In a tangential fired boiler, coal as a fuel can cause a considerable imbalance in the reheat steam temperature on the left hand and right hand sides. It is always misunderstood by many that this kind of difference is mainly due to design and needs to be corrected, even when the steam temperature entering the turbine remains as an average of the two. However, it is a fact that the metal temperature of the reheater tubes on the side where the steam temperature is higher can exceed the allowable specified limit.
There are many reasons that a left-right side steam temperature imbalance can happen in the reheater in a tangentially fired boiler. We can divide the causes resulting this into two major areas, the gas side and steam side parameters.
Gas side
- Fuel
- Organics
- Reactivity
- Fixed carbon / Volatile mater
- Higher heating value
- In-organics
- Ash chemistry
- Ash fusion temperature
- Ash percentage
- Flow dynamics of gas and ash
- Combustion regime
- Design
- Location – Radiant / Convective
- % of radiant surface
- % of convective surface
- Gas velocity
- Spacing
- Free gas area
- Excess air
- Bridging
- Effectiveness
- Deposition
- Combustion completion zone
- Operation
- Fuel outside design
- Fuel different from design
- Blending of fuel
- Highly reactive fuel
- Higher excess air
- Operational
- To reduce high deposition on furnace walls
- Air distribution
- Primary air
- High % of ash
- Operational
- Secondary air
- Reduced amount of secondary air due to high primary air
- Fuel dependent distribution
- Low windbox pressure
- Mills in service
- PC fineness
- Wall blowers
- Number in operation
- Frequency of operation
- Effectiveness
- Optimizing for NOx
All the said factors on the gas side can act independently or in combination. It is also possible that some of these causes can be the result of another factor which can independently affect the steam temperature.
The steam side factors will also affect the reheater outlet steam temperature. Some of the factors listed under steam side can be the combined effect of both gas and steam side.
Steam side
- Cold reheat inlet temperature
- High pressure turbine performance
- Reheater injection
- Reheat steam flow
- Extraction variation
- Reheater injection
- Load
- Header inlet / outlet stub position
- Type of design
- Center inlet
- End inlet
- Quarter point inlet
- Reheater tube diameter
- Design
- Variations due to manufacture based on tube specification
- Distribution of thickness variation across assemblies
- Reheater pressure drop
- Hot reheat / cold reheat pipe pressure drop
- Layout of pipe line
- Length
- Internal diameter of pipe
- Reheater area provided
- Type of heat transfer
- Variation in radiant and convective heat transfer
- Log mean temperature difference
- Gas side temperature
- Deposition in furnace
- Combustion completion
- Steam side temperature variation
- Link tubes and elbows
- Variation in link and elbow internal diameter
- Ovallity in internal diameter
All these factors can be grouped as coal oriented factors, design oriented factors, and operation oriented factors.
There are many methods to correct this temperature difference if needed. Reheater injection is one easy method, but it will result in a marginal drop in plant heat rate. Introducing resistance in the reheater pipes suitably will lead to a good correction in this especially when fuel quality variation is predominant. There is yet another method by which resistance is introduced across the various assemblies after a careful evaluation. However, this method can result in imbalance when fuel quality variation is high and the reactivity of the fuel is changing.
