Waste Plastics injection in a Blast Furnace
Waste Plastics injection in a Blast Furnace
The recycling of waste plastics (WP) by injecting them in a blast furnace (BF) is being practiced in few BFs especially in japan and Europe. The use of plastics in the BF also recovers energy from the WP and so it is sometimes considered as energy recovery. BF based ironmaking processes can utilize WP by any of the following methods.
- Carbonization with coal to produce coke.
- Top charging into the BF, although this generates unwanted tar from the decomposition of the plastics in the shaft.
- Gasifying the plastics outside the BF. The resultant synthesis gas is then injected through the tuyeres.
- Injection as a solid through the tuyeres in a similar way to pulverized coal (PC). Normally it is done as a co-injection of WP and coal into the BF.
The first attempt for the waste plastics injection (WPI) in a BF was made at the Bremen Steel Works in 1994, with commercial injection starting a year later. The first integrated system for injecting plastic wastes was at NKK’s (now JFE Steel) Keihin Works in Japan.
Injecting WP into BF has several environmental, operational and economic advantages. These include the following.
- Reduction in the amount of plastic wastes being landfilled or incinerated.
- Lower consumption of both coke and PC, thus saving coal resources. However, neither WP nor PC can completely replace coke. The amount of coke replaced in the BF is partly dependent on the quality of the WP.
- There is energy resource savings. The benefit of saved resources from mixed WPI is around 11 giga calories per ton (Gcal/t).
- There is decrease in the carbon dioxide (CO2) emissions since the combustion energy of WP is generally at least as high as that of PC normally injected, and their higher ratio of hydrogen (H2) to carbon (C) means less CO2 is produced within the BF from the combustion and iron ore reduction processes.
- There is lower energy consumption since H2 is a more favourable reducing agent than C. The regeneration of H2 is faster and less endothermic than carbon monoxide (CO) regeneration. Thus WPI can lower energy consumption, which also means lower CO2 emissions.
- There is high energy efficiency which is at least 80 %. Around 60 % of the injected plastics are consumed in the reduction of the iron ore, and around 20 % of the energy in the remaining 40 % of the gases is utilized as a fuel within the steel plant. Thus, WP utilization is an efficient process in a BF.
- WP has lower sulphur (S) and alkalis contents than coal. Injectants with low S contents are desired because of the effects of S on the hot metal (HM) quality. Alkalis can contribute to coke degradation, sinter disintegration and deterioration of the refractory furnace lining.
- There are lower emissions of dioxins and furans. Emissions of dioxin at the Bremen Steel Works were 0.0001–0.0005 nano grams per cubic metres (ng/cum) of exhaust gas, values well below the norms. Typically, no additional gas contamination arises so the top BF gas can be used in the steel plant.
The main disadvantages of WPI are the cost of the collection and treatment of the material. WP come from many sources including households, industry and agriculture, and so are widely distributed. Hence, collection and their treatment are expensive. The wastes are highly heterogeneous, consisting of mixtures of various types of plastics. Different plastic wastes need different processing. Plastics with high chlorine (Cl) content, such as polyvinylchloride (PVC), need to be de-chlorinated, adding to the preparation costs. Cl compounds can corrode the BF refractory lining and the pipelines in the top BF gas cleaning plant (GCP). BF performance is predominantly governed by the quality and consistency of the WP, coke and iron ore.
The quality of the WP like PC influences the quality of the HM, stability and productivity of the BF, and the top BF gas composition. Once injected, the combustion performance of the plastics is important as these can adversely influence BF operation.
Types of plastics
Plastics are normally made from simple hydrocarbon molecules (monomers) derived from oil or gas. These undergo polymerization to form more complex polymers from which products are manufactured. Additives, such as antioxidants, colourants and other stabilizers, are added to give the plastics specific properties.
The plastic is a general term describing a range of materials and compounds. There are over 20 distinct groups of plastics with hundreds of varieties. These can be classified into two main types, namely (i) thermoplastics, and (ii) thermosets. The second type consists of plastics which have been hardened by a curing process. Once set they cannot be softened by heating and so are not suitable for BF injection. These include polyurethane, epoxy and phenolic resins.
The main types of plastics suitable for injection in BF are the thermoplastics, which soften when heated and harden on cooling. These consist of five main families are (i) polyethylene (PE), which includes low density polyethylene (LDPE) and high density polyethylene (HDPE), (ii) polypropylene (PP), (iii) polyethylene terephthalate (PET), (iv) polyvinylchloride (PVC), and (v) polystyrene (PS), in the form of solid PS and expanded polystyrene (EPS).
