Flue Gas Recirculation Research Articles

Effects of (Co-)Combustion Techniques and Operating Conditions on the Performance and NO Emission Reduction in a Biomass-Fueled Twin-Cyclone Fluidized-Bed Combustor

This work studied the potential of using two (co-)combustion techniques to reduce NO emission in a novel twin-cyclone combustor with a swirling fluidized bed. Prior to a (co-)combustion study, cold-state hydrodynamic tests were performed to investigate the flow regimes and hydrodynamic characteristics of a silica sand bed employed in this reactor. Static bed height of 20 cm was found suitable to ensure bubbling fluidization of the swirling bed during (co-)firing experiments. In the (co-)combustion study, rice husk (a base fuel) was first co-fired with high-moisture sugarcane bagasse (sharing 15% of the total heat input) using an air staging technique, whereas in the second group of experiments, the base fuel was combusted alone with flue gas recirculation. Detailed investigations of (co-)combustion and emission characteristics of the two proposed techniques was performed with a 100 kW heat input to the combustor for variable operating parameters: 30–60% excess air, 0–0.3 secondary-to-total air ratio in the tests for air staging, and 5– 20% fraction of the recycling flue gas when using the second technique. In all test runs, both (co-)combustion techniques ensured NO reducing conditions in the lower combustor’s chamber, leading eventually to the NO emission reduction, compared to burning the base fuel alone. At optimal operating parameters, the combustor showed high (~ 99%) combustion efficiency and reduction of NO emission: by 30% for husk-bagasse co-firing with air staging, and by 38% when burning rice husk with flue gas recirculation, as compared to conventional combustion of the base fuel.

Numerical Investigation of the Effect of CO2/H2O Composition in Oxidizer on Flow Field and Combustion Behavior of Oxy-pulverized Coal Combustion

ABSTRACT Oxy-coal combustion is the most promising technology for the reduction of greenhouse gases from pulverized coal-fired power plants. Oxy-coal combustion employs recirculated flue gas mainly consisting of CO2 and water vapor as a diluent. Thus, the pulverized coal particles are surrounded and burned under steam rich atmosphere. The addition of H2O would have a significant impact on the combustion characteristics of oxy-coal combustion. The chemical and physical properties of steam are different than the CO2, replacement of CO2 with steam will alter heat capacity, gasification, and radiation properties considerably. The present article numerically compares ideal dry recycle oxy-coal combustion (0% H2O) with wet recycle oxy-coal (10-50% H2O) and oxy-steam combustion (H2O replaces whole CO2 from oxidant) in terms of flow field, temperature distribution, oxidizer distribution, radiative heat transfer, char consumption, and species concentration. Higher flame temperatures under enriched steam oxy-coal combustion cases were found due to lower volume heat capacity of H2O than CO2. Steam enrichment also enhanced char gasification reaction, which has affected temperature distribution and incident radiation profile inside the combustion chamber. Peak temperature obtained under oxy-steam case is around 10% higher than ideal dry recycle case (0% H2O) and 2-5% higher than the wet oxy-coal combustion cases having 50-10% H2O in the oxidizer.

Study of TGM-94 boiler with variable feed water temperature using a calculation model

The article substantiates the need to test performance capability and effectiveness of a steam boiler in the new conditions under any changes in the regenerative heating circuit of the feed water in a steam turbine plant. Such changes may include: the displacement of steam extractions onto high pressure feed water heaters by the sun, by heat from biomass or garbage burning, or by steam from third-party sources. For this purpose we have developed a mathematical model of the steam boiler TGM-94 from steam turbine installation K-150-130. The model was created with Boiler Designer program. Verification of the calculated and experimental data provided by Krasnodar TTP showed sufficient convergence in both the steam-water and gas paths. We observed differences in the following temperatures: feed water temperature after the first stage of the economizer, flue gases and air temperature at the inlet to the furnace. The maximum difference between calculation and experiment here is 6 °C. We studied the boiler operation at a variable load, but with a constant temperature of the feed water equal to the nominal value of 230 °C. It was shown that in order to work in this mode, and in order to maintain the superheat temperature of the primary and secondary steam, it is necessary to turn on the flue gas recirculation smoke exhausters. We studied the operation of the boiler at a reduced feed water temperature. It turned out that such work leads to a decrease in steam production, if we keep the fuel consumption unchanged. In the case of sustaining steam production, it is required to assess the temperature state of the heating surfaces additionally due to a significant increase in the superheat temperature of both the primary and secondary steam.

