Genuinely ‘Clean Coal’ - Solution for Reliable and Cheap Energy Source of Future

Genuinely ‘Clean Coal’ - Solution for Reliable and Cheap Energy Source of Future

Partha Das Sharma, B.Tech(Hons.),
E.mail: sharmapd1@gmail.com
Blog/Website: http://saferenvironment.wordpress.com, www.coalandfuel.blogspot.com

Introduction:
Sufficient, reliable sources of energy are a necessity for industrialized nations. With global energy demand rising at an unprecedented rate, the world's vast coal reserves are attracting growing interest. Despite its low cost and abundance, increasing concerns over greenhouse gases and other emissions from fossil fuels are altering the technological and regulatory environment for coal. Programs are underway to limit, reduce, and capture emissions from coal plants. This may result in the emergence of new generation technologies and the increased use of advanced emissions control technologies. In so doing, the costs and economics of coal use may change.

Environmental Factors:
Coal combustion produces emissions of air pollutants including sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), carbon dioxide (CO2), and mercury (Hg). SO2, NOx, and PM emissions are associated with air quality impacts and acidification of water resources, or acid rain. CO2 emissions contribute to global climate change. Mercury, which can move in multiple environmental pathways, is a neurological toxin in humans and wildlife.

A wide range of control technologies can be employed to reduce emissions of particulates, mercury, sulfur dioxide, and oxides of nitrogen.

Particulates are captured with baghouses (BH), electrostatic precipitators (ESP), and multiclones (MC). Nitrogen oxides (NOx) compliance actions may include a mix of combustion control technologies, such as low-NOx burners (LNB) and overfire air (OFA), and end-of-pipe emission control technologies, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR).

Sulfur dioxide (SO2) compliance actions may include switching to lower-sulfur coal, retirements, and installation of various scrubber technologies, such as flue gas desulfurization (FGD), dry lime injection (DLI), and spray dry injector (SDI). Burning low-sulfur coal can reduce SO2 emissions from an uncontrolled plant by two-thirds; installing a scrubber can reduce emissions by 90 percent or more. Mercury can be reduced to a limited extent by conventional SO2 scrubbers; more advanced controls specifically designed to reduce mercury include carbon injection (CI) and baghouse (BH) equipment.

Carbon capture and sequestration (CCS) technologies are under evaluation for their potential for removing the CO2 emissions from coal-fired power plants. The commercially available method for capturing CO2 from a conventional pulverized coal-fired boiler is the use of an amine-based system to absorb CO2 from the flue gas stream, and its subsequent regeneration to produce a nearly pure product stream. An alternative method, known as oxy-combustion, to capture CO2 is to use oxygen rather than air as the oxidant in the combustion process that yields a flue gas stream comprised primarily of CO2 and H2O. By removing the water, a nearly pure CO2 stream can be produced.

Coal combustion produces significant quantities of solid waste by-products that can be put to beneficial use. Coal combustion waste products can be used as an ingredient in the manufacture of cement, asphalt, roofing shingles, gypsum, calcium chloride, lightweight aggregate, lightweight block, and low-strength backfill.

Emerging Coal Generation Technologies:
Pulverized coal system (PC) is the conventional coal burning technology used in most of the cases. In this, finely ground coal is combusted to make steam that turns turbines and generates electricity. The raw coal is fed into the pulverizer along with air heated to approximately 650˚F from the boiler. As the coal is pulverized, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The powdered coal is then blown directly to a burner in the boiler. The burner mixes the powdered coal in the air suspension with additional pre-heated combustion air and forces it out of a nozzle similar in action to fuel being atomized by an automotive fuel injector. Under normal operating conditions, there is enough heat in the combustion zone to ignite all the incoming fuel.

As environmental emission regulations have been tightened, many coal plants have employed a range of operational modifications and capital equipment investments. In addition to fuel switching, i.e., low-sulfur coal, technologies are available and emerging to reduce emissions from coal burning at three different stages: pre-combustion, combustion, and post-combustion. Pre-combustion cleaning involves the removal of impurities from coal with physical, chemical or biological processes. Advanced combustion processes include improvements in existing coal combustion processes and new processes that remove pollutants from coal as it is burned. Post-combustion cleaning involves the removal of pollutants from the downstream flue gas after combustion and before exiting the stack. Many of the post-combustion pollution control technologies have been widely commercialized and have evolved into proven, mature technologies.

