ) is validated experimentally.

Oxy-fuel co-combustion of sewage sludge and wood pellets with flue gas recirculation in a circulating fluidized bed

Oxy-fuel co-combustion of waste sludge and biomass with flue gas recirculation was conducted to observe the combustion characteristics and enrichment of CO2 using a 30kWth circulating fluidized bed. The combustion reaction was accelerated and the ignition time was shortened by increasing the blending ratio of wood pellets and oxygen mixing ratio. The best combustion performance was observed at a 30% blended biomass ratio, with 23% oxygen mixing rate as oxy co-combustion with respect to heat recovery and enrichment of CO2; this study included experiments with ranges of 0 to 70% for the blending ratio and 21 to 30% for the oxygen mixing ratio. With flue gas recirculation at 60%, the oxy-fuel co-combustion of sludge with biomass was optimized in high enrichment of CO2 (over 90%) with less pollutant emissions, specifically 0.91% CO and 14ppm NO.

NOX reduction in a 40 t/h biomass fired grate boiler using internal flue gas recirculation technology

A decoupled numerical modelling method is developed in this study to simulate the whole combustion process of biomass in a grate firing boiler, which includes the thermochemical conversion of biomass in the fuel-bed and gaseous combustion in the freeboard. With the aid of this modelling method, the objective of this study is to explore the NOX reduction mechanism as well as to investigate the potential of internal flue gas recirculation technology (IFGRT) on the combustion process and emissions formation in a 40 t/h biomass-fired grate boiler. Computational fluid dynamics (CFD) modelling results show that IFGRT can be realized in the grate boiler by establishing intense flue gas recirculation within the boiler, which allows for lower peak combustion temperatures and smaller flame kernel sizes, while improving the overall average gas temperature. Consequently, NOX emission can be reduced mainly via the thermal formation route in comparison with the conventional combustion case. More specifically, the parallel over-fired air (OFA) burner configuration is suggested for implementation to produce even lower NOX emission compared to the staggered OFA burner configuration. To further understand the reasons behind the NOX reduction, NOX formation and destruction mechanisms are also examined through reduced order modelling (ROM) with the help of detailed reaction chemistry. It is revealed that flue gas recirculation inhibits NOX formation from thermal, NNH and N2O routes. Although NOX destruction rate through reburning is suppressed, the net NOX production rate is found to be decreased under the condition of IFGRT. Moreover, as the flue gas recirculation ratio increases, final NOX emission shows a decreasing trend.

Experimental and numerical study on the combustion of a 32 MW wood-chip grate boiler with internal flue gas recirculation technology

The objective of the present work is to evaluate the feasibility of reducing NOX emission while improving thermal efficiency in a 32 MW wood-chip grate boiler with the technology of internal flue gas recirculation (TIFGR). Both experiment and numerical modeling were conducted. Firstly, the experimental case with conventional over fired air (OFA) supply system was modeled as reference. Subsequently, the conventional OFA nozzles was modified to apply TIFGR. Two burner arrangements were considered, i.e. parallel and staggered. The internal flow dynamics, combustion temperature distribution and final NO emission were compared between the original case and the two modified cases. Reasonable agreement is achieved between numerical modeling and experimental measurement for the reference case, proving the adequacy of the present adopted numerical models. As compared to the conventional OFA arrangement, both parallel and staggered burner configurations are able to reduce NOX by establishing intense internal flue gas recirculation. Especially, the parallel OFA arrangement shows a greater potential for NOX reduction while improving the thermal efficiency. As a result, the parallel OFA arrangement is suggested for grate boilers to implement TIFGR.

Combustion measurements of type-1 pulverized coal flames operating under oxy-fired conditions

The present study addresses the impact of oxy-fired conditions on type-1 flames generated by an oxidant-staged burner operating with pre-dried lignite and applying wet flue-gas recirculation. Investigations were carried out in a laboratory facility where the combustion takes place in a furnace with a rated capacity of 0.40MWth. In-flame measurements were performed for oxy-fired conditions operating at three levels of secondary swirl number (1.15, 1.65, and 2.05), while overall O2 fraction upstream of the burner in oxy-firing was kept at 31vol%. One air-fired condition at 1.65 secondary swirl number was also investigated. Measurements of local gas temperature and gas species concentrations were performed using standard water-cooled probes with focus on the near burner region. Results showed evidence of radial flame stratification consistent with gradual O2 admixing to the central fuel jet. Lower temperatures on the flame axis were obtained under oxy-fired condition. In the same region, as a result of CO2 dissociation and/or gasification reactions by water vapor and CO2, higher CO concentrations were also monitored, which contributed to lower temperatures. Experimental data also suggested great potential for NO abatement through flame stratification due to type-1 flame pattern.

