Effects of wind barrier design and closed coal storage on spontaneous ignition of coal stockpiles

Highlights

• Effects of wind barrier design on spontaneous ignition of coal stockpiles are investigated numerically.

• The distance between a wind barrier and a stockpile advances spontaneous ignition.

• Air blowing induced by a barrier installation is effective when wind speed is high.

• A closed stockpile is proposed to root out self-ignition completely.

• A stockpile in a compact silo with a small volume doesn't encounter self-ignition.

Abstract

Effects of wind barrier design on spontaneous ignition of coal stockpiles are investigated numerically. A wind barrier changes air flow pattern around a stockpile and spontaneous heating is affected by design factors of a wind barrier. The distance between a wind barrier and a stockpile is selected as a design factor in the stockpile with both a single and a dual barriers and air blowing from the bottom of the stockpile is applied aerodynamically by installation of a wind barrier. As the distance increases, spontaneous ignition is accelerated slightly with a single barrier and significantly with a dual barrier. A dual barrier with longer distance than a critical one doesn't have retard effect any more compared with the stockpile without any barriers. Air blowing induced by a barrier installation is effective when wind speed is higher than a critical one, but partial air blowing is not. Air blowing should be made from the entire domain of the bottom. As a method to root out spontaneous ignition, i.e., free of self-ignition, a closed stockpile is proposed here. Spontaneous heating depends strongly on the size of the silo covering the pile and it is verified that a compact one with a small volume can suppress ignition completely, where maximum temperature in the pile increases initially and finally, falls down below a critical value for self-ignition.

Introduction

Coal is one of primary fossil fuels and has been widely used as a main energy source in power-plants because its price is relatively low to high heating value. Another advantage is that coal is a storable solid fuel for a long time before it is supplied into a furnace for burning. It is usually stored in the form of stockpile with a large scale in an open coal storage yard. Accordingly, coal is exposed to air and oxidized slowly by oxygen. Although atmospheric temperature is low, low-temperature oxidation of coal leads to spontaneous heating. If heating process continues in a stockpile for a long time, self-ignition can take place and finally, it is followed by violent coal oxidation, leading to combustion (Brooks et al., 1988, Fierro et al., 1999, Moghtaderi et al., 2000). These processes leading to combustion in coal stockpiles have been studied and understood well as described in the literatures (Carras and Young, 1994, Fierro et al., 2001, Shamsi et al., 2004, Turns, 2012, Yuan and Smith, 2009). Spontaneous or self-ignition occurs when heat produced by low-temperature reaction of coal with oxygen is over heat loss or dissipation from stockpiles to the surrounding environment. In the end, rapid combustion is induced by spontaneous ignition of coal in a storage yard and results in economic loss and serious hazard in fire safety.

To predict whether spontaneous ignition occurs or not and when and where it does, large-scale or full-scale experiments have been conducted (Cliff et al., 2000, Kim, 2004). Experimental works are useful and can offer realistic results in finding when and where spontaneous ignition occurs. But, they are limited because a large-scale experiment is formidable and costs much expense and time. Accordingly, a large number of experiments are difficult to be realized in practice. As an alternative to experimental approach, CFD simulations were proposed with simplified models and can be found in lots of literatures (Akgun and Essenhigh, 2001, Jones and Vais, 1991, Kim and Sohn, 2012, Park et al., 2009, Taraba et al., 2014, Salinger et al., 1994, Zarrouk et al., 2006, Xia et al., 2014, Yang et al., 2014, Zhu et al., 2013). Spontaneous ignition is affected by various parameters such as shape and dimensional size of a coal stockpile, porosity of a coal stockpile, external flow or wind, initial temperature of coal, moisture content, and so on. Forced and natural convections of air within the coal stockpiles have been considered in nearly all of previous numerical works. Additionally, several works employed air flow surrounding the piles (Akgun and Essenhigh, 2001, Kim and Sohn, 2012, Salinger et al., 1994). The effect of solar radiation on coal ignition was studied (Krajčiová et al., 2004).

Recently, numerical works showed considerable progress in simulating spontaneous heating and studying retard effect of the parameters aforementioned. Furthermore, new methods to suppress spontaneous ignition were proposed and verified in a quantitative manner by Kim and Sohn (2012). For example, internal walls, forced air blowing from the bottom of a stockpile, and a dual wind barrier were proposed and tested to retard spontaneous ignition and the expected additional delay of each method was calculated to be about 10–30 days. From the comprehensive numerical simulations, theoretical models were suggested in fitted equations as functions of geometric parameters of a stockpile to predict self-ignition time and location with engineering accuracy (Zhu et al., 2013). Recent simulations (Taraba et al., 2014) studied the effect of wind on spontaneous heating of coal stockpiles with the variables of wind speed, direction, and flow fluctuations. From these previous works, information on the effects of the geometry of coal stockpiles and external flow or wind was provided. But, still, suppression effects of the proposed methods are not sufficiently found for design of suppression devices in a complete form and a method to make free of self-ignition has not been suggested.

In this regard, one of effective suppression devices, a wind barrier is selected and its suppression effect is investigated depending on the distance between a barrier and a stockpile. The distance means a secured space to change the size of a stockpile, leading to increase in flexibility for coal storage. And, air blowing is devised by installation of a wind barrier without artificial supply of air, which is called natural air blowing. Finally, closed coal storage with a silo is suggested to root out spontaneous ignition and verified numerically.