Modification of Kiln Plants to Meet Russian Demands

Transcription

1 Modification of Kiln Plants to Meet Russian Demands By Wolfgang Pajonke and Matthias Mersmann, KHD Humboldt Wedag AG, Germany The Russian Cement Industry is About Start Through After the continuous decline of the Russian cement industry in the nineties, from formerly 150 million down to 26 million tons cement per year in 1999, it is to establish a stabilisation and upward surge of the production since the year 2000 again. Especially thanks to the increased demand for capital goods and to the realisation of infrastructural measures, for the coming years it can also be forecasted a growth rate in the cement production. The satisfaction of a climbing cement demand will necessitate primarily the higher utilisation and the increase in efficiency of the existing cement lines. No easy assignment for many Russian cement producers. At present, there are high requirements of modernisation in the cement plants. From the altogether 250 kiln-lines, only 19 lines operate by using the dry process and only one line is equipped with a calciner. KHD Humboldt Wedag offers manufactured to measure engineering solutions for the more effective utilisation and the increase in efficiency of existing cement equipment, based on orientation to the actual necessities of the cement producers, and these solutions have already proven their success in numerous installations world-wide. A Flexible Strategy to Meet Market Demands Investment Limitations and Sales Opportunities In a market situation like in Russia, it is not so easy to find the right balance between satisfying the market demands and overrunning the investment possibilities. Modification of kiln plants, rather than building new ones, is the more realistic way to match sales opportunities to investment limitations. A stepwise approach to increased production and plant availability levels out investment and pay back. The robust machinery utilised in the cement industry allows to keep older equipment in use, even within modified plants forming new process technology. Apart from process relevant modernisation aspects, there is, of course, always the need to check the structural condition of the plant. The following theoretical example illustrates from the process point of view, the sequential upgrading of a typical Russian kiln line, consisting of a long wet kiln (D=4,8 m, L/D= 36) and a grate cooler. The production of this exemplary plant shall be 2000 t clionker /d at a specific heat consumption of 5000 kj/kg clinker (as can be seen in table 1). Switching to Modern Dry Process The main disadvantage of the wet process is the high amount of water that has to be evaporated. The dry process consequently allows a higher specific production (related to kiln diameter) and lower heat consumption. With the invention of the Cyclone Preheater by KHD Humboldt Wedag in 1952, the capacities of the kilns, related to their diameter, have almost been doubled, because the heat transfer into the raw meal could be intensified. In a cyclone preheater the suspended meal offers better accessibility to the heat than in a material layer within a rotary kiln, where the surface layer experiences a high heat flux. The same idea of improving the heat exchange through gas suspension is utilised in the precalcination technology. By simply extending the length of the riser duct between kiln inlet chamber and preheater and introduction of a burner into it, 60 % of the calcination can be realised in the gas suspension. This system does not need a tertiary air duct and is called PYROCLON -S. As a result, the gas load of the kiln can again be reduced due to lower heat demand and lower CO 2 -load. As table 1shows, the benefits in production and heat consumption by switching to preheater plus PYROCLON -S technology are striking. Our exemplary plant now produces 3500 t/d at a heat consumption of 3352 kj/kg cli. Depending on its grate area and fan capacity, the clinker cooler might be capable of handling this production increase. For further capacity increase, however, a retrofit might be mandatory.