The BFs where WPI was first carried out utilized plastics from packaging and containers. BFs inject mixed WP but this is not always defined. One commonly used definition is that mixed WP includes all non-bottle plastic packaging.
WP is highly heterogeneous material. It mostly consists of combustible hydrocarbon polymers and additives. It has been estimated that only 3 % of the total C used as a reducing agent remains non-oxidized. The polymers have different physical and chemical properties. The chemical composition of the main polymer groups are given in Tab 1. Injectants consist of mixtures of these polymer groups (and, in addition, may contain PVC). Hence, the table includes typical compositions of the WP. For comparison purposes, the chemical composition of PC and fuel oil injectant are given.
|Tab 1 Typical chemical composition of waste plastics, PC and fuel oil|
|Element||Unit||PE||PP||PS||PET||PVC||Waste plastic mixture||PC||Fuel oil|
The mixed WP is to meet certain specification requirements. Typical values for these are 3 % or less of moisture, 0.4 % or less of chlorides, and 8 mm or less of particle size. There are limits on the amount of heavy metals and trace metals in the WP mixture, as these can affect the quality of the HM. Typical specification is given in Tab 2.
|Tab 2 Typical specification for heavy metals and trace elements in waste plastics|
|Chlorine||%||Less than 2|
|Sulphur||%||Less than 0.5|
|Mercury||gram/ton||Less than 0.5|
|Cadmium||gram/ton||Less than 9|
|Lead||gram/ton||Less than 250|
|Zinc||gram/ton||Less than 1000|
|Copper||gram/ton||Less than 1000|
|Arsenic||gram/ton||Less than 5|
|Chromium||gram/ton||Less than 500|
|Nickel||gram/ton||Less than 500|
Overall, plastics used for WPI in the BF have the following properties.
- WP has high H2/C ratio (typically higher than coal). Injecting plastics increases the amount of H2 within the BF and in the top BF gas. An increase in the bosh gas H2 content lowers bosh gas density, and hence reduces the pressure drop or allows a greater gas flow for the same pressure. Since reduction by H2 is less endothermic than direct reduction, there is a decrease in the energy requirements. The ability of H2 and water vapour (H2O) to diffuse into and out of individual ore burden particles is significantly higher than CO and CO2. Higher diffusibility promotes faster reduction rates, especially at lower temperatures. The optimum raceway adiabatic flame temperature (RAFT) is also lower because of the higher H2 content in the raceway. However, a higher H2 concentration in the BF shaft can lead to increased amounts of coke fines in the shaft thus decreasing the permeability.
- WP has high calorific value (CV), in many cases larger than PC. The typical CV of PE is around 11 million calories per kilogram (Mcal/kg), PS is around 9.7 Mcal/kg, PET is around 5.6 Mcal/kg, and PVC is around 4.5 Mcal/kg (although there are wide variations between rigid and flexible PVC). The higher is the CV, the greater is the amount of heat supplied by the material, and hence the greater the reduction in coke consumption.
- WP has low S and alkaline contents (often lower than coal).
- WP has low ash if there is no plastic filler (typically lower than coal but higher than fuel oil). , little additional slag is produced. But injecting WP has led to an increase in the pressure drop (drop in the furnace permeability), which has been attributed to the ash component originating from the WP. The high melting point (around 1750 deg C) of the ash means that it does not easily form slag.
- WP has high Cl content if PVC is present. Nearly all of the chlorine leaves the BF as hydrochloric (HCl) acid, which can corrode the pipelines through which the top BF gas flows. PVC is typically removed from the WP although de-chlorination processes have been developed. Chlorine content of the WP mixture is typically limited to less than 2 %, that is, around 3 % PVC. Concern has been expressed about the possible formation of dioxins and furans via the generated HCl, but measurements in the top BF gas have shown low contents.
The strength and hardness of the WP can be an issue. Low strength agglomerated plastics are easily broken during transport (which may lead to blockage and combustion problems and thus lowering of combustion efficiency (CE). The use of WP in BF enables the additional recovery of ferrous materials present in the plastic-rich waste streams. Injection of 1 kg of WP typically replaces around 1.3 kg of PC, and around 1 kg of heavy oil in BF. Substitution of coke by WPI is limited to around 30 %, although values of 40 % have been cited. BF needs a consistent injectant quality for stable operation. Hence the preparation of WP is an essential step.
Preparation of WP for injection
The quality of the WP injectants is important not only in terms of their utilization in the BF itself, but also in the preparation, handling and distribution of the materials to the furnace. WP injectant is prepared and conveyed to a storage hopper. It is then pneumatically transported through individual pipelines or via a distributor to the individual tuyeres.
