Hari Vuthaluru

Thermal behaviour of coal/biomass blends during co-pyrolysis

Abstract Investigations into the thermal behaviour during co-pyrolysis of coal, biomass materials and coal/biomass blends prepared at different ratios (10:90, 20:80, 30:70 and 50:50) have been conducted using a thermogravimetric analysis (TGA) apparatus. Coal sample selected was Collie subbituminous coal from Western Australia, while wood waste (WW) and wheat straw (WS) were used as biomass samples. Three thermal events were identified during the pyrolysis. The first two were dominated by the biomass pyrolysis, while the third was linked to the coal pyrolysis, which occurred at much higher temperatures. No interactions were seen between the coal and biomass during co-pyrolysis. The pyrolytic characteristics of the blends followed those of the parent fuels in an additive manner. Among the tested blends, 20:80 blends showed the lowest activation energies of 90.9 and 78.7 kJ mol −1 for coal/WW and coal/WS blends, respectively. It was also found that the optimum blend ratio for pyrolysis of coal/WS to be 50:50 with a high degradation rate in all thermal events and a higher mass loss over the course of the co-pyrolysis compared to coal/WW blends examined. The reaction orders in these experiments were found to be in the range of 0.21–1.60, thus having a significant effect on the overall reaction rate. Besides the pyrolysis of coal alone, the 50:50 coal/biomass blends had the highest reaction rate, ranging from 1×10 9 to 2×10 9 min −1 .

Effects of moisture and coal blending on Hardgrove Grindability Index of Western Australian coal

Abstract Investigations into the effects of moisture and coal blending on Hardgrove Grindability Index (HGI) were carried out on Collie coal of Western Australia. Experiments were conducted in a standard Hardgrove apparatus on four individual Premier seam coals (namely P2, P3, P4 and Hebe) and several blends (namely Hebe/P2, Hebe/P3, Hebe/P4, Hebe/P2/P4) prepared at various blending ratios. The experiments comprised of 5 days of air-drying followed by oven drying. Among the coal seams tested, Hebe showed the highest HGI (58) whereas P4 was the lowest (47). HGI was found to correlate well with residual moisture, with correlation coefficients ranging from 0.5 to 0.9 depending on the type of coal seam or blend. In contrast, moisture measurements on the samples loaded into the HGI apparatus (size 0.600 to 1.180 mm), referred to as the “coarse fraction” showed erratic trends with HGI. The experimental results suggest that no relationship exist between the coarse fraction moisture and HGI. Measured HGI values of binary and ternary blends were found to correspond well with the weighted average values of HGI within ±2 HGI units. This effect was confirmed by a further investigation with a range of 11 binary (P3/Hebe) blends of various proportions.

A novel correlation for estimation of hydrate forming condition of natural gases

Abstract An inherent problem with natural gas production or transmission is the formation of gas hydrates, which can lead to safety hazards to production/transportation systems and to substantial economic risks. Therefore, an understanding of conditions where hydrates form is necessary to overcome hydrate related issues. Over the years, several models requiring more complicated and longer computations have been proposed for the prediction of hydrate formation conditions of natural gases. For these reasons, it is essential to develop a reliable and simple-to-use method for oil and gas practitioners. The purpose of this study is to formulate a novel empirical correlation for rapid estimation of hydrate formation condition of sweet natural gases. The developed correlation holds for wide range of temperatures (265–298 K), pressures (1200 to 40000 kPa) and molecular weights (16–29). New proposed correlation shows consistently accurate results across proposed pressure, temperature and molecular weight ranges. This consistency could not be matched by any of the widely accepted existing correlations within the investigated range. For all conditions, new correlation showed average absolute deviation to be less than 0.2% and provided much better results than the widely accepted existing correlations.

A Modified Kennard-Stone Algorithm for Optimal Division of Data for Developing Artificial Neural Network Models

Abstract This paper proposes a method, namely MDKS (Kennard-Stone algorithm based on Mahalanobis distance), to divide the data into training and testing subsets for developing artificial neural network (ANN) models. This method is a modified version of the Kennard-Stone (KS) algorithm. With this method, better data splitting, in terms of data representation and enhanced performance of developed ANN models, can be achieved. Compared with standard KS algorithm and another improved KS algorithm (data division based on joint x - y distances (SPXY) method), the proposed method has also shown a better performance. Therefore, the proposed technique can be used as an advantageous alternative to other existing methods of data splitting for developing ANN models. Care should be taken when dealing with large amount of dataset since they may increase the computational load for MDKS due to its variance-covariance matrix calculations.