Flow behaviors, reaction kinetics, and optimal design of fixed- and fluidized-beds for CO2 methanation

The performances of three catalytic reactors for CO2 methanation, i.e., single fixed-bed (1FxB), two-stage fixed-bed (2FxB), and bubbling fluidized-bed (BFB), were evaluated at a feed flow rate of 166.5 Nm3/h. A heterogeneous catalytic reaction model (HCRM) combining the one-dimensional plug-flow model on the reactor scale and the reaction–diffusion model on the particle scale was used for 1FxB and 2FxB. A two-phase reaction model (TPRM), including the mass transfer between the bubble and emulsion phases and axial dispersion, was proposed for the BFB. The results from HCRM and TPRM were in good agreement with experimental and modeling data. The 1FxB, 2FxB, and BFB were optimized via the parametric study considering the hot spot temperature, operating temperature and pressure, space velocity, flue gas recirculation ratio, and operational stability. The catalyst weights of 1FxB, 2FxB, and BFB required for a 90% CO2 conversion were 191, 40, and 34 kg, respectively. The proposed models were able to identify the flow behaviors, mass transfer, and reaction kinetics of the fixed- and fluidized-beds, which are useful for the optimal design of CO2 methanation reactors.

Lower-arch location effect on the flow field, coal combustion, and NOx formation characteristics in a cascade-arch, down-fired furnace

For a cascade-arch low-NOx and high-burnout configuration designated as a solution to the currently incompatible problem of the ultra-low NOx combustion and high burnout in down-fired furnaces, numerical simulations, which were verified by real-furnace measurements of a 600 MWe down-fired furnace with a multiple-injection and multiple-staging combustion technology, were performed to evaluate its in-furnace flow, coal combustion, and NOx formation at various lower-arch location setups of CH = 0.65, 0.6, 0.55, and 0.5 (CH representing the ratio of the lower arch height to the front/rear wall height in the lower furnace). The aim was to evaluate its lower-arch location effect on the furnace performance and confirm its superiority compared with the prior art. With asymmetric combustion developing at extreme settings of CH = 0.65 and 0.5, the combustion symmetry and furnace performance originally improved but then worsened with decreasing CH, resulting in the exhaust gas loss, carbon in fly ash, and CO and NOx emissions all decreasing first and then increasing. Consequently, the CH = 0.6 location setup achieved the optimal furnace performance characterized as NOx emissions of 707 mg/m3 at 6% O2 and carbon content in fly ash of about 5.50%. In comparison to the prior art, the new combustion configuration owned an improved symmetrical combustion pattern, strengthened deep-air-staging conditions, and lengthened flame travel by positioning hopper air, applying flue gas recirculation, and introducing fine pulverized-coal reburning, thereby achieving the apparently improved low-NOx and high-burnout performance with a NOx reduction of 22% without affecting burnout.

