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Energy & Exergy Analyses of Direct Ironsmelting Processes - Coggle…
Energy & Exergy Analyses of Direct Ironsmelting Processes
1- Introduction
Consumption of energy in iron & steel industry is among the largest in the industrial world.
:fountain_pen: Energy (30%) = production cost of steel
:fountain_pen: Exergy (40%) = ironmaking processes
Energy loss
low
,
energy consumption
low
>>
ironmaking effieciency
high
:pen:
1st Thermodynamic Law
(energy management): wastes low, energy efficiency high
:pen:
2nd Thermodynamic Law:
higher level of energy savings including analysis of exergy
Concept of exergy:
to characterize the quality of energy and its ability to perform work.
Energy performance
of the smelting reduction furnace depends on;
:check: post-combustion ratio
:check: heat transfer efficiency
:check: off-gas temperature
6- Conclusion
Industrial processes are
irreversible
& are always accompanied by
exergy losses.
Exergy depends on PCR & HTE:
: :pen:
destroyed:
5460 MJ/THM
:pen:
consumed:
5000-9200 MJ/THM
:fountain_pen: Direct ironsmelting processes (vs blast furnace ironmaking) >> farther from equilibrium
:fountain_pen: Energy & exergy off-gas
constitute
> 1/2 energy & exergy
supplied
to the smelting reduction furnace
If
off-gas utilisation is low
,
fuel efficiency
can be
enhanced
by:
:check:
increase
heat transfer efficiency
:check:
increase
post-combustion ratio
2- Process Irreversibility, Exergy Loss & Production Costs
Process without exergy loss or reversible process is the most exergy efficient process
All process are non-equilibrium and irreversible
The higher the energy efficiency, the lower reaction rate and eventually will decrease the energy loss.
Also, the higher reactor volume, the lower reaction rate. :
Figure 1 shows the graph of production cost against entropy production. The graph tells that when there's an increment of irreversibility of a process and entropy production, then it will cause the decreasing in fuel efficiency as well as its cost while the capital cost decreases.
If fuel cheap- overall production cost will decrease with rising entropy production.
If fuel expensive- highly irreversible process with high entropy production above a certain ideal value will be less cost-efficient.
However in industrial technology, there are other factors which affect production cost too.
Energy loss in ironmaking process is caused by
fuel combustion
heat transfer
reduction reactions
3- Blast Furnace Ironmaking
Table 1 shows the major item of energy and exergy balances for blast furnace ironmaking.
--> THM - ton hot metal
A quarter of the total blast furnace gas energy is used for blast heating.
However it is not included in Table 1 since blast stoves can only be seen as an internal process for energy recovery
37.4 % of energy
32.4 % of exergy
above are outputs with hot metal
The efficiencies are high compared to general chemical process.
It can be proven when efficiency of energy and exergy of blast furnace gas exported for external use reach 67.1 % and 61 % when chemical energy and exergy are included.
The rest of energy is rejected with waste material as well as various heat losses.
Energy which corresponding to exergy accounts- 19.3 %
19.7 % of exergy is destroyed in irreversible process.
Major irreversible process in blast furnace ironmaking:
1) Coke combustion
2) Heat transfer from combustion gas (2270K - 2470K) to burden (1770K-1820K).
3) Formation of metal and slag solutions
4) Blast furnace gas and heat transfer in blast stoves
4- Direct Ironsmelting Process
Figure 2 shows
Direct Iron Ore Smelting
(DIOS) from Nippon Steel Corporation.
Iron ores and fluxes are being added from top of reactor before dissolve into molten slag.
By using carbon dissolved in metal and char in slag --> iron oxide together with other mineral oxides being reduced from the slag.
Amount of temperature of off-gas higher than direct iron smelting process
DIOS really depends on post- combustion ratio and heat transfer efficiency
:fire: Heat
Heat is required for reduction and smelting process
It produced by combustion of CO and Hydrogen in the gas phase
In addition, the Nitrogen also being used where it blown from the bottom as to produce stirring effect.
📌This method involved irreversible process
📌 No reaction in reversible form
The irreversible process include
Coal combustion
Process gas combustion
Heat transfer from gas phase (2070 - 2370K) to bath (1720 - 1820K)
Iron ore dissolution into slag
Iron oxide reduction
Formation metal and slag solution
5- Exergy Loss in Direct Ironsmelting
Figure 8 to Figure 10 shows exergy loss when off gas is not utilised at different HTE
Exergy is calculated with assuming off-gas cooled at 1373K
Conducted in reversible process
At PCR 0.4-0.6 and HTE 0.7-0.8 --> energy & exergy
off gas > requirement for smelting process
Importance of Utilisation of Off Gas
Half energy and exergy of coal can be consumed
Without it, recovery of energy and exergy is low
Can enhance overall fuel effiency
Reduction and Combustion Reaction
Volumentrically expensive
Increase in reaction pressure, decrease irreversibility of reaction so exergy loss is decrease
Ways on Energy Saving
Recovery of heat of high-temp waste from ironmaking
Do feasibility study of methanol synthesis to
--> recover chemical exergy blast of furnace gas
--> decrease greenhouse gas emission
Resulting in 9% of total exergy consumption decreased
Data to calculate exergy (Standard exergy: Szargut et al) (Heat capacity: Knacke et al)
Coal consumption & off-gas exergy has correlation
Enthalpy and exergy of off-gas depend on post-combustion ratio and heat transfer efficiency.
Higher enthalpy/exergy of off-gas, higher the coal consumption.
Exergy loss= Exergy of products - exergy of coal
Cause of exergy loss is irreversible of reduction reaction, combustion and heat transfer.
Gas consumption
--->pre-heat of iron ore / blast
---> electric power generation exportation
Before gas is used, off-gas is clean up & cool down (1070-1370K)
However, exergy loss increase.