Two of the most critical requirements for the successful use of WP in BF are their availability and processing costs. The wastes are often highly heterogeneous and often mixed with other materials. Hence, the collection and sorting of wastes containing plastic residues is expensive. The aim of the processing plant is to provide a feedstock of consistent quality with the requisite particle size and in sufficient quantity. The extent of processing required depends on the condition in which the waste is received.
Foreign materials such as metals and sand need to be removed as they can cause problems, including abrasion in injection systems and of the grinding elements in mills, and a lower quality of HM. Additives added to certain plastic products during fabrication can also lead to abrasion problems. Small amounts of paper, stones and sand included with the plastics cause no problems since they are discharged in the BF slag.
Waste material contains many different types of plastic which requires sorting for separate treatment. This adds to the preparation costs. In addition, costs are influenced by the required particle size, which affects the combustion and gasification efficiency of WP. Automation, where possible, can help to lower these costs. The collected waste material is normally separated into two streams namely (i) solid plastic, which is shredded, the metal contaminants magnetically removed, and then crushed into 6 mm to 10 mm sized pieces, and (ii) film plastic which is cut into pieces, the PVC removed by centrifugal separation, and then melted and agglomerated by the use of the friction heat to form pellets with a particle size of 6 mm to 10 mm.
In case, WPI includes municipal wastes then the waste is treated in a similar manner (as solid and film plastic streams). There may be necessity of a de-chlorination step. The separated PVC pellets are heated with coke in a rotary furnace under a nitrogen (N2) atmosphere to around 300 deg C to 350 deg C, breaking them down into hydrocarbons and HCl acid. The hydrocarbons are separated from the coke and injected into the BF. The recovered HCl acid can either be used within the steel plant or sold. The strength of agglomerated plastics, and their combustibility, can be improved by the addition of calcium carbonate (CaCO3).
In Europe, a process called Redop (REDuction of iron ore in BF by plastics from municipal wastes) has been developed. Slurry of the mixed plastic fraction (separated from municipal wastes) is heated in a stirred reactor at a temperature of 230 deg C to 300 deg C. The released HCl acid is neutralized by the addition of a diluted water-soluble base. The de-chlorinated plastics melt into droplets, the size of which are determined by the stirring and by the traces of the cellulose still present. Upon cooling, the plastic droplets solidify into granules having than 0.15 % Cl which are suitable for injection into BF.
The injection system pneumatically transports and meters the WP from the storage bin through the injectant vessel, where it is pressurized upto or above the BF pressure, to the tuyere injection lances. The lances inject the WP in equal amounts through the tuyeres, which are arranged symmetrically around the circumference of the BF. A critical factor in the distribution system design is to ensure uniform feed of reductant to each tuyere without fluctuations in the WP delivery route. Any interruption in supply of the WP can quickly lead to serious problems. The higher is the injection rate, the more serious are the consequences of an unplanned interruption.
Fig 1 Typical flow sheet for WP injection system
In case WP is being injected along with PC then it can be transported (i) through completely separate injection systems and lances, (ii) through separate injection systems to a common lance, and (iii) as a blend. In most cases coal and WP are transported separately because of their different particle sizes (coal is pulverized while WP is in the size range of 1 mm to 10 mm) and densities.
At least two injection vessels are needed to provide a continuous WP flow to the BF. Basically, there are following two arrangements of these vessels.
- Serial arrangement where the upper vessel periodically replenishes the lower one, which is always kept under pressure, and which injects the WP continuously into the BF. Hence, the injectant vessels are continuously weighed and the flow rate of the WP is carefully controlled. Fouling of the bins by plastic fluff can take place.
- Arrangement where the two vessels inject alternately. An overlapping operation is needed to maintain injection of WP during the change-over period.
WP from the injection vessels can be transported (i) by individual pipes to each tuyere with the quantity of the WP is independently controlled, and charged in each pipe, and (ii) by a common pipeline to a distributor adjacent to the BF which then equally divides the WP into the individual pipes leading to each tuyere. An advantage of the second method is that the distance between the preparation plant and BF can be longer than with the individual pipe system. Differences in the routing of the pipes to the tuyeres and the inevitable uneven splitting of the WP at the splitting points can result in an uneven feed to the tuyeres. Imbalances can also cause uneven wear on the pipes and distributor.
