Question Set 62

1. How you can define coal ?

Ans: Coal is a naturally occurring hydrocarbon that consists of the fossilised remains of buried plant debris that have undergone progressive physical and chemical alteration, called coalification, in the course of geologic time.

2. Which pollutant is major greenhouse gas and what is its effect ?

Ans: CO is major greenhouse gas and it traps the radiation of heat from the sun within earth’s atmosphere.

3. In order to increase efficiency and reduce CO emissions and other emissions, clear coal technologies are receiving major attention. What are these ?

Ans: (i) Advanced pulverised and pressurised pulverised fuel combustion.

(ii) Atmospheric fluidised bed combustion and pressurised fluidised bed combustion.

(iii) Supercritical boilers.

(iv) Integrated gasification combined cycle systems.

(v) Advanced integrated gasification, including fuel cell systems.

(vi) Magneto hydrodynamic electricity generation.

4. What are the important operational performance parameters in design of fuel firing equipment ?

Ans: Fuel flexibility, electrical load following capability, reliability, availability, and maintenance ease.

5. What is the differenc between total moisture and inherent moisture in coal ?

Ans: The moisture content of the bulk as sampled is referred to as total moisture, and that of the air dried sample is called inherent moisture.

6. Proximity analysis of coal provides data for a first, general assessment of a coal’s quality and type. What elements it reports ?

Ans: Moisture, volatile matter, ash and fixed carbon.

7. Ultimate analysis of coal is elementary analysis. What it is concerned with ?

Ans: Carbon, hydrogen, nitrogen, and sulphur in coal on a weight percentage basis.

8. Explain the difference between AFBC, BFBC, PFBC and PCFB in regard to fluidised bed technologies.

Ans: AFBC (Atmospheric fluidised bed combustion) process consists of forming a bed of inert materials like finely sized ash or ash mixed with sand, limestone (for sulphur removal), and solid fuel particles in a combustor and fluidising it by forcing combustion air up through the bed mixture. The gas flows through bed without disturbing particles significantly but gas velocity is high enough to support the total weight of bed (fluidisation). At slightly higher velocity excess gas passes through the bed as bubbles (fluidised bed) and gives the bed the appearance of a boiling liquid.

Bubbling fluidised bed combustion (BFBC) has a defined height of bed material and operates at or near atmospheric pressure in the furnace.

Pressurised fluidised bed combustion (PFBC) system operates the bed at elevated pressure. Exhaust gases have sufficient energy to power a gas turbine, of course, gases need to be cleaned.

In fluidised combustion, as ash is removed some unburned carbon is also removed resulting in lower efficiency. In circulating fluidised bed combustion (CFBC) system, bed is operated at higher pressure leading to high heat transfer, higher combustion efficiency, and better fuel feed. Circulating fluidised beds operate with relatively high gas velocities and fine particle sizes. The maintenance of steady state conditions in a fast fluidised bed requires the continuous recycle of particles removed by the gas stream (circulating bed). The term circulating bed is often used to include fluidised bed sys¬tems containing multiple conventional bubbling beds between which bed material is exchanged.

9. What for Schmidt plot for is used in heat transfer problems ?

Ans: Schmidt plot is a graphical method for determining the temperature at any point in a body at a specified time during the transient heating or cooling period.

10. In which reactor the coolant and moderator are the same ?

Ans: Pressurised water reactor.

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