Innovative system configuration analysis and design principle study for different capacity supercritical carbon dioxide coal-fired power plant

The paper constructs the innovative supercritical carbon dioxide (S-CO2) cycle and boiler coupling thermodynamic model, based on comprehensive system analysis of the S-CO2 cycle, S-CO2 boiler and medium temperature heat recovery. The results reveal that moderate cycle maximum pressure and single reheat cycle configuration show the higher cycle efficiency for S-CO2 cycle with the high boiler pressure drop. High temperature flue gas recirculation mode with large proportion and double rectangular furnace boiler layout can increase the boiler heating surface area and lower the heat duty distribution within S-CO2 boiler, which can eliminate the impact of excessive mass flow and increased average endothermic temperature in the S-CO2 cycle. Besides, the system design method and economic evaluation of S-CO2 coal-fired power plants have been constructed. Single split flow cycle configuration is recommended for the 300 MW S-CO2 power plants. And double split flow cycle configuration is compulsory for 1000 MW power plants, showing 50.13% overall power plant net efficiency and 0.705 CNY/kW·h levelized cost of energy (LCOE) at 620 °C/32 MPa. As for the 600 MW capacity power plants, the former configuration considers the boiler heating exchanger layout simplicity, and the latter configuration increases 0.36% overall power plant net efficiency.

Research into limits of gas-fired burners flame stabilization in the flue gas recirculation

Research has been performed without financial support of any grant or scientific project. The authors are grateful to the reviewers for relevant comments and recommendations on improving quality of this paper.

Key issues and innovative double-tangential circular boiler configurations for the 1000 MW coal-fired supercritical carbon dioxide power plant

When supercritical carbon dioxide (S–CO2) Brayton cycle is introduced into conventional coal-fired power plant, there exists the problems in S–CO2 boiler configurations due to the working medium physical properties’ change. This paper is taking a typical 1000 MW single reheat double-tangential circular boiler as example and investigations focus on the key issues of S–CO2 power plant with traditional boiler configuration from the thermodynamic system, combustion and heat transfer in the furnace, flow and heat transfer in tube. The results show that the studied traditional boiler configuration does not adapt for the 1000 MW S–CO2 coal-fired single reheat power plant for the mismatch heat duty. The effects of boiler size and flue gas recirculation ratio on the heat duty distribution and boiler performance are analyzed. Decreasing the boiler heat duty singly will have a negative impact on boiler performance. The newly developed “partial expansion at the upper zone + double furnace + symmetry flow mode” boiler configurations for the S–CO2 Brayton cycle are presented in order to coordinate the heat duty distribution and boiler performance. The overall net efficiency of the innovative 1000 MW single reheat S–CO2 power plant is up to 48.26%, higher ∼4% than the same-grade traditional steam power plant.

Modelling of oxy-pulverized coal combustion to access the influence of steam addition on combustion characteristics

Oxy-coal combustion is identified as the most powerful technology for the reduction of greenhouse gases from pulverized coal-fired power plants. Under oxy-coal combustion, the recirculated flue gas (RFG) is used as diluent that mainly consists of CO2 and water vapor. Thus, the pulverized coal particles are surrounded and burned under steam rich atmosphere. Addition of H2O would considerably affect combustion characteristics of oxy-coal combustion by varying heat capacity, gasification and radiation due to distinct chemical and physical properties of steam than CO2. This article numerically investigates the effect of steam addition on temperature field, oxidizer distribution, radiative heat transfer, char consumption and species concentration. The mole fraction of steam is varied from 0 to 50% at a fixed oxygen mole fraction of 21 vol% to represent various wet oxy-coal combustion cases. The oxy-steam variant of oxy-coal combustion obtained by replacing CO2 mole fraction by H2O is also compared with the different wet oxy-coal combustion cases. Results revealed higher flame temperature under enriched steam oxy-coal combustion cases due to lower volume heat capacity of H2O than CO2. Steam enrichment also enhanced char gasification reaction, which has affected temperature distribution and incident radiation profile inside the combustion chamber. Peak temperature obtained under oxy-steam case is around 10% higher than ideal dry recycle case (0% H2O) and 2–5% higher than the wet oxy-coal combustion cases having 50–10% H2O in the oxidizer.

