Pilot-scale Fluidized Bed Combustor Research Articles

Eulerian based population balance modeling of agglomeration in a bubbling fluidized bed combustor

Renewable energy sources are contributing to the displacement of fossil fuel use for power generation and biomass combustion is a viable alternative for baseload power. Fluidized bed combustion (FBC) technology has become a favorable option for biomass (including industrial wastes) fired boilers for its benefits including fuel flexibility, high combustion efficiency, lower emissions (NO x , SO x ) in comparison to other solid fuel fired boilers. However, the variation of inorganic species and corrosive deposits lead to bed agglomeration resulting in unscheduled outages. In the present work, biomass combustion and bed agglomeration in a pilot scale atmospheric-fluidized bed combustor is simulated and analyzed. The Eulerian–Eulerian simulation is validated with the experimental data available in literature using a two-dimensional simplified geometry of a pilot scale fluidized bed combustor. Two each homogeneous and heterogeneous reactions are considered for devolatilization and char combustion processes during combustion with the species transport model. Population Balance Model (PBM) is implemented to calculate agglomerate hydrodynamics with air fed as fluidizing medium from the bottom of the combustor. The model results compared well and found consistent with available experimental data. Prediction of defluidization time of the bed showed a good agreement with the results of experiments.

The use of kaolin and dolomite bed additives as an agglomeration mitigation method for wheat straw and miscanthus biomass fuels in a pilot-scale fluidized bed combustor

Renewable biomass fuels are frequently used for power generation. Biomass ash causes bed agglomeration in fluidized bed boilers due to the formation of alkali silicate melts. Very few prior studies have tested dolomite and kaolin bed additives for agglomeration mitigation with agricultural biomasses. In this work, pelletized miscanthus and wheat straw were tested in a pilot-scale 65kWth fluidized bed combustor with varying dosages of dolomite and kaolin on a silica sand bed. Neither additive improved defluidization time with wheat straw, whereas additive use at all dosages prevented bed defluidization with miscanthus. Agglomerates were studied through a novel, detailed SEM/EDX analysis across structural features. SEM/EDX analysis presented evidence of chemical reaction between both additives and fuels. Potassium in ash migrated into kaolin particle at depths of up to 60 μm. With dolomite, calcium and magnesium raised melt temperatures. Thermochemical modelling of the ash and additive combinations predicted that additive use would substantially reduce ash melt formation. It is proposed that the wheat straw pellet acted as a “ready-made” agglomerate structure due to release of molten ash to the pellet surface which bed material then sticks to, hence the lack of change to defluidization time regardless of additive use. Future studies into this behaviour would improve additive use.

Modeling of the Low Grade Coal Combustion in a Fluidized Bed Reactor Using Aspen Plus

With ever-increasing usage of biomass to curb environmental pollution, it is imperative to develop new insights into coal combustion processes, for efficient and optimal usage. Many process models have been developed for combustion process, including fixed and fluidized bed combustion for different types of coal; however, they are designed for specific process and lack generality. More recently, Aspen Plus has introduced new tools, and particularly featured a built-in unit operation model, to design fluidized bed processes. Herein, we comprehensively apply the novel tool to analyze the low grade coal combustion with varying process conditions such as: temperatures, fuel/air ratio, pressure, and other. The model developed in Aspen Plus was validated with our experimental data performed in a pilot-scale fluidized bed combustor. The fuel type of combustion used in the experiments is Ekibastuz coal (a type of low-grade Kazakhstan coal). The data from experiments were used to fine-tune the Aspen Plus model. Eventually, results indicate accuracy of the model in predicting the gas compositions and the reactor hydrodynamics. Furthermore, sensitivity analyses were performed in order to analyze the effect of temperature and feed flow rate on the combustion efficiency and flue-gas compositions. Taken together, the model developed can be ubiquitously used for process design of fluidized bed reactor co-combustion of coal and biomass.

