Abstract
Steelworks require large amount of energy in reduction, fusion and refining processes. A mini-mill is a steelwork that produces steel by melting scrap metal, pig iron and metallic elements in electric arc furnaces. Depending on the desired product, the refining process requires vacuum degassing to remove contaminant gases from the liquid steel. The vacuum in the degassing process can be obtained through steam ejectors, which use superheated steam. Furthermore, environmental legislation requires mini-mills to have dedusting systems over the electric furnaces. In general, a dedusting system generates high flow rates of hot off-gases, which indicates an interesting potential for heat recovering. In this paper, a cogeneration plant to recover heat from the dedusting system of a Brazilian mini-mill is proposed. The actual operation data are considered to calculate the heat available and to conceptualize a bottoming cogeneration plant that generates electric power and superheated steam for the ejectors of the degassing process. Results show that the proposed plant can generate 45.4% of the steam required by the ejectors and up to 2.4% of the power required by the electric arc furnace. Also, the heat recovering from the dedusting system reduces the use of cooling water by 29.3%. From an economic viewpoint, the cogeneration plant decreased the expenses for power, steam and cooling water by 1.5, 32 and 29%, respectively. Overall, there was an expense reduction of 4.8%, resulting in a payback period of 4.1 years in the case base. For a projected best case scenario, the payback period is about 2.3 years.
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Notes
\({\upeta }_{\text{cl}} = { }\left( {{{\dot{W}}}_\text{e} - {{\dot{W}}}_\text{p} } \right)/{q}_{\rm g} .\)
Abbreviations
- \({b}_{\rm p}\) :
-
Bypass ratio [–]
- C :
-
Specific cost [USD/MWh]
- c p :
-
Specific heat [kJ/kgK]
- D t :
-
Tube diameter [m]
- \({E}\) :
-
Expense [USD/year]
- \({f}\) :
-
Annuity factor [–]
- \({H}\) :
-
Annual operating time [h]
- \({h}\) :
-
Specific enthalpy [kJ/kg]
- \({I}\) :
-
Investment cost [USD]
- \({j}\) :
-
Annual interest rate [–]
- \({k}\) :
-
Plant useful life [years]
- \({L}\) :
-
Length [m]
- \({m}\) :
-
Mass [kg]
- \({{\dot{m}}}\) :
-
Mass flow [kg/s]
- \({N}_{\rm t}\) :
-
Number of tubes [–]
- \({P}\) :
-
Pressure [kPa]
- \({\text{pb}}\) :
-
Payback [year]
- \({q}\) :
-
Heat transfer rate [kW]
- \({R}\) :
-
Revenue [USD/year]
- \({r}_{\rm ev}\) :
-
Evaporation rate [–]
- \({s}\) :
-
Specific entropy [kJ/kgK]
- T :
-
Temperature [°C]
- t :
-
Time [s]
- \(\dot{W}\) :
-
Work transfer rate, power [kW]
- Δ:
-
Difference
- η :
-
Efficiency [–]
- ρ :
-
Specific mass [kg/m]
- ap:
-
Approach point
- b:
-
Gas-fired boiler
- c:
-
Colder than average condenser
- cl:
-
Cycle
- cur:
-
Current situation
- cw:
-
Cooling water
- d:
-
Degassing process
- e:
-
Electric energy
- ec:
-
Economizer section
- ev:
-
Evaporator section
- g:
-
Off-gas power generator
- h:
-
Hotter than average
- he:
-
Heat exchanger
- i:
-
Inlet
- i,j,k:
-
Indexes, counting variables
- l:
-
Loss
- ng:
-
Natural gas
- o:
-
Outlet
- om:
-
Operation and maintenance
- p:
-
Pump
- pp:
-
Pinch point
- sat:
-
Saturation
- sg:
-
(heat recovery) Steam generator
- sh:
-
Superheater section
- sl:
-
Salt
- st:
-
Steam
- t:
-
Turbine
- tr:
-
Thermal reservoir
- cg:
-
Cogeneration
- off:
-
Ejector off
- on:
-
Ejector on [–]
- ABNT:
-
Brazilian Association of Tech. Standards
- EAF:
-
Electric arc furnace
- HRSG:
-
Heat recovering steam generator
- LHV:
-
Lower heating value
- NBR:
-
Brazilian standard
- O&M:
-
Operation and maintenance
- TR:
-
Thermal reservoir
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e Silva, C.A.A., Matelli, J.A. Heat recovery of dedusting systems in electric arc furnaces: concept of a bottoming cogeneration plant and techno-economic analysis. J Braz. Soc. Mech. Sci. Eng. 40, 5 (2018). https://doi.org/10.1007/s40430-017-0919-1
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DOI: https://doi.org/10.1007/s40430-017-0919-1