In ceramic production, the high energy expenditures for operating the kiln and drying the wares are a crucial cost driver. In Germany, the financial burden of rising outlays for energy is increasing continuously. Apart from optimizing a plant’s dryers and kilns, the available options include the adoption of combined heat and power generation (CHP) as a way to reduce the impact of rising energy prices on the economic efficiency of production. Considering the high process temperatures required for making structural clay building products, micro gas turbine technology is very well suited for economical application in brickworks. The CPS engineered and marketed by Dürr, a mechanical and plant engineering group with headquarters in Bietigheim-Bissingen, is not only based on micro gas turbine technology, it is also compact, reliable and requires little maintenance. Dürr also helps brickyard operators integrate micro gas turbines into greenfield or brownfield installations according to their own specific needs. Experience to date with the use of Dürr’s CPS for heating dryers in various branches of the industry documents the fact that it reduces the users’ cost outlays for electricity and dryer heating energy. For example in Germany the economic viability of CHP solutions is additionally expanded by a 60% or higher exemption from the EEG surcharge and various other privileges and stimulations.
Continuous-flow and chamber dryers are in widespread use throughout the brick and tile sector, where green products are dried with hot air in several stages with end temperatures ranging between 100 and 200° C. Air circulators provide for intensive air circulation and mixing as due to a quality-assured setting, the climatic conditions at every point within the dryer must be as uniform as possible.
The individual compartments of chamber dryers are filled separately at periodic intervals, and the individual drying processes are started at different times. Nevertheless, most such dryers have so many chambers, that, as far as overall energy expenditure is concerned, practically continuous operation can be assumed.
Usually, kilns and dryers in the brick and tile industry are linked to each other to form a combined energy system. After firing, the bricks are brought down to exit temperature in a single- or multi-stage counterflow cooler, depending on the process details. The resultant heat of the cooling process is forwarded
to the dryer through a feed-air duct and used to help dry the greenware.
In practice, the exhaust air from the kiln’s cooling zone sometimes, depending on the product, does not suffice to provide all the thermal energy the dryer needs. In that case, a gas burner is used to generate the rest of the thermal energy and inject it into the system. The gas burner is customarily located in the supply air duct leading to the dryer. In some cases and for certain products, the burner logs have long running times and consume accordingly, large amounts of gas resulting in high energy expenditures.
Reducing costs with micro gas turbine exhaust
The cost of generating the supplementary heat needed for the dryer can be markedly reduced with the aid of highly efficient combined heat and power generation system, where heat and power for captive use are generated at the same time. Not only at brickworks, but throughout the ceramic industry, micro gas turbines are particularly well suited for this task, because they generate and provide completely gaseous, high-temperature thermal energy for immediate use in the dryer.
If the cooling air from the kiln alone is not sufficient for heating the dryer, a micro gas turbine can be integrated, with the hot exhaust from the turbine being fed into the supply air duct at some point between the kiln and the dryer (»1). The electricity being generated simultaneously can be used mainly for driving the plant’s production equipment. This reduces the rate of purchased power consumption and cuts production costs accordingly. The really high availability of CPS guarantees continous production. Caused by the gas burner nearly 100% of availability could be reached.
Dürr’s micro gas turbine CPS
For such cases, Dürr has an economical solution to offer: a CPS based on micro gas turbine technology. Capable of generating up to 100 kW the CPS modulates within the 30 to 100 kW range, depending on the momentary power requirement. The CPS’s thermal output runs between 210 and 520 kW, subject to the momentary exhaust gas temperature. Contingent on the exhaust-gas exploitation rate, the CPS achieves overall efficiencies up to 98%.
Other CPS characteristics include a minimal maintenance requirement that incurs much lower costs than reciprocating CHP motors, outstanding emission values (CO and NOx < 15 ppm) coupled with high availability. The CPS is therefore very expedient for generating captive power and drawing process heat from dryers in brickworks.
The core component of the system is a turbine-generator unit with an integral compressor (»2). The turbine drives the compressor and the generator via an oil-lubricated shaft. The combustion air for the micro gas turbine is drawn in locally and densified in the compressor, then heated up internally to increase the efficiency. Together with the requisite fuel, the preheated air is fed into the combustion chamber, where it burns with extremely low emissions. At the same time, the turbine is being turned at speeds up to 70 000 revolutions per minute to generate high-frequency electrical current that is first rectified and then put through a power electronic module to emerge as 50 or 60 Hz AC current for synchronization with the local network. As an alternative to mains-synchronized operation, island operation is also possible. The system-specific low-emission exhaust gas can be flexibly utilized for the downstream process. Particularly for drying processes, a micro gas turbine offers the advantage, that the entire useful heat is available from the high temperature exhaust volume. The temperature of the exhaust gas can be regulated between 270 and 650 °C by adjusting the internal heat recovery (recuperation) to the customer’s own needs.
The Dürr CPS can be individually tailored to case-by-case needs in terms of energy requirements and dryer operating mode and neatly incorporated into the overall system (»3). Economically efficient operation of the CPS has been successfully demonstrated on a number of different kilns and dryers in the automotive, textile and ceramic industries.
Autarkic, kiln-decoupled heating
For some time now, the brick and tile industry has been debating the pros and cons of decoupling the kiln from the dryer. If in the future it should become possible to decouple the two, the new systems would not require a closed heat cycle, but new, more economical heating options will become available for use on dryers. In view of the high temperatures prevailing in brick and tile dryers for example, they lend themselves well to being heated exclusively with exhaust heat from a micro gas turbine (»4). This ensures a highly efficient supply of heat.
Dürr has already successfully installed such dryer heating arrangements for customers in the automotive, ceramic and textile sectors.
High Return on Investment
The best service conditions for a CPS are found, when continuous full-load operation is feasible, and a CPS can cater to the base load. The customer’s heat and power requirements must, of course, be analysed at the planning stage. Then, based on the resulting data, a decision must be made as to whether the CPS should be concentrated on producing heat or electricity. That known, the Dürr CPS can be accommodated to the customer’s
own specific requirements.
The rule of thumbs is the more operating hours involved and the greater the spread between the price of electricity and that of gas, the more quickly the investment will show a return. On a case-by-case basis, the payback time for such a system can be expected to run between two and five years. In a number of European countries, the economic efficiency of a CHP investment is further enhanced by stimulation programmes for highly efficient systems. In Germany, for example, a lower EEG surcharge and the CHP bonus is applied, the griduse fee can be suspended and/or the petroleum tax can be refunded.