Effects of Sulfur and Calcium Compounds on Dioxin Reduction in a Fluidized Bed Combustor

ABSTRACT The focus of this study is the effects of sulfur and calcium compounds on reducing dioxins in fly ash collected from the baghouse of a pilot-scale fluidized bed combustor. Pelletized sawdust with various amounts of polyvinyl chloride (PVC), pyrite, and calcium hydroxide was used as the feeding material. The experimental results indicated an extremely low concentration of dioxin in the fly ash from the cyclone (0.025 to 0.284 ng g–1), and the concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the fly ash collected from the baghouse increased by 27.4% as the chlorine content increased from 0.568 to 1.136 wt%. The inhibition of PCDD/Fs by sulfur is not clear and can be attributed to the low content of copper (average 387 ppm), which serves as the catalyst for the Deacon reaction. Octa-chlorinated dibenzofuran (OCDF) was the major congener of the PCDD/Fs. The highest toxicity-equivalent (I-TEQ) of 2,3,7,8-substituted congeners was 2,3,4,7,8-PeCDF, which accounted for 37% of the total congeners (average value). The correlation between the I-TEQ values and 2,3,4,7,8-PeCDF concentration of fly ash is also discussed.

An experimental study of the effect of nitrogen origin on the formation and reduction of NOx in fluidized-bed combustion

The relative importance of char-N and volatile-N on NOx emissions is examined in a pilot-scale fluidized-bed combustor. Coke and toluene are used as the base fuels, and nitrogenous compounds (nitrobenzene, pyridine, and pyrrole) are mixed with them alternatively to study the effect of nitrogen origin on NOx formation and reduction. The results show that NOx evolution would be greatly changed if the nitrogenous species are present in the volatile, and NOx reduction inside the combustor may be the key factor that determines the final emissions. When nitrobenzene is employed, the conversion of volatile-N to NOx is 13.17–15.42%, while that of char-N is 4.49–7.59% in the bed temperature of 750–900 °C, indicating that nitrogen present in volatiles may be much more prone to elicit higher NOx. Moreover, the volatile-N conversion to NOx is always greater than that of char-N, regardless of the nitrogen functionality. However, the degree of formation and reduction of NOx would depend firmly on the functional groups of nitrogen, which are nitro, pyridine, and pyrrole in this case. NOx reduction mainly occurs in the splash zone, and NOx generated from char-N is reduced in a manner dissimilar to that of volatile-N.

Comparison of the thermal behaviors and pollutant emissions of pelletized bamboo combustion in a fluidized bed combustor at different secondary gas injection modes

Staged combustion significantly affects the complete combustion of biomass and reduces pollutant emissions. This study primarily aimed to compare the thermal behaviors and pollutant emissions of different secondary gas injection modes in a pilot-scale fluidized bed combustor (FBC). Pelletized thorny bamboo was used as the fuel, and silica sand was used as the bed material. Flue gas recirculation (FGR) was employed. The experimental results demonstrate that the secondary gas injection modes in the FBC affected the combustion characteristics. The temperature of pelletized bamboo combustion with tangential injection mode (T mode) was lower than that of centripetal injection mode (C mode); however, T mode produced lower CO and NOx emissions than C mode because of the well-swirling effect. The secondary gas velocity and airflow distribution uniformity in the secondary gas layers jointly affect combustion, while T mode with four nozzles or C mode with two nozzles is optimal to reduce emissions. Furthermore, increasing the elevation of secondary air further reduces NOx emissions in C mode but does not significantly affect emissions in T mode.

Combustion of crushed and pelletized peanut shells in a pilot-scale fluidized-bed combustor with flue gas recirculation

Peanut shell generated from peanut harvesting has a great potential to be converted into an energy resource in developing countries such as China, Vietnam, and Indonesia. The main objective of this study is to investigate the combustion characteristics of peanut shell in a pilot-scale vortexing fluidized-bed combustor with flue gas recirculation. Crushed and pelletized peanut shells are used as the feeding materials. The effects of in-bed stoichiometric oxygen ratio (Sb), particle sizes, and particle shapes on the combustion characteristics as well as pollutant emissions are investigated. Results show that similar temperature profiles are observed in all cases. CO emissions from the combustion of both fuels are all less than 70ppm. CO emission decreases, while NOx emissions increase with particle sizes. CO and NOx emissions of pellet combustion show an inverse trend from that of crushed shell combustion at different in-bed stoichiometric oxygen ratios.

