Oxygen Technologies: Reduce Melting Cost and Emissions. * Air Products GmbH, Germany, ** Air Products plc, UK.

Transcription

1 Oxygen Technologies: Reduce Melting Cost and Emissions T. Niehoff *, P. Keena ** * Air Products GmbH, Germany, ** Air Products plc, UK. Abstract This Paper describes the economic and technical case for implementing Air Products' Rapidfire APCOS technology in an iron cupola operation to offset the impact of high and unpredictable costs for coke, steel scrap and alloying agents that all foundries are experiencing. The Paper includes real examples of this technology in operation today, where it typically delivers net savings of between 1 and 3 per tonne of liquid iron produced. The benefits can include dramatically reduced coke rates, higher silicon yields, reduced particulate and gaseous emissions and higher melt rates where required. The option to directly inject solid material (e.g. silicon fines) into the melt zone without impacting the temperature in front of the tuyere provides an additional opportunity for saving raw material and disposal costs. In short, the technology provides operational flexibility for both cold and hot blast cupola operators and is therefore a valuable tool that foundries can use to remain competitive. Key words: Oxygen, process optimization, melting cost, cupola, APCOS. 148/1

2 Introduction The paper briefly gives the latest experience of applying Air Products APCOS (Air Products Cupola Oxy-Fuel System) system in cupola furnaces, which is becoming even more attractive as the cost for coke and metallic charge increase. The APCOS technology combines the injection of supersonic oxygen, natural gas and solid particles such as dust or metallurgical powders through the cupola tuyeres. Recent results demonstrate the system can provide increased melt rates, reduce coke rates, improve silicon yields and enable the injection of silicon fines and iron particles without affecting the temperature in front of the tuyere. APCOS can be applied in cold blast and hot blast cupolas. Coke prices have been high over the last two years and will remain unpredictable in the future. Cost for alloying elements like Silicon (Si) and Manganese (Mn) are high and will remain high for the next years to come. More operational benefits are observed when using APCOS on a cupola these will be described in this paper. APCOS Air Products Cupola Oxy-Fuel System Brief Description Alternative fuels to foundry coke are currently an interesting subject for foundries to keep their cost optimised and low. Natural gas, oil and coal offer potential to replace foundry coke in a cupola melting process. The APCOS technology does allow to inject natural gas, coal and oxygen into a coke filled shaft furnace. The idea to inject solid particles is not new. An English patent from 1831 describes this invention. Dry and powdery materials such as coal, coke breeze, silicon and cupola ash can be injected. Combustion of oxygen and natural gas allows flame temperatures of 2900 C which enables to transfer heat into the coke bed which can have temperatures of 2000 to 2300 C. Hence, injection of solids at higher rates is possible without cooling and freezing of the turyere area. The burners can be operated without solids injection to substitute foundry coke. Substitution rates of 30% have been reached on a modern hot blast cupola. Oxy-fuel firing in a coke bed has the benefits of increased stability of runner iron chemistry, reduced coke rates and operating the furnace at optimum gas velocities. Figure 1 shows the schematic of an APCOS oxy-fuel burner in a cupola tuyere. The burner is typically flanged to the back of the tuyere body and inserted through the water cooled or non water cooled tuyere. At the burner tip oxygen, natural gas and solids are mixed to enhance the effectiveness of the coke bed and the cupola process. 148/2

3 Figure 1: Schematic of APCOS Burner in Cupola Tuyere. Oxy-Fuel Firing Coke is the fundamental fuel for cupola operation that performs the vital task of providing the coke bed structure essential for a cupola to operate. Therefore, if alternative fuels are to be used, they must be used in conjunction with a coke bed or some other structure. The main challenge with using a fuel such as natural gas is that, when burnt in air, the adiabatic flame temperature (~1900ºC) is not high enough to enable sufficient heat transfer to allow iron to be tapped at the temperatures required by most modern foundries (1520 C or higher). The combustion zone of the cupola itself can approach the temperature of the air/gas flame, making heat transfer virtually impossible. The only solution is to burn the fuel in the presence of pure oxygen via an Oxy-Fuel burner, giving a greatly increased adiabatic flame temperature (~2900ºC for Oxygen/Natural Gas). This provides a large enough temperature gradient between the burner and the coke bed/combustion zone to allow heat transfer to take place at a very efficient rate. At higher melt rates only oxygen and oxy-fuel usage allow to operate a given furnace without over blowing. The optimum range of the cupola operation is extended to higher melt rates. With the reduction of coke in the shaft of the furnace the effects of the Boudouard reaction are lessened as there is less carbon available to reduce the products of combustion. The addition of Oxy-Fuel burners in selected tuyeres allows the cupola operator to combine the best features of coke, natural gas and oxygen to give a wide range of practical benefits. A typical APCOS burner installation is shown in the figure 2. Figure 2: APCOS burner installed in a cupola tuyere BMBF Kupolopt Project This project was initiated in 2001 and included several partners: Fritz Winter Eisengießerei GmbH & Co. KG, E.ON Ruhrgas AG, Küttner GmbH, IEHK RWTH Aachen, FHG Umsicht, and Air Products GmbH. 148/3

