Current status and potential of biomass utilization in ferrous metallurgical industry

Renewable and Sustainable Energy Reviews 68 (2017) 511–524

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Current status and potential of biomass utilization in ferrous metallurgical industry ⁎

Rufei Weia,b,c, Lingling Zhanga, , Daqiang Cangd, Jiaxin Lib, Xianwei Lie, Chunbao Charles Xub,c,

⁎

a

School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China School of Metallurgical Engineering. Anhui University of Technology, Ma’anshan 243002, China Institute for Chemicals and Fuels from Alternative Resources (ICFAR), Department of Chemical and Biochemical Engineering, Western University, Ontario, Canada N6A 5B9 d School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China e Baosteel Research Institute, Baoshan Iron & Steel Co., Ltd., Shanghai 201900, China b c

A R T I C L E I N F O

A BS T RAC T

Keywords: Biomass Bioenergy Utilization Iron making and steel making Greenhouse gases Environmental performance

This paper provides a critical review on the current status and potential of biomass utilization in ferrous metallurgical processes, i.e., the blast furnace (BF) – basic oxygen furnace (BOF) route, the direct reduction (DR) – electric arc furnace (EAF) route, the scrap – EAF route and the other routes. In the BF-BOF route, biomass can be used as a fuel for iron ore sintering, or as a raw material for the production of bio-coke, and utilized for blast furnace injection. In the DR-EAF route, direct reduction iron can be produced form iron ore and biomass pellet. In the scrap – EAF route, biomass can be utilized in EAF through a cogeneration system. In addition, biomass can be utilized in magnetic separation of refractory low-grade iron ore, in reuse of iron and steel slag, or as an adsorbent for pollutant control, etc. The challenges and outlook of biomass utilization in metallurgical industry are also discussed in this paper.

1. Introduction Steel production has increased significantly in the past decade, and more than 1.65 billion tonnes of steel were manufactured worldwide in 2013, among which close to 70% was produced in Asia and Oceania with approx. 20% from the European Union and North American countries [1]. In some developing countries, such as countries in Latin America, Asia, Africa and the Indian sub-continent, steel production is still expected to grow [2]. Based on the raw materials utilized and technologies, there are two main routes for steel production [3,4], as shown in Fig. 1. The first route is the primary steel production route using iron ore as main raw material and including the blast furnace (BF) – basic oxygen furnace (BOF) processes [5], the smelting reduction (SR) – direct reduction direct reduction basic oxygen furnace (BOF) processes [6] or the direct reduction (DR) – electric arc furnace (EAF) processes [7]. The second route for steel production uses scrap as the main raw material involving the electric arc furnace (EAF) process [8]. At present, steel production in many countries, such as in China and Japan, mainly uses iron ore as raw materials via the BF – BOF route. Driven by the coupled challenges of the resources availability and the environmental concerns, the SR – BOF and DR – EAF routes have been evolved in recent years, as well as ⁎

the Scrap – EAF route due to the constant accumulation of scrap. Currently, the dominant energy sources in these routes are fossil fuels (e.g. coal, coke and natural gas), contributing to a great amount of CO2 emission. In fact, iron and steel industry is one of the highest energy and emission intensive sectors [9–11]. For instance, iron and steel sector accounts for about 5% of the total CO2 emissions. In particular, steel production via the BF-BOF route consumes 13–14 GJ/ MT tonne of steel produced, accompanied with 1.9 t of CO2 emitted, which has rendered enormous challenges for the iron and steel industry and increased the pressure in seeking clean and renewable energy sources for the industry [12]. Biomass is clean and renewable energy which can partially substitute fossil energy directly, or can be converted into gas, liquid, solid fuels and other chemicals or materials [13,14]. Thus, biomass research has received growing interest due to the increased concerns over fossil resources depletion and enormous environmental issues associated with the use of fossil fuels [15]. In the 21st century, biomass has found more applications in the iron and steel metallurgical processes, mainly as fuels and reducing agents for various metallurgical processes, as overviewed below.

Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (C.C. Xu).

http://dx.doi.org/10.1016/j.rser.2016.10.013 Received 11 February 2016; Received in revised form 22 September 2016; Accepted 14 October 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

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form the loose biomass feedstock into pellets of a certain shape and density in order to reduce the transport costs, increase the combustion intensity and thermal efficiency. Chemical conversion includes transesterification (for bio-diesel production from fat, lipids or oils) and thermochemical conversion of lignocellulosic biomass [23]. The latter produces charcoal, tar (bio-oils) and combustible gases and other highgrade energy products [24]. Biochemical conversion proceeds via the activities of microorganisms mainly in hydrolysis, fermentation, enzymatic synthesis and photosynthesis, producing ethanol, methane gas, bio-diesel, hydrogen and other products [22]. Solid fuel produced by physical conversion (e.g., wood pellets) can be injected into the BF as a fuel or reducing agent [25]. The synthesis gas with a high content of reducing gases (H2 and CO) as well as charcoal and bio-oil (including tar), generated from thermochemical and biochemical conversions, can be directly applied as a fuel or a reducing agent in the BF [4,26,27]. In addition, some new metallurgical technologies using biomass is under development, such as the combined generation technology for co-production of iron and highheating value gas (CO or H2)[28]. 2.3. Biomass pre-treatment Fig. 1. Overview of iron and steel production processes.

Biomass pre-treatment aims to make the biomass polymers more accessible for subsequent processes, e.g. pyrolysis, hydrolysis and densification processes. Torrefaction is a pre-treatment process that removes moisture and some volatile, and improves biomass energy density and grindability. Biomass torrefaction is commonly operated at a low temperature ranging from 250 to 400 °C and medium residence time (15– 30 min), producing a solid residue (i.e., torrefied biomass) with a high mass yield up to 87.5%. When the temperature of torrefaction is very high, over 400 ℃, the LHV of the produced torrefied wood approaches to 26–27 MJ/kg [29]. Moreover, torrefaction of biomass can also improve the quality of syngas in biomass gasification: torrefied woods was found to produce approximately the same quantities of CO2, 7% more H2 and 20% more CO than the parent wood under the same gasification conditions [30]. Syngas with high H2 and CO content is good for reducing iron oxide and can be used in the DR processes. Although steam explosion is a commonly used pre-treatment process for cellulosic ethanol production (treating biomass with hot steam (180–240 °C) under pressure (1–3.5 MPa) followed by explosive decompression), studies have shown that steam explosion of a biomass feedstock can improve the rate, the heat value and charcoal yield during pyrolysis. Han et al. [31] reported that pellets of steam explosion pre-treated straw achieved higher metallization than charcoal based pellets for DRI production.

2. Biomass properties and conversion 2.1. Biomass properties Biomass comprises mainly carbon(C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur(S) as the five organic elements, as well as inorganic elements such as Al, Si, K, Ca, Na, etc. in its ash. The composition of biomass varies depending on the biomass varieties. The ultimate and proximate analyses of different types of biomass are shown in Table 1. Overall, biomass has average C, H, O, N and S contents of 49.3%, 6.0%, 40.5%, 0.8% and 0.2%, respectively, and average volatile matter (VM), fixed carbon (FC) and ash contents (A) of 18.2%, 77.0% and 4.8%, respectively. Irrespective of ultimate analysis or proximate analysis, there are tremendous differences between biomass and the fossil fuels commonly used in iron and steelmaking processes (e.g., coal, coke and heavy oil), as summarized in Table 2. The contents of C, H, and O (on dry basis) can be used for estimating the heating value of fuels via some empirical formula such as the Dulong formula (HHV (MJ/kg) =0.3383 C +1.422 (H-O/8)). A high oxygen content in biomass along with a high water content result in a much lower energy content (usually 14–21 MJ/kg) for biomass compared with that of a fossil fuel (~30 MJ/kg for coal and ~40 MJ/kg for heavy oil) containing generally an oxygen content of less than 5.5 wt% [14]. However, the sulfur content in the biomass (generally ≪1 wt%) is lower than that of fossil fuels (up to 3–6 wt%) [16,17], rendering the environmental dividends by replacing fossil energy with biomass and bioenergy in ferrous metallurgical industry. Proximate composition is very important as it affects the combustion and reduction phenomena of the fuels. Compared with fossil fuels, low ash and high volatility characteristics for biomass offer many advantages in utilizing biomass as a fuel or reducing agent in the ferrous metallurgical processes [18], while biomass's low fixed carbon content and low heating value are disadvantageous to be as a reducing agent and fuel for the traditional ironmaking processes, as will be discussed below.

