Difference between pinch and approach temperature in a boiler / hrsg

Pinch point: It is the difference in temperature of flue gas at evaporator outlet and water temperature at evaporator. The lesser the pinch point, more shall be evaporator surface area (as higher surface area would be required to absorb more heat from flue gas). The decision of pinch point should ideally be made keeping in mind overall costs and boiler efficiency.

Approach temperature: It is the difference in temperature between economiser outlet temperature and saturation temperature. Approach temperature should not be kept very less due to the possibility of steaming.

Types of cfbc boilers

From a neutral perspective following are the types of cfbc boilers:
a) Cold cyclone CFBC: Called as cold cyclone because cyclone temperature is "colder" (approx 400 Deg C) than a hot cyclone cfbc, which is because of some heating surfaces in the first pass. Aim is to have bed temperature at around 850 Deg C to aid in desulphurisation.

b) Hot cyclone CFBC: Heating surfaces in the second pass, hence cyclone is hotter.

c) Compact cyclone CFBC: Cyclone is part of pressure part circuit, done to reduce bed temperature.

d) U beam CFBC: U beams used instead of cyclone for filtering out coarse ash. Similar in nature as hot cyclone cfbc.

Dulongs formula for GCV calculation

If we know the ultimate analysis of fuel, we can easily calculate its GCV.
The basic principle is that there are only 3 components in a fuel which generate heat. These are:
Carbon, Hydrogen and Sulphur.
According to Dulong's formula gross calorific value of a fuel is;
GCV = ((35.5 x C + 114.8 x H + 9.5 x S – 14.5 x O) x 1000) / (100 x 4.1868)
Each multiple of carbon, hydrogen and sulphur represents heat generated by its one mole.
The formula gives GCV in kcal/kg. Simple isnt it !

Boiler Heat Duty

Boiler heat duty = Tonnes per hour of steam generated x (Enthalpy of main steam - Enthalpy of feed water)