INTRODUCTION
One of the most significant scientific challenges in the world today is to efficiently utilize a fuel to generate power at the most economic level. For this purpose various types of techniques of utilizing fuel in solid form (e.g. pulverized coal), liquid form (e.g. petroleum), and gaseous form (e.g. Compressed Natural Gas) have been utilized from the very beginning.
The first oil crisis in 1973 turned all interests on coal converge technologies. The aim was to replace the fossil carbon in mineral oil by the carbon in coal. From this time, extensive studies have been started particularly on coal gasification, liquefaction and combustion. In addition to these existing extensive studies, in recent years, research on coal-water mixtures (CWMs) was commenced.
A typical coal-water mixture consists of:
- 50-75% finely ground coal, with a top size of 250 to 300 microns.
- 25-50% water.
- Approximately 1% chemical additives.
However, when coal in a finely powdered form is mixed with water, the obtained slurry flow behavior generally gets altered depending on the concentration of coal (i.e. solid loading) and the interfacial properties in the water. Ideal CWS with maximum coal loading should be relatively stable at static state and dynamic state with a low viscosity. Rheological behavior will also affect the atomization of CWS to combustion chamber of power plant. The efficient utilization of coal water slurry is possible only when the slurry is prepared such that it permits maximum coal loading with appreciable viscosity and maintains a uniform concentration. Therefore, to study whether a particular coal water slurry is suitable to be inducted as a fuel, rheological studies are often conducted to determine the rheological parameters like viscosity, shear stress etc. by applying a rate of shear on the slurry and the rheograms (flow curves) and viscosity curves obtained thus are then fitted to the different rheological models that best describes the category of fluid in which the slurry in investigation falls in.
1.1 COAL
At various times in the geologic past, the Earth had dense forests in low-lying wetland areas. Due to natural processes such as flooding, these forests were buried under the soil. As more and more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. Under high pressure and high temperature, dead vegetation was slowly converted to coal. As coal contains mainly carbon, the conversion of dead vegetation into coal is called carbonization. Coal is classified as a non-renewable energy source because it takes millions of years to form.
India is the third-largest producer of coal in the world with 589 million tones. Coal is one of the most abundant sources of energy in the country. Coal is accounted for 51 percent of the primary energy consumed in the country.
1.2 TYPES OF COAL
Coal is formed by biological, physical and chemical processes, governed by temperature and pressure, over millions of years on plant remains, deposited in ancient shallow swamps. The degree of alteration (metamorphism), caused by these processes, during the temporal history of development determine their position or rank in the coalification series which commence at peat and extend through lignite to bituminous coal and finally anthracite. The relative amount of moisture, volatile matter, and fixed carbon content varies from one to the other end of the coalification series. The moisture and volatile matter decrease with enhancement of rank while carbon content increases i.e., carbon content is lowest in peat and highest in anthracite.
- Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water. It is also used as a conditioner for soil to make it more able to retain and slowly release water.
- Lignite is the lowest rank of coal, with a heating value of 4,000 to 8,300 British thermal units (Btu) per pound. Lignite is crumbly and has high moisture content Lignite is mainly used to produce electricity. It contains 25 to 35 percent carbon.
- Sub-bituminous coal typically contains less heating value than bituminous coal (8,300 to 13,000 Btu per pound) and more moisture. It contains 35 to 45 percent carbon.
- Bituminous coal is formed by added heat and pressure on lignite. Made of many tiny layers, bituminous coal looks smooth and sometimes shiny. Ir has two to three times the heating value of lignite. Bituminous coal contains 11,000 to 15,500 Btu per pound. Bituminous coal is used to generate electricity and is an important fuel for the steel and iron industries. It contains 45 to 86 percent carbon.
- Steam coal is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea-coal in the U.S. Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating.
- Anthracite is created where additional pressure combined with very high temperature inside the Earth. It is deep black and looks almost metallic due to its glossy surface. Like bituminous coal, anthracite coal is a big energy producer, containing nearly 15,000 Btu per pound. It contains 86 to 97 percent carbon.
- Graphite, technically the highest rank, is difficult to ignite and is not commonly used as fuel — it is mostly used in pencils and, when powdered, as a lubricant.
1.3 COAL-WATER SLURRY
The mixture of solids and liquids is known as slurry. The physical characteristics of slurry are dependent on many factors such as particle size and distribution, solid concentration in the liquid phase, turbulence level, temperature, conduit size, and viscosity of the carrier. Water is the most commonly used fluid.
