“Coal Dust Explosion” in Coal fired Cement plant – Its prevention

“Coal Dust Explosion” in Coal fired Cement plant – Its prevention is necessary for safety and enhancement of environment standards.

The numbers of coal fired industrial systems, particularly cement plants, is rapidly increasing, because of high cost and uncertainty in availability of fuel oil and natural gas. Many of the countries do not have access to sufficient quantity of petroleum products; there by, worldwide growth in coal-fired cement plant is tremendous. There is inherent risk associated with coal pulverizing, drying, blending, transportation and storing.

To understand fully the hazard potential of using pulverized coal as a fuel in a cement plant, one should be familiar with the factors responsible for development of coal dust explosion. A typical pulverized coal fuel system process bulk coal into a form that can be efficiently utilized as a fuel to heat the kiln for calcining the raw material of cement, i.e., clay, limestone, etc., into clinker. This is usually accomplished by grinding and drying the bulk feed in a pulverizer so that coal emerging from pulverizer consist 70 to 80% particulate that passes through 200 mesh screen. High temperature air from clinker cooler is often used to dry the coal and convey it from pulverizer to the burning pipe of the kiln. Coal pulverizing is one of the most hazardous jobs from fire and explosion point of view, as both fuel and oxygen are present at the pulverizer. A coal dust explosion is described as rapid burning of combustible particulate within a confined area; which generates a considerable heat and corresponding pressure rise. The factors responsible for coal dust getting exploded:

• presence of dust in suspension at a concentration above flammability limit,
  
• presence of sufficient oxygen to enable the combustion,
  
• source of ignition of the coal dust air mixture,
  
• a certain degree of confinement to the mixture.

Most coal used in firing system have tendency for spontaneous ignition (Spontaneous heating is an inherent property of coal to get heated on oxidation, when it is kept for prolonged time with air or oxygen. Oxidation of coal is an exothermic process, hence the self-ignition) in the pile. Therefore, care should be taken in storage, handling within firing system and prevention of accumulation of coal dust during system operation.

Special precautions are necessary to ensure safe operation:

• use oxygen-deficient air in the pulverizers under normal operating condition,
  
• use of rock dust, carbon dioxide, water systems in the pulverizers and dust collectors when shutdown occurs,
  

• inerting with water sprays or steam when over-temperature conditions are observed,

• care must be taken to prevent development of coal dust cloud,
  
• use of magnet or metal detection to remove foreign iron substance in the system to prevent occurrences of sparks,
  
• cutting & welding operation should be carried out as per the safety norms,
  
• electric components used in the system should be of non-inflammable type,
• hot coal, if any, should be avoided to charge into pulverizers,
  
• proper control measures should be adopted to prevent spontaneous ignition of coal,
  
• prevention of static electricity discharge into the system should be adopted by grounding dust collector bags,
  
• by properly designing of several coal handling equipments in order to prevent accumulation coal dust at various idle spots of locations.

As coal dust explosion in coal-fired industrial systems (whether in a cement plant or in a thermal power plant) is very much serious in nature and may cause damage to the property and personnel, utmost care should be taken to prevent such occurrences. Last but not least, proper training the personnel is most important to handle such situation most effectively.

Cleaner Coal-fired ‘Supercritical Power plants’

High performance Coal-fired ‘Supercritical Power plants’ – Promotes cleaner environment

As name suggests, Coal-fired Supercritical power plants operate at very high temperature and pressure (580 degree centigrade temp. and at a pressure of 23 MPa) resulting much higher heat efficiencies (46%), as compare to sub-critical coal-fired plants which operates at 455 degree centigrade temp., and efficiency of within 40%. Some of the benefits of advanced supercritical power plants include:

• Reduced fuel costs due to improved plant efficiency;

• Significant improvement of environment by reduction in CO2 emissions;

• Plant costs comparable with sub-critical technology and less than other clean coal technologies;

• Much reduced NOx, SOx and particulate emissions;

• Can be fully integrated with appropriate CO2 capture technology.

In other words, supercritical power plants are highly efficient plants with best available pollution control technology, reduces existing pollution levels by burning less coal per megawatt-hour produced, capturing the vast majority of the pollutants. This increases the kWh produced per kg of coal burned, with fewer emissions.

Because of the above techno-economic benefits along with its environment-friendly cleaner technology; more and new power plants are coming-up with this state-of-the-art technology. As environment legislations are becoming more stringent, adopting this cleaner technology have benefited immensely in all respect. As LHV (lower heating value) is improved (from 40% to more than 45%); a one percent increase in efficiency reduces by two percent, specific emissions such as CO2, NOx, SOx and particulate matters.

"Supercritical" is a thermodynamic expression describing the state of a substance where there is no clear distinction between the liquid and the gaseous phase (i.e. they are a homogenous fluid). Water reaches this state at a pressure above 22.1 MPa. The efficiency of the thermodynamic process of a coal-fired power describes how much of the energy that is fed into the cycle is converted into electrical energy. The greater the output of electrical energy for a given amount of energy input, the higher the efficiency. If the energy input to the cycle is kept constant, the output can be increased by selecting elevated pressures and temperatures for the water-steam cycle.

There are various operational advantages in case of supercritical power plant:

• There are several turbine designs available for use in supercritical power plants. These designs need not fundamentally differ from designs used in sub-critical power plants. However, due to the fact that the steam pressure and temperature are more elevated in supercritical plants, the wall-thickness and the materials selected for the high-pressure turbine section need reconsideration. The supercritical plant needs once-through boiler, where as drum type boiler is required by sub-critical power plant. In fact, once-through boilers are better suited to frequent load variations than drum type boilers, since the drum is a component with a high wall thickness, requiring controlled heating.

• The performance of supercritical plant depends on steam condition. Steam conditions up to 30 MPa/600°C/620°C are achieved using steels with 12 % chromium content. Up to 31.5 MPa/620°C/620°C is achieved using Austenite, which is a proven, but expensive, material. Nickel-based alloys, would permit 35 MPa/700°C/720°C, yielding efficiencies up to 48%. Lot R&D inputs and allying with suppliers are required to achieve higher performance.

• Moreover, fuel Flexibility is not compromised in Once-Through Boilers. A wide variety of fuels have already been implemented for once-through boilers. All types of coal as well as oil and gas have been used.

• Current designs of supercritical plants have installation costs that are only 2% higher than those of sub-critical plants. Fuel costs are considerably lower due to the increased efficiency and operating costs are at the same level as sub-critical plants. Specific installation cost i.e. the cost per megawatt (MW) decreases with increased plant size.

Because of the high performance, efficiency and preservation of much cleaner environments than sub-critical coal-fired power plants, more than 400 supercritical coal-fired power plants are operating in the developed countries like US, Europe, Russia and in Japan. Most of the new power plants coming up now-a-days are of supercritical coal-fired technology.