Emissions and stability performance of a low-swirl burner operated on simulated biogas fuels in a boiler environment

This paper addresses the experimental and numerical modeling of NOx emissions and lean blow off (LBO) stability limits of natural gas and biogas fuels reactions stabilized with a low swirl burner (LSB). The paper presents the methodology to set up a chemical reactor network (CRN) based on experimental results and computational fluid dynamics (CFD) simulations. The CRN is a simplified representation of the fluid dynamics and energy balance of the reactive gases in the boiler environment. The CRN uses a combination of perfectly stirred reactors (PSRs) and plug flow reactors (PFRs) to couple the fluid dynamics with detailed reaction kinetics. By analyzing the CFD and CRN results, it is possible to gain insight into the relationship between internal flue gas recirculation, heat losses, geometric variables and fuel composition on the emission of NOx and the stability of the reactions. In this study, three injector configurations featuring different quarl expansion strategies were considered. Two nozzles were tested experimentally, whereas the third configuration represents an additional hypothetic design used to show the capabilities of the methodology. The numerical results are in good agreement with the experimentally observed trends and show direct relation between the emission of NOx and the lean stability limits. The quarl expansion plays a key role in the interaction of the premixed reactants and recirculated gases within the boiler environment. By varying this interaction, it is possible to attain acceptable tradeoffs between NOx emissions and burner stability. In general, the results demonstrate the ability of this design methodology to efficiently estimate burner performance and evaluate design modifications to accommodate varying fuel compositions.

Effects of reburning mechanically-activated micronized coal on reduction of NOx: Computational study of a real-scale tangentially-fired boiler

The paper presents some results of numerical simulations of the effects of reburning mechanically-activated micronized coal on reduction of NOx and unburnt carbon residual in a tangentially fired boiler of 500 t/h steam production. The conventional RANS approach was used to compute two-phase (reactive dispersed particles in gaseous medium) multi-component system, with some modifications related to particle heat transfer and their reactions. The comprehensive model was verified in simulation of the same boiler with conventional firing, as well as in an experimental pilot-scale combustor using micronized, non-activated and mechanically-activated coal. The comparison with the standard dust-coal firing showed that reburning reduces NOx by 37.5%, but leads to an increase in heat loss due to unburnt carbon residual by 61%, primarily due to the imposed suboptimal stoichiometric ratio. Switching to reburning micronized coal reduced NOx by 49% while heat loss was still substantial, 42% higher than in the conventional firing. Using mechanically-activated micronized coal of the same granulation brought only marginal further improvement in NOx reduction (to 50%), but to a remarkable decrease in heat loss, only 3.3% higher than in the case without reburning. The simulations were performed for a set of plausible operating parameters (stoichiometric ratios, flow rates of reburn coal, flue gas recirculation, overfire air), which all need to be optimized to achieve maximum effects. Nevertheless, the simulations demonstrated that reburning of activated micronized coal is a feasible option to achieve significant NOx reduction without being penalized with excessive unburnt fuel/heat residual.

Effects of internal flue gas recirculation rate on the NOx emission in a methane/air premixed flame

This study fundamentally investigates the effects of internal flue gas recirculation (IFGR) on the combustion characteristics of methane/air mixtures, particularly on the NOx formation. To induce IFGR, a prototype tubular flame burner with internal recirculation has been made, in which 12 recirculation paths are designed. The IFGR rate is controlled by varying the number (n) of opening recirculation paths (n = 0-12). To quantify IFGR rate and furthermore evaluate its influence on the NOx formation, the recirculated gas volume is determined by measurements of velocity distribution with a Particle Image Velocimetry (PIV) system. The results illustrate that by increasing n, the flame length increases while flame luminosity weakens, steady tubular flame range in equivalence ratio shrinks, and the flame temperature decreases. Temperature of recirculated burned gas drops significantly before diluting the fresh reactants. Detailed observations show that the measured IFGR rate is about 13% (convert to STP) when all the recirculation paths are opened (n = 12), in which the NOx concentration significantly decreases to almost a half. Owing to IFGR, the local oxygen mole fraction and equivalence ratio in the reaction zone decrease, which directly reduce the flame temperature and oxygen atom concentration, resulting in a significant decrease of thermal type NO formation. Variations of species concentrations with IFGR rate have been examined, and the obtained relations provide a useful guide for the operation of small boilers adopting IFGR, which can reduce NOx emission without great cost and space.