Factors such as the continued rise in oil prices, the difficulties associated with a stable supply of Crude oil, increased fuel consumption, and the limited oil reserves in recent years have increased interest in and research related to the use of coal, which is relatively cheaper, plentiful, and widely distributed across the globe. Therefore, many studies pertaining to coal slurry as alternative fuel replacing petroleum oil in the liquid state have been carried out. Coal slurry fuel can be divided into CWS (coal-water slurry), COS (coal-oil slurry), COWS (Coal-oil-water slurry), CMS (coal methanol slurry), and CMWS (coal-methanol-water slurry) depending on the type of liquid mixed with the solid coal. Among these different types, CWS fuel is considered by some to have the greatest economic feasibility as a fuel source and the greatest potential for commercialization.
Coal-Water Slurry is slurry of powdered coal and water which maintains a stable state over a long period when a small amount of additive is added. The coal-water slurry can be used as a liquid fuel (CWSF) for boilers and can replace petroleum for energy conversion. These are non-flammable and eco-friendly with good comparable combustion efficiency with that of conventional fuels.
For maximum efficiency as fuel, the coal concentration in coal-water slurry should be as high as possible, maintaining its viscosity at the minimum level simultaneously so that it will be suitable for storage and transportation through pipelines. The primary factors responsible for the optimum stability of coal-water slurry depend on physicochemical properties of coal, such as its (i) surface hydrophobicity, (ii) particle size distribution, (iii) oxygen content, (iv) zeta potential (surface charge), (v) pH sensitiveness, (vi) shear rate- shear stress relation, (vii) porosity, (viii) temperature sensitiveness of the viscosity of the coal-water slurry, and (ix) surface chemistry of coal etc.
1.4 HISTORICAL DEVELOPMENT OF COAL-WATER SLURRY
The first combustion tests of CWS were conducted in the United States, Germany, and the Soviet Union in the 1960s. There was active development of CWMs in the United States in the 1980s, with emphasis on developing technologies to prepare mixtures with desirable physical and chemical properties, demonstrating retrofit in existing boilers, and developing specialized equipment for handling and transporting slurries. During this period, a number of private companies were actively involved in, or abandoned commercialization of slurries as oil prices declined in the early 1980s [Coal Energy for the Future, 1995].
Early efforts in CWS technology development mainly concentrated on the preparation of high energy concentrated on the preparation of high energy density liquid fuel from bituminous coals. The first attempts to utilize low rank coals (LRC) for this purpose consisted of simply mixing the pulverized LRC in its natural state directly with water. Chemical additives were not used and no changes were made to its surface characteristics. The result was that the utilization of LRC in its natural state was not economically feasible due to its extremely low energy content and unfavorable characteristics before and after burning [Witsee et al., 1986]. However the recent introduction of chemical additives has made LRC utilization as CWMs possible and feasible [Uyar et al., 1994].
The particle size distribution of the ground coal must be approximately adjusted to allow high solids loading of the CWM, and the chemical additives should match the interfacial properties of the coal particles, to lower the CWM viscosity and increase its stability. CWM with such properties behaves as a liquid fuel: it is pump able, can be atomized using a specially designed nozzle, ignited in a preheated environment, and hence stably combusted in a boiler or furnace [Wang et al., 1993].
Ideal CWS with maximum coal loading should be relatively stable at static state and during transportation, and exhibit good rheological behavior. Rheological behavior affects the atomization of CWM, which will influence the combustibility of CWM in boiler application. [Li and Li, 2000] The industrially expected value for the viscosity of a CWM, even though not absolute, is Brookfield apparent viscosity of 1000 cp at 100 rpm. [Natoli et al., 1985]
1.5 UTILIZATION AREAS OF COAL-WATER SLURRY:
- CWS can be used in place of oil and gas in any size of heating and power station. CWS is suitable for existing gas, oil, and coal boilers.
- Since the 2004 Russia has continued the development and implementation of CWS technology for heating stations (for district heating) and power stations.
- Countries like China and Indonesia have been using coal water slurry for power generation successfully. More than 90 steam and power generating plants are utilizing CWS in China, with capacity ranging from 1.5 MW to 200 MW.
- CWS have been used in gas turbine and diesel engine for research and pilot plants only, as both of these applications require higher specification of coal than is currently used to produced CWS for boiler and heating applications.
- For gas turbine testing CWS particles five to ten micrometers in size have been used to demonstrate useful substitution for petroleum or natural gas in combined cycle gas turbine power plant applications.
- CWS can be used as fuel in coal gasification process to produce synthesis gas.
1.6 ADVANTAGES OF COAL WATER SLURRY
Ecology:
- WCF is a clean coal technology solution for big and small energy.
- WCF allows utilizing sewage water (from city canalization).
