Circulating Fluidized Bed Boiler Research Articles

Wear Resistant and Refractory Nonmetal Material of CFB Boiler

The key in restricting CFB (circulating fluidized bed) boiler running was material selection, construction fixing, pouring, coating, structure designing and running mode of wear resistant and refractory nonmetal material. Based on the comparison of CFB boiler and PC boiler, this paper expatiated on the failure, abrasion mechanism, material capability and selecting material principle of wear resistant and refractory nonmetal material, pointed out mistakes of utility boilers material selection and arranged reasonable principle of selecting material from the angle of improving working life of boiler and enhancing wearing resistance property of boiler.

Changes in trace element contents in ashes of oil shale fueled PF and CFB boilers during operation

Two oil shale combustion technologies, pulverized firing (PF) and circulated fluidized bed (CFB) were compared with respect to partitioning of selected elements (Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Rb, Sb, Sn, Sr, Th, Tl, U, V, and Zn) in the ashes along the flue gas ducts. The ash samples were characterized by high-resolution ICP-MS. The average contents of toxic heavy metals in fly ash samples from the CFB boiler are lower compared to the PF boiler. Main differences in trace element contents between combustion technologies were as follows: Cd content in the fly ash samples of PF boiler was up to 0.9mg/kg while in CFB boiler it remained below 0.1mg/kg in all analyzed ash samples; Hg was observed in the ashes of electrostatic precipitator (ESP) of CFB boiler while in the PF boiler it was close to or below detection limit. In the PF boiler content of Sn was detected only in the ashes of ESPs, while in CFB boiler it was evenly distributed between bottom and fly ash samples. Highest content among heavy metals in ash samples was observed for Pb in the last field of ESP of the PF boiler (142mg/kg).

Effects of the Updated National Emission Regulation in China on Circulating Fluidized Bed Boilers and the Solutions To Meet Them

The advantage of circulating fluidized bed (CFB) boilers in China is their ability to utilize low rank coal with low cost emission control. However, the new National Emission Regulation (NER) issued in early 2012 brings much more stringent challenges on the CFB industries, which also causes much attention from other countries. Based on the principle of a CFB boiler and previous operating experience, it is possible for the CFB boilers to meet the new NER and maintain the advantage of low cost emission control, while, more influences should be considered in their design and operation. To meet the requirement of the new NER, the fly ash collector should adopt a bag house or combination of electrostatic precipitator and bag filter to ensure dust emissions of less than 30 mg · Nm(-3). For SO2 emission control, the bed temperature should be strictly lower than 900 °C to maintain high reactivity and pores. The limestone particle size distribution should be ranged within a special scope to optimize the residence time and gas-solid reaction. At the same time, the injecting point should be optimized to ensure fast contact of lime with oxygen. In such conditions, the desulfurization efficiency could be increased more than 90%. For lower sulfur content fuels (<1.5%, referred value based on the heating value of standard coal of China), increasing Ca/S enough could decrease SO2 emissions lower than that of the new NER, 100 mg · Nm(-3). For fuels with sulfur content higher than 1.5%, some simplified systems for flue gas desulfurization, such as flash dryer absorber (FDA), are needed. And the NOx emissions of a CFB can be controlled to less than 100 mg · Nm(-3) without any equipment at a bed temperature lower than 900 °C for fuels with low volatiles content (<12%), while for fuels with high volatiles, selective non-catalytic reduction (SNCR) should be considered. Due to the unique temperature in CFB as well as the circulating ash, the efficiency of SNCR could reach as high as 70%. The Hg emission of CFB is very low for the new NER due to its innate property.

An adaptive fuzzy controller based on harmony search and its application to power plant control

Considering the reliability and safety requirements of power stations, a novel adaptive fuzzy controller is presented to implement non-overshoot control in power plants in which the Lyapunov-based adaptive law is employed to guarantee the stability of the controller while a modified Adaptive Binary Harmony Search (ABHS) algorithm is utilized to search the optimal control parameters to improve the dynamic performance. Two strategies are analyzed and used with ABHS to guarantee the non-overshoot of control. Furthermore, a simple but efficient repair operation is developed to deal with the time-varying characteristic of the thermal process. Finally, the proposed ABHS-based Adaptive Fuzzy Control (ABHSAFC) method is applied to the bed temperature control of the Circulating Fluidized Bed Boiler (CFBB). The experimental results demonstrate that ABHSAFC can implement the expected non-overshoot control efficiently and outperforms the classical Lyapunov-based adaptive fuzzy control, ABHS-based fuzzy control and ABHS-based PID control. Considering the characteristics of the easy implementation, robustness and the guaranteed stability, ABHSAFC promises a favorable engineering application prospect.

