Chien-Song Chyang

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.

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 degrees 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.

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.

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.

Gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed

Abstract The gas discharge modes, bubbling and jetting, formed at a single horizontal nozzle in a two-dimensional gas fluidized bed were investigated by visual observation. The inside diameters of the nozzles used in this study were 3.0, 4.5, 6.0, 7.5 and 9.0 mm. The gas velocity through the nozzle was in the range from 1 to 130 m/s. The ratio of the superficial velocity to the minimum fluidizing velocity was in the range from 0 to 4. Gas discharge modes were analyzed by high speed cine photography, and the effects of the nozzle diameter, gas velocity through the nozzle, particle diameter and the static bed height were studied. The results showed that the length of jet penetration increased with the gas velocity through the nozzle and the nozzle diameter. The phase diagram of bubbling, jetting and transition zones were mapped by using a modified Froude number and the ratio of the nozzle diameter, D o , to the particle size, D p .

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.

Radial gas mixing in a fluidized bed using response surface methodology

Abstract Radial gas mixing in a fluidized bed was studied using response surface methodology (RSM), which enables effect examinations of parameters with a moderate number of experiments. All experiments were conducted in a 0.29-m ID fluidized-bed cold model. The gas dispersion process within the bed is described using the dispersed plug flow model. Pure carbon dioxide was used as the tracer gas, continuously injected into the center of the bed by a point source. The downstream radial tracer concentration profile was measured using a gas chromatograph. The radial gas dispersion coefficient, D r , was well correlated with operating parameters and the particle and gas properties: ( U − U mf )/ U mf , H s / d b , φ d , and Ar , with a determination coefficient R 2 of 0.966. Effect test indicates that the dimensionless characteristic velocity, ( U − U mf )/ U mf , has the most significant influence on D r , while the static bed height to bed diameter ratio, H s / d b , is less remarkable. The interactions of ( U − U mf )/ U mf with the distributor open-area ratio, φ d , and with the Archimedes number, Ar , both play important roles. An evolutive response surface model was proposed to describe the radial gas mixing in the bubbling/slugging fluidization regimes.

Pollutant emission characteristics of rice husk combustion in a vortexing fluidized bed incinerator

Rice husk with high volatile content was burned in a pilot scale vortexing fluidized bed incinerator. The fluidized bed incinerator was constructed of 6 mm stainless steel with 0.45 m in diameter and 5 m in height. The emission characteristics of CO, NO, and SO2 were studied. The effects of operating parameters, such as primary air flow rate, secondary air flow rate, and excess air ratio on the pollutant emissions were also investigated. The results show that a large proportion of combustion occurs at the bed surface and the freeboard zone. The SO2 concentration in the flue gas decreases with increasing excess air ratio, while the NOx concentration shows reverse trend. The flow rate of secondary air has a significant impact on the CO emission. For a fixed primary air flowrate, CO emission decreases with the secondary air flowrate. For a fixed excess air ratio, CO emission decreases with the ratio of secondary to primary air flow. The minimum CO emission of 72 ppm is attained at the operating condition of 40% excess air ratio and 0.6 partition air ratio. The NOx and SO2 concentrations in the flue gas at this condition are 159 and 36 ppm, which conform to the EPA regulation of Taiwan.

Particle segregation in a screen baffle packed fluidized bed

The segregation of binary particle mixtures was investigated in a horizontal screen baffle packed fluidized bed, which was operated continuously. In this study, the effects of the gas velocity, baffle spacing, screen size, feeding location, feeding rate, and bed height on particle segregation were examined. The results showed that the efficiency of particle segregation increases as the gas velocity, baffle spacing, and screen hole diameter decrease.

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.