HOTDISC^TM – a perfect combustion device for coarser alternative fuels

1 Introduction

A HOTDISC^TM combustion device is a boon to cement producers, who intend to substitute fossil fuels with solid alternative fuels on a large scale. The apparatus is an integrated part of the pyrosystem and will act as an additional furnace for burning of a wide variety of waste materials. This solution for alternative fuels is the best way to substitute calciner fuel with coarse/lumpy alternative fuels – enabling efficient operation and lower operating costs without compromising performance.

The HOTDISC^TM is a safe, simple and effective combustion device – a large, moving hearth furnace – integrated with the preheater calciner systems that maximizes the substitution of waste derived fuels in an environmentally sound manner. As an integrated part of the kiln system, the HOTDISC^TM is an extension of the calciner and functions by extending the time available for combustion.

When alternative fuel, preheated raw meal and tertiary air are fed into the device, it produces combustion gases, partly calcined meal and combustion residues which are then processed in the calciner along with the other streams that go into the calciner. The result is calcined meal ready for the kiln and well-controlled emissions. A HOTDISC^TM combustion device provides the flexibility to burn all kinds of bulk solid waste in sizes from sludge or grains to large whole truck tyres (max. Ø 1200 x 500 mm). This potentially eliminates the need for expensive shredding of lumpy waste material.

2 HOTDISC^TM – How does it work?

The heat content of the alternative fuels is used for calcination. By providing extra residence time for the fuel, volatile circulation and blockages at the kiln inlet are minimised. Alternative solid fuels are introduced onto the slowly rotating disc, where they start to burn in fully oxidizing conditions when meeting the hot tertiary air. Fuel residence time can be optimised simply by adjusting the disc’s rotational speed. Operation is also controlled by regulating raw meal feed to the HOTDISC^TM. The burning fuel is transported approximately 270 ° on the disc until it reaches the scraper, where the remaining ash and partly calcined materials are discharged into the riser duct. Heavy combustion residues fall down into the kiln inlet, while the lighter fractions and the combustion gases move upwards into the calciner. The heat content of the alternative fuels is used for calcination.

3 HOTDISC^TM components

The HOTDISC^TM consists of the following com­po­nents:

1. An annular, refractory-lined combustion chamber with a stationary casing, a roof, and a central column. The bottom of the combustion chamber is a horizontal, refractory-lined rotary disc. A wall on which scraper segments are mounted. The wall itself divides the annular combustion chamber between the fuel feed inlet and the residual ash outlet.

2. A slewing ring which rests on a support and carries the rotary disc. The slewing ring is toothed on the inside and driven by two geared motors with frequency drive. By means of the frequency drives, the speed can be varied from 1-22 revolutions/hour. The support for the central column, slewing ring and casing rests on a common bottom frame. There is easy access for maintenance and inspection below the disc. The central column is air-cooled by means of natural draught.

3. The casing has inlets in the roof for:

Tertiary air entry, fuel entry and raw meal ­entry typically from the second lowermost cyclone stage in the preheater. This -relatively cold- raw meal is used for controlling the temperature in the combustion chamber during normal operation.

Cold raw meal entry for possible, in case of emergency, cooling of the HOTDISC^TM (not used in normal operation).

4 Leverage the benefits

of coarser alternative fuels

The HOTDISC^TM provides the flexibility to burn a wide variety of solid wastes in sizes up to 1.2 m – from sludge or grains to whole truck tyres. This eliminates the need for expensive shredding of lumpy material and boosts the cement plant’s environmental profile. Since the user is not locked into any one of the fuel suppliers, the user gains a much better position on the fuel market with the HOTDISC^TM.

5 Control and safety

HOTDISC^TM control and safety are simple. Due to the relatively long fuel burnout times in the device, which provide a valuable response buffer, the HOTDISC^TM fits well with the time constants for typical control systems on cement kilns. This makes the apparatus unaffected by the adverse effects of varying fuel feed rates and qualities, which results in effective emissions and process control. The operation is primarily controlled by the split ratio of the raw meal between the HOTDISC^TM and the calciner. It can be controlled automatically or manually to maintain the temperature of the outlet gases from the disc into the riser duct at approximately 1050 °C. This control maintains the appropriate temperature in the HOTDISC^TM despite varying fuel feed rates and quality.

In the event of major changes to fuel properties or feeding capacities, adjustment of the disc speed optimises the overall performance of the HOTDISC^TM and kiln system. The temperature in the lowermost cyclone stage of the preheater is conventionally controlled by a control loop with the calciner’s “control fuel” stream. In case of power failure or during other situations where the kiln or the ID fan suddenly stops, the fuels on the disc will continue to burn. To stop the combustion process, cold raw meal can be introduced to the disc from a bin placed above. This makes it possible to quickly extinguish the combustion process at any time and avoids any unacceptable emissions during upset conditions.