Study of Flue-Gas Recirculation Sintering

The sintering process in a sinter machine with a sintering area of 224 m2 involving recirculation of flue gas to reduce the emission of harmful substances into the atmosphere and its flow through a bag filter to be installed after the existing electrostatic precipitator is analyzed. Alternatives of supplying a portion of the flue gas alone or mixed with atmospheric air or hot air from the sinter cooler to the bed to increase the oxygen content of the gas are considered. It is shown that forcing atmospheric air into the bed is inappropriate. It is recommended to recirculate a mixture of 70% flue gas and 30% air. Implementing the recommended alternative will reduce the gas flow through the bag filter by 30–35%, the CO emissions into the atmosphere by 8136 tons/yr, and the solid fuel consumption by 5600 tons/yr.

Pure Oxy-Fuel Circulating Fluidized Bed Combustion by Controlling Adiabatic Flame Temperature Using Fuel Staging

In the present study, a new method is proposed for temperature controlling by combustion staging. Two combustion stages can be used with two stages of fuel feeding. A high stoichiometric ratio (SR) gimel > 1 is used at the first stage to mitigate adiabatic flame temperature (AFT) in case of high O2% in the oxidant. For validation, a series of experiments are conducted using mini-CFB (circulating fluidized bed), and an oxidant of 100% O-2 concentration is used with three SR ratios, i.e. gimel = 1.25, 2.0 and 3.0. The resulting average temperatures along the riser for biomass are 1031 degrees C, 950 degrees C and 798 degrees C; and for coal 1129 degrees C, 1051 degrees C and 961 degrees C respectively. The controlling of AFT with pure oxy-fuel combustion eliminates the recycled flue gas in oxy-fuel CFB combustion and flue gas recirculation section; this simplifies design, fabrication and installing-operating costs of the power plants. Familiarizing this concept can accelerate adapting oxy-fuel combustion in CFB power plants for carbon capturing and sequestration. This study can help commercialize the third generation of oxy-fuel CFB combustion with zero RFG. Finally, the concept of controlling AFT by SR is validated experimentally.

Convective recirculation effect on the selective non-catalytic reduction behavior in an industrial furnace

Abstract Selective non-catalytic reduction (SNCR) has become an important technique for reducing NOx emissions in industrial furnaces. The occurrence of convective recirculation can cause significant changes in SNCR performance. This paper presents the results of a study performed by Computational Fluid Dynamics (CFD) considering: (i) changes of initial NOX; (ii) injection plan and (iii) NH3/NO molar ratio. The simulations have demonstrated that the convective recirculation (resultant from different injection plans and high temperature gradients) of flue gas assumes a role of vital importance for performance analysis of existing plants and the potential NOX reduction in furnaces during the design phase.

AN ALTERNATIVE SOLUTION TO ESP RECONSTRUCTION FOR THE COAL FIRING THERMAL POWER PLANTS

The mathematical modeling is performed of the efficiency of flue gas cleaning from fly ash particles of coal-fired thermal power plants, upon installation of a preliminary flue gas cleaning system that consists of a louvered dust concentrator and a battery cyclone, with the recirculation of flue gas from the battery cyclone outlet to the electrostatic precipitator pre-chamber. Based on the available experimental data for the fractional composition of fly ash downstream the boilers of coal-fired TPPs, the size distribution functions were calculated, of fly ash particles at each stage of the preliminary dust-cleaning process, as well as concentrations and modified particle size distributions, to be further used as the input data for designing options and scope of the reconstruction of existing electrostatic precipitators. Bibl. 13, Fig. 3.

Recent Advances and Research Status in Energy Conservation of Iron Ore Sintering in China

For the ferrous burden of blast furnaces in China, sinter generally accounts for more than 70% and the sintering process accounts for approximately 6–10% of the total energy consumption of the iron and steel enterprise. Therefore, saving energy during the sintering process is important to reduce the energy consumption in the iron and steel industry. This paper aims to illustrate recent advances and the research status of energy conservation of iron ore sintering in China. It focuses on the development and application of energy-saving technologies such as the composite agglomeration process, sintering with high-proportion flue gas recirculation sintering, recovery of sensible heat from the sinter cooling process, homogeneous deep-bed sintering technology, and comprehensive treatment technology of leakage of sintering. Moreover, some suggestions for the future development of energy-saving technologies are put forward.