Depending on the ratio of WP to conveying gas, the WP is pneumatically transported from the injecting vessel to the tuyeres in either (i) dilute phase, or (ii) dense phase. The carrier gas for WP is usually compressed air. Gas velocity for WP in dense phase systems is 3 metres per second (m/s) to 8 m/s. Some plants use dilute phase conveying for the plastic pellets (up to 10 mm).
The carrier gas velocity is always to be higher than the minimum transport velocity in order to prevent blockages. This minimum velocity depends on a number of parameters including the system pressure and pipe diameter, and these variables interact with each other. The low velocity in dense phase systems means low pipeline and component wear, whereas the high velocity of dilute phase systems can lead to wear, particularly at pipe bends. The wear rate is determined by the hardness, shape and velocity of the particles. Plastic agglomerates have an irregular particle shape which can cause erosion, whereas extruded plastic pellets have a regular shape. Crushed plastic particles are harder than the agglomerated pellets. Lining the parts of the pipes prone to erosion with, for example, a urethane elastomer material will provide abrasion resistance, as well as retarding the build-up of fines that can lead to blockages.
WP properties which are related to transfer line blockages include (i) moisture content which needs to be controlled to prevent blockages, and (ii) the presence of ultrafine particles. The particle size distribution of agglomerated mixed plastics is important. The proportion of particles of size less than 250 micro meters (micron) is to be limited to 1 % when the particle size specification is 0 mm to 10 mm and the granules are conveyed in a dilute phase. Also, for stable injection, it is essential that around 50 % of the injected plastics have an upper particle size of 6 mm. The use of fibrous plastic particles is difficult because the fibres agglomerate to form larger particles blocking the pipes. Plastic fluff can also jam the pipes. Plastic particles can become electrostatically charged during their transport through pipelines causing them to adhere to the walls. In severe cases the pipes may block, especially at bends. The addition of a free-flowing fine grained material can influence the effect.
Blockages can be lessened by improvements in the pipe layout and distribution systems. The injection system has methods for detecting and clearing blockages. Transfer lines include purge ports where blockages are cleared, typically with high pressure air. A simple and practical test is needed to assess the flowability and handleability of WP. This enables problematic materials to be identified before they are utilized.
The injection lance injects WP into the blowpipe which leads up to the tuyere. The particles are immediately heated by the hot blast, ignited, gasified and burned. The design and placement of the lance influences the CE of the WP. Problems of lance and tuyere blockages and melting of the lance tip can occur. Blockages are mostly due to the WP being heated to a temperature where they become sticky and adhere to the surface of the injection lances and tuyeres. There are set procedures for detecting and clearing these blockages before they can cause any problems.
Raceways are vital regions of the BF even though their total volume usually does not exceed 1% of the inner volume of the BF. They supply the process with heat and reducing agents. Injection of WP affects raceway conditions which, in turn, have consequences outside the raceway. Unburnt particles leaving the raceway can cause operational problems such as reduced permeability, undesirable gas and temperature distributions, excessive coke erosion, and an increase in char carryover. The amount of unburnt char increases with increasing injection rates. Thus the combustion and gasification behaviour of the injected plastics in the raceway is an important factor for stable furnace operation. The combustion behaviour of waste plastics is given in Fig 2.
Fig 2 Combustion behaviour of WP in BF
It is obvious that the BF can consume more injected WP than that combusted within the raceway since the unburnt material is consumed elsewhere in the BF. The combustion of plastics follows a similar path to PC except that some types of plastic thermally decompose into a combustible liquid and volatile gas. Less char is formed from those plastics that have lower ash content than coal. Hence, gas combustion can be more important than char combustion. Plastic particles have a low thermal conductivity and hence heat transfer in the raceway is high. Combustion behaviour is dependent on the type of plastic, its properties (such as hardness/density) and size. Larger particles have a longer residence time in the raceway for example around 4 seconds to 6 seconds for particles of 7 mm.
It is the combustion characteristics of WP rather than coke combustion which governs the gas composition and temperature distribution in the raceway since they are preferentially combusted. Fig 2 shows how the gas composition (including H2) varies in a simulated (hot model) raceway when waste plastics are injected. For comparison, the Fig 2 includes the gas composition for all coke operation when only blowing hot air through the tuyere.
The extent of combustion (CE), and hence the amount of unburnt material transported out of the raceway, depends on several factors which include (i) properties of WP, such as volatile matter (VM) content, particle size, and density, and (ii) operating conditions, for example, BF gas composition and temperature, and lance position and design. The combustion and gasification behaviour of waste plastics in the raceway is influenced by their properties.