4 Fritz Winter Eisengießerei GmbH & Co. KG operates two hot blast cupola furnaces in Stadtallendorf (Germany). The cupola used for this project is a modern hot blast cupola furnace (24 t/h) of Küttner design, which mainly produces grey iron. The charge consists mainly of steel scrap (70%) and in house foundry returns (30%). Fritz Winter Eisengiesserei GmbH & Co. KG produces high quality iron castings for the automotive industry. Six oxy-fuel burners are installed in cupola No. 2 at Fritz Winter Eisengießerei GmbH & Co. KG. Each burner has a firing capacity of 1,000 kw = 33 therms/h (total 6,000 kw = 205 therms/h). The burners have been in use since 2002 and results regarding melt rates, energy and combustion efficiency are now available. In particular, the melt rate has been increased by more than 20%, based not only on short term monitoring and observation but also on long-term results. The increase in melt rate is limited to operating conditions at Fritz Winter Eisengießerei GmbH & Co. KG, as the automated charging system is operated at its capacity limit. Figure 3 shows relative melt rate scenarios for a variation of total oxygen flow rate through the cupola furnace. The total oxygen is the sum of oxygen introduced via hot blast air, oxygen lancing and oxy-fuel burner operation. Figure 3: Melt Rate as a Function of Total Oxygen Flow. The melt rate increases due to several factors including the packing of the cupola charge in the furnace shaft and the altered way in which the coke (carbon) is oxidized and gasified. First about 6% less coke and about 10% less silicon (Si) containing alloying briquettes is charged. This gives room for more iron and steel mass per furnace volume. Hence, the melt rate has to increase. Less coke and less silicon briquettes lead to reduced ash and slag of about 8% ( 6 kg slag /t Fe ). The energy to melt and vitrify this 8% of slag is available to heat and melt the metal. However, the energy that is provided from the burn up of silicon needs to be replaced by oxy-fuel energy. Overall results are summarised in Table 1. Table 1: Two Examples of Results of APCOS Oxy-Fuel Burner Operation with different objectives. The use of hydrocarbon fuels shifts the equilibria of various gas to gas and gas to solid systems (as described in reference [4]). The combination of an increase in temperature and the availability of hydrogen molecules (due to the addition of natural gas) impacts the water gas shift reaction with the presence of coke. This will result in an increase of Carbon Monoxide (CO) and Hydrogen (H 2 ) in the raw top gas of the cupola operation. For hot blast cupolas the chemical energy contained in the top gas is partially recovered in the post combustion chamber followed by a recuperative heat exchanger to pre heat the blast air. The effect on the cupola energy balance of the cupola is detailed in Table 2, while Figure 4 illustrates the changing composition of the top gas. 148/4

5 Table 2: Results Cupola Energy Balance before and after application of the APCOS system The modified top gas composition has several benefits for the process. First the Hydrogen (H 2 ) and (CO) increases the reducing potential in the hot areas of the cupola shaft. This effect reduces the oxidation process of metallic charge components like: Iron (Fe), Si and Mn. Results are reduced losses of metals like iron and alloying agents. To high temperatures and to much oxygen can have negative effects on iron quality and cupola operation. The cooling effect of the dissipating gases controls this in a smart way. Now this allows using higher quantities of oxygen and natural gas to achieve high rates of coke substitution and production rates. Now the blast rate becomes the parameter to decide on the objective which can be coke replacement and melt rate optimisation. Figure 4: Changing top gas composition before and after application of the APCOS oxy/fuel burners. Many more positive side effects can be realised when using APCOS oxy/fuel burners in cupola tuyeres. The bed coke which is charged for cupola downtimes can be reduced or even eliminated. When starting up the desired iron temperature and chemistry is reached sooner with the burner. APCOS can enable to run with only half the blast (and top gas) rate at unchanged production rates. This has consequences on dust carryover and dust generation of the cupola process. In addition it leads to underutilised flue gas cleaning system which then allows to conduct maintenance and servicing of heat exchangers and other components which the production continues at 100%. Oxy/fuel assisted iron production leads to more consistent runner iron chemistry particular carbon and silicon. It also reduces the sulphur content of the iron as the coke rate can be reduced. Solids Injection Once oxy/fuel firing is established the burners can be used to inject powdery dusts and fines directly into the hot coke bed through the tuyeres, utilising a specialised materials handling machine. In the traditional cupola most materials, if injected in any quantities, would cause cooling of the melt zone, with consequences for metallurgy and temperature and even freezing and blockage of a tuyere. If the material is introduced through the centre of an oxy/fuel flame at about 2600 C it is superheated and therefore has no undesired cooling or metallurgical effects and causes no tuyere blockage. If the material injected is a foundry waste product such as bag house fines, fettling shop waste, etc. then increasingly expensive disposal costs can be saved. Table 3 summarises several types of solids that have tested to date. 148/5