2 Cooler Modification The grate coolers of the 1970 s were designed to be operated at a specific grate load of around or below 30 tons of clinker per day and square meter. Today s modern cooler technology allows to raise this parameter up to around 50 t/(dm 2 ) at even higher availability and efficiency. Hence, the typical Russian grate coolers hold strong potential for a modernisation, because the existing grate area can handle a considerably larger production what makes expensive replacement of civil structures and transport devices unnecessary. Retrofitted with features like stationary inlet section and controlled aeration devices, part of the production increase can be generated by the modified cooler itself. By raising the recuperation efficiency from 60 to 75 %, as shown in this example, the fume gas flow the kiln can be reduced. This not only puts fan capacity on disposal for a higher production, but it also reduces the specific thermal heat consumption at the same time. As can be seen in table 1, the production of our exemplary plant has now been raised by 7 % to 3750 t/d, while the specific heat consumption has been lowered accordingly down to 3070 kj/kg cli. Even stronger argumentation for a cooler retrofit comes at last when further production increase is targeted. In this case, additional grate surface comes together with higher efficiency and availability. Additional Tertiary Air Duct The final step in process optimisation consists of the installation of a tertiary air duct. The consideration behind this is, that only about 40 % of the total heat requirement for burning clinker is needed to convert the precalcined meal into cement clinker in the rotary kiln. Consequently, the kiln could be released from a major portion of the total gas load, if the combustion air for the precalcination burner would not be led through the kiln itself, but through a tertiary air duct alongside the kiln. To realise this, the kiln hood has to be modified to contain a tertiary air tapping duct. Adjacent to that a dedusting chamber separates about 85 % and routes it back into the cooler. The dust free tertiary air duct serves for optimum process operation of the precalciner and reduces heat consumption because only a very small amount of clinker dust is recirculated into the burning process. With this technology, precalcination degrees of more than 95 % are possible without operation problems, however, the kiln itself should not be longer than 13 times its diameter. This is important in order not to keep the hot meal too long in the transition zone of the kiln, which could lead to operation problems because the meal has already been prepared for sintering and tends to form kiln rings in the transition zone. At a given length to diameter ratio (L/D) of 14 in our example, the precalcination degree is limited to 90 % at maximum deliberately. This leads to a fuel portion of 50 % in the calciner. In order to ensure appropriate burnout of the calciner fuel as well as decarbonisation in the calciner, the length of the duct has to grant a retention time of approximately 3 seconds, depending, of course, on the type of fuel. If an respective extension of the calciner is not possible, the PYROTOP, a circular mixing chamber, can be introduced into the duct. The PYROTOP (see picture 1) serves for efficient burnout and reactivity. From table 1 can be seen, that our kiln line is now producing 6500 t/d at a heat consumption of only 2970 kj/kg cli. This represents an increase of 325 % in production compared to the initial situation. Benefit in Production Benefit in Heat Consumption before after this step accumulated before after this step accumulated Modification t/d t/d % % kj/kg cli kj/kg cli % % Preheater Cooler Tert. Air Duct table 1: Overview of achievable benefits with suitable modification measures

3 picture 1: The PYROTOP compact swirl chamber serves for efficient mixing and burn-out References KHD Humboldt Wedag looks back at a history of more than 140 years delivering equipment and machinery to the process industry. Besides complete new plants, tailor-made plant modifications have always been integral part of the company s business. From the comprehensive reference-list, some of the latest examples shall be given below. Modification of a ZAB Grate Cooler In 1997 ZAB, a subsidiary of KHD Humboldt Wedag, has retrofitted stationary inlet sections into two Grate coolers at the Rüdersdorf plant close to Berlin, Germany. These coolers had initially been delivered in 1968 by ZAB. The data in table 2 underline the attractive return on investment of these retrofits. Lepol kiln no. Heat before after improvement consumption modification modification 3 kj/kg cli kj/kg cli = 9 % 4 kj/kg cli kj/kg cli = 6 % table 2: Comparison of efficiency data of the Rüdersdorf lines 3 and 4 before and after retrofit of stationary inlet sections. Shortening of a Rotary Kiln At the Gorazdze plant in Poland, KHD Humboldt Wedag presently modifies a rotary kiln in order to adapt it to a precalcination process. The kiln was initially operated in a preheater-process without precalcination. According to the reasons described above, the kiln had to be shortened from L/D 21,8 down to 16,3. In order to reduce project costs, maximum possible re-use of existing parts and preservation of the roller stations with their foundations was mandatory. To ensure lifetime of the kiln

4 shell, the kiln was not just shortened, but actually re-engineered(see picture 2). The mechanical strength properties of all tube sections have been checked for suitability not only in their present, but also in their possible future positions. picture 2: schematic view of the kiln shortening of the Gorazdze plant. Roller stations have been kept in use Installation of a New Preheater Tower Modifications like the installation of a new or additional preheater has become a usual procedure for major plant suppliers in recent years. KHD Humboldt Wedag has delivered more than 30 additional preheaters in the last 25 years. The latest reference is made in Ünye, Turkey (see picture 3). In order to keep kiln downtime at minimum, the new preheater is installed parallel to the existing structures during normal operation of the kiln. Depending on the specific situation in the respective plant, only few days are needed to connect the new tower to the kiln.

5 picture 3: View of the Ünye plant in Turkey with its new preheater Plant Retrofits Require Experience Compared to newly built plants, retrofit modifications impose additional challenges on the project, because the new equipment has to fit to the existing. On closer look, it becomes obvious, that every project features it s own specifics. Consequently different conditions and targets have to be considered regarding design and implementation. In the process of defining suitable scenarios for future modification options, special attention should be given to the implication which the single modification steps impose on future options. Close investigation is mandatory and the findings are different in every specific project. In this important pre-project phase, close co-operation between supplier and cement producer helps safeguarding the investment.