Vortexing Fluidized Bed Combustor Research Articles

Investigation of Combustion Behaviors in a Vortexing Fluidized-Bed Combustor with a Contraction Chamber

Investigation of Combustion Behaviors in a Vortexing Fluidized-Bed Combustor with a Contraction Chamber

Combustion and pollutant emissions characteristics of Camellia oleifera shells in a vortexing fluidized-bed combustor

Because of their saponin content, Camellia oleifera shells cannot be directly discarded. However, this eco-unfriendly agricultural waste is suitable for use as a fuel. A comparison between the combustion of crushed C. oleifera shell (CCS) and pelletized C. oleifera shells (PCS) was carried out in a pilot-scale fluidized-bed. Both directly combustion and flue gas recirculation (FGR) combustion modes were employed. The effects of particle size and the FGR ratio on the combustion behavior and pollutant emissions characteristics were also investigated. Results show that the combustion efficiencies for both CCS and PCS are higher than 99%. The combustion fraction in the bed region of CCS and PCS-6 are 55% and 85%, respectively, resulting in different combustion and pollutant emission characteristics. CCS has smaller CO and NO emissions and higher PCDD/Fs concentration of the fly ash compared with those of pelletized fuels. However, considering the material bridging in the feeding process of CCS, using pellets as the fuel is a better choice. The CO emissions increase while dioxin and furan emissions decrease with an increase in FGR ratio. The lowest NO emissions (about 150 ppm) of the three pelletized fuels combustion were achieved at an FGR ratio of 42%. In addition, all the pollutant emissions are lower than the minimum standards required for municipal solid waste (MSW) incinerators in Taiwan.

Application of response surface methodology to determine effects of operational conditions on in-bed combustion fraction in vortexing fluidized-bed combustor using different fuels

Distinct secondary gas injection modes of a vortexing fluidized-bed combustor (VFBC) affect combustion fraction distribution, resulting in different combustion and pollutant emissions characteristics. To determine the important operational conditions, correlations that consider most of the VFBC parameters were derived to predict the in-bed combustion fraction in VFBC. The predictions obtained using the regression correlation analysis were in good agreement with the results of present experiments and with previously published results. Response surface methodology (RSM) was used to analyze the sensitivity of different coded factors based on established model. A new correlation equation, with a fewer parameters, based on the sensitivity analysis, was also developed and exhibited a positive predictive accuracy. Results demonstrate that the primary gas ratio, particle size, and fuel type significantly affect the combustion fraction. The in-bed combustion fraction increases with increasing particle size, and decreases with increasing primary gas ratio and the ratio of volatile to fixed carbon.

Investigation of the Vortexing Effect on Sawdust Combustion in a Fluidized Bed Combustor

The vortexing effect on reducing pollutant emission was verified via the combustion of three groups of sawdust with different diameters in a vortexing fluidized bed combustor (VFBC). The temperatures and pollutant emission distributions along the combustor were measured simultaneously. The combustion fraction in four zones of the VFBC were also calculated on the basis of the oxygen consumption. Additionally, the effects of operating variables, such as the in-bed stoichiometric oxygen ratio and the excess oxygen ratio, on the pollutant emissions were also investigated. The results show that the combustion fractions in the bed and freeboard zones increase with larger particle sizes. However, an inverse trend occurred in other zones. The CO emissions from the three sawdust combustions decrease with an increase in the particle size, excess oxygen ratio, and in-bed stoichiometric oxygen ratio, whereas the NOx emissions exhibit an inverse trend. The vortexing effect, which is caused by secondary air injection, ...

The Combustion Performance of Pelletized Jatropha Seed Residue in a Vortexing Fluidized-Bed Combustor

Jatropha curcas is considered a promising energy crop. The response surface methodology is introduced to study the combustion performance of pelletized Jatropha seed residue in a vortexing fluidized-bed combustor, and regression models for the bed temperature (Tb), the combustion fraction (Yb), and the NOx emissions are provided. The result shows that the combustion efficiency is above 99.5%, indicating that JSR pellets burn very well in the combustor. CO can be eliminated by stage combustion accompanied by a controlled freeboard temperature, but NOx increases correspondingly. Tb and Yb have similar trends, showing the interaction between them. The variation in the NOx emissions can be attributed to the shift in volatile-N combustion. In summary, the optimum operating conditions lie slightly above the line of Tb equal to 800 °C.

