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Energy and the New Reality, Volume 1: Energy Efficiency and the Demand for Energy Services Chapter 6: Industrial L. D. Danny Harvey harvey@geog.utoronto.ca.
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Energy and the New Reality, Volume 1:Energy Efficiency and the Demand for Energy ServicesChapter 6: Industrial L. D. Danny Harveyharvey@geog.utoronto.ca This material is intended for use in lectures, presentations and as handouts to students, and is provided in Powerpoint format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see www.earthscan.co.uk for contact details. Publisher: Earthscan, UKHomepage: www.earthscan.co.uk/?tabid=101807
Major Industrial Sectors - Iron & Steel - Aluminum - Copper - Cement - Glass - Pulp & Paper - Plastics - Petroleum refining - Chemicals (including fertilizers – Chapter 7) - Food processing (Chapter 7) - General manufacturing
Figure 6.1 Industrial Energy use in 2005 as a percent of total energy use in various regions
Figure 6.2a Industrial energy use in OECD countries in 2005 Note: “Non-metallic minerals” includes the production of cement
As seen in the preceding two slides, a large part of total industrial energy use (~ 2/3) is in the production of processed materials from raw materials (steel, aluminium, cement, chemicals, plastics, paper products)
Figure 6.3 Global primary energy use for production of the 12 commodities (other than the production of fuels) using the most energy
Definitions: • Primary metals: made from virgin ores (raw materials) • Secondary metals: recycled from scrap • Feedstock energy: The energy content of fossil fuels that become part of the material in a commodity (such plastics). It is equal to the heating value (that is, the heat released when it is burned)of the final product • Process energy: energy (in the form of heat or electricity) used to power a chemical transformation. It is equal to the total energy inputs to the production process minus the heating value of the final products • Embodied energy: the total amount of energy (process + feedstock) that went into making something
Figure 6.4: Trends in production of major commodities (solid lines use the left scale, dashed lines the right scale)
Processing of Minerals • Most minerals of interest occur as oxide minerals in ores (rock bodies with various minerals mixed together, besides the ones of interest) • For example, iron ore consists of the iron oxide minerals hematite (Fe2O3) and magnetite (Fe3O4) mixed in with mineral grains consisting of alumina (Al2O3) and silica (SiO2) and possible others • The “grade” of an ore is the mass fraction of the metal in the ore. For example, iron ore might consist of 80% iron oxide minerals, and within these minerals, the mass fraction of iron is 69% for hematite and 73% for magnetite, so the grade of the iron ore would be ~ 0.8 x 0.7 = 56% (typical grades for iron ore are 30 to __70%, but only 1-2% for copper ores)
Processing of Minerals (cont’d) • The steps in processing minerals are thus – separation of the minerals of interest from the other minerals in the ores, done by grinding followed by some separation technique (higher grade ores tend to have larger mineral grains, so less grinding is needed to separate them and less energy is required) - removal of oxygen from the minerals (reduction) - purification (easier if we’re starting from higher grade ores)
Reduction of oxide minerals to make pure metals produces CO2 because a C-containing fuel is used (today) as the reducing agent. • Iron: 2Fe2O3 +3C → 4Fe+3CO2 • Aluminum from alumina (made first from bauxite): 2Al2O3 + 3C → 4Al+3CO2 • Copper from cuprite (made first by roasting Cu-containing minerals): 2CuO+C→2Cu+CO2
In the case of iron, aluminum and copper, the C used for reduction comes from fossil fuel inputs, or from materials (such as C anodes) made from fossil fuels, and so is accounted for in the energy use data combined with the emission factors (kgC/kg fuel) for these energy inputs.Thus, fossil fuel energy inputs play two roles in producing Fe, Al, or Cu: – as a source of C for the reduction reaction and as a source of heat (through combustion) to drive the reaction.
Calcination of limestone to make cement: CaCO3→CaO+CO2 Production of glass: nSiO2 + mCaCO3 + xNa2CO3 + .... → Glass + CO2 In the case of calcination of limestone in making cement or in transformation of the raw materials for glass into glass, there is C in the raw materials that reacts to form CO2 and is released to the atmosphere:
As this CO2 comes from non-fuel inputs rather than from the fossil fuel inputs, it is not accounted for in the energy use data. Thus, you will find that national CO2 emission data are given separately for coal, oil, natural gas, and production of cement.In sum, there are 2 sources of CO2 emissions associated with the production of cement and glass: - from the chemical reactions involving non-fuel inputs, and from the burning the fossil fuels used to power the chemical reactions.