Plastic types vary in composition, structure and degree of order (crystallinity). For example, the structures of PP, PS and PVC differ from that of PE as these contain methyl (CH3), benzene and Cl respectively, as the repeating unit. PE consists of a long chain of aliphatic hydrocarbons made from ethylene monomer. Both HDPE and LDPE essentially have a similar molecular structure except the chain branching which is responsible for the density differences. Thus, the thermal decomposition behaviour of the various WP constituents differs. Thermal decomposition of PE, as an example, favours greater H2 release compared to CO.
Injecting plastics lowers the RAFT as they promote endothermic reactions. WPI has a stronger cooling effect than PCI, and the effect is dependent on the type of plastic. Poly butylene terephthalate (PBT) has a higher cooling effect than PE which, in turn, is larger than PS. Injection rates of 100 kg for low grade plastics and up to 170 kg for PS are theoretically possible under constant tuyere conditions without incurring a flame temperature drop to below 2000 deg C (Fig 3). Increasing the blast temperature and/or O2 enrichment, and/or decreasing blast moisture can compensate for the cooling effect of the WP.
Fig 3 Effects of plastics and other injectants on flame temperature
Thermal gravimetric analysis (TGA) studies have shown that the pyrolysis behaviour of PS, PP, PBT, LDPE and HDPE are similar, with a rapid weight loss of hydrocarbons occurring within a narrow temperature range of around 80 deg C to 100 deg C. The pyrolysis of PS begins and finishes before PP which, in turn, begins and finishes before PE. The thermal degradation behaviour of PVC is more complex. First benzene (C6H6) and then Cl are released, followed by degradation of the remaining hydrocarbons which occurs at a similar temperature to the other plastics. Additionally, PVC produces a char fraction, unlike the other plastics. It has a more complex structure. In general, PE (and some other types of plastics) thermally decomposes into a combustible liquid and volatile gas.
Some of the studies have shown that PVC produces a char unlike LDPE, HDPE, PP and PS (without a colouring agent). Which is nor coloured yields no solid residue after pyrolysis, it generates a large amount of soot. Also, PVC produces a lot of soot followed, in order, by PS, PP, and PE. PVC also shows a faster ignition and shorter pyrolysis and combustion times than similarly sized PE, PP and PS. The faster ignition is attributed to the lower ‘activation energy for thermal degradation’ of PVC (20 kcal/mol to 33.5 kcal/mol for PVC compared to 48 kcal/mol to 72 kcal/mol for the other plastics). Differences in the pyrolysis behaviour between the various plastics are also due to differences in their chemical structure which can alter their reactivity. The reactivity of PS is greater than the reactivity of PP which, in turn, is higher than LDPE and HDPE.
The combustion performance of WP is influenced by their particle size. For complete conversion, and thus effective utilization of the WP, the heating up, devolatilization, pyrolysis, and combustion of the particles is to take place between their entry to the hot blast and the raceway boundary.
The combustion behaviour of the different WP is varying. PE is regularly used as a substitute material for investigating WPI in a BF because of the abundance of its derivatives in WP. The ignition temperature of PE increases with increasing particle size (360 deg C with 3 mm to 5 mm compared to 380 deg C with a 6 mm to 10 mm particle size) when combusted in air. This is attributed to the larger contact surface area of the finer particles to O2. Hence finer plastic particles are expected to have a higher CE than coarser ones.
An analysis of the CO2 concentration in the generated gas (often used as a measure of CE) shows that the larger PE particles undergo combustion further away from the tuyeres, and hence take longer to combust in BF than finer ones. This is a because of the low thermal conductivity of plastics. The analysis also shows that, besides having a lower ignition temperature, PE has a shorter burning time and higher burning rate compared to the coal with a particle size of 0.6 mm to 0.7 mm. This is since PE decomposes to combustible gas at high temperatures. The combustion of the pyrolysis gas with O2 is a gas-gas reaction, which is a faster reaction.
A study with combustion of PE and mixed WP in an electric furnace under a flow of hot air and measurement of the CO and CO2 contents of the generated gas, has shown that the combustion rate of the smaller particles of both the materials is faster than the larger particles at 1200 deg C, but at 1250 deg C, particle size had little influence on the combustion process. As expected, CE (termed combustion ratio and defined as the ratio of C content to the original C content) of particles with the same size is better at the higher temperature. Smaller particles has a higher CE during the initial 200 seconds to 600 seconds, but after this period the CE is reversed in that the larger particles had a higher CE.
Addition of CaCO3 improves the strength of agglomerated plastics, allowing the particles to circulate for a longer time within the raceway. It additionally lowers the melting point of the formed slag, thereby lessening the pressure drop in the furnace caused by deterioration in permeability.
