Evaluation of brick kiln performances using computational fluid dynamics (CFD)

Modern history of civilization is concurrent to the use of brick and its manufacturing. Brick kiln is the most important component in the manufacturing of clay-burnt bricks. Poorly operated brick kilns are considered as the major sources of greenhouse gas (GHG) emission nowadays. Various types of brick kilns are in operation throughout the world. Tunnel kiln is the most widely used technology in developed countries as it is highly automated. Other technologies which are quite popular in developing countries are: Hoffman kiln, Vertical Shaft kiln, Fixed Chimney kiln, Zigzag kiln, etc.<br><br>Computational Fluid Dynamics (CFD) software, ANSYS CFX is being applied to evaluate performance of Tunnel kiln using natural gas as its fuel. The idea of a typical Tunnel kiln layout geometry has been envisaged from local brick industries. The length, width and height of the Tunnel kiln geometry are taken as 100 m × 3.24 m × 1.48 m. The length and width of the brick stack is taken as 920 mm × 440 mm. With a gap of 400 mm and 100 mm between two brick stacks longitudinally and laterally respectively, a total of 450 (6 × 75) stacks can be accommodated inside the kiln at a time. Brick stack height including the kiln car height is taken as 1.38 m. There is a clearance of 100 mm between the stack and the kiln roof. To produce certain quality bricks/ceramics, a particular temperature distribution throughout the kiln needs to be maintained. This temperature distribution with respect to the kiln length is known as Tunnel kiln curve.<br><br>To achieve the Tunnel kiln curve obtained from industry for ordinary brick type, some design parameters need to be optimized for a given geometry. Selection of these optimized design parameters are obtained through a series of trial and error runs of the CFD model. The total length of the tunnel can be divided into pre-heating, firing and cooling zones. Green bricks pass through the pre-heating, then firing and finally the cooling zones, while fresh air flows in opposite direction of the brick stack move. It is to be noted that brick industries are very reluctant to disclose any of those technical secrets related to their brick kiln design. In this regard, this design is based on initial guesses of those parameters and slowly come up with best performing scenario with respect to considered Tunnel kiln curve.<br><br>To achieve the Tunnel kiln curve, the design parameters that need to be played around are gas and air flow rates, flow directions, their inlet-outlet number, spacing and placements at different locations of the kiln are considered very crucial. Other important parameters that are varied include brick stack placement with respect to air and gas inlet-outlets, gaps between iv kiln roof and stacks and gaps between two consecutive stacks. <br><br>To supply adequate air, a large rectangular air inlet with an area of 0.8 m2 is placed at the roof of the exit end of the kiln. To maintain the air temperature distribution as given in Tunnel kiln curve, one intermediate size air outlet with an area of 0.4 m2 and a series of 13 rows × 12 columns small air inlet-outlets (openings) are also placed at the roof of the kiln in the cooling zone. All these heated air has been transferred to dryer to dry the green bricks. A series of 12 rows × 12 columns of gas inlets are placed in the roof of the firing zone. At the entry end of the tunnel, a flue gas outlet with an area of 0.8 m2 is placed in the roof. Due to three dimensional nature of the kiln geometry, the CFD simulation of the whole system would be very time consuming. A close look of the geometry dictates that, a one-sixth slit of the total geometry (100 m × 3.24 61 × m) containing 1 row × 75 stacks of bricks is enough to simulate the whole geometry of the kiln. <br><br>This modelled geometry is meshed and mesh independency is checked using ANSYS Mesh. Turbulence, combustion and radiation models are adopted to simulate a realistic Tunnel kiln environment using ANSYS CFX Pre. Several model runs are performed until the simulated temperature distributions obtained closely replicate the Tunnel kiln curve of the industry. From these simulations, the optimum Tunnel kiln design is suggested. The resulting CO2 and NO emissions are also obtained from these simulations. Gas inlet velocity is proposed to be 6 m/s with an inlet diameter of 25 mm. Gas velocity direction is suggested to be normal to the kiln roof. Air flow direction should be at 14o with kiln roof towards firing zone.<br><br> Gaps between brick stacks and the kiln roof should be about 200 mm. To get a uniform distribution of heated gases, positions of the brick stacks are such that on each occasion of its changed position it would be just directly below the inlet jets. Gaps between two consecutive brick stacks should also be reduced to 200 mm instead of the initially assumed 400 mm spacing. Hence additional number of bricks could be accommodated inside the kiln at a time which will result higher production of bricks with the same amount of fuel.