Figure 6.5a: World production of primary + secondary raw steel
Traditional Steps in Making Steel: • Beneficiation of iron ores (removal of impurities) • Agglomeration of fine particles • Reduction of iron ore to make crude iron • Refining of crude iron to make steel (removing impurities, adding trace elements) • Shaping of steel into final products
Reduction of iron ore • Commonly done in a blast furnace • C from coke (which is like charcoal, and made from coal by driving off volatile materials) and coal is used as a reducing agent • Theoretical minimum energy requirement is 6.8 GJ/t • Practical lower limit is 10 GJ/t, best blast furnaces use about 12 GJ/y, world average is about 14.4 GJ/t • The output is pigiron, which is contaminated with about 4-5% C and other impurities
Blast furnace and related units: Blast furnace placed in an installation 1. Iron ore + limestone sinter 2. Coke 3. Elevator 4. Feedstock inlet 5. Layer of coke 6. Layer of sinter pellets of ore and limestone 7. Hot blast (around 1200 °C) 8. Removal of slag 9. Tapping of molten pig iron 10. 17. Coke oven
An alternative to reduction of liquid iron ore in a blast furnace is to reduce fine iron minerals in the solid state by reacting them with a mixture of CO and H2 that is produced from natural gas or coal. This produces direct reduced iron (DRI).One can go one step further and use H2 as the reducing agent, with the H2 produced electrolytically from renewably based electricity. The net reduction reaction for hematite would be:Fe2O3 + 3H2 2Fe + 3H2O
Production of DRI using natural gas requires about 25% less energy than producing pig iron in a blast furnace, and further reduces CO2 emissions because the emission factor for natural gas is ~ 13 kgC/GJ vs ~ 25 kgC/GJ for coal.Using pure H2, the energy requirement is reduced by 40% and, as long as the H2 is produced from C-free energy sources, eliminates CO2 emissions both for the reduction reactions and in supplying the heat to drive the reactions.
Refining (purification) of iron The iron is turned into crude steel by refining it in a basic oxygen furnace, in which the C in the iron is oxidized to CO2 and various impurities are removed. This step actually generates energy – the oxidation of the C contamination in the iron releases energy. To avoid overheating, steel scrap will be added and melted along with the iron, so we get some “free” processing of scrap too.
Source: da Costa et al. (2013, J . Cleaner Prod)
Scrap steel is already in elemental form (~ pure Fe) and so does not need to be reduced, and instead needs only be melted and poured into new shapes to be re-used. The melting is done in an electric arc furnace (EAF) which, as the name suggests, largely uses electricity. The energy requirement (as electricity) is quite small (< 4 GJ/t in the most-efficient EAFs, compared to 15-20 GJ/t to make crude steel from virgin iron ore in a blast furnace). However, scrap steel invariably has contaminants or other problems, so some pig iron or DRI has to mixed in with the scrap steel in the EAF.
Trend in best-practice EAF energy use (GJ/t) compared to theoretical and practical minimums and average use in India and US (early 2000s) Source: IIASA, Global Energy Assessment (2014)
Shaping of Steel, Traditional Method Produce steel in cubical blocks, small bars, or slabs using a continuous caster, then convert into final products using various hot mills (heating and cooling occurs between steps, with an energy loss each time) This step uses ~ 2 GJ/t, mostly as heat from burning fossil fuels
Shaping of Steel, Alternative approaches:Cast the molten steel closer to the desired final shape, using thin-slab casting, thin-strip casting, or powder metallurgy Thin-strip casting has the potential to reduce energy use for shaping by 90-95% In thin-strip casting, the length of the production line has been reduced from 500-800 m to 60 m – about a factor of ten reduction!
Steel Summary: Primary Energy Requirements • Primary Steel (from virgin ores): - 36 GJ/t world average today, assuming electricity supplied at 40% efficiency • Secondary Steel (from pure scrap): - 7 GJ/t world average today – a reduction by about a factor of 5 compared to primary steel
Steel Summary (continued): • Current average with 32% recycled steel: 26.3 GJ/t • Future average with 90% recycled steel and current best practice as average: 6.9 GJ/t This is a reduction by almost a factor of 4 • Future average with 90% recycled steel, best projected energy intensities for primary and secondary steel: 5.9 GJ/t This is a reduction by a factor of 4.5 • All of the above plus 60% electricity supply efficiency instead of 40%: 4.5 GJ/t This is a reduction by a factor of ~ 6 • Thus, the overall potential reduction in the average primary energy intensity of steel is a factor of 4.5 to 6
Production of Aluminium • Mining of bauxite (mostly Al(OH)3 and AlO(OH)) (most of the mining is through strip mining) • Refining of bauxite into alumina (Al2O3) -grinding, then digestion with caustic soda at high temperature and pressure • Smelting of alumina into aluminium, through electrolysis of alumina that has been dissolved into cryolite (Na3AlF6) at 900oC -both the cathode and anode are made of C -the net reaction is 2Al2O3+3C→4Al+3CO2, so this is the reduction step – occurs after refining rather than before it (as in production of iron)