Decreased PCDD/F formation when co-firing a waste fuel and biomass in a CFB boiler by addition of sulphates or municipal sewage sludge

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are formed during waste incineration and in waste-to-energy boilers. Incomplete combustion, too short residence times at low combustion temperatures (<700°C), incineration of electronic waste and plastic waste containing chlorine are all factors influencing the formation of PCDD/Fs in boilers. The impact of chlorine and catalysing metals (such as copper and iron) in the fuel on PCDD/F formation was studied in a 12MWth circulating fluidised bed (CFB) boiler. The PCDD/F concentrations in the raw gas after the convection pass of the boiler and in the fly ashes were compared. The fuel types were a so-called clean biomass with low content of chlorine, biomass with enhanced content of chlorine from supply of PVC, and solid recovered fuel (SRF) which is a waste fuel containing higher concentrations of both chlorine, and catalysing metals. The PCDD/F formation increased for the biomass with enhanced chlorine content and it was significantly reduced in the raw gas as well as in the fly ashes by injection of ammonium sulphate. A link, the alkali chloride track, is demonstrated between the level of alkali chlorides in the gas phase, the chlorine content in the deposits in the convection pass and finally the PCDD/F formation. The formation of PCDD/Fs was also significantly reduced during co-combustion of SRF with municipal sewage sludge (MSS) compared to when SRF was fired without MSS as additional fuel.

Gas emissions from a large scale circulating fluidized bed boilers burning lignite and biomass

Previously established and validated coal combustion model in a circulating fluidized bed (CFB) was employed to predict co-combustion of lignite and biomass processes. The validity of the model was successfully performed on a large-scale 261MWe COMPACT CFB boiler. Forest biomass, sunflower husk, willow and lignite coal were applied in co-combustion tests with different shares of biomass and lignite. The energy fraction of biomass in fuel blend was: 7%, 10% and 15%. Emissions of CO2, CO, SO2 and NOx (i.e., NO+NO2) from the co-combustion tests, measured during experiments and predicted by model were compared. The gaseous pollutant emissions, evaluated using the developed model were in a good agreement with experimental results.

Design optimization of the bell type blast cap employed in small scale industrial circulating fluidized bed boilers

Blast cap is the key component of air distributor in circulating fluidized bed (CFB) boilers. Its configuration and performance affects the fluidization quality of the bed, as well as the boiler performance. With the advantage of controlling the jet penetration characteristics and the resistance coefficient individually, bell type blast cap has been widely applied in big-scale utility CFB boilers. Moreover, the inner circuitous gas pass of the bell-type blast cap is able to prevent the bed material from flowing backward into the air chamber effectively, and its tube shield is also convenient to be maintained and replaced. Consequently, the bell-type blast cap is also fit for the small scale industrial CFB boilers with lower operation and maintenance levels in spite of higher manufacturing cost. At present, bell type blast cap is mainly applied in the large scale utility CFB boilers, seldom employed in the small scale industrial CFB boilers with lower bed pressure drop. In the study, aim to acquire favorable fluidization quality and reasonable pressure drop of the bed of small scale industrial CFB boiler, the air jet penetration characteristics of the bell type blast cap were investigated in static beds composed of two typical solids in CFB individually, and the impacts of the inner configuration on blast cap’s resistance characteristics were studied via numerical simulation. The detailed design principle and approach of bell type blast cap was proposed finally based on the study results.

Identification and predictive control for a circulation fluidized bed boiler

This paper introduces the design and presents the research findings of the identification and control application for an industrial Circulation Fluidized Bed (CFB) boiler. Linear Parameter Varying (LPV) model is used in the model identification where steam flow is selected as the operation-point (scheduling) variable. Three kinds of weighting functions, namely linear, cubic splines and Gaussian functions are compared. LPV model based Model Predictive Control (MPC) is also simulated. Test results show that LPV model is more accurate than linear model, and LPV MPC yields a better control effect than linear MPC.

