Selecting fines recycle methods to optimize fluid bed combustor performance

The effects of gas velocity to draft tube (3–6 Um), bed temperature (800–900°C) and excess air ratio (0–30%) on the total entrainment rate, overall combustion efficiency and heat transfer coefficient have been determined in an internally circulating fluidized bed combustor with a draft tube. The total entrainment rate increases with an increase in gas velocity to draft tube, but decreases with increasing bed temperature and excess air ratio. The overall combustion efficiency increases with increasing excess air ratio, but decreases with increasing gas velocity to draft tube. The overall combustion efficiency obtained in internally circulating fluidized beds was found to be somewhat higher than that in a bubbling fluidized bed combustor.

Limiting NO x and SO 2 emissions from an industrial-size fluidised-bed combustor

A new method to measure apparent reactivity, which accounts for all the effects of coal type on reaction kinetics in fluidized bed combustion (FBC), has been developed to characterize and rank various coals according to overall burning rates in a fluidized bed environment. This method estimates FBC reactivity by means of a parameter called mean carbon conversion time, which can be related to the carbon inventory of practical FBC systems. An easy and rapid experimental technique, which burns coal batchwise in a bench-scale fluidized bed combustor, has been developed to determine the mean carbon conversion time. This technique was employed to examine five different coals and rank them according to FBC reactivity. To differentiate between devolatilization and char combustion, a turning point on the experimental burning rate curve was used instead of the common method of observing the volatile flame. Char mean carbon conversion times were also measured and compared. The effects of coal particle size and bed temperature on FBC reactivity and combustion efficiency and the effects of particle fragmentation on experimental results were investigated. Efforts were made to correlate the FBC reactivity with other coal parameters such as volatile content, carbon content and free swelling index.

Onakawana lignite containing up to 47% moisture has been burnt in two pilot scale atmospheric fluidized bed combustors. Bed parameters including fluidizing velocity, bed temperature and fuel moisture were varied to study their effect on combustion efficiency and emissions. Carbon combustion efficiency varied between 95 and 98% without recycle, the highest value being achieved above 900 ° C. With fly ash recycle, combustion efficiencies of >99% were achieved at temperatures as low as 800 ° C, providing that the ratio of "fines to lumps" in the coal feed stock did not exceed 2:1. NO x emissions typically varied between 17 and 90 ng/J input, well within the limit of 258 ng/J input specified by the National Guidelines for Stationary Sources. On the other hand, S0x , varied from 270 to 1400 ng/J depending on operating conditions. The specified limit of 258 ng/J can be achieved by adding small amounts of limestone at about 850 ° C.

Experiment and grey relational analysis of seaweed particle combustion in a fluidized bed

Study on the emission characteristics of nitrogen oxides with coal combustion in pressurized fluidized bed

The feasibility of converting waste preparation plant coal streams into marketable fuels with good combustion and emissions properties has been investigated in this project. Coal pellets containing both hydrated lime and limestone as sorbents were made from a flotation column waste feedstock under a related Illinois Clean Coal Institute project. These pellets, which contain sorbent with Ca/S ratio varying from 0.8 to 2.4 were successfully burnt in a 4-inch internal diameter circulating fluidized bed combustor. Emissions levels of sulfur dioxide, oxides of nitrogen and hydrogen chloride were measured as a function of bed temperature. Mineral matter analyses of the combustion generated ash was made, and combustion efficiencies were studied. The pellet coals combustion performance was compared to that of an Illinois No. 6 run-of-mine coal from the same preparation plant. Results show that the coal pellets, produced from the flotation column waste stream, are excellent fluidized bed combustor fuels. Calcium hydroxide impregnated pellets yielded lower sulfur dioxide emissions than limestone sorbent pellets for the same Ca/S ratio. Increase in Ca/S ratio with the hydrated lime sorbent produced a more rapid decline in sulfur dioxide emissions than with the limestone sorbent. Oxides of nitrogen emissions were generally on the order of 0.3 to 0.4 lbs/10{sup 6} Btu. Hydrogen chloride emissions were found to be influenced more by Ca/S ratios than by bed temperature, and varied in the range of 0.0075 to 0.055 lbs/10{sup 6} Btu in the present tests. Combustion efficiencies of the pellet fuels were about 98% without secondary cyclones recycle. EDX analysis of the mineral matter in the raw coal and the combustion-generated ash samples sheds light on the mineral transformations during combustion.

An approach to estimate the depth of oxidizing and reducing regions in a fluidized bed combustor with staged combustion

These studies support the development program in atmospheric and pressurized fluidized-bed coal combustion. Laboratory and process development-scale studies are aimed at providing information on limestone utilization, control of the emission of alkali metal compounds and SO/sub 2/ during combustion, particulate loading in flue gas, and other aspects of fluidized-bed coal combustion. This report presents information on the effect of bed temperature, superficial gas velocity and gas hourly space velocity on the removal of gaseous NaCl from hot gas streams by a granular bed of diatomaceous earth; results of experiments in which the adsorption capacity for gaseous NaCl of activated bauxite was measured after regeneration by an aqueous leaching process; and an estimate of the cost of using these two sorbents for alkali metal control in a large PFBC. Results are also presented for studies aimed at the development of an acoustic dust conditioning system, and the evaluation of a high-efficiency cylone for removal of particles from a high-pressure, high-temperature gas stream. The relationship of the SO/sub 2/ reactivity of limestones to internal surface area, pore diameter, and pore volume are examined.