6 High energy efficiency

The HOTDISC^TM’s use of clinker cooler tertiary air and preheated raw meal, along with alternative fuel, ensures complete energy efficiency. This results in stable production capacity and minimum fuel and power consumption from the pyrosystem.

Conventional feeding of alternative fuels into the pyrosystem (pneumatically or mechanically) has several bottlenecks. A pneumatic feeding system requires a high grade processing of alternative fuels (good PSD of AF and no off-spec material) and a considerable volume of conveying air. Settling of heavy fuel material in the kiln inlet followed by CO peaks, sulphur recirculation and coating formation in the kiln riser area are some of the common problems associated with the conventional feeding. Such problems often limit the feed rate of solid alternative fuels or sometimes lead to decreased clinker production levels and thus the system becomes inefficient. HOTDISC^TM , to a great extent, helps to avoid the unwanted downtime of the system.

In the case of the HOTDISC^TM combustion device, it is not necessary to inject false air into the pyrosystem as the pre-heated tertiary air rich in oxygen is used for burning solid alternative fuels. The heavier and lumpier alternative fuels burn on the HOTDISC^TM table and residence time can be adjusted for different alternative fuel types, resulting in the complete combustion of solid alternative fuels. Only the residual ashes fall into the kiln inlet and are incorporated in the clinker. High energy efficiency is achieved because the HOTDISC^TM does not rely on external media without recuperated process heat such as air or water. In addition, negligible fuel pre-processing requirement saves power and costs.

The HOTDISC^TM does not require much maintenance and is actually maintenance friendly. Installation of the device can be split into three phases wherein the kiln downtime can be restricted to two phases only (phase 1 and phase 3 below, overall downtime of about 5 to 6 weeks). The kiln can be kept running during installation phase 2.

Phase 1, kiln shutdown Preparation of space for preassembly of the complete HOTDISC^TM. Additional civil structure for the HOTDISC^TM etc. This may include re-routing of material chutes.

Phase 2, kiln operation Preassembly of the complete HOTDISC^TM including lining. The mechanical test run can be made in this position.

Phase 3, kiln shutdown Connection of the HOTDISC^TM to the riser as well as installation of related equipment.

7 Case study – Vasavadatta Cements, Sedam

Vasavadatta Cements had an ambitious target on substituting fossil coal with alternative fuels. The company wanted a robust and versatile combustion solution to co-process a wide variety of solid alternative fuels. Earlier in 2006, Vasavadatta C­ements had installed the clinker production line 3. The contract was awarded to FLSmidth, wherein FLSmidth had to supply low a NO_x in-line calciner system with provision for the HOTDISC^TM. Line 3 began successful working without a HOTDISC^TM after commissioning.

Vasavadatta Cements wanted to co-process whole tyres, shredded tyres, cut tyres, RDF and several other alternative fuels which are available in the plant’s vicinity. Later in 2010, FLSmidth was approached to supply the first HOTDISC^TM combustion device within India with complete solid alternative fuel feeding system. As wanted, FLSmidth supplied the complete solid alternative fuel feeding system, the whole tyre feeding system and the latest generation HOTDISC^TM combustion device.

The solid alternative fuel feeding system consists of equipment for a reception and an extraction unit (unloader feeder) for solid alternative fuels, dosing unit and material transport unit (belt conveyors). The whole tyre feeding system consists of a set of roller conveyors for dosing and feeding of whole tyres on the transport conveyors.

Since the date of inception, Vasavadatta C­ements could successfully co-process different types of wastes such as whole tyres, shredded tyres, reject sandals etc., resulting in an average thermal substitution rate (TSR) of around 35 % in the calciner. It is important to note that the TSR achieved at Vasavadatta is limited by the challenging availability of the AF material and as such the HOTDISC^TM can help to achieve up to 60 % TSR in the calciner. The company is working with many different agencies to ensure availability of AF material on a sustainable basis for increased TSR. The thermal substitution rate achieved in line 3 using the FLSmidth HOTDISC^TM combustion device is the highest among all cement plants in India, making it a benchmark.

Apart from higher thermal substitution rates, other benefits to Vasavadatta Cements include reduction in the requirement of iron content in the raw mix, reduction in NO_x and SO_x emissions, reduction in the tonnage of coal feed and thereby savings in coal mill power etc.

8 Summary

Complete combustion of waste derived materials in an oxygen rich environment, variable residence time for different types of alternative fuels, bigger surface area to accommodate higher alternative fuel feed rates, unsurpassed fuel flexibilty, higher thermal substitution rates (even with difficult fuel), emission reduction are the important factors which makes the HOTDISC^TM a perfect combustion device for coarser, lumpier alternative fuels.