Solid Fuels Combustion of Coal

Combustion of Coal When a solid fuel particle is exposed to a hot gas flowing stream it undergoes three stages of mass loss Drying Devolatilization Char combustion The relative significance of these three is indicated by proximate analysis of coal

Combustion of Coal Drying DEVOLATILIZATION The combustible material generally constitutes water e.g. lignites up to 40 % Upon entry into the gas stream, heat is convected and radiated to the particle surface and conducted into the particle The drying time of a small pulverized particle is the time required to heat up the particle to the vaporization point and drive off the water DEVOLATILIZATION When the drying of a solid fuel particle is complete, the temperature rises and the solid fuel begins to decompose Devolatilization or pyrolysis is the process where a wide range of gaseous products are released through the decomposition of fuel. The volatile matter (VM) comprises a number of hydrocarbons, which are released in steps Since the volatiles flow out of the solid through the pores, external oxygen cannot penetrate into the particle, hence the devolatilization is referred to as the pyrolysis stage

Combustion of Coal DEVOLATILIZATION The rate of devolatilization and the pyrolysis products depend on the temperature and and the type of the fuel The pyrolysis products ignite and form an attached flame around the particle as oxygen diffuses into the products While water vapour is flowing out of the pores, the flame temperature will be low For lignite coals, pyrolysis begins at 300-400 C releasing CO and CO2 Ignition of the volatiles occurs at 400-600 C CO, CO2, chemically formed water, hydrocarbon vapours, tars and hydrogen are produced as the temperature reaches 700-900 C Above 900 C pyrolysis is essentially complete and the char (fixed carbon) and ash remain

Combustion of Coal DEVOLATILIZATION For other types of coal, devolatilization proceeds differently Although the proximate analysis provides an estimate of the VM, the actual yield of VM and its composition may be affected by a number of factors like: Rate of heating Initial and final temperature Exposure time at the final temperatures Particle size Type of fuel Pressure

Combustion of Coal CHAR COMBUSTION The devolatilized fuel, known as char, burns rather slowly. For example, it would take 50–150 sec for a char of size less than 0.2 mm to burn out Since it takes this long to burn completely, some of the particles may not burn out in the bed before leaving. The elutriation of these unburnt, fine char particles results in combustion losses.

Combustion of Coal CHAR COMBUSTION The combustion of a char particle generally starts after the evolution of volatiles from the parent fuel particle, but sometimes the two processes overlap. The char, being a highly porous substance, has a large number of internal pores of varying size Surface areas of the pore walls are several orders of magnitude greater than the external surface area of the char. Oxygen diffuses into the pores and oxidizes the carbon on the inner walls of the pores.

Combustion of Coal During the combustion of a char particle, oxygen from the bulk stream is transported to the surface of the particle. The oxygen then undergoes an oxidation reaction with the carbon on the char surface to produce CO. The CO then reacts outside the particle to form CO2. The mechanism of combustion of char is fairly complex. Some factors which effect the burning rate are: Oxygen concentration Gas temperature Reynolds number Char size and porosity

Combustion Systems for solid Fuels Fixed-bed combustion Suspension Firing/Pulverized Coal combustion Fluidized-bed combustion Fixed-bed Combustion: Fixed-bed systems require least fuel size reduction compared to other two systems mentioned above Crushed coal up to 4 cm in size is used Solid fuel handling and feeding are the focus of much effort compared with gas or liquid fuels A stoker type of boiler is an example of shallow fixed- bed combustion system

Combustion Systems for solid Fuels Fixed-bed Combustion: A continuous fuel feed system is referred to as a stoker Air flows up through the grate and through the bed of ash, char and fuel Since the bed is thin, the pressure drop is less and the blower costs are reduced There are three types of stokers based on the way the coal is fed onto the grate Overfeed stokers Underfeed Stokers Cross feed stokers

Spreader stoker with travelling grate

Combustion Systems for solid Fuels Overfeed stokers: The flow of fuel and air is counter current. Fuel is fed onto the top of the bed and moves downward as it is consumed Air flows up through the layers of ash, char and fresh fuel Volatile gases burn above the bed and some fine fuel particles burn above the bed Overfire air is supplied to complete the combustion Ash is removed by dumping, shaking, vibrating or continuously moving the grate The bed is usually 10-20 cm deep Fresh fuel is heated by the upward moving gases and by radiation from the flame above the bed The speed of the grate is adjusted so that the coal burns out before it reaches the edge of the grate and the ash dumps nto the ash pit below

Combustion Systems for solid Fuels Underfeed stokers: The flow of fuel and air is upward The evolved moisture, volatile matter and air pass through the burning fuel layer The bed is up to 1 m deep near the centre. Fresh fuel is forced from below by a screw conveyor The grate is usually inclined so that the ash automatically moves outwards as the fresh fuel is forced from below