Combustion of Municipal Solid Waste in a Pilot Scale Fluidized Bed Combustor

Combustion study of municipal solid waste combustion in a pilot scale fluidized bed combustor had been carried out. The work was aimed at demonstrating sustainable combustion of municipal solid waste by employing operating parameters gained from previous studies. The primary and secondary air factor used were AF = 0.8 and AF = 0.6 respectively. The fluidization number was 5Umf and both in-bed and freeboard region temperature distributions were monitored continuously. Results on the combustion studies revealed that the initial bed temperature could be sustained due to high thermal capacity of sand but later dropped due to problem related to the mixing of bulky and heterogeneous components of municipal solid waste and sand.

Use of chemical fractionation to understand partitioning of biomass ash constituents during co-firing in fluidized bed combustion

Three species of biomass origin (straw pellets, olive cake and wood pellets) and two coals from different countries (Coal Polish and Coal Colombian) have been studied to understand the fate of their ash forming matter during the combustion process and to investigate the influence of co-firing biomass with coal. Three different approaches to investigate the ash behaviour were employed: (1) chemical fractionation analysis to evaluate the association/reactivity of ash forming elements in the fuels as a prediction tool, (2) establishment of elements partitioning in ash streams produced in the combustion and co-combustion trials, and (3) evaluation of enrichment factors of elements in the ash streams. The chemical fractionation analysis was applied to all fuels used to evaluate how the association/reactivity of elements making up ash may influence their behaviour during combustion. Combustion tests were carried out on a pilot scale fluidized bed combustor (FBC). Four ash streams were obtained at different locations. The uncertainty of measurements was estimated allowing a critical evaluation of mass balances over the combustion system and the partitioning of elements in the ash streams. The enrichment factors of elements in the several ash streams were estimated, incorporating uncertainties associated with analytical measurements. Results obtained showed that for FBC the relation between the chemical fractionation and the experimental partitioning is strongly affected by elutriation of particles. The element enrichment factor estimated for each ash stream, using Al as a reference element, revealed better correlations with the elements reactivity obtained by chemical fractionation because it overcomes particles elutriation effects. Nevertheless, it was observed that the reactivity estimated by chemical fractionation could not be solely interpreted as tendency of the elements to volatilize on FBC system, as reaction in bed zone of boiler may also occur retaining reactive elements.

Investigation of ash deposition in a pilot-scale fluidized bed combustor co-firing biomass with lignite

This study presents the results from investigation of ash deposition characteristics of a high ash and sulfur content lignite co-fired with three types of biomass (olive residue, 49 wt%; hazelnut shell, 42 wt%; and cotton residue, 41 wt%) in 0.3 MW t Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig. Deposit samples were collected on an air-cooled probe at a temperature of 500 °C. Samples were analyzed by SEM/EDX and XRD methods. The results reveal that co-firing lignite with olive residue, hazelnut shell and cotton residue show low deposition rates. High concentrations of silicon, calcium, sulfur, iron, and aluminum were found in deposit samples. No chlorine was detected in deposits. Calcium sulfate and potassium sulfate were detected as major and minor components of the deposits, respectively. High sulfur and alumina-silicate content of lignite resulted in formation of alkali sulfates instead of alkali chlorides. Therefore, fuel blends under consideration can be denoted to have low-fouling propensity.

Trace elements partitioning during co-firing biomass with lignite in a pilot-scale fluidized bed combustor

This study describes the partitioning of 18 trace elements (As, Ba, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and 9 major and minor elements (Al, Ca, Fe, K, Mg, Na, S, Si, Ti) during co-firing of olive residue, hazelnut shell and cotton residue with high sulfur and ash content lignite in 0.3 MW t Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) test rig with limestone addition. Concentrations of trace elements in coal, biomass, limestone, bottom ash, cyclone ash and filter ash were determined by inductively coupled plasma optical emission and mass spectroscopy (ICP-OES and ICP-MS). Partitioning of major and minor elements are influenced by the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Co, Cr, Cu, Li, Mn, Mo, Ni, Pb, Tl, V and Zn) are recovered in fly ash when firing lignite only. Co-firing lignite with biomass enhances partitioning of these elements to fly ash. Co-firing also shifts the partitioning of Cd, P, Sb and Sn from bottom to fly ash.

