Pressurized Fluidized Bed Combustion Research Articles

Erratum to: Managing of agglomerate formation in a utility scale pressurized fluidized bed combustor. Chem. Process Eng., 2010, 31, 767–788

Erratum to: Managing of agglomerate formation in a utility scale pressurized fluidized bed combustor. Chem. Process Eng., 2010, 31, 767–788

Preliminary Life Cycle Assessment of a Net-zero Power Plant Co-fired with Waste Coal and Biomass

In this study, we apply life cycle assessment (LCA) to a novel power plant being designed to run on waste coal and biomass. LCA is critical in helping guide the plant engineering design towards net neutral or net negative emissions. The power plant will utilize an advanced power generation technology, Pressurized Fluidized Bed Combustion (PFBC) to co-fire waste coal and biomass. The ability of PFBCs to combust fine, wet, waste coal with a high efficiency helps deliver low emissions while avoiding the need to mine new coal.. The objective of this study is to identify the ratio of biomass required to achieve net neutrality so that the power plant can be designed with the necessary infrastructure. Our results show that firing with 4–13% biomass by mass is expected to achieve net-zero emissions.

Particulate matter formation mechanism in pressurized fluidized bed combustion of various solid fuels

Pressurized fluidized bed combustion (PFBC) is an advanced clean coal technology, and co-combustion of coal and biomass can effectively reduce pollutant and greenhouse gas emission during power generation. However, studies concerning the particulate matter formation during coal and biomass co-firing in PFBC are very rare. In this work, experiments on combustion of different solid fuels (two lignites, one bituminous coal, rice husk) and co-combustion of coal and biomass were carried out at 10 kW pressurized fluidized bed with system pressure of 0.5 MPa. Despite the different fuel types, pressurized combustion produces PM1 with higher content of easily vaporized elements (Na, K, and S), while super-micron particles are mainly composed of refractory elements (Ca and Si). For the cases of cofiring coal with biomass, it was found that the mineral interaction can reduce the yield of submicron particles, such as PM0.4 formation in Zhundong lignite/rice husks and Xiaolongtan lignite/rice husks were reduced by 28% and 47%, while the reduction is 61% and 64% for PM1, and 80% and 75% for PM2.5. Overall, this work provides the insight on particulate matter formation during the combustion of various solid fuels in pressurized combustion conditions, which hopefully can help the development of PFBC technology.

Permeation Behavior of Oxide Bonded SiC Ceramics at High Temperature and Prediction of Pressure Drop in Candle Filters

Oxide-bonded silicon carbide supports of porosity ranging from 33% to 47% were prepared by heating powder compacts (SiC, clay and alumina) at 1400°C in air with graphite acting as a pore former. The supports were spray-coated with an aqueous slurry of fine SiC powder (d50=15 μm), then sintered to produce a filtering layer with thickness ranging from 116 to 200 μm and average pore size ranging from 5 to 20 μm. Airflow tests were performed on both supports and coated filters at temperatures ranging from 25° to 700°C and superficial velocities ranging from 0.02 to 0.9 m.s−1. Experimental permeability coefficients were used to simulate the pressure drop behavior of hypothetical candle filters for industrial combustion/gasification processes (biomass combustion in water-tube steam boilers (BCSB), pressurized fluidized-bed combustion (PFBC) and integrated gasification combined cycle (IGCC)). The simulated permeation properties of the hypothetical candles were compared to those of commercial hot gas filters.

Thermodynamic analysis of the 2nd generation pressurized fluidized-bed combustion cycle utilizing an oxy-coal boiler and a gasifier

This study proposes the 2nd generation hybrid pressurized oxy-coal combustion power cycle that utilizes both fluidized-bed combustion coupled with the Rankine cycle and gasification connected to the Brayton cycle. The fluidized-bed boiler and gasifier are thermally connected by a flue gas stream flowing from the former to the latter. This improves thermal integration carbon conversion in the gasifier. The Rankine cycle is thermally connected to the Brayton cycle through the reheating process of intermediate pressure steam. This hybrid cycle enables the system to recover thermal energy from the pressurized flue gas with effective thermal integration in a way that it balances heat flow rates between hot and cold streams and increases the degree of freedom for heat flow control. The proposed system also reuses cold boiler exhaust gas as a diluent of the Brayton cycle to control its gas turbine inlet temperature, which raises the gas turbine flow rate and hence the gross power. The results indicate more than 9%-point and 1%-point increases in the gross and net efficiency, respectively, as compared to a single Rankine cycle system. The proposed hybrid system exhibits the net efficiency of 34.39% (HHV) higher than 33.04% (HHV) of the single oxy-PFBC cycle.