Bed agglomeration characteristics of rice straw combustion in a vortexing fluidized-bed combustor

To investigate bed agglomeration characteristics, the combustion of pelletized rice straw was conducted in a bench-scale vortexing fluidized bed. Effects of bed temperature, superficial velocity, secondary gas velocities, and mass blended ratio of coal on the defluidization time were investigated. The alkali concentrations in different sections of the bed zone were also studied. The bed materials and agglomerates were analyzed using SEM/EDX to obtain the surface morphology and the compositions. The results revealed that the defluidization time is increased with superficial gas velocity and is decreased with bed temperature. Eutectic composition with low melting point materials promote defluidization at high temperatures. Effect of the secondary gas velocity on the defluidization time indicates different trends at different bed temperatures. The highest value of alkali concentration appears at upper bubbling zone. Coal ash can avoid the existence of a certain eutectic composition, and increases its melting point.

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.

Comparison of combustion behaviors and pollutant emissions using bituminous coal and corncob in a fluidized bed combustor

ABSTRACTThe combustion behaviors and pollutant emissions of corncob and bituminous coal in a pilot scale vortexing fluidized bed combustor was compared. The flue gas recirculation combustion mode was used in this study. The effects of operating parameters, such as stoichiometric oxygen ratio, and bed temperature on the pollutant emissions were investigated. The results show that the combustion fractions vary with different kinds of fuels, resulting in different CO and NOx emission characteristics. Corncob combusts primarily in the freeboard zone, whereas bituminous coal combusts primarily in the bed zone. Therefore, the CO emission decreases with the increasing bed temperature in bituminous coal combustion, whereas the effect of bed temperature on CO emission is not obvious in corncob combustion. The CO emission of corncob combustion decreases with the stoichiometric oxygen ratio, whereas the CO emission of bituminous coal combustion has the inverse trend. For both fuels, the NOx emissions increase with the bed temperature but decrease with the stoichiometric oxygen ratio. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

Effect of secondary gas injection on the peanut shell combustion and its pollutant emissions in a vortexing fluidized bed combustor

Peanut shell is a common agricultural waste in Asia, and its high calorific value is suitable to be used as a fuel. In this study, a vortexing fluidized bed combustor (VFBC) with silica sand as the bed material was used for peanut shell combustion. There was no indication of bed agglomeration during combustions for as long as 12h. The temperatures and gas concentrations were measured along the axial direction at various operating conditions, including excess oxygen ratio and secondary gas flow rate. Results show that CO emission decreases with rising excess oxygen ratio and secondary gas flow rate, while NOx emissions show a reverse trend. To meet the minimum CO and NOx emission standards of Taiwan EPA, excess oxygen ratio ranging from 40% to 55% and secondary gas flow rate ranging from 1.56 to 2 Nm(3)/min are found optimal for crushed peanut shell combustion in a VFBC.

Combustion behavior and pollutant emission characteristics of RDF (refuse derived fuel) and sawdust in a vortexing fluidized bed combustor

This paper presents the results obtained from RDF and sawdust combustion in a pilot scale vortexing fluidized bed combustor (VFBC). Flue gas recirculation (FGR) combustion mode was employed. The effects of operating parameters such as bed temperature, excess oxygen ratio, and in-bed stoichiometric oxygen ratio on the combustion behavior and pollutant emission characteristics were investigated. The experimental results show the different combustion characteristics with different fuels in the VFBC. For sawdust combustion, CO emission decreases with bed temperature, excess oxygen ratio, and in-bed stoichiometric oxygen ratio. The NOx emission shows an inverse trend. Combustion using sawdust as the fuel has difficulty meeting the CO emission regulation requirement. The CO and NOx emissions of RDF combustion show a similar trend as that of sawdust; however RDF combustion can significantly decrease the CO emission due to its distinct pellet structure and burning pattern.

Combustion behavior and pollutant emissions of batch fluidized bed combustion

The combustion experiments were carried out in a pilot-scale vortexing fluidized-bed combustor (VFBC) with a batch fuel feeding system. Effects of particle sizes and feeding intervals on the temperatures within the combustor and the pollutant emissions at the exit of the combustor were studied. Rubber balls were used as the fuel to simulate the hazard waste. Silica sand was used as the bed material. The experimental results show that the combustor temperatures change periodically with the feeding cycles and the highest temperature appears in the splashing or freeboard regions. The mean bed and freeboard temperatures increase with the particle size, and decrease with the feeding interval. However, the mean bed and freeboard temperatures show a inverse trend. The mean CO concentration in the flue gas decreases with fuel particle size, but increases with feeding interval. There is an inverse dependency between the CO and NOx concentrations at the furnace exit. Results from this study would be helpful in developing more comprehensive physical model for VFBC with batch feeding system and setting optimal operational parameters in practical applications.