Cofiring Lignite with Hazelnut Shell and Cotton Residue in a Pilot-Scale Fluidized Bed Combustor

In this study, cofiring of high ash and sulfur content lignite with hazelnut shell and cotton residue was investigated in 0.3 MWt METU Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig in terms of combustion and emission performance of different fuel blends. The results reveal that cofiring of hazelnut shell and cotton residue with lignite increases the combustion efficiency and freeboard temperatures compared to those of lignite firing with limestone addition only. CO2 emission is not found sensitive to increase in hazelnut shell and cotton residue share in fuel blend. Cofiring lowers SO2 emissions considerably. Cofiring of hazelnut shell reduces NO and N2O emissions; on the contrary, cofiring cotton residue results in higher NO and N2O emissions. Higher share of biomass in the fuel blend results in coarser cyclone ash particles. Hazelnut shell and cotton residue can be cofired with high ash and sulfur-containing lignite without operational problems.

Pilot-Scale Fluidized-Bed Combustor Testing Cofiring Animal-Tissue Biomass with Coal as a Carcass Disposal Option

This study was performed to demonstrate the technical viability of cofiring animal-tissue biomass (ATB) in a coal-fired fluidized-bed combustor (FBC) as an option for disposing of specified risk materials (SRMs) and carcasses. This is a potential option for beneficially utilizing SRMs and carcasses that must be removed from the food chain, allowing them to be used as an energy feedstock rather than disposing them via landfilling or incineration. The purpose of this study was to assess the technical issues of feeding/combusting ATB and not to investigate prion deactivation/pathogen destruction. Overall, the project successfully demonstrated that carcasses and SRMs can be cofired with coal in a bubbling FBC. Feeding ATB into the FBC did, however, present several challenges. Specifically, handling/feeding issues resulting from the small scale of the equipment and the extremely heterogeneous nature of the ATB were encountered during the testing. Feeder modifications and an overbed firing system were necessary. Through statistical analysis, it was shown that the ATB feed location had a greater effect on CO emissions, which were used as an indication of combustion performance, than the fuel type due to the feeding difficulties. Baseline coal tests and tests cofiring ATB into the bed were statistically indistinguishable. Fuel feeding issues would not be expected at the full scale since full-scale units routinely handle low-quality fuels. In a full-scale unit, the disproportionate ratio of feed line size to unit diameter would be eliminated thereby eliminating feed slugging. Also, the ATB would either be injected into the bed, thereby ensuring uniform mixing and complete combustion, or be injected directly above the bed with overfire air ports used to ensure complete combustion. Therefore, it is anticipated that a demonstration at the full scale, which is the next activity in demonstrating this concept, should be successful. As the statistical analysis shows, emissions cofiring ATB with coal would be expected to be similar to that when firing coal only.

Dioxins/furans emissions from fluidized bed combustion of salt-laden hog fuel

Polychlorinated dibenzodioxins/furans (PCDD/F) were formed in substantial quantities in a pilot-scale fluidized bed combustor burning salt-laden waste wood, a common fuel for Canadian coastal pulp and paper mills. Formation of PCDD/F increased with increasing chloride content in the feed, and appeared to correlate with the chlorine content in the fly ash. It took a very long time for the ash chlorine content to stabilize, suggesting that chlorine transferred slowly from the flue gas to the ash. The baghouse may contribute largely to formation of the PCDD/F, owing to its temperature range and the potentially long residence time for ash particles. Controlling the baghouse temperature to reduce the PCDD/F formation in the baghouse should be effective in reducing the total emission level. While sulphur addition was found to reduce the emission level by as much as 90%, the emission level was still above the regulated level for the mills burning salt-laden wood under the conditions of the present study. No relation between the emission level and CO concentration in the flue gas was observed.