Experimental study and modeling of oxy-char combustion in a pressurized fluidized bed combustor

Pressurized oxy-coal combustion has received wide attention recently due to its potential of high efficiency and low cost in CO 2 capture. However, there is still a significant knowledge gap on the effects of pressure on the oxy-coal and oxy-char combustion behaviors. In this study, oxy-char combustion experiments were carried out in a pressurized fluidized bed combustor, and the effects of pressure and oxygen concentration on the burnout time, reactivity and temperature of the char particles were investigated. The experimental results showed that the burnout time of the char particles decreased significantly with an increase in pressure from 0.1 MPa to 0.5 MPa. Besides, an increase in either the combustion pressure (within the range of 0.1 – 0.5 MPa) or the oxygen concentration (within the range of 10 – 30% O 2 in CO 2 ) led to increases in the reactivity and temperature of the char particles, while the promotional effects of the pressure increase were weaker at the higher pressures. In order to better interpret the experimental results, a combustion model considering reaction kinetics, mass and heat transfer, and energy balance was developed and validated by the experimental results. The model simulation results showed that an increase in pressure would reduce the burnout time of char particles only when the combustion was kinetically controlled or combined kinetic-diffusion controlled. Once the combustion became diffusion-controlled, a further increase in pressure would have an insignificant effect on the char burnout time. Although the values of kinetic constants were different when different kinetic models, the global power-law model and Langmuir-Hinshelwood model, were adopted, the simulation results of the burnout time of the char particle under the pressurized fluidized bed combustion conditions were almost the same since the char combustion was mainly diffusion-controlled. The elevated combustion pressure accelerated both the char combustion rate and the char-CO 2 gasification rate, while the carbon consumption ratio by gasification increased with pressure. Moreover, the enhanced gasification reaction had a great impact on the particle temperature especially under the higher oxygen concentration combustion conditions.

Condensed phase corrosion of P91 and DSS 2205 steels at advanced oxygen‐fired pressurized fluidized bed combustion plants

AbstractTo achieve the targets of high energy efficiency and reduced CO2 emission, advanced oxygen‐fired pressurized fluidized bed combustion technology is being developed. The generated flue gas condensates are very corrosive, but very limited information is available to select appropriate alloys for the cost‐effective construction and long‐term safe operation of flue gas components. Thus, this study investigated the corrosion performance of P91 and DSS 2205 steels in the simulated condensates at 60°C–150°C. The dominant reactions on the two steels were considerable oxide formation and high chemical dissolution of the formed oxides instead of localized pitting. The increase in temperature leads to an exponential increase in the long‐term corrosion rates of the steels. Benefited from its high Cr and Mo contents, DSS 2205 steel exhibited much better corrosion resistance, and the formed surface scales consisted of inner Fe‐enriched and outer Cr‐enriched oxides in which Cr2O3 was transformed into Cr(OH)3 with the increase in temperature. The corrosion products on P91 steel consisted of inner Cr–Fr–Mo oxides and outer Fe‐enriched oxides, which were porous and unable to protect the steel.

Oxy-coal combustion in a 30 kWth pressurized fluidized bed: Effect of combustion pressure on combustion performance, pollutant emissions and desulfurization

Oxy-coal combustion with pressurized fluidized beds has recently emerged as a promising carbon capture and storage (CCS) technology for coal-fired power plants. Although a large number of energy efficiency analyses have shown that an increase in combustion pressure can further increase the net plant efficiency, there are few experimental studies of pressurized oxy-coal combustion conducted on fluidized bed combustors/boilers with continuous coal feeding. In this study, oxy-coal combustion experiments with lignite and anthracite were conducted with a 30 kWth pressurized fluidized bed combustor within the pressure range of 0.1 MPa to 0.4 MPa. The investigation focused on the elucidation of the impacts of combustion pressure on the combustion performance, pollutant emissions and desulfurization of oxy-coal combustion in fluidized beds. The results showed that an increase in pressure increased the combustion efficiency and combustion rate of coal particles, and the promoting effect of pressure increase was more significant for the high rank coal with smaller particle size and the high O2 concentration atmosphere. For both coals, NOx emissions decreased with pressure but N2O emissions increased with pressure and accounted for a considerable part of the nitrogen oxide pollutants under high pressure oxy-coal combustion conditions. The pressure had insignificant impact on the SO2 emissions of oxy-coal combustion but an increase in pressure enhanced the direct desulfurization of limestone.

