Cement Kilns

Firing systems

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"Firing" in cement industry parlance is the supply of heat to a kiln by use of a burning fuel. Throughout the early history of the industry, fuel was added to static kilns in lump form, pretty much in its "as-received" state. With the advent of the rotary kiln, it was necessary to develop sophisticated means of preparing the fuel and injecting it into the kiln system. Indeed, these were the critical processes without which the rotary kiln could not have developed.

The nature of the processing equipment is to a large extent dependent upon the nature of the fuel chosen, and so historical developments are in part a function of trends in fuel usage, but certain common features in rotary kiln firing systems can be identified.

Fuels used

In the context of rotary kilns, fuels can for convenience be organised under five main headings, and typical data on these is given in separate pages:

Fuel Data
Coal Data
Coke Data
Oil Data
Gas Data
Waste Fuel Data

The history of the usage of these fuels is discussed in the fuel data pages and in the article on historical trends

Common features

In rotary kilns, irrespective of the fuel used, the most obvious common feature of the firing system is the firing pipe (burner pipe) which passes through the kiln hood at the kiln's hot end, arranged more or less along the kiln's centre line, and through which the fuel is projected into the kiln at high velocity. The effect is rather like a bunsen burner, producing a flame that extends some way up the kiln. Primary air is high-velocity air that passes up the firing pipe with the fuel, and gives the fuel momentum. Generally speaking, the primary air stream contains only a small fraction of the air needed to burn the fuel. The air-stream passing into the kiln outside the firing pipe is the secondary air, and this supplies the bulk of the oxygen needed for combustion. Although this was not always appreciated in the early days of rotary kiln operation, it is important that the flame should not impinge upon the clinker bed in the bottom of the kiln, or on the walls of the kiln. The flame should be throughout its length aligned with the centre line of the kiln. The flame consists of a mass of incandescent gas, the outside of which contains excess oxygen, which transfers heat solely by radiation. The primary air is pressurized by the firing fan (or sometimes in the early days by compressors). The secondary air usually comes mainly from the clinker cooler, and is at a high temperature (300-800°C). The ignition distance (the distance between the end of the firing pipe and the start of the flame) and the flame length are controlled by the flowrates and temperatures of the primary and secondary air.

Solid fuels

In the case of liquid and gas fuels, the firing equipment consists of not much more than a firing pipe and firing fan. However, solid fuels have to be dried and finely ground before they can be used, and are conveyed into the kiln as fine powder suspended in the primary air stream. Drying and grinding were performed separately in the early days, but fuel mills gradually developed in which drying and grinding are performed simultaneously. The systems employed fall into two broad categories - indirect firing and direct firing, although a hybrid of the two - semi-indirect firing - is also encountered.


Direct Firing

direct firing

Direct firing uses a fuel mill swept by the primary air, which then immediately conveys the fuel into the kiln without any intermediate storage of the finely ground fuel. Thus, with only a small time delay, the fuel feed to the kiln is controlled by the feed-rate of raw fuel to the mill.


Indirect Firing

indirect firing

Indirect firing uses a fuel mill that is not connected to the firing fan. The fine fuel is stored in a hopper for use as required.

wilmington firing

The firing floor of Wilmington kilns 3 (further away) and 4, around 1920. Indirect firing with a common firing fan. The fine coal hoppers (top left) deliver fuel to the firing pipe along variable speed screws. The fuel drops into the pipe by a weighted flap valve, which prevents the pressurized primary air from blowing the fuel back up the screw. The coal mills were on the floor below.


Semi-indirect Firing

semi-indirect firing

Various hybrid systems are possible: in the system shown, part of the fine fuel (depending on the efficiency of the cyclone) goes directly to the kiln, but most is stored and metered back into the firing pipe as required.

The earliest systems were indirect, if only because no mills suitable for direct firing were available. From around 1910, various direct firing mills were tried, and direct firing became progressively more popular until by 1970 nearly all new installations were direct fired. However, the development of precalciner kilns reversed this trend because in these fuel is fed to the system at two or more points, and this is most conveniently achieved with a common source of fine fuel.

Direct fired systems have a number of advantages:

  • The system is simpler in design.
  • Finely ground fuel is hazardous: the air/fuel mixture is explosive, and the stored fuel is susceptible to spontaneous combustion. Immediately conveying the fine fuel into the kiln largely avoids these problems.

On the other hand, the direct system has distinct disadvantages:

  • Control of the fuel feed to the kiln is inferior because of the time lag, because the feed of fuel in lump form is more crude, and because wet fuel can flow erratically.
  • Much more cool primary air is needed to convey the fuel through the system, restricting flame temperature.
  • The mill must operate whenever the kiln operates, and unless multiple mills are used, a fuel mill breakdown leads to a kiln stoppage, whereas in an indirect system, the fuel mill can be operated more-or-less independently of the kiln.

Modern indirect systems used on precalciner kilns solve the explosion hazard problem by heating and sweeping the mill with preheater gases, which are high in CO2 and low in oxygen.

In development . . . .

© Dylan Moore 2011: last edit 26/08/14.