The Emissions of VOCs during Co-Combustion of Coal with Different Waste Materials in a Fluidized Bed

The combustion of different fuels gives rise to the formation of small but appreciable amounts of volatile organic compounds (VOCs). They basically result from incomplete combustion and their emissions have negative repercussions on health and on the environment in general. As their measurement is difficult, costly, and very time-consuming, very little is reported on the emissions of VOCs from combustion installations. In this study, various blends of two different coals with several wastes were burned in a pilot-scale fluidized bed combustor and measurements of VOCs at several locations along the combustor height as well as just before the stack were carried out. The results demonstrate that the parameters important for the formation of VOCs are temperature, excess air levels, and the effectiveness of the mixing of air with fuel. Furthermore, it was observed that coal was the principal source of VOCs, but the combustion of volatiles from fuels such as biomass, occurring in the freeboard, was important in reducing the emissions of VOCs to almost zero.

Testing of a mathematical model for the combustion of lignites in an AFBC

A mathematical model, previously developed and tested for the modelling of continuous fluidized-bed combustors burning good-quality coal, has been modified to incorporate combustion of lignites having much higher VCM FC ratios and ash contents. Modifications of the model for volatiles combustion and the incorporation of overflow ash into mass and energy balances are described. Concentration and temperature profiles and combustion efficiencies predicted for 0,19 and 48% excess air levels are compared with experimental data from a pilot-scale fluidized-bed combustor. It is shown that oxygen consumption in the freeboard is mainly due to the oxidation of volatile matter released by lignite particles splashed into the freeboard. The predictions show reasonable agreement with the experimental data.

Reduction of nitrogen oxides by injection of urea in the freeboard of a pilot scale fluidized bed combustor

The ‘thermal deNO x’ process using urea has been investigated in a 1 MW fluidized bed combustor. NO x reductions of up to 76% were obtainable by using this method. The experimental results show that urea is at least as active as NH 3, which is commonly used in this application, but which is far more toxic and corrosive. Emission levels of 200 mg m −3 for NO x could be achieved by injecting the urea at a height of 2 m above the distribution plate in a molar ratio urea: NO x = 1.5. The SO 2 emission value also appeared to be reduced when the urea was injected at a urea: NO x molar ratio > 4.

Spatial distributions of O 2, CO 2 and SO 2 in a pilot-scale fluidized bed combustor operated with different coals

Axial concentration profiles of O 2, CO 2 and SO 2 have been measured at different radial positions in a coal-fired fluidized bed combustor. The profiles show some spatial inhomogeneity in the bed, the degree of which depends on fluidizing velocity and amount of excess air. Comparison of SO 2- with CO 2-concentration profiles reveals that SO 2 is formed in proportion to CO 2 only with anthracite as fuel. With bituminous coals, SO 2 is preferentially formed near the coal feed point. These results are discussed with regard to characteristic times of mixing and chemical reaction in fluidized beds, and SO 2-formation characteristics of the employed coals obtained in a thermobalance.

HEAT TRANSFER BETWEEN HORIZONTAL TUBES AND FREEBOARD REGION OF A LOW-RANK COAL FLUIDIZED BED COMBUSTOR

Experiments are conducted to measure the heat transfer rates to air-cooled horizontal tubes of diameter 38.1mm in the freeboard region of a pilot-scale fluidized-bed combustor burning North Dakota lignite- Silica sand with diameter ranging from 883 to 973 μim is used as bed material. The tests are carried out without limestone addition and ash recycle, at average bed temperatures ranging from 698 to 1117K, freeboard temperatures of 700 to 1139 K and superficial velocities of 1.74 to 1.86 m/s. Among the existing correlations, those proposed by George and Grace (1982), and Byam et al. (1981) are found to predict the experimental results quite well and their root-mean-square deviations are found to be 15.8 and 18.4 percent respectively. @KEYWORDS: Heat transfer, Fluidized bed Coal combustor

Testing of a model for fluidized bed coal combustors

A system model, developed previously for the modelling of continuous fluidized bed combustors burning coal with wide size distribution, has been applied to the prediction of the behaviour of a pilot-scale fluidized bed combustor and to the investigation of the effect of excess air on its performance. The predictions have been tested, using previously reported experimental values. Expected trends are predicted correctly and reasonable agreement is obtained between the measurements and the predicted bed concentration profiles, carryover and total losses. The model should prove useful to design engineers in evaluating the effects of variation of excess air and recycle on the concentration and temperature profiles, combustion efficiency and size distribution of particles in bed and carryover in coal-fired fluidized bed boilers.