Corrosion of duplex stainless steel 2205 in hot flue gas environments produced at advanced oxy-fired pressurized fluidized bed combustion plants

The selection of constructional material of flue gas components is crucial for the successful deployment and long-term safe operation of the newly developed oxy-fired pressurized fluidized bed combustion plants. In this study, the corrosion performance of a candidate steel, duplex stainless steel 2205 (DSS 2205) was investigated in simulated flue gas mainly composed of CO2/H2O/O2/N2/SO2/HCl with a total pressure of 1.50–1.64 MPa at 270−320 °C. The oxidation kinetics of DSS 2205 followed parabolic law, and the formed surface oxide was a double-layer structure with outer Fe-rich oxide and inner Cr-rich oxide. Besides general oxidation, SO2 and HCl-induced pitting also occurred. Increasing temperature led to enhanced general oxidation and accelerated pitting. Extending exposure duration resulted in increased pit density, but did not noticeably affect the maximum pit depth. These results indicate that the steel is a promising candidate for further pilot- and industrial-scale assessments.

3D simulation on pressurized oxy-fuel combustion of coal in fluidized bed

Pressurized oxy-fuel combustion technology is considered as a perspective carbon capture technology in industrial process. A computational fluid dynamics (CFD) model based on Multi-Phase Particle-In-Cell (MP–PIC) method was developed to predict pressurized oxy-coal combustion process in fluidized bed. The heterogeneous and homogeneous combustion reactions of coal were considered in this model. The predicted results were validated the accuracy of this model with experimental data from a 15 kWth pressurized fluidized bed combustor in terms of the gas component and temperature characteristics. The characteristics of gas–solid flow and combustion under different pressure (0.1–2 MPa) and oxygen atmosphere were studied in this work. The predicted results show that the intensity of particle motion and the expansion degree in the fluidized bed was gradually decreased with an increase in pressure. A correlation was proposed based on the simulation results to maintain suitable fluidization conditions in pressurized circulating fluidized bed at different pressures. The temperature of particle phase region gradually increased with combustion pressure and inlet O2 concentration increased. In addition, the CO2 concentration in outlet increased while the emission of CO and NOx decreased as the combustion pressure increased.

Research and Development of Supercritical Carbon Dioxide Coal-Fired Power Systems

Using supercritical carbon dioxide (S-CO2) Brayton cycle instead of the traditional steam Rankine cycle is a promising technique to enhance the coal-fired power generation efficiency. Researchers from all over the world are actively designing and exploring efficient S-CO2 coal-fired power plants in recent years with great efforts made to overcome the significant technical challenges in the cycle layouts of S-CO2 and its specific thermal integration with coal-fired heat resources. This paper provides a detailed review of the research progress on the coal-fired power generation using S-CO2 Brayton cycles. The basic knowledge of S-CO2 properties, the promising S-CO2 power cycles and the conceptual designs for S-CO2 coal-fired power plants are comprehensively summarized, with some key issues in the constructing process and the corresponding engineering solutions being emphatically discussed. Based on the current achievements, the overall technical and economic evaluations on the S-CO2 coal-fired power system are figured out. Furthermore, the specific integration applications of S-CO2 cycles with different coal firing devices and modes including pulverized coal combustion, circulating fluidized bed combustion, oxy-coal combustion, pressurized fluidized bed combustion, chemical looping combustion are discussed. Finally, the main challenges requiring further studies are highlighted.

