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Evaluation of new equipments for utilization of waste heat in sponge iron industry

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Abstract

The present work investigates the design modifications which can lead to efficient energy integration in coal-based sponge iron plant with a capacity of 500 t/day. For the present energy integration investigations, two scenarios: 1 and 2 are proposed and compared with the existing one. During the operation in coal-based sponge iron plant, a tremendous amount of heat is generated and a significant part of this heat, associated with the waste gas, remains unutilized. In scenario 1, the feed materials are preheated in a rotary drier with waste gas outside the rotary kiln. As a result of preheating, the intake capacity of the kiln is enhanced. The author takes this opportunity and increased the feed rate of iron ore by 10 % in scenario 2. For the purpose of energy integration, design and development of duct carrying waste gas and rotary drier is carried out. The comparative study based on capital investment, coal and energy consumption, water requirement, profit, and payback period, shows that scenario 2 is the best one. In scenario 2, production rate is increased by 10 % which consumes 17 % less coal, generates 24 % less waste gas, and consumes 30 % less water to produce 1 t direct reduced iron than that of the existing one and gives a profit of $31,000/day with a payback period of 34 days.

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Abbreviations

A :

Heat transfer surface area of the duct/drier, m2

C :

Specific heat capacity of waste gas, J/kg °C

D :

Internal diameter of the duct/drier (meters)

d :

Mean particle diameter of the feed materials (micrometers)

F :

Flow rate of the feed materials (kilograms per hour)

G :

Drying gas flow rate (kilograms per hour)

g :

Earth acceleration constant (meters per square second)

h :

Individual heat transfer coefficient (watts per squared meter Kelvin)

k :

Thermal conductivity of the liner/duct/drier plate (watts per meter Kelvin)

L :

Length of the duct/drier (meters)

\( \overset{\cdotp }{m} \) :

Mass flow rate of the waste gas through the drier/duct (cubic meter per hour/kilograms per seconds)

m :

Mass flow rate of iron ore/coal/air in the kiln (tons per hour)

N :

Rotational speed of the drier (revolutions per minute)

Nu :

Nusselt number

n :

Number of refractories including the duct plate

Pr :

Prandlt number

Q :

Heat lost during the waste gas flowing through the duct (watts)

r :

Inside radius of the duct/ drier (meters)

R :

Drying rate of feed materials inside the drier (kilograms H2O per kilogram dry matter per hour)

Re :

Reynold’s number

T :

Temperature (degrees Celsius)

\( \overline{T} \) :

Average temperature of waste gas flowing inside the duct/drier (degrees Celsius)

t :

Tonne/Thickness of the duct plate including liner of the duct (meters)

U :

Overall heat loss coefficient from within the duct/drier to outside (watts per squared meter Kelvin)

\( \overset{\cdotp }{v} \) :

Volumetric flow rate of the waste gas flowing through the duct/drier of radius (r; cubic meter per second)

v :

Velocity of the waste gas inside the duct/drier (meters per second)

x :

Along the length of the duct (meters)

z :

Mean free falling distance (meters)

X :

Ratio of feed materials volume to drier volume

a :

Ambient

b :

Bulk

coa:

Convective heat transfer coefficient from outer surface of the duct/drier to outside atmosphere

di:

Duct plate including liner of the duct

d :

Drier

f :

Waste gas

gl:

Heat transfer coefficient from waste gas to the duct lining (watts per square meter Kelvin)

i :

Inside/inlet

L :

Loss

o :

Outside/outlet

p :

Particle

roa:

Radiative heat transfer coefficient from outer surface of duct/drier to outside atmosphere

r :

Residence

s :

Soaking

υ :

Kinematic viscosity (square meters per second)

α :

Slope of the drier (degrees)

ρ :

Density (kilograms per cubic meter)

τ :

Time (minutes or seconds)

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Authors and Affiliations

  1. National Institute of Technology Jamshedpur, Jharkhand, 831014, India

    Anil K. Prasad, Nishant R. Dey & Shravan K. Singh

Authors
  1. Anil K. Prasad
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  2. Nishant R. Dey
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  3. Shravan K. Singh
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Corresponding author

Correspondence to Anil K. Prasad.

Appendixes

Appendixes

Appendix A: calculation of U L for the duct

The overall heat transfer coefficient is given below is nothing but Eq. 2

$$ {U}_L={\left[\frac{1}{h_{\mathrm{gl}}}+{\displaystyle \sum \left\{\frac{t_{\mathrm{di}}}{k_{\mathrm{di}}}+\frac{1}{{h_{\mathrm{ro}}}_{\mathrm{a}}+\frac{1}{h_{\mathrm{coa}}}}\right\}}\right]}^{-1} $$

Value of h gl, h coa, and h roa

It is the heat transfer coefficient from waste gas to duct lining which is a function of Re and Pr.

$$ \mathrm{Re}=\frac{D_i\;\upsilon }{\upsilon } $$

Where, υ = 131.8 × 10−6 m2/s and D i  = 1.2 m;

