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Memorandum 

To: Jeff Telander, EPA 

From: Garth Tayler: Acme Brick Company 

Date: June 23, 2010 

Re: Discussion of why there are no HF and HCl emissions for brick 

plant dryers 

BACKGROUND

 

There is a concern that certain amounts of HF, HCl and particulate matter may 

be emitted from the dryer stacks of clay brick operational processes. The 

purpose of this memo is to discuss the design and operation of the brick drying 

and firing system and to explain why the emission of HAPs and PM from the 

dryer stack is highly unlikely with a correctly designed and operated clay brick 

tunnel kiln/dryer system. 

DESIGN FEATURES OF THE TUNNEL DRYER/KILN SYSTEM

 

Extruded clay brick that are set onto kiln cars, pass through the tunnel dryer to 

bring the moisture down to less than 1%, prior to firing. The firing process takes 

place through the tunnel kiln, which is designed to heat up the brick load 

gradually through the various stages in order to develop the desired fired color 

and required degree of vitrification to render it durable and fit for purpose. 

The typical tunnel kiln firing process is made up of the following stages: 
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1. Kiln exhaust section; consisting of exhaust ports, ducting and 

exhaust fan. 

2. Early pre-heat zones; consisting of burner and/or recirculation 

sections to maximize the convective heat transfer process. 

3. Main pre-heat zones; utilizing convective and radiant heat transfer 

to facilitate oxidation and load preparation for the final high 

temperature firing stage. 

4. Furnace zones; designed to develop the final vitrification and color 

development of the material. 

5. Rapid cool zone; the load is cooled rapidly down to the quartz 

inversion temperature of approximately 1100F. 

6. Ware cooling zone; the load is gradually cooled to final exit 

temperature. 

In order to maximize efficiency and energy recovery, the tunnel kiln is designed 

with a heat recovery scheme that extracts thermal energy from the ware and the 

kiln cars via a hot air recovery system in the rapid and ware cooling sections of 

the kiln. These sections are connected to the drying process and are the cooling 

sections (stages 5 and 6) of the tunnel kiln. This part of the kiln operates 

independently of the firing zones of the kiln. Between it and the firing zone is an 

area of the kiln known as a neutral zone. 

This area is a virtual area of the kiln where the pressure balance on either side is 

such that no air movement occurs through this area. Subsequent to a pressure 

balance scenario some kilns also use the quick cooling air supply volume to 

create an air curtain to further insure that airflow occurs as required. Downstream of the neutral zone (with respect to airflow as in towards the kiln exhaust 

zone) is where the combustion gasses and any off gassing from the brick body 

travel, along the length of the tunnel to the exhaust section. 
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The ware, moving through the tunnel, passes through the preheating and final 

firing zones and into the above mentioned cooling zones, where cooling down 

takes place under controlled conditions. Ambient air is drawn into the exit and 

intermediate parts of the cooling section. This ambient air cools the mass of the 

load; the resulting heated air is drawn from the kiln by means of a centrifugal fan 

and transported through a ducting system to the dryers. This design ensures 

that no products of combustion or HAPs emissions are present in the waste heat 

circuit supplying the dryer air when the correct operation of the kiln is taking 

place.