Propane--air mixtures alone and with added nitric oxide, ammonia, or dimethylamine were burned in a fluidized bed. Bed temperature, flow rate, fuel--air ratio, particle size, bed depth, and the amount of fuel bound nitrogen were varied. Much less nitric oxide is formed from atmospheric nitrogen than is formed in conventional combustion processes. The amounts formed are higher than equilibrium amounts at each bed temperature with stoichiometric and rich mixtures and lower with lean mixtures. In lean flames, when nitrogen is chemically bound to fuel, virtually all of the fuel-bound nitrogen is converted to nitric oxide. Powdered nickel almost completely eliminated nitric oxide from the products of rich mixtures.

Enhanced combustion behavior and NOx reduction performance in a CFB combustor by combining flue gas recirculation with air-staging: Effect of injection position

Bottom ashes collected from ESP and baghouse of seven pulverized coal-fired power plants using subbitumi- nous and bituminous coal and a fluidized bed combustor using crushed bituminous coal were examined for their mineral- ogy and elemental composition. The results presented in this paper are based on the average of three samples collected from each power plant. The mineralogy of each sample was determined using XRD and SEM/EDX; Elemental content was determined using INAA, ICPMS, and CVAAS; and the speciation of As, Cr, and Ni was determined by XANES. Bottom ash from the pulverized power plants consists of granular particles with a minor input of melted glassy fragments, while the bottom ash from the fluidized bed combustor consists entirely of granular particles. The sulphur and carbon con- tents of pulverized bottom ashes range from 0.03 to 2.32 wt % and 0.19 to 6.62 wt %, respectively. For the fluidized bed combustor, the sulphur and carbon contents were 5.27 wt % and 10.72 wt %. The concentrations of As, Cr, Hg, Ni, and Pb in bottom ash are related to sulphur content of coal and are higher for bottom ashes from high sulphur feed coals. Most of the elements associated with S (As, Hg, and Pb) are captured from fluidized bottom ash, more so than by the correspond- ing ESP fly ash. Most of the elements in bottom ash have enrichment (RE) factors of less than 0.7 indicating that they are not enriched in the bottom ashes as compared to the feed coals. Arsenic, Cd, Cr, Ni, and Pb have higher concentrations in granular bottom ash as compared to glassy bottom ash of the same power plant. The feldspars and quartz of feed coal are health hazards and are captured mostly by bottom ash and therefore prevent their emission from stack. The As, Cr, and Ni in form of: non-toxic As +3 , mostly beneficial Cr 3+ , and non-carcinogenic Ni 2+ are in coordination predominantly with oxygen. Mercury and Pb are low and have very little environmental impact.The hazardous elements (As, Cr, Ni, and Pb) are only leached by HCl, indicating that under normal condition they remain immobile and their impact on environment can be considered low.

The fluidized bed combustion (FBC) process, used in power generation, can handle a variety of fuels. However, the range of fuels that can be accommodated by an FBC boiler system is affected by the ability of the fuel, sorbent, and ash-handling equipment to move the required solids through the boiler. Of specific interest is the bottom ash handling equipment, which must have sufficient capacity to remove ash from the system in order to maintain a constant bed inventory level, and must have sufficient capability to cool the ash well below the bed temperature. Quantification of a fuel’s bottom ash removal requirements can be useful for plant design. The effect of fuel properties, on the rate of bottom ash production in a laboratory FBC test system was examined. The work used coal products ranging in ash content from 20to40+wt.%. The system’s classification of solids by particle size into flyash and bottom ash was characterized using a partition curve. Fuel sizing was compared to the partition curve, and fuels were fractionated by particle size. Fuel fractions in the size range characteristic of bottom ash were further analyzed for distributions of ash content with respect to specific gravity, using float sink tests. The fuel fractions were then ashed in a fixed bed. In each case, the highest ash content fraction produced ash with the coarsest size consist (characteristic of bottom ash). The lower ash content fractions were found to produce ash in the size range characteristic of flyash, suggesting that the high ash content fractions were largely responsible for the production of bottom ash. The contributions of the specific gravity fractions to the composite ash in the fuels were quantified. The fuels were fired in the laboratory test system. Fuels with higher amounts of high specific gravity particles, in the size ranges characteristic of bottom ash, were found to produce more bottom ash, indicating the potential utility of float sink methods in the prediction of bottom ash removal requirements.

Attrition of chars in fluidized bed combustion

Combustion of refuse-derived fuel in a fluidised bed