C. Jim Lim

CO2 absorption by NaOH, monoethanolamine and 2-amino-2-methyl-1-propanol solutions in a packed column☆

Detailed concentration and temperature measurements are reported for the absorption of CO2 from air into aqueous NaOH, monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP) solutions. An absorber (0.1 m i.d. packed with 12.7 mm Berl Saddles up to heights of 6.55 m) was used which operated in countercurrent mode and near ambient conditions. The measurements for the CO2NaOH and CO2-MEA systems are compared with predictions from a mathematical model and gave generally good agreement, except at high CO2 loadings of MEA solutions. Compared with MEA, AMP was found to have superior CO2 absorption capacities and inferior mass transfer rates.

HYDRODYNAMICS OF SPOUTED BEDS AT ELEVATED TEMPERATURES

A review of the literature reveals little information on the hydrodynamics of spouted beds at high temperature. Most existing correlations are based upon experiments done at ambient conditions; the...

Characterization and kinetics study of off-gas emissions from stored wood pellets

The full potential health impact from the emissions of biomass fuels, including wood pellets, during storage and transportation has not been documented in the open literature. The purpose of this study is to provide data on the concentration of CO(2), CO and CH(4) from wood pellets stored in sealed vessels and to develop a kinetic model for predicting the transient emission rate factors at different storage temperatures. Five 45-l metal containers (305 mm diameter by 610 mm long) equipped with heating and temperature control devices were used to study the temperature effect on the off-gas emissions from wood pellets. Concurrently, ten 2-l aluminum canisters (100 mm diameter by 250 mm long) were used to study the off-gas emissions from different types of biomass materials. Concentrations of CO(2), CO and CH(4) were measured by a gas chromatograph as a function of storage time and storage temperature. The results showed that the concentrations of CO, CO(2) and CH(4) in the sealed space of the reactor increased over time, fast at the beginning but leveling off after a few days. A first-order reaction kinetics fitted the data well. The maximum concentration and the time it takes for the buildup of gas concentrations can be predicted using kinetic equations.

Rate and Peak Concentrations of Off-Gas Emissions in Stored Wood Pellets—Sensitivities to Temperature, Relative Humidity, and Headspace Volume

Wood pellets emit CO, CO(2), CH(4), and other volatiles during storage. Increased concentration of these gases in a sealed storage causes depletion of concentration of oxygen. The storage environment becomes toxic to those who operate in and around these storages. The objective of this study was to investigate the effects of temperature, moisture, and the relative size of storage headspace on emissions from wood pellets in an enclosed space. Twelve 10-l plastic containers were used to study the effects of headspace ratio (25, 50, and 75% of container volume) and temperatures (10-50 degrees C). Another eight containers were set in uncontrolled storage relative humidity (RH) and temperature. Concentrations of CO(2), CO, and CH(4) were measured by gas chromatography (GC). The results showed that emissions of CO(2), CO, and CH(4) from stored wood pellets are more sensitive to storage temperature than to RH and the relative volume of headspace. Higher peak emission factors are associated with higher temperatures. Increased headspace volume ratio increases peak off-gas emissions because of the availability of oxygen associated with pellet decomposition. Increased RH in the enclosed container increases the rate of off-gas emissions of CO(2), CO, and CH(4) and oxygen depletion.

Spouted, fluidized and spout-fluid bed combustion of bituminous coals

Abstract Spouted, fluidized and spout-fluid bed combustion of three bituminous coals or coal rejects of differing ash contents was carried out in a 0.3 m internal diameter combustor. Coal feed rates were between 6 and 22 kg h −1 , excess air was usually 15–20%, the internal coil coolant was either air or water, and the average bed temperature ranged from 740 to 980 °C. Each of the three contacting modes was able to burn the coal to which it was subjected. Although differences in combustion performance between the three modes were generally small, the experimental results indicated that the spout-fluid bed tended to give somewhat higher combustion efficiencies at the lower temperatures, greater temperature uniformity and improved bed-to-immersed-surface heat transfer compared with the other two modes of operation. For all three modes, combustion efficiencies were over 90%, provided that bed temperatures were higher than 870 °C and that fines captured in the primary cyclone were recycled to the bed. The size and density of inert bed materials played a significant role. Denser and coarser inert particles tended to segregate, leaving burning char particles at or near the bed surface. Bed-to-cooling-coil heat transfer coefficients were of similar magnitude to those measured in earlier studies.