Experimental study of SO2 emissions and desulfurization of oxy-coal combustion in a 30 kWth pressurized fluidized bed combustor

Pressurized oxy-fuel combustion (POFC) has attracted wide attention due to its potentials of high efficiency and low cost in CO2 capture. Compared with numerical simulation and system analysis, there are few experimental studies about POFC. In order to investigate the SO2 emissions and desulfurization behaviors of limestone under continuous fuel-feeding POFC conditions, a series of oxy-coal combustion experiments were conducted with a 30 kWth pressurized fluidized bed combustor. The results showed that the SO2 emission was almost independent of combustion pressure and excess oxygen coefficient, while it was higher in air than in O2/CO2 atmosphere because less sulfur was retained by the coal ash. Although the higher CO2 partial pressure caused by an increase in combustion pressure from 0.1 MPa to 0.4 MPa had negative effects on the calcination of limestone and inhibited the indirect desulfurization, the higher combustion pressure was beneficial to improve the direct desulfurization efficiency. An increase in combustion temperature from 850 °C to 950 °C significantly improved the desulfurization efficiency of limestone with both atmospheric and pressurized oxy-coal combustion. SEM images were obtained and used to show the surface morphology of limestone products under different combustion conditions, which was helpful to the understanding of the desulfurization behaviors of limestone.

Experimental study of NOx emissions in a 30 kWth pressurized oxy-coal fluidized bed combustor

As one of the most promising carbon capture technologies for coal-fired power plants, oxy-coal combustion has attracted wide interests during the last two decades. In comparison to atmospheric oxy-fuel combustion, pressurized oxy-fuel combustion has the potential to further reduce the energy penalties caused by the carbon capture and storage and improve the net power plant efficiency. Although many researchers have investigated the NOx emissions of atmospheric oxy-coal combustion, the NOx emission behaviors under pressurized oxy-coal combustion conditions are much less understood and further comprehensive experimental investigations with continuous fuel-feeding pressurized oxy-coal combustion systems are needed in order to fill this knowledge gap. In the present study, a series of oxy-coal combustion experiments were conducted in a 30 kWth pressurized fluidized bed combustor. The effects of combustion pressure, bed temperature and excess oxygen on the NOx emissions were investigated systematically. The experimental results have shown that an increase in combustion pressure from 0.1 MPa to 0.4 MPa leads to a significant reduction in NOx emissions. An increase in bed temperature or excess oxygen results in higher NOx emissions under the higher combustion pressure conditions, which is consistent with what is observed under the atmospheric pressure combustion condition. Besides, it is found that the promoting effect of temperature increase on NOx emissions under the higher combustion pressures is weaker than that under the atmospheric pressure.

Concept design of supercritical CO2 cycle driven by pressurized fluidized bed combustion (PFBC) boiler

Boiler is key to influence supercritical carbon dioxide (S-CO2) cycle performance. Here, S-CO2 cycle driven by pressurized fluidized bed combustion (PFBC) boiler is investigated. The proposed systems A and B include recompression cycle with reheating (RC + RH), air compression-gas turbine (AC-GT), simple Brayton cycle (SRC), and RC + RH, AC-GT, respectively. A simulating code is developed for system analysis. We show that system A has higher efficiencies than system B. Larger flue gas pressure drop in PFBC boiler apparently worsens system performance. Effect of combustion pressures of PFBC boiler (Pfur) is thoroughly analyzed. Energy distributions in the system is affected by Pfur. With Pfur increase from 1.0 MPa to 3.5 MPa, boiler efficiencies are increased due to the decreased flue gas heat discharged to environment. Power generation efficiencies are increased but approach constant value. To overcome the challenge of heating surface arrangement in boiler, bubbling PFBC boiler is recommended. Compact heat exchangers are suggested in the top “pure” flue gas region. To further make it easier for heating surface arrangement, excess air coefficient is suggested to be increased to decrease thermal load fraction in furnace αfurnace. Flue gas recirculation is a better way to decrease αfurnace, which is applicable for system B.