Multi-environment Probability Density Function Modeling for Turbulent CH4 Flames under Moderate or Intense Low-Oxygen Dilution Combustion Conditions with Recirculated Flue Gases

The present study has adopted the multi-environment probability density function (MEPDF) approach to simulate the turbulent CH4 flames under flameless combustion conditions with recirculated flue gases. The MEPDF approach is based on an Eulerian PDF formulation with computational efficiency. Micromixing can be represented via the IEM (interaction by exchange with the mean) model, and the chemistry is based on the GRI 2.11 mechanism including the NOx chemistry. Special emphasis is given to the effects of the micromixing model constants on the structure and characteristics of recirculated moderate or intense low-oxygen dilution (MILD) combustion processes. In terms of the temperature and species mole fraction, the multi-environment PDF model with a micromixing constant of 0.5 yields reasonably good agreement with experimental data. In terms of the integrated NO production rate for the MILD combustion condition, the N2O path yields the highest level, followed by the prompt, reburn, NO2, thermal, and NNH mech...

Effect of flue gas recirculation on nitric oxide (NO) emissions during the coal grate-fired process

ABSTRACTFlue gas recirculation (FGR) is a low nitrogen oxide (NOX) combustion technology. The present study used standard gas to simulate the cycle gas (the main ingredients of which are oxygen (O2), nitrogen (N2), and carbon dioxide (CO2)). The coal grate-fired process was divided into three zones, namely (1) volatilization zone, (2) main combustion zone, and (3) char combustion and burn-out zone. The effects of FGR on coal combustion and NO emissions were investigated in these zones of a unit-boiler experimental system. An industrial test was then conducted on a chain boiler that previously used FGR. Data showed that if the cycle gas was directed into the furnace from the volatilization zone, the curve of the coal surface temperature moved backwards, the temperature peak increased, and coal ignition was delayed. When the FGR rate was 20%, NO emissions/g coal was 41.8% less than in the absence of FGR, in the overall combustion process except for the volatilization zone. An industrial test demonstrated that FGR decreased the NO emissions and incomplete-combustion loss of gas. NO and carbon monoxide (CO) emissions were reduced by 26.9 and 38%, respectively. These observations may prove to be beneficial in reducing ambient air pollution and saving energy.

Optimization of Oxygen-based CFBC Technology with CO2 Capture

O2GEN project was running during more than three years and it was successfully finished in January 2016. The main target was to develop the 2nd generation oxyfuel circulating fluidized bed (CFB) power plants based on higher oxygen concentrations with the aim of decreasing flue gas recirculation and the energy penalty. Remarkable advances have been achieved in Air Separation (ASU) and Compression and Purification Units (CPU) reducing significantly their energy consumption. CFB boiler concept was proposed by scaling-up from past designs. No special drawbacks were found regarding combustion, heat transfer and emissions. Finally, a process integration methodology was applied and overall efficiency was increased by heat integration. Energy penalty was reduced from 10.5 to 7.3 efficiency points. The new power plant lay-out avoids technical restrictions in the use of complex heat exchangers and facilitates the operational flexibility of the system.

Modelling and Dynamic Simulation of the 2nd Generation Oxy Fired Power Plant – Oxidant Fan Failure Case

In large and complex processes use of simulators will take place more and more in the future to avoid and learn about transients and unexpected situations. There are plenty of dynamic simulation tools in the markets but most of them have been developed and used for quite specific and limited application areas. At the moment there are just few simulation tools which are capable to handle dynamically in the same simulation case challenging process areas like combustion, distillation, chemistry and control of very large process areas. For this reason, in this study, there was a need to carry out co–simulation where two dynamic simulation tools Apros and Aspen Plus Dynamics were combined together by using Matlab Simulink as an interface. The co–simulation approach was a way to construct large dynamic process simulation environment, where typical operational transients and failure situations of an oxy fired power plant were possible to test and analyze.Under EU 7th Framework Programme (FP7) the FLEXI BURN CFB project an oxy firing concept based on circulated fluidized bed (CFB) combustion and supercritical once-through (OTU) water steam cycle was developed and successfully demonstrated [1]. This project also generated a dynamic simulation model of the 1st generation oxy firing concept, as we call it. In the EU 7th Framework Program's project O2GEN (Optimization of Oxygen-based CFBC Technology with CO2 capture), the 2nd generation concept of CFB based oxy firing was developed. The target of the project was, at first, to increase O2 content in recirculated flue gas to see what kind of effect it has to the heat exchange and corrosion in the boiler. Secondly, the target was to carry out exergy analysis to increase the whole concept efficiency by decreasing energy consumption in the boiler, Air Separation Unit (ASU) and CO2 -purification unit (CPU). Based on these steady state analysis means the refined concept was then modelled for dynamic simulations which were carried out by using Apros and Aspen Plus Dynamics dynamic modelling tools in a co–simulation mode.This paper deals with dynamic simulation of the 2nd generation 600 MW oxy fired CFB once trough boiler. In this paper we will present the dynamic model which includes oxy fired CFB boiler, turbine island, ASU and CPU with all needed equipment like heat exchangers, pipes, pumps, air and fuel feeding systems, distillation columns and sequestration vessels. Automation like measurements, controllers and actuators are included in the model. The simulation case selected for this paper presents an analysis what happens when one of the oxidant fans trips on boiler full load operation and how to handle this kind of failure by decreasing the boiler load to sufficient level of regarding the available oxidant feeding capacity. There were several items which were taken into account in the boiler, turbine island, ASU and CPU side when this kind of failure takes impact. The main findings in the failure analysis will be presented. Finally, discussion and future aspects of the simulation approach described will be given.