Waste-to-energy technology integrated with carbon capture – Challenges and opportunities

Carbon dioxide emission is a serious environmental issue that humankind must face soon. One of the promising technologies for reducing global CO2 emissions is oxy-fuel combustion (OFC) technology, which belongs to the carbon capture methods. OFC involves the use of oxygen and recirculated flue gas as an oxidizer in the combustion process. Application of oxy-fuel combustion in waste incineration can result in negative CO2 emission since some part of the carbon in municipal solid waste is biogenic. Such technology is often described as BECCS or Bio-CCS and it has attracted the attention of scientists recently. In addition to easier CO2 capture, oxy-fuel combustion of municipal solid waste offers other advantages, such as reduced flue gas volume, increased combustion temperature and the possibility of retrofit existing incineration plants. In the present paper, studies of oxy-fuel combustion of waste materials, in particular, municipal solid waste and sewage sludge are presented and summarized. The study shows the opportunities and challenges that have to be addressed to fully exploit the potential of the oxy-fired incineration plant.

Influence of flue gas recirculation on the performance of incinerator-waste heat boiler and NOx emission in a 500 t/d waste-to-energy plant

The flue gas composition and the flue gas temperature at outlet of the economizer were tested, and the influence of flue gas recirculation (FGR) on the efficiency of the incinerator-waste heat boiler and NOx emission in a waste incineration power plant with a waste disposal capacity of 500 t/d was explored experimentally. The results indicate that the largest proportion of the total heat loss is the exhaust heat loss under different loads, and the next is the heat loss of slag. Within the test range, the efficiency of the incinerator-waste heat boiler increases from 80.26% to 80.42% as the ratio of the recirculating flue gas increases from 0 to 16.43%. The oxygen content in the flue gas and FGR have significant influence on NOx emissions. The NOx concentration at outlet of the economizer increases from 209.54 mg/m3 to 307.30 mg/m3, that is an increase of 46.65%, when the oxygen content at outlet of the economizer increases from 4.52% to 8.00%. Compared with the shutdown of FGR system, the NOx concentration at outlet of the economizer decreases from 209.54 mg/m3 to 126.15 mg/m3 when the FGR valve is fully opened. The results have important reference significance for the design of incinerator-waste heat boiler and the optimal operation of power plant.

Energy and exergy analysis of oxy-fuel combustion based on circulating fluidized bed power plant firing coal, lignite and biomass

Oxy-fuel combustion with circulating fluidized bed (CFB) is a promising technology applied in carbon capture and sequestration. In this work, a supercritical oxy-fuel combustion system based on CFB boiler firing coal, lignite and sawdust is established to evaluate the system performance. Five primary units are arranged including an air separation unit, a CFB combustor considering the fuel combustion characteristics, a steam generation cycle, a simplified power island and a CO2 purification and compression unit. After the validation, the influence of fuel types, oxygen concentration, flue gas recirculation forms and exhaust flue gas temperature on the system performance including adiabatic flame temperature, flue gas loss, net efficiency and exergy efficiency are evaluated. The results showed that compared to lignite and sawdust combustion, oxy-fuel combustion with coal has the highest net efficiency and exergy efficiency which is mainly because of the lower moisture and ash. However, the drying process will not significantly increase the net efficiency of sawdust combustion according to the electricity consumption of biomass drying. Also, a dry cycle is preferable in high moisture fuels combustion due to the better burnout performance and less recirculation fan work. To increase the net efficiency, reducing the exhaust flue gas temperature and increasing the combustion pressure are both effective.