6 Table 3: Composition of several types of solid material injected into the cupola. In all cases, the injection was performed with no loss of temperature in the combustion zone in front of the tuyeres. Injection of SiC was particularly successful with immediate reaction and Si pickup in the runner iron. This confirms the results previously achieved at Tatra (Tafonco) in the Czech Republic where Si fines have been continuously injected via an APCOS system for 5 years. Similarly, the injection of iron particles (brake disc fettling dust) was also successful. The particles were successful recycled in the cupola melting process with no evidence of the dust in the off-gas system. However, injected coke breeze and sand recycling dust was found in the flue gas due to the combination of low density and high velocities. Further testing is required to enable these materials to provide an economic benefit when they are injected. Conclusions The application of APCOS delivers reliable results that allow cupola operators (cold blast and hot blast) to increase melt rate, reduce coke rates, achieved improved Si yields, reduce manganese losses and reduce slag formation. Injection of silicon fines and iron particles has also been demonstrated to deliver positive, repeatable results without impacting the temperature of the melt zone. As a result, the technology offers iron foundries the opportunity to increase their cost competitiveness and flexibility at a time when the costs of crucial input charge materials are rising. References 1) T. Niehoff, H. Strüning, O. Frielingsdorf, M. Wilczek, T. Wieting, J. Schäfer, M. Lemperle: Oxy-fuel burner technology for cupola melting, 2nd International Cupola Conference, Trier March 18/19, ) D. Saha, T. Niehoff, S. P. Smith and O. Frielingsdorf: Oxygen a versatile tool to enhance cupola operations - 2nd International Cupola Conference, American Foundrymen s Society, Cincinnati, 7 October ) M. Adamec - Tavení šedé litiny na modernizované kupolové peci Slévárny TATRA, a.s. Kopřivnice, Slévárenství ) R. H. Nafziger - Alternate Fuels for Cupola Operations, AFS transaction ) W. J. Peck Supplementary Cupola Fuels, AFS CMI Joint Conference, ) AFS - Cupola handbook, 5th edition 7) F. Neumann, Technologie des Schmelzens für Eisen und Stahlguss Dortmund /6

7 Tables Objective Maximise Melt Rate Coke Replacement Melt Rate + 21% 0% Coke Reduction - 6% - 28% Silicon Savings - 10% 0% Manganese Savings - 12% 0% Table 1: Two Examples of Results of APCOS Oxy-Fuel Burner Operation with different objectives. Baseline Operation APCOS HEAT INPUT [%] [%] Foundry Coke Silicon burn up Hot Blast Natural Gas Other HEAT OUTPUT [%] [%] Runner Iron Top Gas Water Cooling Slag Other Table 2: Results Cupola Energy Balance before and after application of the APCOS system 148/7

8 Material Composition Injection Rate SiC 47% SiC, 13% SiO2, 10% CaO Grain Size: mm Density: 1.36 g/cm3 150 kg/hr Coke Breeze 93% C Grain Size: mm Density: g/cm3 300 kg/hr Sand Recycling Dust SiO2 60%, clay 25%, C 17% Grain Size: mm Density: 0.71 g/cm3 400 kg/hr Iron Particles Brake disc fettling dust 93% iron content Grain Size: mm Density: 1.43 g/cm3 600 kg/hr Table 3: Composition of several types of solid material injected into the cupola. 148/8

9 Figures hot wind belt fuel solids oxygen Figure 1: tuyere with oxy-fuel burner Schematic of APCOS Burner in Cupola Tuyere. Figure 2: APCOS Burner installed in a Cupola Tuyere. 148/9

10 Melt Rate [%] With Oxy-Fuel Burners Reference Operation Total Oxygen Flow [m 3 /h] Figure 3: Melt Rate as a Function of Total Oxygen Flow CO and H2 [vol.-%] CO H Total Oxy-Fuel Burner Firing Rate [kw] Figure 4: Changing top gas composition before and after application of the APCOS oxy/fuel burners. Contacts: Thomas B. Niehoff niehoftb@airproducts.com Pete Keena keenap@airproducts.com 148/10