3. Biomass applications in BF – BOF processes BF – BOF processes is the major route for iron and steel making, produce 64.4% of crude steel, but are associated with high energy consumption and emissions from the sintering, coking and BF processes [1]. In order to reduce BF coke rate and CO2 emissions, significant amount of effort has been in exploring biomass applications in the BF – BOF processes, as detailed below. 3.1. Injection of charcoal into blast furnaces

2.2. Biomass conversion

BF is one of the major emission sources in steel production, and the use of biomass in this process could reduce 22–32% of carbon dioxide emissions [32]. Biomass can be injected into BF in three forms, i.e., biomass char, bio-oils and biomass-derived synthesis gas (syngas) [25,33,34]. For the economy, operability and other reasons, the previous studies are mainly on injection of charcoal into BF [19,32,35–37]. Charcoal is not suitable for replacing lump coke directly in a blast furnace due to its insufficient strength compared with coke

Biomass can be converted into various energy and solid, liquid and gaseous fuel products via three main processes, i.e., physical conversion, chemical or thermochemical conversion, and biochemical conversion. Thermochemical biomass conversion involves direct combustion, gasification, liquefaction and pyrolysis [22]. Physical conversion involves process such as palletization to trans512

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Table 1 Ultimate analysis of different biomass [14,16,17]. Biomass

Alder fill sawdust Alfalfa stalk Almond hulls Almond shell Balsam bark Bamboo whole Barley straw Beech bark Beech wood Birch bark Christmas tree Coconut shells Coffee husks Corn stover Corncob Cotton gin Cotton husks Elm bark Eucalyptus Hazelnut shell Kenaf grass Mustard husks Miscanthus Neem wood Olive husk Olive wood Palm kernels Peach pit Pepper residue Pine pruning Pistachio shell Plum pits Poplar Red oak wood Rice husk Rice straw Sawdust Sorghastrum grass Soya husks Spruce bark Spruce wood Sugar cane bagasse Sugarcane bagasse Sunflower Sunflower shell Switch grass Tamarack bark Tea waste Tobacco leaf Tobacco stalk Walnut hulls & blows Walnut shell Wheat straw Wheat straw Willow Wood chips Wood yard waste Average a b

Ultimate analysis (wt%)a

HHVb (MJ/kg)

C

H

O

N

S

53.2 45.4 50.6 47.9 54 52 49.4 51.4 49.5 57 54.5 51.1 45.4 49.4 49 42.8 50.4 50.9 48.3 50.8 48.4 45.8 47.8 48.3 49.9 49 51 53 45.7 51.9 48.8 49.9 48.5 50 47.8 38.5 46.9 49.4 45.4 53.6 51.4 49.8 44.8 50.5 47.4 46.7 57 48 41.2 49.3 55.1 53.6 49.4 41.8 49.8 48.1 52.2 49.3

6.1 5.8 6.4 6 6.2 5.1 6.2 6 6.2 6.7 5.9 5.6 4.9 5.6 5.4 5.4 8.4 5.8 5.9 5.6 6 9.2 5.1 6.3 6.2 5.4 6.5 5.9 3.2 6.3 5.9 6.7 5.9 6 5.1 5.3 5.2 6.3 6.7 6.2 6.1 6 5.4 5.9 5.8 5.9 10.2 5.5 4.9 5.6 6.7 6.6 6.1 5.5 6.1 6 6 6.0

4.2 36.5 41.7 41.7 39.5 42.5 43.6 41.8 41.2 35.7 38.7 43.1 48.3 42.5 44.2 35 39.8 42.5 45.1 41.1 44.5 44.4 42.6 43.5 42 44.9 39.5 39.1 47.1 41.3 43.4 42.4 43.7 42.4 38.9 – 37.8 44 46.9 40 41.2 43.9 39.6 34.9 41.3 37.4 32 44 33.9 42.8 36.5 35.5 43.6 35.5 43.4 45.7 40.4 40.5

0.5 2.1 1.2 1.1 0.2 0.4 0.7 0.7 0.4 0.5 0.5 0.1 1.1 0.6 0.4 1.4 1.4 0.7 0.2 1 1 0.4 0.8 – 1.6 0.7 2.7 0.3 3.4 0.5 – 0.9 0.5 0.3 0.1 0.9 0.1 0.3 0.9 0.1 0.3 0.2 0.4 1.3 1.4 0.8 0.7 0.5 0.9 0.7 1.6 1.5 0.7 0.7 0.6 – 1.1 0.8

0 0.1 0.1 6 0.1 0 0.1 0.1 – 0.1 0.4 0.1 0.4 0.1 0 0.5 0 0.1 0 0 0.2 0.2 0.2 – 0.1 0 0.3 0.1 0.6 0 – 0.1 0 – – – 0 0.1 0.1 0.1 0 0.1 0 0.1 0.1 0.2 0.1 0.1 0 0 0.1 0.1 0.2 0 0.1 – 0.3 0.2

