Consideración de Nuevas Inversiones

The amount of electrical and other energy inputs (oxygen, natural gas, carbon) will vary from furnace to furnace and between different melts in the same furnace depending on the amount and type of scrap metal used. In this case study, the energy consumption figures were not available for commercial reasons, hence typical figures required for the analysis have been sought from the literature. The majority of heat input is from electricity, but there are also significant chemical energy inputs (mainly oxygen and natural gas) as shown in Sankey diagrams of Figure 5-9 for typical EAF operations.

Figure 5-9: Energy balances for a typical modern electric arc furnace. Sources: (a) Schliephake et al. (2011); (b) Jones (1997).

Around 50- 55% of the energy input leaves as sensible heat energy in the steel and Figure 5-9 (a) shows that around half of the energy escaping in the off-gas is chemical energy, mainly carbon monoxide and

Chapter 5 Industrial Heat Recovery hydrogen produced in the furnace. Higher electrical inputs for some furnaces are often due to the lower use of oxygen (Jones, 1997). Figures for electricity consumption from the literature show some

variation with examples given in Table 5-1.

Table 5-1: Electrical energy input for electric arc furnaces. Note: tls = tonne of liquid steel output.

Arc furnace electrical energy input (kWh/tls) Reference

404 – 748 European Commission (2013)

360 – 400 Jones (1997)

560 – 680 Environment Agency (2004)

400 Zuliani et al. (2010)

The UK’s four EAF sites are listed in Table 5-2 with their production capacities, recent actual production, and 2004 carbon dioxide emissions figures. Together, the EAF sites in Sheffield and

Rotherham produce over a million tonnes of steel per year. The UK produced 1,942,000 tonnes of crude steel via the EAF route in 2013 (Worldsteel Association, 2014), equivalent to 51% of the overall

production capacity listed in Table 5-2.

Table 5-2: Electric Arc Furnace plants in the UK with production and emission statistics.

Source: Capacities from McKenna (2009), emissions from Entec (2006), production levels from Koh et al. (2011), ABB (2011), ABB (2014) and Celsa (2012).

Operator Melt Shop capacity (kt/year)

Melt Shop recent production (kt/year)

CO2 Emissions in 2004 (kt

CO2/year)

Celsa UK, Cardiff 1,200 1,140 101,500*

Outokumpu, Sheffield 540 260 50,800

Tata Steel**, Rotherham 1,250 750 298,649

Forgemasters, Sheffield 130 60.2 69,162

Total 3,840 2,210 550,877

* - Since 2006 rebuild, new off-gas analysis saves 18,000 t CO2/year (Celsa, 2012). ** – Formerly Corus UK Ltd.

Dryden (1975) illustrated a typical power input programme for an arc furnace as shown in Figure 5-10. The suspension of power input is usually to allow the addition of more scrap to the furnace or for sampling of the steel to occur. Dynamic energy balance models were generated by Kleimt et al. (2005) showing the energy losses to each waste heat stream including cooling water and their variation though time. Example results for one modelled cycle are shown in Figure 5-11 and Figure 5-12.

Figure 5-10: Typical power input programme for an arc furnace. Source: Dryden (1975).

Figure 5-11: Modelled energy inputs for an arc furnace cycle. Source: Kleimt et al. (2005).

Figure 5-12: Modelled energy output variation for an arc furnace cycle. Source: Kleimt et al. (2005).

EAF flue gases

A schematic of the de-dusting system for the EAF is shown in Figure 5-13, with most of the flue gas flowing to a primary duct while escaping gases are captured by an extraction canopy inside the building. Gas velocities in the main duct can be up to 40 m/s while the secondary duct can capture as much as 30% of the flue gas energy (Kirschen et al., 2006). There can be a heavy dust load in the off- gas fumes from 9.5g/m3 _{(Brandt et al., 2014) up to 200 g/m}3 _{(Dixon and Bramfoot, 1985) before flue}

gases reach the on-site plant for filtering and treatment.

