VSBK - for cleaner brick production

Conventional burnt clay bricks will continue to be the main walling material for meeting the huge demand for housing in the foreseeable future; notwithstanding substantial efforts to develop alternatives. Current technologies for brick production such as clamps, downdraught kilns and Bull’s Trench Kilns consume large quantities of fuel such as coal, firewood and other biomass. The devastating effect of the pollution caused by huge amount of emissions from the brick industry has attracted the attention of regulatory agencies who have issued deadlines after which the polluting kilns are to" clean up or close down". Subsequent to the notification in 1996, several thousand polluting movable chimney BTKs have been converted to fixed chimney BTKs conforming to CPCB – MOEF – Supreme Court directives. Still several thousand BTKs especially smaller ones have not been converted and face threat of closure after June 2001.

The Vertical Shaft Brick Kiln technology developed in China is an energy efficient, environment friendly and economically viable alternative means to produce quality bricks, especially for mid range brick makers, producing 10 to 40 lakh bricks per year.

VSBK Construction - basic features

The VSBK consists of one or more shafts located inside a rectangular brick structure. The shafts are 1 to 1.25 metres wide with nominal lengths of 1 m, 1.5m, 1.75m or 2.0 m. In India, shaft heights have been varied to hold from 8 to 13 batches. The inside surface is a brick wall, often lined with refractory fireclay bricks. The gap between the shaft wall and outer kiln wall is filled with insulating materials - clay and rice husk, etc. (see diagram). Provision for peep-holes and thermocouple probes are provided along the shaft height to monitor the position of fire as well as temperature profile of the kiln.

The shaft is loaded from the top in batches of green bricks. Each batch typically contains four layers of bricks set in a predetermined pattern. The stack of bricks rest on square support bars (which can be removed or inserted) and supported in turn by a pair of horizontal beams across the arches in the unloading tunnel. For evacuation of exhaust gases, typically, two rectangular chimneys are provided at opposite corners of each shaft. Lids are provided to cover the shaft top, which direct the gases to the chimney through the flue system.

VSBK - How it works

During initial firing operation, a fire (with wood and charcoal briquettes) is lit in the firebox at the bottom of the brick setting (or top). During continuous operation, one batch of dried green bricks is loaded in layers at the top at a time. A weighed quantity of sized coal (5-15 mm) is spread on each layer uniformly to fill the gaps. The brick unloading is done from the bottom using an unloading trolley, which runs on rails along the length of the unloading tunnel. Lifting and lowering of the trolley is done using a single screw unloading mechanism. For unloading, the trolley is lifted so that the whole stack of bricks in the shaft rests on it. The support bars are taken out, when the load is released. The whole stack is then lowered till the layer with openings appears, through which the support bars are then reinserted. On further lowering, the load of the stack is taken by the support bars except for the batch being unloaded which comes down along with the trolley finally resting on a pair of rails. The trolley is later pulled out along the rails laid out on the floor of the tunnel. The bricks subsequently unloaded and sorted out for dispatch. The next batch is loaded at the top from the green bricks lifted onto the loading platform. The frequency of unloading - loading varies from 90 to 150 minutes. The residence time of a batch in the kiln typically varies from 26 to 30 hours.

The skill in operation is to keep the firing zone in the middle of the shaft. The draught of air moving up from the bottom cools the fired bricks in the cooling zone and itself gets heated. Maximum temperatures of upto 1000⁰C are attained in the central firing zone. The hot gases moving upwards dry and heat up the green bricks in the preheating zone. This recovery of sensible heat accounts for the high energy- efficiency of the VSBK technology.