Heat transfer in a pressurized fluidized bed with continuous addition of fines

The goal of this work was to investigate the impact of pressure and the presence of fine particles (i.e., a surrogate for pulverized fuel) on the overall surface-to-bed heat transfer coefficient in relation to an oxygen-fired pressurized fluidized bed combustor. Experiments were conducted in a pilot-scale fluidized bed with an inner diameter of 0.15 m under cold flow conditions. A tube bundle consisting of five horizontal staggered rows was completely submerged in the bed. One of the tubes was replaced by a heating cartridge housed in a hollowed copper rod. Five thermocouples distributed at 45° intervals along the copper rod circumference measured the surface temperature and ensured that local effects were included. The bed material was glass beads of 1.0 mm in diameter while the fines were glass beads of 60 μm in diameter and thus susceptible to entrainment. The fine particles were continuously fed to the fluidized bed and then captured downstream by a filter system. Fluidization was conducted at 101, 600 and 1200 kPa (abs) with excess gas velocities (Ug-- Umf) of 0.21, 0.29 and 0.51 m/s. Fine particle feed rates were 0, 9.5 and 14.4 kg/h. Two heating rod positions (2nd row and 4th row) were studied. Overall, the heat transfer coefficient approximately doubled when pressure was increased from 101 to 1200 kPa. At atmospheric conditions, where the slug flow regime occurred, the maximum heat transfer coefficient was at the bottom of the rod, while it moved to the side of the rod at high pressures where the bubbling regime occurred. When the heating rod was moved from the 2nd row to the 4th row, the averaged heat transfer coefficient increased by 18%, 9% and 6% at 101, 600 and 1200 kPa. The addition of fine particles decreased the average heat transfer coefficient by 10 to 20 W/m2 K, where the averaged coefficients were approximately 220 and 450 W/m2 at 101 and 1200 kPa respectively, but there was no effect on the angular heat transfer profile around the tube surface. The results showed that average heat transfer coefficients matched the correlation developed by Molerus et al. (1995) within a 5% difference across all conditions when fines were not present.

Simulation of a calcium looping CO2 capture process for pressurized fluidized bed combustion

The Canadian regulations on carbon dioxide emissions from power plants aim to lower the emissions from coal‐fired units down to those of natural gas combined cycle (NGCC) units. Since coal is significantly more carbon intensive than natural gas, coal‐fired plants must operate at higher net efficiencies and implement carbon capture to meet the new regulations. Calcium looping (CaL) is a promising post‐combustion carbon capture (PCC) technology that, unlike other capture processes, generates additional power. By capturing carbon dioxide at elevated temperatures, the energy penalty that carbon capture technologies inherently impose on power plant efficiencies is significantly reduced. In this work, the CO2 capture performance of a calcium‐based sorbent is determined via thermogravimetric analysis under relatively high carbonation and low calcination temperatures. The results are used in an aspenONE™ simulation of a CaL process applied to a pressurized fluidized bed combustion (PFBC) system at thermodynamic equilibrium. The combustion of both natural gas and coal are considered for sorbent calcination in the CaL process. A sensitivity analysis on several process parameters, including sorbent feed rate and carbonator operating pressure, is undertaken. The energy penalty associated with the capture process ranges from 6.8–11.8 percentage points depending on fuel selection and operating conditions. The use of natural gas results in lower energy penalties and solids circulation rates, while operating the carbonator at 202 kPa(a) results in the lowest penalties and drops the solids circulations rates to below 1000 kg/s.

Study of particle residence time in a pressurized fluidized bed with in-bed heat exchanger tubes

Anthropogenic climate change is amongst the greatest of challenges to human civilization. A key area that will play a large role in mitigating its effects are clean fossil fuel applications. Clean coal combustion can be achieved with an oxygen-fired pressurized fluidized bed combustor incorporating carbon capture and storage. In relation to pressurized fluidization processes, understanding the influence of pressure on fluidized bed hydrodynamics and, in turn, their effect on parameters including fuel residence time is essential. For the combustor under consideration here, a fraction of the heat exchanger boiler tubes are submerged in the fluidized bed such that the effect of the horizontal tube bundle on the fuel residence time is of great importance. The main focus of the present work was to evaluate the impact of gas velocity, pressure, presence or absence of a horizontal tube bundle and fuel feed rate on the average fuel residence time in a dense gas-solid fluidized bed. Experiments were conducted under cold flow conditions in a pressurized fluidized bed with an inner diameter of 0.15 m. The fluidization material was large glass beads (1.0 mm in diameter) while fuel particles were simulated with smaller glass beads (64 and 83 μm in Sauter mean diameter) that were susceptible to entrainment. Operating pressures and superficial gas velocities were maintained between 101.3 and 1200 kPa and 1.5 and 3.2 Umf, respectively. To simulate continuous fuel injection, experiments were conducted with the fuel surrogate particles being continuously fed to the fluidized bed of large particles over a desired period of time. Downstream, entrained particles were captured to determine the average entrainment rate and average mass of fuel particles inside the fluidized bed at steady state, which yielded the average fuel residence time. The combination of elevated pressure with the tube bundle present was found to have the most influential impact when compared to base conditions of atmospheric pressure and with no tube bundle present. It was found to enhance gas bubble break up and reduce the average gas bubble size substantially. In turn, this increased the average residence time of 83 μm particles by nearly three-fold in comparison to the case of atmospheric pressure with no tube bundle present. The effect of gas velocity on particle residence time was not found to be statistically significant under the range tested. Similarly, the effect of increasing fuel feed rate by 50% had no statistically significant impact.