Sensitivity Analysis and Optimization of Operating Parameters of an Oxyfuel Combustion Power Generation System Based on Single-Factor and Orthogonal Design Methods

The main purpose of this paper is to quantitatively analyze the sensitivity of operating parameters of the system to the thermodynamic performance of an oxyfuel combustion (OC) power generation system. Therefore, the thermodynamic model of a 600 MW subcritical OC power generation system with semi-dry flue gas recirculation was established. Two energy consumption indexes of the system were selected, process simulation was adopted, and orthogonal design, range analysis, and variance analysis were used for the first time on the basis of single-factor analysis to conduct a comprehensive sensitivity analysis and optimization research on the changes of four operating parameters. The results show that with increasing oxygen purity, the net standard coal consumption rate first decreases and then increases. With decreasing oxygen concentration, the recirculation rate of dry flue gas in boiler flue gas ( χ 1 ) and an increasing excess oxygen coefficient, the net standard coal consumption rate increases. The net electrical efficiency was just the opposite. The sensitivity order of two factors for four indexes is obtained: the excess oxygen coefficient was the main factor that affects the net standard coal consumption rate and the net electrical efficiency. The influence of oxygen concentration and oxygen purity was lower than that of excess oxygen coefficient, and χ 1 has almost no effect.

Numerical study of methane combustion under moderate or intense low-oxygen dilution regime at elevated pressure conditions up to 8 atm

This paper investigates the effect of operating pressure (up to 8 atm) on the moderate or intense low-oxygen dilution (MILD) combustion of methane. Computational fluid dynamics (CFD) modeling was firstly conducted for a laboratory-scale combustor to obtain the distributions of temperature and minor species (CH2O and OH), and pollutants (CO and NO) emission under elevated operating pressures. To provide further insight into the heat release characteristics, flame structure, CO formation mechanism as well as extinction limit, a counterflow diffusion flame with hot oxidizer diluted oxidizer configuration was also modeled using detailed chemistry. The CFD modeling results show that increasing the operating pressure tends to increase both peak value and spatial gradient of the combustion temperature, and reduce the width of the reaction zone, indicating the departure of MILD combustion regime when using the identical burner. However, enhancing the internal flue gas recirculation by minimizing the air nozzle diameter can help sustain the MILD combustion regime. The kinetic modeling shows that MILD combustion features only positive heat release region, while traditional combustion exists both negative and positive heat release regions, where multiple peaks are observed. In addition, MILD combustion shows a dual oxygen consumption stage regardless of operating pressures. Simultaneously, MILD combustion exhibits a stretched S-curve of flame response without extinction and re-ignition processes, while traditional combustion presents obvious extinction and ignition points. These unique phenomena can be used for identifying MILD combustion regime in future work.

Oxy-Fuel Combustion Characteristics of Pulverized Coal under O2/Recirculated Flue Gas Atmospheres

Oxy-fuel combustion is an effective technology for carbon capture and storage (CCS). Oxy-combustion for coal-fired power stations is a promising technology by which to diminish CO2 emissions. Unfortunately, little attention has been paid to the oxy-combustion characteristics affected by the combustion atmosphere. This paper is aimed at investigating the oxy-fuel combustion characteristics of Australian coal in a 0.3 MWth furnace. In particular, the influences of various oxygen flow rates and recirculated flue gas (RFG) on heating performance and pollutant emissions are examined in O2/RFG environments. The results show that with increases in the secondary RFG flow rate, the temperatures in the radiative and convective sections decrease and increase, respectively. At a lower oxygen flow rate, burning Australian coal emits lower residual oxygen and NO concentrations. In the flue gas, a high CO2 concentration of up to 94.8% can be achieved. Compared to air combustion, NO emissions are dramatically reduced up to 74% for Australian coal under oxy-combustion. Note that the high CO2 concentrations in the flue gas under oxy-coal combustions suggest great potential for reducing CO2 emissions through carbon capture and storage.

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.