Smelting step (continued) • Like any electrolysis process, an electrolyte is required – the material that conducts ions between the anode and cathode in order to complete an electrical circuit • The electrolyte used is CaF2, which can react at the anode (when there are problems in the operating conditions) to produce CF4 and C2F6 (referred to as PFCs or perfluorinated hydrocarbons) • PFCs are extremely strong GHGs (about 7000 and 19000 times stronger than CO2 on a molecule-by-molecule basis) and have average lifespans in the atmosphere of 50,000 and 10,000 years, respectively • Strong measures have been taken to reduce PFC emissions per tonne of primary aluminum • There are no PFC emissions in producing secondary Al (recycled from scrap)
Figure 6.12 Aluminium Mass Flow in 2005 From this diagram it can be seen that a little over 5 t bauxite are mined for every tonne of primary aluminium that is produced Source: IAI (www.world-aluminium.org)
Figure 6.14: World average electricity use for the production of aluminium
Figure 6.15 Efficiencies of individual processes in producing aluminium Source: Thekdi (2003, Aluminum 2003, The Minerals, Metals & Materials Society, 225–237)
Figure 6.16: World production of primary and secondary aluminium, and the secondary share of total production
Aluminium Primary Energy Requirements • Primary aluminium: - 193 GJ/t world average today, assuming electricity supplied at 40% efficiency • Secondary aluminium: - 17 GJ/t world average today – more than a factor of 10 smaller than for primary aluminium • Average of the above (with 18.7% recycled) is 160.3 GJ/t (more than 5 times that of steel)
Aluminium Summary (continued): • Future average with 90% secondary and current average energy use separately for primary and secondary Al: 34.5 GJ/t This is a reduction by a factor of 4.6 • Future average with 90% secondary, best projected energy intensities for primary and secondary steel: 23.3 GJ/t This is a reduction by a factor of 6.9 • All of the above plus 60% electricity supply efficiency instead of 40%: 19.1 GJ/t This is a reduction by a factor of 8.4 • Thus, the potential reduction in the average primary energy intensity of aluminium is a factor of 5 to 8
Production of Copper Metal • Copper minerals occur either as oxides (combined with CO3 or SiO2) or as sulfides (combined with S). • A given ore body tends to have oxide minerals in the upper zone (close to air) and sulfide minerals in the lower zone • There are two different production routes: Hydrometallurgy (acid related) – tends to be applied to oxide minerals Pyrometallurgy (heat related) – tends to be applied to sulfide minerals • In the transition zone of the ore body, either technique can be applied, but there has been a shift to more use of hydrometallurgy
Steps in Pyrometallurgy (1): • Extraction from the mine, crushing, and grinding • Froth flotation – mix with chemical foaming agents, as ore minerals adhere to bubbles and float to surface and can be skimmed off. Produces a concentrate of 25-30% copper, mostly CuFeS2. • Smelting – heating the concentrate in oxygen-enriched air to 1200-1250oC, with addition of silica (SiO2), partially oxidizing the Fe and S, releasing SO2 gas, and producing a molten copper matte (Cu2S*FeS) and molten slag (FeO*SiO2): CuFeS2 + O2 + SiO2→ Cu-Fe-S + FeO*SiO2 + SO2 + heat
Steps in Pyrometallurgy (2): • Converting – separating the Cu2S from the FeS in the copper matte and oxidizing the S. Produces blister copper (99% copper) and further iron slag Cu2S + O2 → 2Cu° + 2SO2 + heat 2FeS + 3O2 + SiO2 → 2FeO•SiO2 + 2SO2 + heat The smelting and converting reactions are exothermic –and the heat released is sufficient to maintain the required temperature once the process has started. This eliminates the need for fuel energy in state-of-the-art smelters & converters, but requires continuous rather than batch processing.
Steps in Pyrometallurgy (3): • Fire refining. This is a process for removing most of the remaining O and S and, like previous steps, is carried out at a temperature of about 1200oC. The O is removed as CO2 through reaction with a hydrocarbon reducing agent (typically 5-7 kg per tonne of copper), while S is removed as SO2 through reaction with atmospheric oxygen. Fire refining is carried out in special rotating furnaces that are heated by combusting hydrocarbon fuels. The liquid product is directly cast into thin anodes that are interleaved with cathodes in electro-refining cells. The copper anodes still contain about 0.15% O and 0.002% S.
Steps in Pyrometallurgy (4): • Electro-refining. This is an electrolysis process that involves electrochemically dissolving copper from impure copper anodes into a CuSO2-H2SO4-H2O electrolyte and electroplating pure copper from the electrolyte onto a cathode without the impurities. After 7-14 days the cathodes are removed from the cell and the pure metal is scrapped off. The reactions are: Cuºanode → Cu 2+ + 2e- at the anode, and Cu 2+ + 2e- → Cuº at the cathode.
Steps in Hydrometallurgy: • Leaching – excavate ores, pile in a heap, and add acid to dissolve the ore, or drill holes into the ore body and pump in acid, and weeks to months later, pump out the leachate • Concentration – add organic solvents to the acid solution, to selectively absorb copper from the solution • Refining – an electrolysis process called electro-winning, similar to electro-refining except that the anode consists of an inert Pb-Sn-Ca mixture. The copper is electroplated onto the cathode from the Cu solution supplied from the concentration step rather than supplied by dissolution of a copper anode. The combination of acid leaching and electro-winning is called the solvent-extraction electro-winning process, or SX-EW.