In recent years, technological advancements have led to substantial reductions in the cost of controlling SO2 and NOx emissions. Some of the most successful advancements are low-NOx burners, Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR), and scrubbers. Advanced pollution controls installed on existing power plants or engineered into new facilities can provide effective and low cost ways to reduce sulfur dioxide and nitrogen emissions.

Advanced power generation technologies are complete electric power generating systems that offer superior efficiency and environmental performance over conventional coal-burning systems. These new processes, such as circulating fluidized bed (CFB) combustion, can improve both efficiency and emission control. Another category of advanced coal technologies involves the conversion of coal into another form of fuel, e.g., gas or liquid. In most of these cases, the new fuel form provides both energy and environmental benefits by reducing the pollutants emitted from combusting the new fuel as compared to coal. Integrated gasification combined cycle (IGCC) is an example of this type of technology.

a. Circulating Fluidized Bed (CFB) - CFB combustion evolved from efforts to control pollutant emissions without external emission controls, such as scrubbers. The CFB technology suspends solid fuels on upward-blowing jets of air during the combustion process, resulting in a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer. The technology allows burning at temperatures well below the threshold where NOx form. In addition, the mixing action of CFB brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite, capturing more than 95 percent of the sulfur pollutants inside the boiler. The popularity of fluidized bed combustion is due not only to its capability of meeting SO2 and NOx emission standards without the need for expensive add-on controls but also technology's fuel flexibility. Almost any combustible material, from coal to municipal waste, can be used for fuel.

b. Integrated Gasification Combined Cycle - Another emerging combustion technology, integrated gasification combined cycle (IGCC), converts coal to a gaseous form similar to natural gas before being burned. This advanced technology converts coal into a combustible synthetic gas by reaction with oxygen and heat/steam. Emissions from these plants are very low compared to other coal technologies because the gas is cleaned prior to combustion, burned in a gas turbine, and the resulting exhaust gases are used to produce steam that then drives a steam turbine. Typically 60 to 70 percent of the power comes from the gas turbine with IGCC. The result is an IGCC configuration that provides ultra-low pollution levels and, in addition, carbon-capture technologies can more readily be built on to the back end of IGCC plants than traditional pulverized coal combustion technologies.

On the front end of IGCC is a gasification technology. Worldwide, there are 117 operating plants that include 385 gasifiers. Products from the syn-gas produced from gasification include chemicals, liquid fuels, and electric power.

c. Super Critical Steam - The use of supercritical (SC) and ultra-supercritical (USC) steam, heated to a higher temperature than conventional boilers, has the potential to achieve greater generation efficiency, resulting in more output per unit of fuel as well as fewer pollutants. Efficiencies of 40 percent and higher have been demonstrated. SC and USC plants require the use of more durable metals and alloys in order to withstand the higher operating temperatures. Although several SC and USC coal plants have been constructed and operated in the United States, some have experienced operating difficulties due to the high tolerances required. In more recently constructed plants in Japan and elsewhere, anecdotal reports indicate that SC and USC plants have operated more reliably with fewer outages than earlier designs.

d. Oxy-combustion (Oxy-Coal) – Oxy-combustion, or oxy-coal involves the combustion of coal in a mixture of oxygen and re-circulated flue gas. The main benefits of oxy-combustion technology with CCS are:
* Reduction of carbon dioxide emissions up to nearly 96.9 percent removal
* Reduction of SO2
* Potential for enhancement of mercury removal in the baghouse and advanced SO2 controls

Because it uses conventional equipment already proven in the power generation industry, the oxy-combustion technology can readily be applied to new coal-fired power plants. Plant control during startup, shutdown, and load following is very similar to a conventional PC plant. Finally, the key process principles have been proven in the past including air separation and flue gas recycle (FGR).

However, several challenges to oxy-combustion have also been identified:
* Air infiltration into the boiler dilutes the resulting flue gases. This could potentially be minimized by improved boiler materials, sealants, control technologies, and membranes.
* Combustion of fuels in a purified oxygen stream would occur at temperatures too high for existing boiler or turbine materials. This issue is being addressed by diluting the oxygen via the FGR, which results in an increase of the auxiliary power load and decreases efficiency. Further developments aim at increasing the efficiency of the FGR and improved boiler materials.
* The current capital and operating costs of specialized components are high.
* Plant efficiency is reduced by the use of the auxiliary load of FGR and air separation equipment.