Chapter 5 Industrial Heat Recovery The amount of oxygen and natural gas injected into the furnace has a big effect on the volume of off- gases and so is important for energy losses and heat recovery (Pfeifer and Kirschen, 2002). Temperature peaks for off-gas correlate with concentration of carbon monoxide and the occurrence of post-

combustion reactions described by equations 3.16 and 5.55 (Kirschen et al., 2006). Post-combustion of gases leaving the furnace can liberate some of this chemical energy raising it to well over 25% of the energy input (Zuliani et al, 2010).

CO + 0.5O2 → CO2 ∆H=-282.98 kJ/mol (5.54)

H2+ 0.5O2→ H2O ∆H=-241.81 kJ/mol (5.55)

Born and Granderath (2013b) estimate that off gases contain 170 to 240 kWh of energy per tonne of liquid steel depending upon the level of chemical energy input (i.e. natural gas and oxygen injection volumes). Further details on flue gas properties are noted by Brand et al. (2014).

Conventional EAFs have water-cooled flue ducts that bring gas temperatures down to around 700o_C

(Zuliani et al., 2010); one such system uses pressurised hot water at 220o_{C and 23 bars for evaporative}

cooling to recover 30-37 kWh energy per tonne of steel with high efficiency (Kirschen et al., 2006). Most steelworks inject water spray into the flue gas while others inject excess air to lower the temperature and reduce the risk of dioxin production (Born and Granderath, 2013b). The solution chosen depends on various factors including the filtering methods used for dust capture (Kirschen et al., 2006).

Dixon and Bramfoot (1985) recorded temperatures in a UK-based EAF flue duct over a melt lasting approximately two hours. The gas temperatures were highly variable as shown in the first graph of Figure 5-14. The period of low flue gas temperatures around 50 minutes corresponds to the adding of a second basket of scrap metal (ibid.). The melting practice will vary between different steel mills and perhaps between melts.

Figure 5-14: Measured flue gas properties during a cycle of a 180t British Steel furnace. Source: Dixon and Bramfoot (1985).

EAF scrap preheating

Commonly, but not in the UK, the sensible heat in the EAF off-gas is used to preheat scrap to around 800°C; this can save 100 kWh of energy consumption per tonne of steel produced (European

Commission, 2013). It lowers energy inputs by 15-20%, and helps to achieve shorter furnace cycle times (Jones, 1997). Scrap preheating can cause problems with formation of dioxin gases from warm scrap metal. A residence time for off gases of at least two seconds over 800°C with oxygen over 6% is deemed necessary to destroy dioxins, with the chance of reformation between 550 and 250°C if cooling rate is below 300°C/s (Frittella et al., 2015). The mechanisms for reformation were identified by Taylor and Lenoir (2001).

Scrap preheating techniques include basket preheaters, the rotary kiln process and the twin shell method (Bramfoot et al., 1985). Experience has shown the need to avoid overheating of the scrap basket to prevent welding and bridging in the charge (ibid.) and the savings represent some 10 to 25% of the energy in the flue gas. Reasons for not preheating scrap can include inconvenience in the shape of scrap and concerns over side-effects on the treatment of furnace off-gases.

EAF cooling water

Recovering heat from cooling water will usually require the use of heat pumps to raise temperatures, as explained in Section 5.3.4. However, there are examples where heat exchangers have been used to direct heat from cooling water to DH. For example, in Graz, Austria heat is recovered from a gas-fired reheat furnace and cooling water at around 90°C from two electric arc furnaces (Schlemmer, 2011). This project was recovering 40 GWh of heat per year, and with a 67m3 _{buffer this could be raised to 60}

GWh per year (ibid.). One foundry in Alvesta, Sweden is using a lower-temperature parallel DH network in order to deliver 7-8 GWh of heat caught from cooling towers as metals cool (SFS AB, 2002).