Combustion Systems for solid Fuels Crossfeed stokers: An older type of stoker and grate arrangement This type of system is often used for hard-to-feed fuels such as unprocessed refuse, bagasse, lignite, wood pulp etc The fresh fuel is moved to a horizontal platform where it ignites When the next charge of the fuel enters, the ignited fuel moves across a sloping vibrating grate The air flows upward through the grate Such stokers operate with high excess air and considerable fuel loss in the ash pit

Fluidized-bed combustion A fluidized bed is a bed of solid particles which are set into motion by blowing a gas stream upward through the bed at a sufficient velocity to suspend the particles. The bed appears like a boiling liquid. The fluidization occurs when the drag force on the particles in the bed due to the upward flowing gas just equals the weight of the bed. There are two principal types of fluidized bed boilers: 1. Bubbling fluidized bed (BFB) 2. Circulating fluidized bed (CFB)

Quality of fluidization

Fluidized-bed combustion Bubbling fluidized bed (BFB) A bubbling fluidized bed boiler comprises a fluidizing grate through which primary combustion air passes and a containing vessel, which is either made of (lined with) refractory or heat-absorbing tubes. The vessel would generally hold bed materials. The open space above this bed, known as freeboard, is enclosed by heat-absorbing tubes. The secondary combustion air is injected into this section The boiler can be divided into three sections: 1. Bed 2. Freeboard 3. Back-pass or convective section.

Fluidized-bed combustion Bubbling fluidized bed (BFB) As the velocity is increased above minimum fluidization, bubbles are formed The bubbles are referred to as the dilute phase The size of the bubbles depend upon the type of the distributor plate A plate with a few large orifices inlets will have larger bubbles while a plate with many small inlets will have many bubbles near the plate In fluidized bed combustion the bed temperature is maintained well below the melting point of the ash To capture SO2 in the bed, limestone CaCO3 which is calcined in the bed to form CaO The optimum temperature for CaO reaction with SO2 to form CaSO4 at atmospheric pressure is 815-900 C

Fluidized-bed combustion Circulating Fluidized Bed Boiler In a CFB boiler furnace the gas velocity is sufficiently high to blow all the solids out of the furnace. The majority of the solids leaving the furnace is captured by a gas–solid separator, and is recirculated back to the base of the furnace. A CFB boiler is shown schematically in Figure The primary combustion air (usually substoichiometric in amount) is injected through the floor or grate of the furnace The secondary air is injected from the sides at a certain height above the furnace floor. Fuel is fed into the lower section of the furnace, where it burns to generate heat. A fraction of the combustion heat is absorbed by water- or steam-cooled surfaces located in the furnace, and the rest is absorbed in the convective section located further downstream, known as the back-pass.

Bubbling Fluidized Bed (BFB) FACTORS AFFECTING COMBUSTION EFFICIENCY The combustion efficiency of a bubbling fluidized bed (BFB) boiler is typically up to 90% without fly-ash recirculation and could increase to 98–99% with recirculation . The efficiency of a circulating fluidized bed (CFB) boiler is generally higher due to its tall furnace and large internal solid recirculation. The efficiency depends to a great extent on the physical and chemical characteristics of the fuel as well as on the operating condition of the furnace. Factors affecting the combustion efficiency can be classified into three categories: 1. Fuel characteristics 2. Operational parameters 3. Design parameters

EFFECT OF Fuel characteristics ON COMBUSTION EFFICIENCY The fuel ratio of a fuel is the ratio of fixed carbon (FC) and VM contents of the fuel. This ratio has an important effect on the combustion efficiency of coal in a CFB boiler Higher ratios possibly leading to lower combustion efficiencies A high rank fuel like anthracite has a higher fuel ratio than a low rank fuel like lignite. For this reason low-rank fuels (or low fuel ratio) like lignite and bituminous have higher efficiencies than anthracite. The fuel ratio is easily computed from the proximate analysis of a fuel

Effect of Operational parameters ON COMBUSTION EFFICIENCY Fluidizing Velocity The combustion efficiency generally decreases with increasing fluidizing velocity due to higher entrainment of the unburnt fines and oxygen by-passing. Excess Air The mixing between fuel and air is never perfect. Some areas will be oxygen-deficient and some areas even oxygen-starved. Ultimately, all fuel particles must have the necessary oxygen to complete their burning; thus, extra oxygen is always provided in FB boilers in the form of excess air. The combustion efficiency improves with excess air, but this improvement is less significant above an excess air of 20%. Bubbling bed boilers may need a slightly higher amount of excess airthan CFB boilers. Combustion Temperature The combustion efficiency generally increases with bed temperature because the carbon fines burn faster at high temperatures. The effect of temperature is especially important for less reactive particles, which burn under kinetic-controlled regimes.

EFFECT OF DESIGN PARAMETERS ON COMBUSTION EFFICIENCY The combustion efficiency of bubbling FBs is affected by several design parameters : Bed height Freeboard height Recirculation of unburnt solids Fuel feeding Secondary air injection