Conclusion:
With introduction of improved and emerging technology, the efficiency and cleanliness of coal-fired power stations is improving. Genuinely clean coal - i.e.: one that emits close to zero CO2 thanks to carbon capture and storage technology - is not expected to become economically viable before twenty years, are now viable. Thus, coal is making a comeback as a cheap and reliable source of energy.

Solar Cell – Clean Energy Source:

Solar Cell – Clean Energy Source:

Solar cell is a semiconductor device that converts the energy of sunlight into electric energy. These are also called ‘photovoltaic cell’. Solar cells do not use chemical reactions to produce electric power, and they have no moving parts.

Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. In large arrays, which may contain many thousands of individual cells, they can function as central electric power stations analogous to nuclear, coal-, or oil-fired power plants. Arrays of solar cells are also used to power satellites; because they have no moving parts that could require service or fuels that would require replenishment, solar cells are ideal for providing power in space.

A. Most photovoltaic cells consist of a semiconductor pn junction, in which electron-hole pairs produced by absorbed radiation are separated by the internal electric field in the junction to generate a current, a voltage, or both, at the device terminals. Under open-circuit conditions (current I = 0) the terminal voltage increases with increasing light intensity, and under short-circuit conditions (voltage V = 0) the magnitude of the current increases with increasing light intensity. When the current is negative and the voltage is positive, the photovoltaic cell delivers power to the external circuit.

The mechanism of producing electricity from light is:

(a) Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon;

(b) Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel;

(c) An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.

B. Characteristics of a Solar Cell - The usable voltage from solar cells depends on the semiconductor material. In silicon it amounts to approximately 0.5 V. Terminal voltages is only weakly dependent on light radiation, while the current intensity increases with higher luminosity. A 100 cm² silicon cell, for example, reaches a maximum current intensity of approximately 2 A when radiated by 1000 W/m². The output (product of electricity and voltage) of a solar cell is temperature dependent. Higher cell temperatures lead to lower output, and hence to lower efficiency. The level of efficiency indicates how much of the radiated quantity of light is converted into useable electrical energy.

C. Cell Types: One can distinguish three cell types according to the type of crystal: monocrystalline, polycrystalline and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production process guarantees a relatively high level of efficiency.

The production of polycrystalline cells is more cost-efficient. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. As a result of this crystal defect, the solar cell is less efficient.
If a silicon film is deposited on glass or another substrate material, this is a so-called amorphous or thin layer cell. The layer thickness amounts to less than 1µm (thickness of a human hair: 50-100 µm), so the production costs are lower due to the low material costs. However, the efficiency of amorphous cells is much lower than that of the other two cell types. Because of this, they are primarily used in low power equipment (watches, pocket calculators) or as facade elements.

D. Efficiency: Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 42.8% with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-16%. The highest efficiency cells have not always been the most economical — for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power.

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the energy that is not needed immediately.

E. Advantages of solar energy: Solar cells are long lasting sources of energy which can be used almost anywhere. They are particularly useful where there is no national grid and also where there are no people such as remote site water pumping or in space. Solar cells provide cost effective solutions to energy problems in places where there is no mains electricity. Solar cells are also totally silent and non-polluting. As they have no moving parts they require little maintenance and have a long lifetime. Compared to other renewable sources they also possess many advantages; wind and water power rely on turbines which are noisy, expensive and liable to breaking down.

Rooftop power is a good way of supplying energy to a growing community. More cells can be added to homes and businesses as the community grows so that energy generation is in line with demand. Many large scale systems currently end up over generating to ensure that everyone has enough. Solar cells can also be installed in a distributed fashion, i.e. they don't need large scale installations. Solar cells can easily be installed on roofs, which mean no new space is needed and each user can quietly generate their own energy.

F. Disadvantages of solar cells: The main disadvantage of solar energy is the initial cost. Most types of solar cell require large areas of land to achieve average efficiency. Air pollution and weather can also have a large effect on the efficiency of the cells. The silicon used is also very expensive and the problem of nocturnal down times means solar cells can only ever generate during the daytime. Solar energy is currently thought to cost about twice as much as traditional sources (coal, oil etc). Obviously, as fossil fuel reserves become depleted, their cost will rise until a point is reached where solar cells become an economically viable source of energy. When this occurs, massive investment will be able to further increase their efficiency and lower their cost.