Article pubs.acs.org/EF
Comparison of Superheated Steam and Air Fluidized-Bed Drying Characteristics of Victorian Brown Coals David Stokie, Meng Wai Woo, and Sankar Bhattacharya* Department of Chemical Engineering, Monash University, Clayton, Victoria 3150, Australia ABSTRACT: Brown coals represent a large resource for Victoria, with reserves of over 500 years at the current rate of consumption. However, its high moisture content (∼60%) impedes its utilization in an efficient manner. Information on drying kinetics of these coals is still scarce, affecting the development of economic and efficient drying technology. This experimental and modeling study presents a comparison of the drying kinetics between steam fluidized-bed drying and air fluidized-bed drying of three Victorian brown coals: Loy Yang, Yallourn, and Morwell. The effects of gas temperature (100−200 °C), gas velocity (0.32−0.61 m/s), and particle size (0.5−1.7 mm) on the drying kinetics have been examined. Both air and steam fluidized bed drying has been found to result in similar trends, with air drying having shorter drying times compared to steam drying. Increase in temperature and velocity, and a reduced particle size decreases the length of time required for complete drying to occur. Comparing air to steam fluidized bed drying, the relative drying ratios for a similar conditions (such as 130−170 °C) remains consistent, with the exception of particle size. The resultant 130 °C dried coal was used to analyze the moisture readsorption properties of the coals. The data show that steam fluidized-bed dried coals readsorb less moisture than air fluidized-bed dried coal, regardless of the coal type, with an average difference in moisture of 1.6%. The physical and chemical characterization of dried coal shows that moisture readsorption is a function of the oxygen functional group content. Several drying models available in the literature were compared against the experimental data acquired during this study. The results indicate that the Midilli− Kucuk model accurately describes the drying kinetics of the three investigated Victorian brown coals.
1. INTRODUCTION Brown coal is an inexpensive and readily available form of fuel in Victoria, with reserves that are estimated to last over 500 years.1With energy demand in Victoria projected to increase by up to 15% in the next 10 years, the demand on Victoria’s energy infrastructure will continue to increase, with up to 90% of Victorian power being currently generated from brown coal.1,2 Victorian brown coal has a moisture content of >60% on an as-mined basis and up to 25% of the coals internal energy may be required to remove the moisture before combustion.3−6 This results in lower overall efficiencies and higher carbon emissions for power generation.7 Therefore, this is a strong incentive to predry the coal efficiently before further use. Many countries are reliant on varying ranks of coal for power generation, including countries such as South Africa (93%), Poland (88%), China (78%), India (68%), and the United States of America (45%).8 Many of these countries encounter similar problems as Australia does with large moisture contents in their coal. The efficiency of a coal combustion power plant drops by ∼4% when the moisture content is within the range of 10%− 40%. If the moisture content reaches 60%, a 9% reduction in efficiency is seen.8 This is significant, since a 1% increase in efficiency can result in up to a 2.5% reduction in CO2 emissions. While many different technologies have been successfully commercialized or are in late stages of development, none have succeeded in completely addressing the environmental, economic, and efficiency needs.2 The reason that such a range of technologies exists is because of the heterogeneous nature of coal.9−11 The moisture content and composition of coal varies from region to region3 and, © XXXX American Chemical Society
depending on the product desired by using the dried coal, can influence the effectiveness and applicability of specific technologies. No single drying method can efficiently dry every type of coal, which is why many drying technologies are still being evaluated to determine the best possible method for the drying of each individual coal. This paper examines the characteristics of two specific drying processes: steam fluidized-bed drying (SFBD) and air fluidizedbed drying (AFBD). Fluidization occurs when a gas medium is pushed through a distributor plate and a particle bed at a sufficient velocity to cause the particles to move and mix, causing the bed to obtain fluidlike properties. SFBD has advantages over traditional forms of drying as the drying occurs in an inert atmosphere, which removes the potential for oxidization and spontaneous combustion, with other advantages in the use of steam including an increased heat recovery.6,12 SFBD was originally developed at Monash University by Professor Owen Potter and in the 1970s.13,14 Initially envisioned a single-stage fluidized bed, many variations have resulted in trying to increase drying efficiency. This includes the development of a mutli-level cascade system12 and the implementation of the internal waste heat utilization systems (WTA) at the RWE facility in Germany.15 SFBD can also be integrated into a power station by using low-grade steam, resulting in a cycle that has comparable or lower energy requirements for moisture removal on a wt (H2O)/wt (coal) Received: August 18, 2013 Revised: October 4, 2013
A
dx.doi.org/10.1021/ef401649j | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
basis.6,12 Compared to SFBD, AFBD is more susceptible to spontaneous combustion, especially when drying highly reactive coals such as Victorian brown coals. However, it has been successfully applied at the Great River Energy site, with typical moisture reduction from 38% to 29%.8,16,17 Currently neither superheated steam nor air fluidized-bed drying technology are commercially utilized for Victorian brown coal. A small steam fluidized-bed plant was opened in 1992 at the La Trobe valley, in Victoria, to supply auxiliary fuel for the Loy Yang A power station. Designed by Rheinbraun and Lurgi, the 150 000 tonne per year (dry coal) plant used steam supplied from Loy Yang A power station as a fluidization medium. Although the plant was run successfully, the facility was closed in 2003 because it was perceived to be expensive compared to the available alternatives such as coal briquettes or natural gas.18 Information on drying kinetics, chemical composition, physical and structural changes as well as resultant combustion properties are not readily available and investigating these factors will give a greater understanding of the practicality of fluidized bed drying of Victorian Brown coals.19 Such information is also required when scaling up the fluidized-bed drying process for Victorian brown coals and eventually aiding efforts to commercialize the technology. This paper addresses some of these issues by investigating key differences between air fluidized bed drying and superheated steam fluidized bed drying. This is achieved by examining the effect of initial particle size, fluidization velocity, and bed temperature on drying times. These values are then used to compare the differences in drying times of air and steam fluidization media. The paper goes on to determine which model can best represent the drying of Victorian brown coals under both air and steam fluidization, model that then can be used for scaling up purposes. Finally, the moisture readsorption properties of the resultant coal have been analyzed, with the results justified through physical and chemical analysis.
The moisture readsorption experiments analyze the uptake of moisture by exposing recently dried coal to the atmosphere. By taking a sample at set times and determining the moisture content, the moisture readsorption profile is found. 2.3. Minimum Fluidization Velocity Calculation. The minimum fluidization velocity was calculated using the Wen and Yu correlation:19−21 Ar =
Re =
volatile matter, VM (%)b
functional carbon, FC (%)b
Yallourn Loy Yang Morwell
61.2 58.1 59.9
5.0 1.5 4.6
45.7 49.5 41.0
49.3 49.0 54.4
a
(2)
umf d pρg μ
(3)
where Re is the Reynolds number. Using an average sieved coal particle size of 1.2−1.7 mm, a minimum fluidization velocity (Umf) of 0.23 m/s is calculated. The velocities used for the drying experiments vary between ∼1.4Umf and ∼2.7Umf. With the variation in particle size, an absolute velocity was used and not changed, based on the different minimum fluidization velocities, as other researchers have.19 This was chosen to present conditions typical of industry,18 where the particle size range in generally described as