A study was carried out in Germany for the combustion behaviour of WP with the same composition (76 % C, 10 % H2, 8 % O2, and 5 % ash) and particle size (3 mm to 6 mm) but prepared in different ways. Three plastic types were studied. They were agglomerate (fraction after crushing and removal of unwanted substances), granulate (after smelting at 100 deg C) and re-granulate (after additional pressing, having the highest density). The agglomerated (crushed) plastic had the highest CE due to its larger surface area and lowest density, followed by granulate and then the re-granulate. The CE of all three plastic types was low since the large particles could not completely burn out in the available residence time.
The effective use of WP needs operational changes to compensate for alterations in the raceway parameters and their effect elsewhere in the BF (such as the thermal state, slag regime and gas dynamics). Injecting WP up to 10 kg/tHM is not expected to disturb BF operation. Measures to intensify the combustion of WP in the tuyere/raceway region, and hence increase injectant rates, include (i) increase of the amount of O2 in the tuyeres, and (ii) adjustment of the blast temperature and moisture.
Oxygen can be added to the tuyere by (i) enrichment of the hot air blast, (ii) injection through the WP lances, and (iii) separate O2 lances. The addition of O2 means more O2 is available for the participation in the combustion of WP in the raceway. Thus the CE increases. However, the influence of O2 enrichment on the CE is limited.
Oxygen enrichment of the hot air blast produces both a reduction in bosh gas flow and a rise in flame temperature. The former effect can help counteract the increase in burden resistance (lower permeability) and the pressure drop associated with high injection rates. The latter effect can help compensate for the cooling effect of the decomposition of the WP volatiles. The CO and H2 contents also increase with O2 enrichment, resulting in improved reduction of the iron ores in the central shaft. The CV of the top BF gas normally improves with the O2 enrichment. The lower limit of O2 enrichment is generally determined by the amount needed to maintain the required RAFT, with more O2 required as the volatile content of the WP increases. If the flame temperature becomes too high, then burden descent can become erratic. Too low a flame temperature hinders WP combustion and melting of the ore burden. The upper limit is dependent on maintaining a sufficient top gas temperature. As O2 is increased, the gas mass flow within the BF decreases, which decreases the heat flow to the upper region of the BF for drying the burden. The upper limit of the top gas temperature may also be governed by the need to protect the top gas equipment. Other limitations to O2 enrichment include its cost and availability.
The key measure for combustion at high injectant rates is a high blast temperature. O2 enrichment plays a more important role as a means of controlling gas flow in the BF rather than controlling the WP combustion. Generally, a higher hot blast temperature is an inexpensive measure than O2 enrichment since it allows a lower O2 consumption. Increased blast temperatures also reduce coke consumption. WP has a stronger cooling effect on flame temperature than coal.
Although increasing the blast temperature raises the RAFT with waste PE injection, it has been found in a study that regardless of the blast temperature (900 deg C, 1000 deg C, and 1100 deg C) and O2 enrichment (0.7 % and 1.2 %), the maximum RAFT which can be achieved is around 1950 deg C. This suggests that blast temperature and O2 enrichment only affect the combustion kinetics (rates), and not the thermodynamics, as long as the plastic particles start burning, the maximum temperature related to the enthalpy of combustion remains constant.
Lowering blast moisture can help to compensate for the cooling effects of WPI. If the RAFT becomes excessive, then blast moisture can be increased. Raising hot blast moisture means more H2 in the bosh gas for iron ore reduction. The optimum RAFT in the BFs operating with higher H2 contents can be lower than those operating with lower H2. In addition, the blast velocity can be adjusted to not only improve waste plastics combustion, but to maintain the required length of the raceway zone which is critical for obtaining good conditions in the hearth.
With the increase of the injection rate, the combustibility of WP has a tendency to decrease resulting in unburnt material (char, fines and fly ash) leaving the raceway. Some of this material, along with coke debris, accumulates at the back of the raceway, in the bird’s nest, hampering the rising gas flow and entrained solids in this area. The majority are swept upwards where they can accumulate under the cohesive zone, decreasing permeability and hence BF productivity. Changes in the lower BF zone permeability can also affect the HM quality and slag viscosity.
The unburnt material tends to accumulate at positions where large changes in gas flow occur. Finally it is entrained into the gas flow, passing through the cohesive zone coke slits, and up the BF shaft, where it can influence burden permeability, and is finally released with the top BF gas. Higher WP injection rates also increase the volume of combustion gases, and hence the gas flow, and change the heat load in the lower part of the BF. In addition, more slag is produced.