For flow rate of waste gas = 54,400 m3/h,

$$ v=\frac{54,400}{1.131\times 3600}=13.4\;\mathrm{m}/\mathrm{s} $$

From the above, the figure of Re is computed as 1.12 × 105 which indicates that the flow of the waste gas inside the duct is turbulent. Hence, the following expression is used to evaluate the figure of h gl:

$$ Nu=0.023\;R{e}^{0.8}P{r}^{0.33};\mathrm{and}\;{h}_{gl}= Nu{k}_f/{D}_i, $$

Considering Pr and k f to a value of 0.6 and 0.09 W/m °C at the 800 °C, respectively, the value of h gl comes to be 16 W/m2 °C. The values of h coa and h roa are considered to be 5 W/m2 °C (Arora and Domkundwar 1987).

Value of k di

It is predicted by considering heat loss through the duct which is given as:

$$ {Q}_L=\frac{\overline{\mathrm{T}}-{\mathrm{T}}_{\mathrm{a}}}{\frac{1}{{\overline{h}}_{gl}2\pi {r}_iL}+\frac{ \log {r}_o/{r}_i}{2\pi L{k}_{\mathrm{di}}}+\frac{1}{{\overline{h}}_{\mathrm{coa}}2\pi {r}_oL}} $$

Further, Q L  = m · f c f (T fi  − T fo )

Considering c f , ρ f , t di, and L to a figure of 12,60 J/kg °C, 1.002 kg/m3, 0.225 m, and 43 m, respectively, the value of k di is found to be 30.58 W/m °C. The properties of the waste gas are computed at the average temperature of 800 °C (Arora and Domkundwar 1987). Putting all the above values in Eq. 2; U L is computed to a figure of 3.8 W/m2 °C at 850 °C, and 3.9 W/m2 °C at 900 °C.

Appendix B: cost analysis

Capital cost

All data of cost are taken from different manufacturing units to evaluate the cost of rotary drier and its accessories.

Cost of duct from ABC outlet to rotary drier

The density, weight, and cost of duct material are 7,876 kg/m3, 10.24 t, $717/t, and cost of bricks used in the duct as $38,266 for 2,780 number bricks and commissioning charges as $8,333, the total cost for installation of duct are estimated $54,900. Cost1 = $54,900.

Cost of returning duct (from rotary drier to ESP input)

The length, thickness, and internal diameter of non-insulated returning duct is 20 m, 6 mm, and 1.6 m. Its capital cost is computed in a similar manner as above and found $4,083. Cost2 = $4,083.

Cost of rotary drier with all accessories

Considering cost of rotary drier with accessories such as girth gears, support rollers, drier mounted instruments, weigh feeders, conveyors, and vibrofeeder to an amount $716,666, cost of induction motor used to drive the drier (capacity, 55 kW) as $36,666, cost of gear box to drive girth gear as $51,666, installation cost of rotary drier as $12500; total cost of rotary drier with all accessories are estimated as $817,500. Cost3 = $817,500.

Cost of ID fan with all accessories

Considering cost of ID fan of capacity 60,000 m3/h to an amount $53,333, cost of three phase induction motor for ID fan of rating 39 kW as $33,333, cost of bearing for ID fan as $9,166, cost of coupling used to couple fan and motor as $1,283, approximate installation cost as $125, approximate cost of electrical works including material cost as $43,333; total cost of ID fan are estimated as $140,575. Cost4 = $140,575.

Erection cost

Considering cost of concrete work for 800 m3, structural erection of 320 t and equipment cost to an amount $8,600, $17,600, and $8,800, respectively; total cost of erection work comes to an amount $35,000. Cost5 = $35,000.

Total capital cost for modification

$$ {\mathrm{Cost}}_1+{\mathrm{Cost}}_2+{\mathrm{Cost}}_3+{\mathrm{Cost}}_4+{\mathrm{Cost}}_5=\$1,052,000. $$

Operating cost

Comparative analysis of production cost and selling price of 1 t DRI for existing one and scenarios 1and 2

All data of cost to prepare the Tables 8 and 9 are taken from a typical sponge iron plant in India of capacity 500 t/day operating with SL/RN process. The cost analysis of the DRI in existing one is presented in Table 8, which shows that production cost of 1 t DRI is $272. In similar lines, the production costs of DRI in scenarios 1 and 2 are computed to an amount of $261 and $253, respectively. The selling price for different sizes of DRI is mentioned in Table 9 which is $316. From above, profit to sell 1 t DRI for existing one, scenarios 1 and 2 are coming to an amount $44, $55, and $63, respectively.

Table 8 Production cost to produce 1 t DRI in existing one
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Table 9 Average selling price of DRI
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Prasad, A.K., Dey, N.R. & Singh, S.K. Evaluation of new equipments for utilization of waste heat in sponge iron industry. Energy Efficiency 7, 591–608 (2014). https://doi.org/10.1007/s12053-013-9242-6

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