The present invention relates to a variable fluid bed for optimizing superheater/reheater surfaces in a bubbling fluidized bed combustion boiler.
Fluidised Bed Combustion boiler is a system consisting of fluidised bed enclosure, where the solid particles are in a suspended state. The state is achieved by controlling the amount of air passing through the bed. At low velocities, pressure drop across the bed is negligible and buoyant force derived from low velocities is not sufficient to suspend the particle in the bed. The bed remains static and undisturbed.
As the flow (velocity) is increased gradually, the buoyant force overcomes the gravitational force exerted on the particle and suspends in the state of fluidised bed. The pressure drop increases till a point, where it matches with the weight of particles per unit area. At this stage, the bed particles are in suspended state and it is called "minimally fluidised bed".
When the flow is increased further, the bed becomes highly turbulent as the interstitial spaces between the bed particles has increased allowing high velocity gas streams promoting rapid mixing of the bed. At this stage the bed of solids attain pseudo fluid properties.
In the BFBC boiler, the above mechanism is employed to suspend the solid coal particles. The entire bed is preheated to ignition temperature of the fuel (coal). On reaching the ignition temperature, the coal particles are fed. They undergo combustion and release heat which is imparted to the whole volume of bed uniformly due to high turbulence and rapid mixing characteristics of fruidising process.
In addition to convective surfaces employed, surfaces are employed in side the bed to maintain desired temperatures in the bed to promote good combustion and retard slagging of ash and curb NOX generation. The bed temperatures are maintained around 750 to 900°C to achieve the above stated objectives.
The heat transfer to these Inbed Surfaces (Superheater / Reheater / Evaporative Circuit) are predominantly through conduction (by contact with solid bed particles) and convection. The radiation component is insignificant as they are immersed inside the dense bed.
In the prior art of controlling bed temperatures and maintaining steam parameters involves attemperation to control the amount of heat absorbed in the Inbed surface. At low loads, ftuidizing air is cut off to those compartments where surfaces are located, creating static bed thereby making the surface ineffective.
STATEMENT OF INVENTION
According to this invention there is provided a variable fluid bed for optimizing superheater/reheater surfaces in a bubbling fluidized bed combustion boiler comprising of evaporative, superheat and reheat surfaces (9) immersed in the said fluidized bed and in the convention zone located above said fluidized bed characterized in that the heat transfer vary in the active fluidized bed by changing the fluidized nozzle (14) elevation.
When the solid particles in a fluidized bed are preheated to the ignition temperature of fuel, the fuel fed will burn and impart heat to the whole volume of the bed. The temperature in the entire bed would be uniform. This process is called bubbling fluidized bed combustion.
Bubbling Fluidized Bed Combustion (BFBC) boilers operate at lower temperatures in the range of 750 to 950°C unlike conventional pulverized fuel fired or stoker fired boilers.
The heat transfer to the superheaters and reheaters in BFBC boilers is primarily by conduction and convection and very little by radiation.
The present invention provides a newly designed variable fluid bed for optimizing superheater / reheater surfaces in a bubbling fluidized bed combustion boiler comprising multiple surfaces positioned inside said fluidized bed and in the convention zone located above said fiuidized bed.
In the newly designed variable fluid bed for optimizing superheater / reheater surfaces in a bubbling fluidized bed, the bed zone is capable of providing different heights with the change in fluidized nozzle elevation and overflow elevation to ensure required superheat or reheat surface.
The invention further provides a method of superheating and reheating fluidized bed at different loads and for widely varying fuel quality by providing surfaces inside fluidized bed and the conventionione located above said fluidized bed.
In the aboyte method the active fluidized bed zone is moved to different heights by altering the fluidized nozzle elevation and the overflow elevation. Further, in the fluidized bed the superheat is ensured by immersing the teheat surface in said active bed or by varying boiler load or changing fuel qualities.
In the fluidized bed of the present invention the heat transfer to superheat or reheat surfaces in the convention zone above said fluidized bed is altered due to change in gas temperature entering said zone.
In order to achieve the required superheat and reheat temperatures at different loads or for widely varied fuel quality it becomes necessary to provide surfaces inside the fluidized bed as well as in the convention zone located above the fluidized bed. Figure 1- Shows a typical arrangement of variable fluid bed
Figure-2- Shows variable fluid bed utilizing static and variable distributor plate assembly
A typical arrangement of Variable Fluid Bed is illustrated in Figure 1 This comprises a bed ash drain pipe (8), plurality of bed coil surfaces (superheater/ reheater) (9), a variable bed with nozzle assembly (10), a fixed bed plate (11), the fluidised bed enclosure (12), a wind box (13) and a plurality of fluidising nozzles (14).
The invention proposes Variable Fluid Bed utilizing static and variable distributor plate assembly to alter the active fluidised zone as elucidated in Figure 2 The modus of supporting Variable Distributor Assembly is using rectangular frame of plates on which the distributor plates and fluidizing nozzles are welded on to and is supported from the bottom through a motor driven gear and screw jack mechanism to adjust the location.
The Variable Fluid Bed adjusts the active fluidization part by altering the fluidised nozzle elevation. The covering of the immersed surfaces by static bed hinders the conduction mechanism of heat transfer rendering the surface ineffective and therefore the control of heat duty and hence superheat and reheat is achieved without affecting the basic process of fluidization.
The advantage of the invention lies in fact that the fluidisation process over entire bed is not disturbed ensures a ideal scenario for turn down operation of the boiler. This results in more uniform bed temperatures improving fuel utilisation and better control over thermal NOX production.
The other benefits associated with Variable Fluid Bed are the abilities to control heat duty and ensuring spray requirements are minimal for reheat circuit and improves overall cycle efficiency.
A significant benefits stem from this unique control allows adjusting for wide varying fuel quality by going in for deeper bed for low volatile fuel to ensure good combustion efficiency.
The schematic arrangement of design feature of the invention is illustrated in the accompanying drawing.
The invented design feature optimizes the utilization of superheat / reheat heat transfer surfaces in large BFBC boilers.

WE CLAIM;
1. A variable fluid bed for optimizing superheater/reheater surfaces in a bubbling fluidized bed combustion boiler comprising of evaporative, superheat and reheat surfaces (9) immersed in the said fluidized bed and in the convention zone located above said fluidized bed characterized in that the heat transfer vary in the active fluidized bed by changing the fluidized nozzle (14) elevation.
2. A variable fluid bed as claimed in claim 1 wherein said fluidized bed zone provide different height with the change in nozzle (14) elevation and overflow elevation to ensure required superheat or reheat surface (9).
3. A variable fluid bed as claimed in claim 1 wherein said active fluidized bed zone is moved to different heights by varying the fluidized nozzle (14) elevation and the overflow elevation.
4. A variable fluid bed as claimed in any of the preceding claims wherein superheating is carried by immersing the reheat surface (9) in said active bed or by varying load or changing fuel qualities.
5. A variable fluid bed substantially as herein described and as illustrated by the drawings accompanying the Specification.