Study on Air Preheater Corrosion Problem of CFB Biomass Directed-Fired Boiler in Zhanjiang Biomass Power Plant

Biomass has attracted more and more attention in the world as renewable energy. Biomass direct-fired technology is a relatively mature technology with widely application in China. Due to the high content of alkali metals and chlorine in the biomass fuel, biomass direct-fired power plant suffers from ash-deposition and corrosion problems of low-temperature heating surface in boiler, accordingly impact the security and economy of boiler. Based on the literature survey, on-site detection, as well as EDXRF experimental analysis, reasons which resulted in ash-depostion and corrosion problems of air preheater are analyzed for 2×50MW CFB (Circulating Fluidized Bed) boilers in Zhanjiang Biomass Direct-fired Power Plant by Guangdong Yudean Group. Suggestive measures to prevent ash-depostion and corrosion are also proposed in this paper by the analysis of biomass combustion characteristics, ash-depostion and corrosion formation mechanism.

LOW GRADE FUEL OIL SHALE AND BIOMASS CO-COMBUSTION IN CFB BOILER

Estonia has two of the world's largest oil shale firing circulating fluidized bed (CFB) units with a designed electrical capacity of 215 MW each. The units are based on double boiler CFB technology provided by Foster Wheeler Energia OY. The units are located at Eesti and Balti power plants (EPP and BPP). The paper presents analyses of data obtained from tests of oil shale and biomass co-combustion in the full-scale CFB boiler located at BPP. The tests were conducted at nominal boiler load: 100% (314 t/h), with a biomass thermal input of 15%. During the experiments ash samples from the furnace chamber (bottom ash), INTREX, super-/reheater (SH, RH), economizer (ECO), and air preheater (APH), and from all four fields of the electrostatic precipitator (ESP) were taken. Samples of fly ash for determining the mass division (total suspended particulates PM10 and PM2.5) were taken after the ESP. The gas analysis was performed at the ESP inlet. Analysis of the chemical composition of ash was carried out. The specific consumption of oil shale per useful heat and gross electricity were found and other techno-economic characteristics determined. It was found that oil shale and biomass co-combustion reduced CO2 emission by 14.6% and ash formation by 16% when compared with con- ventional oil shale CFB combustion. The SO2 emissions remained in the limits of 20-30 mg/Nm 3 . Total suspended particulates PM10 and PM2.5 did not change compared to conventional oil shale CFB firing. The CFB boiler efficiency even increased slightly, when it is known that in case of coal and biomass co-combustion it decreases. Therefore, oil shale and biomass co- combustion can be considered as a viable option and near-term solution for reducing the environmental impact of oil shale-based power production.

Influence of Sorbent Characteristics on Fouling and Deposition in Circulating Fluid Bed Boilers Firing High Sulfur Indian Lignite

125 MWe circulating fluidized bed combustion (CFBC) boiler experienced severe fouling in backpass of the boiler leading to obstruction of gas flow passage, while using high sulfur lignite with sorbent, calcium carbonate, to capture sulfur dioxide. Optical microscopy of the hard deposits showed mainly anhydrite (CaSO4) and absence of intermediate phases such as calcium oxide or presence of sulfate rims on decarbonated limestone. It is hypothesized that loose unreacted calcium oxides that settle on tubes are subjected to recarbonation and further extended sulfation resulting in hard deposits. Foul probe tests were conducted in selected locations of backpass for five different compositions of lignite, with varied high sulfur and ash contents supplied from the mines along with necessary rates of sorbent limestone to control SO2, and the deposits build-up rate was determined. The deposit build-up was found increasing, with increase in ash content of lignite, sorbent addition, and percentage of fines in limestone. Remedial measures and field modifications to dislodge deposits on heat transfer surfaces, to handle the deposits in ash conveying system, and to control sorbent fines from the milling circuit are explained.