Research on Nox Formation and SCR Denitration System Control by Smoke Mixing Under Low Load

With 2 × 330MW subcritical coal-fired boiler as the research object, the influence of two kinds of primary air flue gas recirculation schemes on the combustion of pulverized coal and the formation of NOx were studied by numerical simulation software. The variation law of SCR inlet fume temperature was studied with boiler thermodynamic calculation. The calculation results show that the reasonable smoke mixing method can effectively improve the SCR inlet flue gas temperature and reduce the furnace outlet NOx concentration. The optimal proportion was 20% with the primary air mixing smoke while maintaining constant air mass. The results of numerical simulation and thermodynamic calculation show that SCR inlet flue temperature can be increased by 15°C, the concentration of NOx emission can reduce by 18.29mg/Nm3.

Novel Test Bench for the Active Reduction of Biomass Particulate Matter Emissions

This paper introduces an experimental plant specifically designed to challenge the main operating issues related to modern biomass combustion systems (mainly NOx, particulate matter, and deposition phenomena). The prototype is an 11–18 kW overfed fixed-bed burner with a modular configuration, and the design considers the implementation of certain strategies for improving combustion: (1) a complete refrigeration system that also includes the fuel bed; and (2) an air injection control through flue gas recirculation. First, the stability and repeatability of the facility were successfully tested, establishing the duration of transient periods in the phase of experiment design. The results revealed similar effects in temperature and particulate emissions when comparing the use of the cooling bed and recirculation techniques. Reductions of 15% and up to 70% were achieved for the exhaust temperature and particulate matter concentration, respectively. Otherwise, the refrigeration considerably reduced the bed temperature, especially in its core, which enhanced the condensation of volatile salts and therefore the fouling phenomena. Although the viability of using both techniques as temperature control methods is demonstrated, further studies are needed to clarify the specific effects of each technology and to clarify the possible significance of a hybrid solution that combines both strategies.

Quantitative analysis of the impact of flue gas recirculation on the efficiency of oxy-coal power plants

Oxy-combustion typically consists of burning coal with a combination of oxygen and a large amount of recycled flue gas (60–70%) to obtain a similar heat flux profile to that of air-fired systems. As the cost of electricity from first-generation oxy-combustion is relatively high, several new oxy-combustion process concepts have been proposed in recent years, and within these, the proposed amount of flue gas recycle (FGR) has varied from near-zero to 80%. To better understand the fundamental impact of FGR on the efficiency of oxy-combustion systems, a thermodynamic approach is used herein. Second-law losses associated with flue gas recycle are found to be significant and highly non-linear with recycle ratio. A difference in efficiency of up to 10 %-points can be realized, with a maximum efficiency occurring at zero FGR. Furthermore, due to the non-linear relation of plant efficiency with recycle ratio, processes with low recycle (< ∼33%) experience only a small efficiency penalty, compared to no recycle. Additionally, fan power requirements also scale non-linearly with recycle ratio, resulting in significantly lower FGR fan power requirements for low recycle processes as well. These results suggest that for systems employing cold recycle, FGR should be kept below 33%.Due to the recent interest in developing pressurized oxy-combustion (POC) for efficient, low-cost carbon capture, the impact of flue gas recycle on POC systems is also presented, with a discussion on the valorization strategies for the latent heat of flue gas moisture recovery.

Optimization and control of NOx in coke oven vertical flue under flue gas recirculation

The orthogonal experimental design with CFD simulation was carried out to simulate combustion in the coke oven vertical flue at external flue gas recirculation. The control of nitrogen oxides (NOx) generation was analysed by different factors such as air excess coefficient, oxidant preheating temperature, flue gas recirculation and oxidizer inlet cross-sectional area ratio. The simulation results showed that the the orthogonal experimental design with CFD had a significant effect on the combustion process. The flue gas recirculation ratio effectively improved the temperature distribution uniformity in the vertical flue, and had a significant effect on NOx production in the vertical flue. Within the parameters of the orthogonal design, when the air excess coefficient was 1.1, the oxidant preheating temperature was 1200 K, the flue gas recirculation ratio was 30%, and the oxidant inlet cross-sectional area ratio was 0.5, the NOx production in the coke oven vertical flue was the smallest, lower than the industry emission standard.