Equilibrium modelling of catalytic steam reforming of methane in membrane reactors with oxygen addition

It has previously been demonstrated that permselective membranes can increase the yields of hydrogen, improve the product quality and minimize or even eliminate the adverse effect of system pressure in catalytic steam methane reforming. At the same time, oxygen addition can lead to autothermal operation by providing some or all of the required endothermic heat of the reforming reaction. Previous comparison of reactor model predictions with experimental results has demonstrated, at least for the fluidized bed membrane reactors (FBMRs) investigated so far, that the product distribution in the non-permeate stream approaches very closely to the shifted chemical equilibrium after allowance is made for the removal of hydrogen. In this paper, we adapt an equilibrium model devised for gasification to predict the influence of various process parameters. It is shown that it should be possible to operate autothermally and free of coke formation over a considerable range of temperature, pressure and steam-to-methane ratio.

Characterization of steam reactivation mechanisms in limestones and spent calcium sorbents

Three limestones and three spent calcium sorbents with differing sulfation patterns were subjected to sulfation and steam hydration at 250 and 450°C in an effort to identify the optimum conditions for increasing their ability to capture sulfur. Visual and chemical investigations of the hydrated and resulfated samples using a scanning electron microscope indicate that the sulfation pattern controls whether or not a limestone can be reactivated significantly using steam. The increasing calcium utilization with decreasing hydration temperature results from fracturing during both hydration and de-hydration. Spent sorbents were harder to reactivate than pure limestones. Only low temperature (250°C) steam hydration led to an increase in utilization for two of the spent sorbents. Problems reactivating the spent sorbent may be related to the higher temperatures encountered in the boilers in which they were produced compared to the limestones prepared in the laboratory. The sulfation ‘history’ of the spent sorbents, as well as reactions between the limestone and the coal (both the organic and inorganic fractions), may also contribute in reducing the response to hydration of spent sorbents.

Co-gasification of biosolids with biomass: Thermogravimetric analysis and pilot scale study in a bubbling fluidized bed reactor

This work studied the feasibility of co-gasification of biosolids with biomass as a means of disposal with energy recovery. The kinetics study at 800°C showed that biomass, such as switchgrass, could catalyze the reactions because switchgrass ash contained a high proportion of potassium, an excellent catalyst for gasification. However, biosolids could also inhibit gasification due to interaction between biomass alkali/alkaline earth metals and biosolids clay minerals. In the pilot scale experiments, increasing the proportion of biosolids in the feedstock affected gasification performance negatively. Syngas yield and char conversion decreased from 1.38 to 0.47m(3)/kg and 82-36% respectively as the biosolids proportion in the fuel increased from 0% to 100%. Over the same range, the tar content increased from 10.3 to 200g/m(3), while the ammonia concentration increased from 1660 to 19,200ppmv. No more than 25% biosolids in the fuel feed is recommended to maintain a reasonable gasification.

Effects of Headspace and Oxygen Level on Off-gas Emissions from Wood Pellets in Storage

Few papers have been published in the open literature on the emissions from biomass fuels, including wood pellets, during the storage and transportation and their potential health impacts. The purpose of this study is to provide data on the concentrations, emission factors, and emission rate factors of CO(2), CO, and CH(4) from wood pellets stored with different headspace to container volume ratios with different initial oxygen levels, in order to develop methods to reduce the toxic off-gas emissions and accumulation in storage spaces. Metal containers (45 l, 305 mm diameter by 610 mm long) were used to study the effect of headspace and oxygen levels on the off-gas emissions from wood pellets. Concentrations of CO(2), CO, and CH(4) in the headspace were measured using a gas chromatograph as a function of storage time. The results showed that the ratio of the headspace ratios and initial oxygen levels in the storage space significantly affected the off-gas emissions from wood pellets stored in a sealed container. Higher peak emission factors and higher emission rates are associated with higher headspace ratios. Lower emissions of CO(2) and CO were generated at room temperature under lower oxygen levels, whereas CH(4) emission is insensitive to the oxygen level. Replacing oxygen with inert gases in the storage space is thus a potentially effective method to reduce the biomass degradation and toxic off-gas emissions. The proper ventilation of the storage space can also be used to maintain a high oxygen level and low concentrations of toxic off-gassing compounds in the storage space, which is especially useful during the loading and unloading operations to control the hazards associated with the storage and transportation of wood pellets.

ADVANCES IN MODELING OF FLUIDIZED-BED CATALYTIC REACTORS: A COMPREHENSIVE REVIEW

A number of fluidized-bed catalytic reactor models have been proposed during the past half-century based on conservation equations as well as empirical relations. This article presents a comprehensive review of these models, ranging from the classic and pioneering reactor models found in the literature to the current state-of-the-art. Each model incorporates a different set of assumptions, leading to different expressions for simulating reactor performance. Forty models are analyzed depending on the characteristics of their conservation equations and their underlying assumptions by reducing each model to a sequential combination of assumptions. This review contributes to the elucidation process for choosing the appropriate model to simulate a specific fluidized-bed reactor.