Mineralogical influence of mining intrusions in CFB combustion of Indian lignite

Lignite has emerged as an additional important fuel source for thermal power generation in India. Circulating fluid bed combustion technology is applied considering the impurities, moisture, ash and sulfur content, and wide variations for large units. Lignite mineralogy greatly influences combustion behavior. Agglomeration and clogging/blockage are experienced due to sintering of lignite at lower temperature regime in which circulating fluid bed boilers operate (800°C to 900°C). At this low temperature range, the extensive knowledge built for pulverized coal combustion with respect to slagging, fouling, and high-temperature corrosion is not useful. Sintering studies using the heating microscope’s potential are applied for understanding this phenomenon. The gray clay, which occurs as intrusions/thin bands in the lignite mine, is sampled at the mine site and taken up for analysis. Morphology of sintered deposits in the bottom ash is matched with the properties of clay which is very close to halloysite mineral (kaolinite group). Sintering is avoided by selecting the operating temperature range of combustion on either side of peak sintering temperature in the sample case. With higher ashing temperature, the lignite ash loses part of its sintering tendency. This indicates to a new hypothesis that once the lignite ash undergoes transformations resulting in sintering, its sintering tendency is lowered. Conventional slagging indices are also analyzed, and correlation was derived for sintering behavior of lignite.

Research Self-Adjusting T-S Fuzzy Control System Based on LS-SVM Online Prediction on CFBB

Automatic control system of circulating fluidized bed boiler (CFBB) is considered that has long combustion lag characteristic, instable parameters and difficult to formulate a mathematical model. A T-S fuzzy controller according to the study of fuzzy control based on proportional factors is put forward. The value of each proportional factor is verified by experiment and an online adjusting rules are made. The controller can distinguish the forecast output of least square support vector machine (LS-SVM) and adjusts the proportional factors by the rules. To verify the control effect, the model is simulated with matlab. The results reveal that the control effect is superior to normal control system.

Simulation of Circulating Fluidized Bed (CFB) Boiler Oxygen-Enriched Combustion

In this paper, we focus on the relationship between oxygen-enriched combustion efficiency and oxygen content of primary air under N2 /O2 atmosphere combustion on CFB boiler. Firstly, an entirely possible of CFB boiler oxygen-enriched combustion model was proposed. Secondly, a platform was built for simulation of CFB combustion process on XD-APC configuration software. Finally, industrial simulation with industrial data was going on to prove the platform was reasonable. The simulation results were consistent of industrial data. It shows the simulation platform reliability, and the model accuracy. On this basis, coal combustion efficiency was simulated. It shows that the combustion efficiency increases following by oxygen content increasing. It’s economic for real process when oxygen content chooses from 25% to 30%.

Process Simulation of Oil Shale Circulating Fluidized Bed Boiler Based on Aspen Plus

A model for the combustion of oil shale in the 65t/h circulating fluidized bed (CFB) boiler was developed, consisting of oil shale combustion, steam-water and ash circulation system, calculating the O2 and RO2 content of flue gas emission under three kinds of oil shale combustion in 65t/h CFB boiler. The calculated results indicate that the simulation values are consistent with the experimental values. Effect of boiler load on the temperature of furnace, flue gas emission, inlet and outlet flue gas of economizer was discussed based on the model. Boiler load on the increase results in a increase in temperature of furnace, flue gas emission, inlet and outlet flue gas of economizer. The main performance parameters of 65t/h oil shale CFB boiler system were discussed and preliminarily predicted by the model.

The Exploration and Practice of Using Additive to Inhibit the Heating Surface Deposition in Biomass-Fired Boiler

In response to the deposition problem in biomass-fired boiler, inert addictive was tested to inhibit the possibility of coking. The deposition samples collected from final and primary superheater of biomass-fired CFB (circulating fluidized bed) boiler were grinded into fine powder. The inhibitive ability of additive was tested by mixing the deposition sample with different proportions of inhibitor under high temperature. By means of scanning electron microscope (SEM), X-ray diffraction (XRD) and other analytical procedures, the properties of the samples with inhibitor such as microstructure, composition were studied. The inhibitor was also introduced to the industrial biomass CFB boiler to verify its inhibition effect on superheater deposition. Unfortunately, the effect of adding inhibitor in industrial boiler is not certain, though the inhibitor added to the samples is obviously effective in weakening coking in the laboratory scope.

