thermal power plant pulveriser

Thermal Power Plant Pulveriser: An Overview

The pulveriser, or coal mill, is a critical component in the fuel preparation system of a conventional coal-fired thermal power plant. Its primary function is to grind raw, mined coal into a fine powder, known as pulverized fuel (PF), to facilitate efficient combustion in the boiler furnace. This process increases the surface area of the coal exponentially, allowing for rapid and complete burning, which is essential for maximizing heat release and minimizing unburned carbon losses. The performance and reliability of the pulveriser directly impact plant efficiency, emissions, and operational stability. This article delves into the types, operating principles, key challenges, and technological advancements associated with this vital equipment.

Types and Operating Principles

Pulverisers are classified based on their grinding mechanism and speed. The three predominant types used in power generation are:

Type	Operating Principle	Key Characteristics	Typical Application
Ball-Tube Mill (Low Speed)	Rotating cylinder filled with steel balls; coal is crushed by tumbling action.	Robust, high capacity, high power consumption, slower response to load changes.	Often used in older plants or for grinding harder coals.
Vertical Spindle Mill (Medium Speed)	Grinding rollers press against a rotating table; coal is crushed by roller pressure.	Compact design, good fuel fineness control, lower power consumption than ball mills.	The most common type in modern large utility boilers (e.g., MPS, HP, RP mills).
Bowl Mill (Medium Speed)	A subtype of vertical spindle mills where rollers (or balls) rotate within a stationary bowl.	Similar to other vertical mills; known for reliable operation and maintainability.	Widely used across various plant sizes and coal types.
Impact Mill (High Speed)	Uses beater plates or hammers rotating at high speed to shatter coal by impact.	Simple design, fast response, but higher wear rates and less precise fineness control.	Often used for softer coals or as a component in direct-firing systems.

The ground coal is dried using a mixture of primary air (pre-heated from the boiler) and hot gas from the air preheater before being transported pneumatically through pipes (fuel lines) to the burners.

Key Challenges and Technological Solutions

Operational challenges include grinding element wear (rollers, tires, tables), which reduces efficiency and increases maintenance downtime; achieving consistent optimal fuel fineness; and managing mill fires or explosions due to volatile accumulation.

A prominent real-world case of addressing these issues is the retrofit project at the Jhajjar Power Plant (India). The plant faced persistent issues with excessive vibration and low throughput in its existing coal mills. The solution involved upgrading to advanced MPS-type vertical roller mills from Gebr. Pfeiffer. These modern mills featured improved grinding geometry, dynamic classifiers for better fineness control (~75% passing 200 mesh), and robust construction materials for wear parts like rollers and tables. The retrofit resulted in a documented 15-20% increase in grinding capacity per mill, reduced specific power consumption by approximately 10%, extended maintenance intervals due to superior wear resistance.

Frequently Asked Questions (FAQs)

1. Why is coal pulverized to such a fine powder?
   Pulverization dramatically increases the surface area-to-volume ratio of the coal particles. This allows for intimate mixing with combustion air, leading to near-instantaneous ignition and more complete combustion at high temperatures (>1300°C). This maximizes heat release per unit mass of fuel and minimizes waste like unburned carbon in ash.

2. What is "coal fineness," why does it matter?
   Coal fineness refers to the particle size distribution of the pulverized coal—typically measured as the percentage passing through standard mesh screens like 200-mesh (74 microns). Optimal fineness ensures complete combustion within the short residence time in the furnace boiler tubes (<2 seconds). Poor fineness leads to higher unburned carbon loss (reducing efficiency) and can cause slagging issues.

3. How does mill performance affect NOx emissions?
   The pulveriser directly influences burner performance through PF fineness distribution velocity A uniform fine PF stream enables stable low-NOx burner operation by creating controlled staged combustion zones If mill performance degrades producing coarse particles it can disrupt this staging leading unstable flame higher NOx formation potential increased carbon monoxide CO

4.What are common signs of pulveriser problems?
Key indicators include: rising mill motor current for same coal feed rate indicating worn grinding elements; decreasing outlet temperature differential suggesting poor drying or airflow issues; increasing "rejects" spillage from bowl mills; abnormal vibrations often linked mechanical failure imbalance Foreign material ingress also frequent cause sudden severe damage requiring shutdown

5.Can pulverisers handle different types/qualities of coal?
Yes but within design limits Mills designed specific Hardgrove Grindability Index HGI moisture content abrasiveness Switching significantly different feedstock e.g., softer lignite harder anthracite without modification can cause severe throughput capacity problems excessive wear Modern dynamic classifiers adjustable grinding force help accommodate wider range but fundamental redesign usually required major fuel switch