Heat production for the thermal drying ­process in the industry

Hot gas generators are usually in use when drying raw materials such as ore, limestone, phosphates, gypsum or coal, in order to reduce the moisture for the further production or refinement of products (Figs. 1–2). The hot gas generator consists of a combustion chamber to burn fuels and allows the mixing of process air or recirculation gas with the combustion gases in order to obtain the required amount of hot gas flow with the necessary exit temperature (Fig. 3). In the past normally the fuels gas and oil were used as the energy source.

1 Hot gas generation with coal

Today with increasing fuel costs, the 100 % coal-fired hot gas generators (HGG) are a very attractive alternative to oil- and gas-fired HGG’s. Compared to oil or gas and with a compensation of the heat value, the fuel costs of coal are only 25–30 % of the costs of oil and gas. The fuel cost savings are so high, that it pays off to invest in this proven technology. Even pulverized alternative fuels such as for example wood dust or animal meal can be co-incinerated to reduce fuel costs even further.

When burning solid fuels such as coal, the size of the combustion chamber is larger in comparison to oil and gas firing, because the combustion time for burnout of coal particles requires a longer retention time. Of course, coal combustion with ash as the combustion residue requires the vertical installation of the combustion chamber, in order to remove the part of the ash which is not conveyed by the hot gas flow. Therefore, the installation costs are more expensive then with ho­rizontally arranged combustion chambers. Nonetheless, the fuel savings are much higher than the increased installation costs.

Naturally, the combustion of coal is harder to control compared to oil and gas. Three decades of research work and practical experience led Greco to a design, which allows safe operation with hot gases for most drying processes.

2 Adapted plants for solid fuel combustion

The fundamental features of the technology are the burner design and the control of the temperature profile in the individual sections of the combustion chamber:

Very high temperatures near the burner, in order to create a high radiation of fuel and the refractory lining for volatilization and gasification of carbon elements. A fast temperature increase (fuel reaction velocity) and high radiation allows a fast ignition of the solid fuel. This creates a stable flame formation.

A high temperature in the upper 1/3 of the combustion chamber controls the combustion time and therefore the flame length. It is essential that the ash melting point and ash melting temperature are higher than the surrounding conditions, to avoid molten ash build-ups on the refractory bricks.

In the middle to lower section of the combustion chamber, normally the mixing air (or dilution air) is introduced into the combustion process to control the required HGG outlet temperature. The mixing area for the mixing air, which is normally colder than the combustion gas temperature, needs to be well designed in order to avoid a prolonged burn-out time.

By taking into consideration these combustion processes and the combustion chamber thermal stress [MW/m³], a stable combustion of the solid fuels is achieved. The refractory linings of the Greco hot gas generators are designed to last for a complete lifetime without re-lining work.

The grinding fineness of the fuel depends mainly on the fuel characteristics as volatile matter, and influences the flame length and therefore the combustion chamber design length. It should be avoided that the flame burn-out continues in the ductwork to the heat consumer (e.g. the coal mill). A group of grinding specialists are supporting the design and preparation for the required grinding specification.

3 Starting phase

The starting procedure of the HGG requires a classic fuel such as gas and/or fuel oil in order to reach the necessary surrounding refractory temperature for coal combustion. The combustion chamber lined with refractory bricks needs to be preheated for several hours to reach the required radiation for the release of coal combustion which usually starts at a brick temperature of 750 °C. The hot gas flow of the HGG is normally kept constant as drying processes are not controlled by a variation of the hot gas flow. The power is controlled by the burner performance. The thermal power of the burner is regulated in a range of 20–100 %. The fuel dosing system is therefore an important piece of equipment to allow precise and accurate feeding of coal dust to the burner.

The oxygen content in the flue gas can be designed and adjusted in operation in a range between 3–8 %, especially when a coal grinding mill is connected to the HGG in order to use inert hot gas for explosion protection.

The entire system (HGG outlet temperature, tem­perature monitoring system, burner, fuel control and fuel dosing system) is controlled by a fail-safe PLC system to achieve a safe operation and to comply with the safety standards as described in DIN, GOST or NFPA.

4 Complete drying plant

The complete industrial processes which are required for the drying of coal- petcoke or limestone in cement- and lime plants, which are operated with coal can be offered as a turnkey system by Greco as one contractor for the main equipment shown in Figure 4. The possibility of delivery of the complete equipment and engineering supply is an advantage for all the plant operations. No interfaces disturb the planning and therefore a highly safe project integration into a running production line is possible.
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