18.5 17 16.7 15.8 20.1 19.9 19.2 17.1 19.4 25.9 17.3 16.4 21.7 11.6 17.9 15.7 17 21.9 16.5 17.9 17.8 16.2 16.3 19.8 17.6 16.5 15.9 18.2 14.9 17.6 – 18.2 16.5 20.7 19.3 – 17.5 17.2 21.2 17.8 13.7 19.5 17.6 17.8 16.9 28.1 17.1 17.6 16.7 17 18.8 17.3 18.9 17.8 16.3 18.7 22.5 18.0

Proximate analysis (wt%)a FC

VM

Ash

18.5 – 17.2 – 20 20.7 15.1 10.9 – 19.2 20.7 11.5 17.5 27 21.9 17.7 17 12.4 20.3 18.8 14.2 13 13.1 23.4 12.2 17 12.4 17 – 22.1 15.9 26.1 14.3 23.3 19.8 13.6 14.3 17 37.9 13.3 20.7 17.5 17.7 28.3 19.8 26.3 15.8 12 – 12.3 20.1 17.5 17.8 15.9 17.2 18.3 20.1 18.2

73.7 – 78 – 77.4 74.2 82.2 84 – 76.6 76 87.4 79.6 64.8 77.6 75.3 79.4 81 74.3 73.1 81.6 85.5 85.2 73.4 85.9 61.8 81 82.5 – 77.2 65.5 70.3 84.6 72.6 76.2 66 76.7 81.6 59.3 82.4 76.5 77.3 75.3 69.3 76.2 69.5 78.9 85.6 – 85.6 73.8 81.6 80.8 82.5 79.6 81.2 73.8 77.0

7.8 – 4.8 – 2.6 5.1 2.7 5.1 – 4.2 3.3 1.1 2.9 8.2 0.5 7 3.6 6.6 5.4 8.1 4.2 1.5 1.7 3.2 1.9 21.2 6.6 0.5 – 0.7 18.7 3.6 1.1 4.1 4 20.4 9 1.4 2.8 4.3 2.8 5.2 7.0 1.4 4.0 4.2 5.3 2.4 – 2.1 6.1 0.9 1.4 1.6 3.2 0.5 6.1 4.8

On dry basis. Higher heating value estimated by Dulong formula: HHV (MJ/kg) =0.3383 C +1.422 (H-O/8).

lower than that of a coal, whereas the volatile content of former is higher than the latter. Chemical analyses of a charcoal and two coals and their reactivity for CO2 gasification reactions are compared in Table 3. The reaction characteristics of the charcoal are better than those of the pulverized coals [36]. The reaction rate (defined as mass percent per min (%/min)) of the charcoal with CO2 was as high as 22%/ min, suggesting a high reactivity, compared with only 6–16%/min for the two pulverized coals tested. It was believed that the higher reactivity of the charcoal is mainly due to its larger pore volume and

[32]. Accordingly, charcoal is more often used as a blast furnace injection fuel. As reported in some previous studies, charcoal powder injection has demonstrated in a mini-blast furnace in Brazil at an injection rate of 100–190 kg/t [19,35,36,37]. It has also been estimated that charcoal injection rates of 200–225 kg/t would also be feasible for large blast furnaces [38,39]. 3.1.1. Reactions and combustion of charcoal Fixed carbon content and heating value of a charcoal are generally 513