Exergy Analysis of Captive Power Plant

Energy sector is one of the important areas in the economic development of any country. Meeting the growing demands at acceptable costs in various sectors like industries, commercial, transport etc. is challenge to energy planner. The present work is an account of energy and exergy losses and distribution in Heat Exchanger of captive power plant of M/S Vardhaman Acrylics ltd, Jhagadia, Bharuch, Gujarat, India. This plant consists of 5.5 MW turbine-generator set with two extraction points which supply the heat energy to meet process demand. The steam is generated in 1?42 TPH capacity 45 kg/cm2 pressure, atmospheric pressure fluidized bed combustion boiler supplied by M/S Thermax Babcock & Wilcox Ltd,Pune ,India. The complete energy & exergy analysis of Heat Exchanger is carried out based on massive data collection over a period of three months. The energetic and exergetic losses of various components & suggestion for improving energetic & exergetic efficiency of the plant form core of this work.

Thermodynamic assessment of a hybrid particle-based concentrated solar power plant using fluidized bed heat exchanger

In this study, we modify a practical hybrid system developed by National Renewable Energy Laboratory (NREL) and analyze it thermodynamically through energy and exergy methods for analysis, performance assessment and evaluation purposes. In this system, the solid particles are used as the direct heat transfer medium in solar receiver, thermal energy storage and fluidized bed heat exchanger. Furthermore, the hydrodynamic aspects of the fluidized bed (FB) are conducted to reveal the pressure drop and the fluidizing-gas compression power as two important factors in the FB consideration. The present results show that a application of the particle FB heat exchanger in the Rankine power cycle and the pressurized fluidized bed combustion (PFBC) in the air Brayton cycle enable the whole solar plant performance to achieve the energy and the exergy efficiencies of 50% and 53.8% respectively. In addition, the particle energy storage and the solar tower sub-systems potentially can achieve the high energy efficiencies up to 88% and 44.2% and the exergy efficiencies of 96% and 35.1%, respectively. Moreover, highest exergy destructions take place in the PFBC due to chemical reaction and heliostat field and particle receiver due to the high-temperature gradients.

Experimental investigation on the coal combustion in a pressurized fluidized bed

Pressurized oxy-coal combustion is considered as one of the most promising carbon capture technologies in terms of high carbon capture efficiency and low cost. However, practical operational experience with pressurized coal combustion in a fluidized bed, especially with continuous coal feeding, is still very limited. In this study, a pressurized fluidized bed combustion system was developed and a series of coal combustion experiments were carried out with continuous coal feeding under the pressures from 0.1 to 0.4 MPa. The effects of the combustion pressure and stoichiometric air coefficient on the fluidized bed combustion performances of a Chinese lignite in terms of the temperature distribution profile, apparent solid holdup, combustion efficiency, conversion ratio of carbon in coal to CO2 and ash composition were investigated. The experimental results show that an increase in pressure is beneficial to the improvement of combustion efficiency, and the positive effect of increasing stoichiometric air coefficient on the conversion ratio of carbon in coal to CO2 is more obvious with a lower combustion pressure. The bottom ash and fly ash have similar chemical compositions under different pressures. Based on the experimental data, a correlation as the functions of pressure and stoichiometric air coefficient is proposed to predict the conversion ratio of carbon in coal to CO2.