CFD analysis on bioliquid co-firing with heavy fuel oil in a 400 MWe power plant with a wall-firing boiler

Liquid biomass derived from food processing, biodiesel production, or fast pyrolysis has good potential for use as fuel to displace diesel or heavy fuel oil (HFO) for heat and power, and a major demonstration was successfully conducted in a 400MWe HFO-fired power plant with a wall-firing boiler configuration using a BL blend of palm oil, its residue, and animal fat at a 20% co-firing ratio. When compared to a condition with HFO as fuel, the heat transfer rate on the membrane wall significantly decreased, and that of the convective heat exchangers in the upper furnace increased. This trend was different from what was observed at two smaller plants of 100 and 75MWe, both with a tangential-firing boiler.This study uses computational fluid dynamics (CFD) to investigate the reason for the difference in the heat transfer characteristics. In both boiler types, bioliquid (BL) combustion led to lower soot concentrations and a corresponding decrease in radiation. This directly reduced the heat absorption on the furnace wall by approximately 5% at 20%-BL co-firing in the wall-firing boiler, and it was predicted further decrease linearly by 14% at 100%-BL firing. However, the tangential-firing boilers had less than a 1% decrease in measured heat absorption on the furnace wall at 100%-BL firing. Such differences were attributable to the gas flow pattern of the two boiler configurations. The burner tilting and flue gas recirculation of the tangential-firing boilers also contributed to successful operation at 100%-BL firing. The conclusions indicate that without effective measures to control the heat absorption distribution, the capability of a wall-firing boiler for BL combustion is limited to low co-firing ratios.

Dual-Stage Drying Process of Lignite Using Pilot Scale Coal Rotary Dryer

ABSTRACTIn conventional rotary drying process, lignite is fed into the drum with rotary-valve feeder and hot flue gas is injected concurrently at the bottom of the feeder. It is called single-stage drying since lignite is passed through one-step drying process. This study presents dual-stage drying process in which the primary dried coal is reintroduced into the rotary drum which results in secondary dried coal. Its effect on particle size distribution and total moisture at various feed mass flow rates ranging from 325 to 500 kg/h were investigated. The results show that total moisture of lignite decreased significantly for both single-stage (23.9%) and dual-stage drying (11.7%) without any spontaneous combustion even at excessive temperature as high as 188°C. Recirculation of flue gas creates less oxidative air which ranged from 16.5% to 19.7% inside the rotary drum. Standard deviation of moisture content implies that dual-stage drying process is able to reduce variation in moisture content which means better drying consistency.

A numerical investigation of the effect of flue gas recirculation on the evolution of ultra-fine ash particles during pulverized coal char combustion

Flue gas recirculation (FGR) has been widely employed for NOx reduction. However, because PM removal in flue gas is never 100% efficient, a gradually accumulated particle concentration (particularly the ultra-fine particles, smaller than 1.0μm in aerodynamic diameter) may occur in the furnace, affecting PM emission and power plant operation. Experimental measurements cannot provide detailed information on the evolution of ultra-fine particle (including formation, and time-dependent growth and number size distribution). In this work, an improvement over our previous effort to model ultrafine particle formation during coal char combustion is developed and applied to elucidate the effects of various FGR parameters on ultrafine particle formation, including FGR ratio, dust removal efficiency, and particle size distribution in FGR. Model results show that FGR without recirculated particles results in the nucleation of fewer but larger particles. However, the presence of diluting FGR gases results in a low particle collision frequency and consequently yields small particle size and high number density after coalescence. The recirculated particles not only provide available surface area for condensable mineral vapors but also take part in coalescence via collision; consequently, larger particles are formed at a lower overall number density. The ultra-fine particle size first increases slowly with an increase in dust removal efficiency, but then drops rapidly after reaching a peak; An increase in recirculated particle size and a decrease in FGR results in increased particle size. For all cases, the particle number density shows opposite trends to particle size. Overall, the results derived here provide meaningful guidelines for practical application.