Experimental study on rice husk combustion in a vortexing fluidized-bed with flue gas recirculation (FGR)

Vortexing fluidized-bed combustor (VFBC) has been proven to be an effective equipment in converting biomass wastes into clean energy. This study conducted experiments on rice husk combustion in a VFBC with FGR. The effect of FGR on combustion characteristics is investigated. In addition, the effect of operating variables such as excess oxygen ratio, and in-bed stoichiometric oxygen ratio on the temperature distributions, pollutants emissions, and combustion efficiency are also studied. The results show that the combustion efficiency of rice husk can reach 99% at optimal operation condition. CO emission increases with the in-bed stoichiometric oxygen ratio, but decreases with excess oxygen ratio. NOx emissions show inverse trend, and it can be effectively reduced by using FGR in the VFBC.

Effect of Operating Conditions on NOx and CO Emissions in a Pilot‐Scale Vortexing Fluidized‐Bed Combustor with Flue Gas Recirculation

AbstractIn order to study the effect of flue gas recirculation (FGR) by primary air mixed with FGR or nitrogen on NOx emissions in a fixed total primary air flow rate, the bed temperature, stoichiometric oxygen ratio in the combustion chamber, and excess oxygen ratio were investigated in a pilot‐scale vortexing fluidized‐bed combustor (VFBC). As a result, the NOx emissions could effectively be decreased with FGR in the VFBC. The lower NOx emissions were attributed to the fact that the FGR allows NOx to enter the reactor again with the chance of being reduced and that the FGR contains more CO2 to suppress NOx formation or promote NOx reduction through its conversion to CO by reaction with char.

A study on fluidized bed combustion characteristics of corncob in three different combustion modes

This paper presents results obtained from corncob combustion in a pilot scale vortexing fluidized bed combustor (VFBC). Three combustion modes including direct combustion, staged combustion and flue gas recirculation (FGR) combustion were employed, and their combustion and pollutant emission characteristics were studied. In addition, the effects of combustion fraction and bed temperature on pollutant emission characteristics were investigated. The experimental results show that the combustion fractions vary with different combustion modes, resulting in different CO and NO emission characteristics. Staged and FGR combustions can reduce the NO emission concentration. Under similar working condition, NO concentration decreases by 30% in FGR mode, while 15% in staged mode compared with direct mode.

Effect of Flue Gas Recirculation (FGR) on NOx Emission in a Pilot-Scale Vortexing Fluidized-Bed Combustor

The combustion experiments were carried out in a 0.75 m inner diameter and 4.6 m high pilot-scale vortexing fluidized-bed combustor (VFBC). The secondary air was injected tangentially at the bottom of the freeboard, 2.05 m above the distributor plate. Coal of particle sizes from 1.4 to 8 mm was used as the fuel. Silica sand was used as the bed material. The effects of operating conditions, such as the bed temperature, excess oxygen ratio, secondary air preheating, and stoichiometric oxygen in the combustion chamber, on the temperature distribution within the VFBC, CO emission, and NOx emission were investigated. The experimental results show that NOx emission can be effectively decreased using flue gas recirculation (FGR) in the VFBC. The lower NOx emission can be attributed to the following reasons: first, FGR sends NOx back to the reactor and increases its chance of further reduction, and second, FGR supplies more CO2 to help suppress NOx formation or promote NOx reduction through its conversion to CO f...

Characteristics of a Pilot-Scale Vortexing Fluidized-Bed Combustor with Flue Gas Recirculation (FGR): Effect of Operating Conditions on the Combustion Behavior

The effects of operating conditions, such as the fraction of fine coal in the feeding material, the stoichiometric oxygen in the combustion chamber, and the excess oxygen ratio, on the combustion e...

Combustion and NO emission of high nitrogen content biomass in a pilot-scale vortexing fluidized bed combustor

The combustion of biomass of various nitrogen contents and its NO emission were investigated experimentally in this study. All the experiments were conducted in an I.D. 0.45 m pilot-scale vortexing fluidized bed combustor (VFBC). Rice husk, corn, and soybean were used as feeding materials. Urea was added into the feeding materials for the purpose of adjusting nitrogen content. The effects of various operating parameters on NO emission, such as bed temperature, excess air ratio, and flow rate of secondary air, were investigated. The effects of nitrogen content of fuels on NO emissions were also investigated by using the mixtures of rice husk/soybean, rice husk/urea, corn/soybean, and corn/urea in various weight ratios. The NO concentrations at various positions in the combustor were sampled and recorded. The experimental results show that most nitric oxide is formed at just above the bed surface. Temperature and excess air ratio are the major operating parameters for NO emission. For biomass with high nitrogen content, NO emission decreases with excess air, and increases with bed temperature. Compared with char-N, volatile-N is the more dominant reactant source for NO emission.