- WCF ash is an ideal additive to concrete mixtures.
Economy:
- Reduces cost price of Gcal and kWh.
Technology & Handling:
- WCF is an explosion-proof.
- WCF is easy to storage and pumping.
- WCF minimum number of non standard elements for preparing and combustion.
Autonomy:
- Fully autonomous energy source.
- Almost any type of coal could be used for WCF preparation
- WCF combustion reduces on 30-50% NOx emission comparing to regular pulverized coal combustion.
Coal utilization while WCF combustion is about 99%. It means that only 1% of uncombusted coal exists in ash.
1.7 ROLE OF RHEOLOGY IN THE DESIGN OF COAL-SLURRY TRANSPORTATION SYSTEM
Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. It applies to substances which have a complex microstructure, such as mud, sludge, suspensions, polymers and other glass formers (e.g., silicates), as well as many foods and additives, bodily fluids (e.g., blood) and other biological materials or other materials which belong to the class of soft matter.
Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature. Although this viscosity will change with temperature, it does not change with the strain rate. Only a small group of fluids exhibit such constant viscosity, and they are known as Newtonian fluids. But for a large class of fluids, the viscosity changes with the strain rate (or relative velocity of flow) are called non-Newtonian fluids. Rheology generally accounts for the behavior of non-Newtonian fluids, by characterizing the minimum number of functions that are needed to relate stresses with rate of change of strains or strain rates. For example, ketchup can have its viscosity reduced by shaking (or other forms of mechanical agitation, where the relative movement of different layers in the material actually causes the reduction in viscosity) but water cannot. Ketchup is a shear thinning material, as an increase in relative velocity caused a reduction in viscosity, while some other non-Newtonian materials show the opposite behavior: viscosity going up with relative deformation, which is called shear thickening or dilatant materials.
The Rheological properties of particle suspensions are of great importance in numerous industrial applications including pipeline transportation of slurries. Rheological data can be used to find the relationship between flow rate and pressure drop. Rheological parameters can also be used to determine the power required to agitate the slurry in the tank, and to determine the wear rate and life of the pipeline. Knowledge of suspension rheology is also important to ensure a stable/energy efficient pipeline transportation system. The rheological characteristics of a slurry depends on several parameters such as shape, size and size distribution of particles, solids concentration, carrier fluid properties etc. By suitably manipulating the particle size distribution, if other parameters are same, it is possible to obtain a stabilized slurry suspension. The particle size is also important from the dewatering view point. If the solids are coarse then the cost of dewatering is less but the flow becomes more heterogeneous whereas if the particles are fine then the flow is homogenous but the slurry becomes non-Newtonian and the cost of dewatering also increases. Thus in a slurry transportation system, a compromise has to be made between the particle size and the cost of dewatering. The rheological behavior of the slurry is also required to predict the head requirement for pumping the slurry. The presence of solid particles in the slurry affects the performance characteristics of the pump. In addition, surfaces of the impeller and the walls of the casing wear more rapidly due to solid particles. The characterization of rheological behavior of slurry is complicated due to the fact that a large number of factors influence it. Historically, the rheology of suspension has been investigated mostly through experimentation on equalized particulate suspension. Correlations have been derived, on the basis of the above data, to predict the Newtonian viscosity of suspension. However, in commercial slurry, all the particles of the material will not be equalized and the ratio of the size of the largest particles to that of the smallest particles may be of the order of 1000 or even more.
Further, the particles of different materials will differ in various properties like density, shape etc. Thus the actual flow pattern that exists in a slurry pipeline will differ from material to material. Also the behavior of the slurries is generally non- Newtonian at the concentrations that are commercially used.
Non-Newtonian slurries make the principles of fluid mechanics more complex since the resistance to flow ‘viscosity’ now must be defined through a physical model reflecting process conditions. It is essential that a good understanding of the methods of characterizing rheological properties and extrapolating these characteristics to commercial slurries be obtained. In the absence of any suitable correlations for predicting the rheological parameters of non-Newtonian slurries contains large sized particles and wide particle size distribution, viscometric tests are unavoidable. For slurries containing large sized particles in a low viscosity carrier liquid, viscometric measurements are difficult because the large particles tend to settle down during measurements thus affecting the homogeneity of the suspension. Also the geometric interference of particles with the walls of the viscometer places a limit to the largest size particles that can be accommodated during tests. To overcome these problems the large sized/heavy particles may have to be scalped (removed) from the original sample and rheomtetric tests are performed with the remaining fine particulate slurries. At present, the effect of removal of large sized particles is not fully understood.
Source: Deepak Kumar - Department of Mechanical Engineering Thapar University, Patiala
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