Hybrid neural network in circulating fluidized bed boiler based on information fusion clustering control

Circulating fluidized bed boiler (CFBB) is regarded as a clean and highly efficient coal combustion technology, and recently, this combustion technology is rapidly developing. However, CFBB is a complicated control object with distributing parameters, nonlinearity, time varying, large time-delay, and strong coupling. Traditional control methods cannot obtain good control effect, which becomes one of the bottlenecks for its extending application. In this paper, we design a three-level ART2-BP–BP of data fusion—cluster control system based on methods of multi-sensor information fusion and cluster analysis. We complete data fusion from the data level, feature level, to the decision level. By this method, more complete description of the conditions is obtained, and appropriate control strategies according to each category are taken, describing behavioral characteristics in the process. Then, the potential situation threat in working boiler is estimated, and the concept of situation threat space is proposed. Results of simulation show that the control system in this paper is feasible and effective; in particular, the control system still has more satisfactory control effect in the case of a variety of sensor failures.

NOx Emission from a Circulating Fluidized Bed Boiler Cofiring Coal and Corn Stalk Pellets

The NOx emission from a circulating fluidized bed (CFB) boiler with 50 MW rated thermal output (50 MWth) during the cofiring of anthracite coal and pelletized corn stalk biomass was investigated. Cofiring could improve combustion efficiency and, thus, the thermal efficiency of the boiler. However, contrary to most results found in bubbling fluidized bed (BFB) combustors and the one found in a CFB boiler burning rice husk, NOx concentration in flue gas and fuel nitrogen conversion ratio increased with the biomass mass fraction. Several causes for such a contradiction were discussed and the configuration in combination with feedstock location of biomass was regarded as the most important one. In a CFB boiler, fuel is mostly burnt above the dense bed, opposite to that in a BFB boiler, and, thus, the average oxygen concentration above the dense bed, where biomass was fed, was higher. High bed temperature and high primary air ratio used for burning the anthracite coal increased NOx emission and enhanced the variation trend. In addition, the catalyst-contained ash also could promote NOx formation. The results indicated that biomass is not necessarily an effective cofiring fuel to reduce NOx emission in a CFB boiler and in order to reduce NOx emission, low primary air ratio, and locating the feedstock ports of biomass below the secondary air should be considered.

Heavy metal characterization of circulating fluidized bed derived biomass ash

Although the direct combustion of biomass for energy that applies circulating fluidized bed (CFB) technology is steadily expanding worldwide, only few studies have conducted an environmental assessment of biomass ash thus far. Therefore, this study aims to integrate information on the environmental effects of biomass ash. We investigated the concentration of heavy metal in biomass ash samples (bottom ash, cyclone ash, and filter ash) derived from a CFB boiler that combusted agricultural and forest residues at a biomass power plant (2×12MW) in China. Ash samples were gathered for the digestion and leaching test. The heavy metal content in the solution and the leachate was studied via an inductively coupled plasma-mass spectrometer and a Malvern Mastersizer 2000 mercury analyzer. Measurements for the chemical composition, particle size distribution, and the surface morphology were carried out. Most of the metals in cyclone ash particles were enriched, whereas Ti and Hg were enriched in filter ash. Residence time contributed most to heavy metal enrichment. Under HJ/T 300 conditions, the heavy metals showed serious leaching characteristics. Under EN 12457-2 conditions, leaching behavior was hardly detected.

The Application of Regression Analysis in the Characteristics Research of Adjustment to Flue Gas Temperature at Furnace Outlet of CFB Boiler

A type of HG-1025/18.64-L.PM41 circulating fluidized bed boiler (CFB) has been put into production since 2009, a lot of significant experiences have been accumulated by the equipment improvement and operation exploration. To further understand the adjustment characteristics of flue gas temperature at furnace outlet, the operation data of 30 operating conditions have been measured, and also the method of regression analysis is applied to research and analyze the correlative factors composed of boiler load rate, ash content as received basis, volatile content as received basis, net calorific value as received basis, particle diameter of feeding coal, bed pressure, primary air ratio, upper secondary air ratio, lower secondary air ratio and oxygen content at furnace outlet.