Modeling the Woody Biomass Combustion in a Vortexing Fluidized-Bed Combustor

This study develops an isothermal one-dimensional model to easily predict the performance of biomass combustion in a vortexing fluidized-bed combustor (VFBC). This model comprises a series of three reactor zones: a fluidized-bed combustion chamber, a lower freeboard, and an upper freeboard. This model can calculate the composition of the flue gas exhausted from a combustor and the concentration profiles of individual gas species at various combustor heights. This study verifies the proposed model by comparing its predictions with actual experimental data obtained from a series of systematic combustion experiments burning pinewood sawdust in a pilot-scale VFBC. This comparison revealed an air-leakage problem in the pilot-scale VFBC. The simulated results of CO2 and O2 concentrations are most sensitive to EA but almost non-sensitive to λ. As for the operating temperature, the simulation results for CO, CO2, and O2 are only sensitive at temperatures lower than 750 °C. The CO concentration in the exhaust is more affected than CO2 and O2 by the secondary air added. The fresh O2 added by secondary air can enhance CO consumption, but too much secondary air shortens the residence time of CO in the upper freeboard. Although there are obvious gaps between experimental and simulation results, the predicted change tendencies of gas concentrations correspond to the actual data. This model makes it possible to obtain more information in the preliminary steps of designing a larger scale VFBC for burning highly volatile fuel, e.g., biomass.

An investigation on pollutant emissions from co-firing of RDF and coal

Pollutant emissions from co-firing of refuse derived fuel (RDF) and coal were investigated in a vortexing fluidized bed combustor (VFBC). RDF-5 was made of common municipal solid waste (MSW). CaCO 3 was injected in the combustor to absorb HCl at 850 °C. The results show that NO x and HCl emissions increase with RDF-5 co-firing ratio. The NO x concentration in flue gas at the bottom of the combustor is higher than that at the top. However, the trend of HCl released is reverse compared with NO x emissions. It was found that the HCl concentration decreases with increasing the molar ratio of Ca/Cl. However, the effect of CaCO 3 addition on HCl retention is not significant when the molar ratio of Ca/Cl is higher than 5. The chlorine content in fly ash increases obviously with the molar ratio of Ca/Cl. PCDD/Fs emissions decrease slightly with an addition of CaCO 3. In this study incomplete combustion is regarded as the main cause for polychlorinated dibenzo- p-dioxins and dibenzofurans (PCDD/Fs) formation.

Effects of Acetic Acid Injection and Operating Conditions on NO Emission in a Vortexing Fluidized Bed Combustor Using Response Surface Methodology

The effects of acetic acid injection and operating conditions on NO emission were investigated in a pilot scale vortexing fluidized bed combustor (VFBC), an integration of circular freeboard and a rectangular combustion chamber. The dimension of the freeboard is 0.75 m I.D. and 4.6 m in height. The cross section of the combustion chamber is 0.8 × 0.4 m2, and the height of the combustion chamber is 1.47 m. The secondary air injection nozzles were installed tangentially at the bottom of the freeboard. Coal was used as the fuel. Silica sand was employed as the bed material. Acetic acid was used as the reductant to reduce NO emission. Operating conditions, such as the stoichiometric oxygen in the combustion chamber, the bed temperature and the injecting location of acetic acid, were determined by means of response surface methodology (RSM), which enables the examination of parameters with a moderate number of experiments. In RSM, NO emission concentration after acetic acid injection and NO removal percentage at the exit of the VFBC are used as the objective function. The results show that the bed temperature has a more important effect on the NO emission than the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber. Meanwhile, the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber have a more important effect on the NO removal percentage than the bed temperature. NO emission can be decreased by injecting the acetic acid into the combustion chamber, and NO emission decreases with the height of the acetic acid injecting location above the distributor. On the other hand, NO removal percentage increases with the height of the acetic acid injecting location, and NO emission increases with the stoichiometric oxygen in the combustion chamber and the bed temperature. NO removal percentage increases with the stoichiometric oxygen, and increases first, then decreases with the bed temperature. Also, a higher NO removal percentage could be obtained at 850 °C.