Air Pollution due to Lime and Brick-Kilns | Environmental Engineering

In this article we will discuss about the occurrence of air pollution due to lime and brick kilns.

Lime-Kilns:

Quick lime manufacture involves calcination of lime stone at temperatures exceeding 900°C. Calcination is an endothermic process and the required heat energy is supplied usually by firing coal along with the lime stone.

There are heaped and rectangular trenched kilns commonly-known as country kilns, vertical shaft kilns of KVIC designed and CBRI designed vertical shaft country kilns and Belgian vertical shaft kilns. The lime-stone to coal ratio (depending upon the quality of coal and its calorific value) varies from 17 to 30 percent, with least coal consumption in semi-continuous type kilns.

Lime-kilns produce SO₂, CO about 50 percent un-burnt volatile matter of coal and lime­stone coal dust, emitting from the top layer of the kiln. Most of the kilns do not have any kind of stack for release of flue gases.

Except for Belgian type kilns, the other kilns, namely, KVI, CBRI and country type vertical kilns have batch feeding twice a day. Belgian kilns have automatic mechanical feeding every 10 minutes. Based on feeding pattern, coal, varying amounts of SO₂, are emitted in the flue gases. CaO formed during calcination is very reactive and, as the temperature increases the reactivity increases with increased equilibrium and SO₂ concentration.

It has been noted that Oxygen concentration and temperature of the flue gases are important parameters scavenging SO₂ by CaO. The temperature range where CaO is effective as an effective capturing agent of SO₂ can be taken from 960 to 1230°C. The lower temperature down to room temperature. Fig. 13.1. Presents the equilibrium concentrations of SO₂ in the gas in contact with CaO as a function of Oxygen concentration in the flue gas.

Fig. 13.2 presents a typical pattern or SO₂, concentration in the flue gas at different times after the batch tiring of coal and lime stone mix feed. After the lime removal from the kilns the batch feeding is done.

During this period, the feed gets slightly heated and SO₂, and volatile matter is released clearly excessively, with higher concentration of O₂ and CO. As the time progresses, temperature of the kiln rises and the SO₂ released gets captured in the partially calcined lime stone and lime. This is clearly explained in Fig. 13.2, where SO₂ concentration starts rising within the first 2 hours of batch feeding. After that SO₂ concentration reduces considerably.

The emission inventory of particulate matter and gaseous constituents are given in Table 13.3. This table indicates the approximate range of values obtained on number of different kinds of kilns.

Brick Kilns:

Three types of brick kilns are in operation in India namely, the Bull’s trench kiln. CBRI High draught kiln and heap type country kilns. In brick-kilns, the moulded raw bricks are fired at temperature around 950-1050°C. This heating is required to remove the physically and chemically combined water and for the decomposition of calcium carbonate content in the clay.

The setting of raw moulded bricks is one of the most important parameters effecting the thermal efficiency of the kilns. The CBRI high draught kiln requires only around 11 to 12 tonnes of coal whereas Bull’s trench kilns require 15-20 tonnes of coal for manufacturing one lakh bricks.

In country type kilns, the energy needs are met by coal and wood. Locally available raw materials like cow-dung cake, arhar sticks, jute sticks and rice-husk are also used in country type heaped brick-kilns. Use of rice husk as fuel for burning of bricks in Bull trench kiln has also been suggested. It has been found that coal and rice husk in the ratio of 1: 2: 5 may be used for firing of the kiln.

The emission level of various pollutants form brick-kilns depend upon the type of the kiln, mode of tiring and the type and size of fuel used. The SO₂ content of the flue gas will be directly proportional to the sulphur content of the fuel and air to fuel ratio. CO content will depend upon the combustion efficiency of the kiln. NO_x will be a function of temperature.

It is found that the emission of the pollutants have a definite concentration time history with peak emissions during the first 1/2 to 1 hr after the firing of the fuel in Bull’s trench kilns.

Emission level varies with the:

(i) Passage of time after the firing of each batch,

(ii) Type of kiln i.e., CBRI, KVIC or country type,

(iii) The height to diameter ratio of the kiln, (h/d) and

(iv) The particle size and quality of coal and lime stone.