The deposition of unburnt fine material is a complex phenomenon consisting of several generation mechanisms, reactions, multiphase flow, accumulation and re-entrainment. Different gas flow models have been developed to understand and predict the behaviour of fine material within the BF. With suitable burden charging patterns (such as central coke charging) and the use of stronger coke many of the problems relating to gas flow have been solved.
The experience has shown that most of the unburnt char is consumed within the BF. The three mechanisms for this are (i) gasification with CO2 and H2O, (ii) reaction with liquid iron (carburization), and (iii) reaction with slag. It is advantageous if the unburnt char participates in the ore reduction reactions, thus replacing more of the coke and lowering the amount of unburnt solids in the top BF gas.
The reaction of chars with CO2 and H2O begins in the raceway, but since the residence time for fine particles is very short for appreciable reaction, gasification mainly occurs in the BF shaft. The reactions of char C with CO2 (Boudouard reaction) and H2O are slower than char combustion. The chars resulting from WP and coke compete with each other for CO2 and H2O. Chars from WP are more reactive than those from coke and thus are preferentially gasified. Thus coke degradation by the Boudouard reaction decreases with increasing WPI rates.
It has been reported that the CO2 gasification rate of char from agglomerated WP (particle diameter 400 micron to 500 micron) is around 10 times higher compared to the PC char (50 micron), despite its larger size. The rates have been determined using a thermo-balance. The CO2 gasification rate of PVC char is also slightly higher than PC char. It has also been reported that the reaction rate of unburnt char from WP (300 micron to 400 micron) is around half that of PC char (50 microns). Though WP char has a longer residence time in the packed coke bed due to its larger size, it has a small gasification rate due to its fairly small specific surface area. Hence, it can accumulate in the lower part of the BF, decreasing permeability, unless CE in the raceway is high.
The reactivity of C in the unburnt char to CO2 and H2O is dependent not only on its surface area (particle size) but also on its structure and composition, as well as operating conditions. Also, since the residence time for particles at high temperatures is too short in a BF, char gasification mainly occurs at decreasing temperatures in the furnace shaft. The properties of char change as it moves up the BF, and hence its reactivity to CO2 and H2O. The reacting atmosphere is not uniform, for example, the concentrations of CO, CO2, H2 and H2O vary at different locations within the BF. Normally higher H2 and CO concentrations are found at the periphery compared to the centre of the BF for waste plastics with a particle size of 0.2 mm to 1 mm but the reverse takes place with the injection of larger particles size (less than 10 mm).
Injection of WP increases the bosh gas H2 concentration. Since the chemical reaction rate of H2 reduction is higher than that of CO, the extent of Boudouard reaction reduces as bosh gas H2 increases. CO2 and H2O are present in the upper part of the BF due to the reduction of iron oxides. Under the conditions here, char gasification by CO2 is likely to be controlled by the rate of the chemical reactions. In the lower part of the BF, char gasification is partly diffusion controlled. Hence the overall reaction rate of char gasification is likely to be influenced by the chemical reactivity of char to CO2 in this region. Char reactivity towards CO2 is influenced by its chemical structure, with less ordered structures being more reactive. The char structure from agglomerated WP has an isotropic texture with high CO2 reactivity.
The presence of certain minerals in the char ash, such as Fe and alkalis, can catalyze the CO2 gasification reaction, whereas other minerals, such as silica (SiO2) and alumina (Al2O3), can slow down the reaction. Depending on its composition, ash can also retard the C conversion due to the blockage of char particles as a result of increased proportion of slag formation in the char particle. In the lower part of the BF, condensed alkalis from the recirculating gases can have a catalytic effect. The loss of C by gasification increases the char ash content. In general, WP has a lower ash (mineral) content than PC and thus are more likely to be consumed within the BF.
Carburization of the HM begins in the solid phase within the cohesive zone of the BF, and continues during descent of the metal droplets through the active coke, deadman and hearth zones. Unburnt char and fine material leaving the raceway can contact the dripping liquid metal in the bosh and hearth zones. C and other elements, such as Fe, Si and S, dissolve from the char into the liquid Fe influencing the composition of the HM. The dissolution of C contributes to the carburization of liquid Fe, and commands the level of char consumption by the HM. It is critical where CE is low. If the HM is close to saturation when it reaches the areas of deadman and hearth, the unburnt material cannot be consumed, thus diminishing permeability in these regions. The C comes from unburnt WP materials, as well as coke.