Behaviour of inorganic constituents and ash characteristics during fluidised-bed combustion of several Australian low-rank coals

Abstract Behaviour of inorganic constituents during fluidised-bed combustion of several Australian low-rank coals was studied using a laboratory-scale spouted-bed combustion system. Coals from Victoria (Loy Yang and Morwell) and South Australia (Lochiel and Bowmans) were chosen for the present study. Characteristics of ash buildup on bed material and bed defluidisation were compared for the coals tested at temperatures between 800°C and 900°C. Samples of ash-coated bed particles and fly ash withdrawn from the system were subjected to various analytical techniques including chemical analysis, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Experimental results indicated that the coal type has a significant effect on the ash characteristics and buildup on bed material particles. Coals with high contents of sodium and sulphur have resulted in the formation of low-melting-point compounds (viz. alkali sulphates) in the ash coating on bed material particle surfaces, rendering them more sticky at fluidised-bed temperatures. This led to increased ash buildup on bed material particle surfaces with such low-rank coals. In contrast, for coals with low sodium and sulphur contents, the combustion lasted for longer periods without any particle agglomeration and defluidisation. The varying content of sodium and sulphur in these coals were identified to be responsible for the different ash buildup and defluidisation behaviour observed.

Effect of coal blending on particle agglomeration and defluidisation during spouted-bed combustion of low-rank coals

The possibility of blending coals to alleviate particle agglomeration and bed defluidisation during fluidised-bed combustion (FBC) of several low-rank coals was exploited. A laboratory scale spouted bed combustor was employed to fire coal blends from two lignites with a sub-bituminous coal at ratios of 50:50 and 90:10, at temperatures ranging 800°C. Experiments showed significant improvements in FBC operation with the coal blends compared to the raw lignites. No particle agglomeration and bed defluidisation were evident after 15 h of operation with the blends at 800°C. Chemical analyses indicated that the formation of low temperature eutectics was suppressed by calcium aluminosilicate phases from the sub-bituminous coal, rendering the surface of ash-coated particles dry and less sticky. This was identified as the key mechanism for the control of particle agglomeration and bed defluidisation in FBC, which led to extended combustion operation with the coal blends.

Effect of Ca- and Mg-bearing minerals on particle agglomeration defluidisation during fluidised-bed combustion of a South Australian lignite

Abstract Behaviour of calcium and magnesium during fluidised-bed combustion (FBC) of a South Australian lignite was investigated using a laboratory scale spouted bed combustion system. Combustion experiments were aimed at investigating the effectiveness of Ca- and Mg-bearing minerals (as alternative bed materials) in controlling particle agglomeration and bed defluidisation during FBC combustion of low-rank coals. Additional experiments performed with a Ca-treated coal investigated the role of Ca in agglomeration and defluidisation process. Experimental results indicated that both Ca/Mg-bearing minerals and Ca-treated coal were effective to different extents in reducing bed defluidisation. Tests with calcite (as the bed material) and Ca-treated coal runs (with sand as the bed material) showed trouble free operation for 8–10 h before bed defluidisation incurred. Tests with magnesite (as the bed material) showed no agglomeration and defluidisation tendencies for longer operating periods (∼12 h at 800°C). Mg-bearing compounds have been found to be effective in controlling defluidisation and allowed extended combustion operations. On the other hand, high levels of Ca either in coal or in bed material have been found to delay and decrease the severity of agglomerates formed.

Predicting hydrate forming pressure of pure alkanes in the presence of inhibitors

An inherent problem with natural gas production or transmission is the formation of gas hydrates, which can lead to safety hazards for production/transportation systems, and substantial economic risks. Hydrate inhibition with different inhibitors such as, methanol, ethylene glycol (EG), triethylene glycol (TEG), and sodium chloride solution continues to play a critical role in many operations. An understanding of when the hydrates form in the presence of these hydrate inhibitors, is therefore necessary to overcome hydrate problems. Several thermodynamic models have been proposed for predicting the hydrate formation conditions in aqueous solutions containing methanol/glycols and electrolytes. However, available models have limitations that include the types of liquid, compositions of fluids, and inhibitors used. The aim of this study is to develop a simple-to-use correlation for accurate prediction of hydrate-forming pressures of pure alkanes in the presence of different hydrate inhibitors, where the obtained results illustrate good agreement with the reported experimental data.

Method Accurately Predicts Water Content of Natural Gases

Abstract An accurate prediction of natural gas water content is an important parameter in proper designing of natural gas production, transmission, and processing facilities. The aim of this study is to describe an accurate method for predicting water content of sweet and sour natural gases, where the obtained results by the proposed method show good agreements with the experimental data. The proposed method provides good estimates of the equilibrium water vapor content of sour natural gas for a range of conditions, including H 2 S contents of 0–50 mole % with CO 2 contents of 0–40 mole %, pressures from atmospheric to 100,000 KPa for sweet gas and 70,000 KPa for sour gas, and temperatures from 10°C–150°C for sour gases and 10°–200°C for sweet gases.

Analyzing solubility of acid gas and light alkanes in triethylene glycol

Physical solvents such as ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG) are com- monly used in wet gas dehydration processes with TEG being the most popular due to ease of regeneration and low solvent losses. Unfortunately, TEG absorbs significantly more hydrocarbons and acid gases than EG or DEG. Quantifying this amount of absorption is therefore critical in order to minimize hydrocarbon losses or to optimize hydrocarbon recovery depending on the objective of the process. In this article, a new correlation that fully covers the operating ranges of TEG dehydration units is developed in order to determine the solubility of light alkanes and acid gases in TEG solvent. The influence of several parameters on hydrocarbon and acid gas solubility including temperature, pressure, and solvent content is also examined.