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IT TAKES GREAT ENGINEERING TO MAKE
THINGS THIS SMALL.( CHARCOAL BRIQUETTES)
Loesche vertical roller mills deliver cutting-edge throughput rates and operating efficiency for the cement, iron & steel, energy and minerals industries. We set benchmarks with the performance and productivity of our machinery and the quality of our service.
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OFTENUNDERRATEDTobias Korz, Loesche GmbH, Germany,
considers coal preparation in gasication plants.
he coal gasification industry has experienced enormous growth over the last few years, and consequently the demand for coal grinding and drying
has also increased. However, coal preparation has often been neglected in comparison to the importance given to the gasifying method, even though it is an integral part of a gasification plant. The grinding and drying process always stands at the very beginning of the coal gasification process and the entire process is influenced by the quality of the feedstock produced in this first step.
In 1990, Loesche GmbH supplied the first coal grinding plant for an IGCC power plant in Buggenum, the Netherlands, with three LM 26.3 D
4-roller Loesche Mill
in modular design with
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WorldCoal Reprinted from March2009 www.worldcoal.com
Figure 2. Loesche roller module.
Figure 1. IGCC plant in Buggenum, Netherlands.
(Figure 1). The customer was Demkolec B.V., which was founded by the Association for Electricity Industry of the Netherlands to focus on the execution of this IGCC power plant. The plant is now owned by Nuon, a Dutch energy company, which bought it in 2001.
The scope of supply included the complete equipment from raw material feed to downstream of the bag filter, including the grinding mills, all the necessary bunkers, ducts, flaps, electrical drives and insulation material.
The development of coal millsThe development of Loesche coal mills began with two-roller mills and progressed to three and then to at least four-roller mills. Due to the fact that the industry was asking for even higher throughputs, the company designed a LM 43.4 D as the worlds biggest coal mill, which successfully went into operation in two cement plants in India in August 2008. Even though this four-roller coal mill was a new development, the required throughput was reached within a few days. This type of mill is able to
WorldCoalwww.worldcoal.com Reprinted from March2009
produce up to 120 tph of pulverised coal for gasification purposes. This production rate is sufficient to feed one 2000 tpd gasifier train with only one mill.
As the industry is asking for even bigger mills than the LM 43.4 D, Loesche is able to design them without the need for a long developmental period, due to the modular system of Loesche mills. Figure 2 shows a module consisting of a roller with rocker arm, a pedestal and complete hydro-pneumatic spring unit. The modular structure of large roller grinding mills enables the use of the same components in different mill sizes. The principle behind this structure was patented in the 1960s and is employed for coal grinding mills and mills in the cement and industrial minerals industry. Two, three, four or even six grinding roller configurations for a single grinding table can be realised. This concept, which has already been successfully established in the cement industry by using one raw material mill per cement kiln, has also being realised for coal grinding.
The particular value of a four-roller mill is that operation can be maintained
even with only one pair of rollers. In case of damage to one grinding roller, one pair of rollers, linked in one hydraulic system, could be lifted up, and grinding can be continued with the remaining rollers making up 60% of the required capacity (Figure 3). This means high availability and considerable flexibility with regard to the machines control behaviour. This flexibility would be enhanced further by a six-roller coal mill, e.g. an LM 53.6 D with 5.3 m diameter grinding table. In the case of a failure of a grinding roller the possible capacity would be 80% of normal capacity (Figure 4).
This in-built redundancy is an opportunity to save investment costs as it means that the building of a complete stand-by plant is not required. This is an important development as gasification projects all over the world are postponed or even cancelled due to high CAPEX costs by reason of overdrawn redundancy.
The present state-of-the-art technology of gasifiers with a capacity of 3000 tpd could be fed with only one six-roller mill.
The demands of the gasification industry have led Loesche to realise throughputs with more than 180 tph. The issue of variability is of enormous importance for coal mills, particularly with regards to the cement industry and, recently, the gasification industry. In recent years, Loesches coal mill has undergone constant development in order to meet the requirements. The combination of standard features with more unusual design characteristics within the mill enables the economic and fail-safe preparation of hard coals, lignites and petcokes.
Plant layout and safety aspectsFigure 5 shows a typical flow sheet for a coal gasification plant with the possibility of feeding the mill with different types of material beside coal. It is a self-inert grinding plant, meaning that inert recirculated gases occur in the installation system itself. The companys hot gas generator, an intrinsic part of the grinding plant, provides the required process heat.
The description below indicates and explains the main control loops:
The raw coal feed is controlled by a level measuring system of the coal dust silo and by mill differential pressure. The materials are fed in a certain mixture, with the main material content being raw coal.The gas volume at the mill outlet must be constant in order to achieve perfect drying and constant fineness of the product.The pressure at the mill inlet has to be kept constant by means of the exhaust air regulation flap.Dust control in the circulation gas ducting is necessary to give a warning in case of filter leakages and to shut off the plant in case of too high dust content in the waste gases.The temperature at the mill outlet must be controlled and regulated to avoid a temperature exceeding 120C at filter inlet. The oxygen content at the filter outlet needs to be controlled and kept below 12%, normal operation approximately 10%.
The safety of coal grinding plants must be ensured in all operating
Figure 3. Part load conditions of four-roller mill.
Figure 4. Part load conditions of six-roller mill.
WorldCoal Reprinted from March2009 www.worldcoal.com
situations. Safety aspects are laid down in regulations and ordinances. Nowadays, inert and pneumatically-operated grinding plants can be designed with incorporated operational safety characteristics. The operator decides on the ultimate plant layout.
Solid fuels can, under certain circumstances, combust, flash ignite or explode as a result of the following conditions:
An increased oxygen content (>12%) in the process room.The presence of ignition sources.The presence of large quantities of volatile material.An increased carbon monoxide content in the process air.The fineness of the grinding material.The temperature of the material and process air.
As a result, there is a higher demand for effective safety measures. These include the following:
An inert operating status with a maximum oxygen content of approximately 10% in damp process gas.Pressure-shock resistant structure (only some parts of the plant).Oxygen monitoring.Carbon monoxide monitoring (e.g. to detect smouldering when the grinding mill is stationary).Quick-closure gate valves to isolate plant components.
Avoidance of ignition sources (e.g. stop buffers in the mill preventing metal-to-metal contact).Avoidance of deposit areas and coal dust deposits for a certain period of time.Plant safety design in compliance with local regulations.
Due to the inert process gases, special explosion pressure relief systems are not necessary, which helps provide additional costs savings.
Today, safety and monitoring measures are automated, monitored and remotely controlled through simple systems of control consoles supported by process computers.
BenetsThe plant configuration for a coal dry grinding plant for the gasification industry is the same as in ironworks. In this industry, coal is injected into the blast furnace in a procedure called Pulverised Coal Injection (PCI). A large part of the coke needed for the iron making process can be substituted with pulverised coal, which is much cheaper. Therefore the need for PCI is very high due to the quick return on investment.
As Loesche sold more than 55 coal mills for PCI, the expertise and experience for coal dry grinding purposes is existent and available. These mills are located all over the world, reaching throughputs from 20 tph up to 100 tph. Since interest in PCI escalated in
the 1980s, the company has become one of the market leaders in selling coal mills to this industry.
There are a number of concerns within the industry ove