Carbon dissolution from unburnt char into liquid metal is influenced by the operating conditions and the following factors.
- Char particle size – Unburnt chars which maintain their original form react very little with the liquid Fe and slag as they cannot penetrate into the liquids. However, if they are agglomerated into larger particles or captured by the larger pieces of coke, then they behave like bosh coke and carburize the metal up to saturation.
- Char structure – Generally, the rate of dissolution improves as the C structure becomes more ordered.
- Char mineral matter – In general, SiO2, Al2O3, and magnesia (MgO) slow the C dissolution kinetics, while calcium fluoride (CaF2) and Fe oxides enhance the rate. The effect of lime (CaO) is less clear. The reaction of calcium (Ca) with S in the metal produces a CaS (calcium sulphide) layer which hinder C transfer. The ash fusion temperature (AFT) is also one of the controlling mechanisms which limit C dissolution. The formation of an ash layer on the carbonaceous material reduces the surface area available for dissolution, hence retarding C dissolution rates. Low AFT allows easy removal of the ash, in the form of liquid slag. This results in constant exposure of fresh C surface to the HM, permitting the mass transfer of C to the liquid iron.
- Liquid metal composition – It changes over time. The C dissolution rate typically decreases as the C content of the liquid metal increases. Higher S content also retards C dissolution. Combustion of WP and coke releases sulphur oxides (SOx) which can react with the descending liquid metal and slag. This is less of a problem with WP since they typically have a lower S content than coal and coke.
Unburnt char, ash, fines, and coke can interact with the dripping liquid slag. The slag composition changes as it moves down the BF, with the Fe oxide concentration being continuously lowered as it is reduced. The reactions at the interface between the solid char and liquid slag play a major role in char consumption since they influence the kinetics of the reduction reactions and the contact area between the slag and char available for reaction.
Factors influencing unburnt char interactions with the slag include the slag composition, char C content, and char ash content and composition, as well as the operating conditions. Basically, char consumption by slags occurs through the following.
- Reduction of the Fe oxides in slags by C in the char – The wetting characteristics have a significant effect on the dominant reduction mechanism taking place. The wetting characteristics of slags vary with slag composition, temperature, time, and carbonaceous material. Wetting varies as a function of time since the reduction of Fe oxide in the slag by char, and the dissolution of the char ash components into the slag, results in continuous variations in the slag and char compositions. An increase in temperature normally results in improved wettability at the slag/C interface. Reduction rate usually increases with increasing slag FeO (2 % to 10 %) content and with increasing reaction temperature (1300 deg C to 1600 deg C).
- Reduction of SiO2 in slag by C of char – This is a function of temperature. At temperatures less than 1500 deg C, only reduction of Fe oxide occurs. At higher temperatures, both SiO2 and Fe oxides in the slag are reduced, resulting in increased consumption of the char. SiO2 is reduced by C, via gaseous SiO, to Si or silicon carbide (SiC). Self-reduction of SiO2 in the char ash by C can also occur, resulting in further consumption of the char. The reduction kinetics of SiO2 is influenced by the wettability of chars by the slags. Wetting behaviour improves with an increase in slag SiO2 content, and with an increase in temperature (1500 deg C to 1700 deg C). Higher amounts of SiO2 and Fe oxides in the char ash facilitate the slag/C interactions, leading to improved consumption of these oxides through reduction reactions.
- Interaction between components in the slag and char – This interaction leads to the assimilation of char ash components such as S.
In addition, the reduction of MgO in slag by char C can lead to further consumption. Self-reduction of the oxides in the char ash by C can also contribute to char consumption.
The presence of unburnt char in the slag can interfere with tapping by increasing slag viscosity, while assimilation of char normally increases the fluidity of the bosh slag. Changes in slag mobility can affect the position and shape of the fluid and cohesive zones. A high viscosity slag around the tuyeres also leads to serious gas flow problems. Slag viscosity is a complex function of slag composition, temperature and O2 partial pressure. As well as unburnt char and coke, unburnt ash from WP can interact with the slag. All of these carbonaceous materials contribute oxides to the slag. In general, higher amounts of SiO2 or Al2O3 (acidic components) increase slag viscosity, whereas a higher basicity (higher CaO or MgO) lowers slag viscosity because of de-polymerization of the silicate network. Slag viscosity decreases with increasing FeO (0 % to 20 %) content at a fixed basicity. Basicity is generally determined by the CaO/SiO2 ratio. Since the slags do not fully assimilate the char and ash in the bosh region, bosh slag normally has a higher basicity than tapped slag. The addition of fluxes can help solve slag formation problems.