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lator, the peak temperatures of charcoal injection was only 16 °C lower than that of coal injection (1698 °C vs. 1714 °C) under the same injection rate and concentration in the blast [19]. Moreover, oxygen and CO2 contents in exhaust gas were very similar with charcoal and coal, suggesting that there is no need to significantly change the oxygen supply system when replacing coal with charcoal for BF injection. Generally, effects of charcoal injection on BF operation parameters are obtained via theoretical calculation of heat and mass balance. Blue Scope Steel analyzed the BF operating parameters for injection of different fuels using a heat and mass balance model, and the results are shown in Table 4 [41]. From the Table, it can be seen that injecting different fuels leads to obvious different impacts on blast furnace operation. Injection of various kinds of charcoal substituting pulverized coal results a decrease in coke ratio by 20–30 kg/t. In addition, when keeping the injection ratio (the mass of the fuel injected into blast furnace via the tuyere in relation to one tonne of pig iron production) and fuel ratio (the mass of all fuel injected for each tonne of pig iron produced) at 140 kg/t and 492 kg/t, injecting charcoal into a BF could decreases coke ratio, slag ratio, compared with coal injection, and the sulfur content in pig iron decreased with increasing charcoal injection [32,42]. Babich et al. [19] also calculated the mass and heat balance of BF with charcoal injection using a joint model based on Ramm theory. It was found that using charcoal (e.g. wood charcoal) with low ash content and high alkalinity to replace coal could reduce slag amount, coke consumption, air input and heat loss, and increase pig iron productivity. However, using the charcoal (e.g. grasses charcoal) with high ash content and low alkalinity decreased the pig iron productivity and increased the coke ratio. In addition to injecting charcoal solely to replace coal completely, co-injection of coal and charcoal or other alternative fuels was found to be more practical and beneficial for BF operation. Castro et al. [43] studied different co-injection of coal and charcoal into BF using a mathematical model and implemented industrial tests on a BF with working volume of 3800 m3 under three injection conditions, i.e., coals alone injection, charcoals alone injection and co-injection of both. Interestingly, the results of the industrial tests and mathematic simulation unanimously indicated that co-injection of coals and charcoals at 200 kg/t and 50 kg/t injection ratios, respectively, led to the best performance of the BF operation with respect to various BF indicators, suggesting synergistic effects of the charcoal and coal for BF injection. In a summary, charcoal injection into BF can reduce coke and slag ratios, and improve the quality of pig iron. However, the ash content of charcoal is harmful for BF operation due to the operational problems related to corrosion and deposition of alkaline ash compounds inside BF [19]. As such, low ash charcoals are highly desirable for BF injection.

Table 2 Composition analysis of biomass, coke [19,20], coal [19] and oil [20,21]. Ultimate analysis Ultimate analysis (wt%)a

Biomass Charcoal Coke Pulverized coal Heavy oil

C

H

O

N

S

49.3 74.7 88.0 80.6 87.0

6.0 5.0 0.4 4.4 10.5

40.5 1.4 0.5 5.4 –

0.8 1.0 0.4 1.7 –

0.2 17.9 0.6 0.5 2.01

HHVb (MJ/ kg)

18.0 32.1 30.3 32.6 ~40.0

Proximate analysis (wt%)a FC

VM

Ash

18.2 38.7 – 81.5 –

77.0 61.3 1.26 18.5 –

4.8 15.7 11.7 10.3 –

a

On dry basis. Higher heating value estimated by Dulong formula: HHV (MJ/kg) =0.3383 C +1.422 (H-O/8). b

a higher specific surface area (155 m2/g for the charcoal vs. 89 m2/g for the coal). As a result, carbonization temperature for charcoal preparation has an important influence on its reactivity due to the dependency of the charcoal textual properties on the temperature [36]. On the other hand, charcoals prepared at a low temperature contain a higher volatile content, and the continuous release of the volatiles (reductant) upon heating would positively contribute to reduction of iron ores in a wider range of reaction temperatures [19]. Similarly, the combustion rate of charcoal is faster than that of coal. For example, the rate of a charcoal determined was 3.6 mg/min, compared with 2.4 mg/min only for a coal [40]. In BF injection, the influence of oxygen enrichment on combustion of coal is also different for coal and charcoal. At a higher O/C ratio, the combustion rate of both fuels increase as expected, but the increase was much less with the charcoal injection, compared with that with the coal injection, due to the already very high mass transfer rate for charcoal, hence for charcoal injection there is no need to use too high oxygen enrichment percentage [19]. A study [32] on charcoal injection into BF carried out on a combustion facility, made by Blue Scope Steel and BHP Billiton, simulating BF tuyere and raceway led to some interesting findings as follows. Burning charcoal could form a stable flame more easily than burning coal, with no ash accumulation in the tuyere. The combustion performance for charcoals greatly exceeded that of some coals even with similar volatile contents as the charcoals. Softwood charcoals appeared to exhibit significantly better combustion performance than hardwood charcoals, and they attained good combustion performance even without any oxygen enrichment. The oxygen enrichment tests demonstrated very little enhancement in the combustion performance of a softwood charcoal, while in contrast the oxygen enrichment resulted in great enhancement in the combustibility of a medium volatile hardwood charcoal. This is likely due to the softwood charcoal generally has a higher volatile content and superb textural properties (specific surface area and pore structure) [32].

3.2. Bio-Coke production 3.1.2. Influence of charcoal on blast furnace operation Compared with coal injection, charcoal injection does not cause much change of temperature. According to injection-coke-bed-simu-

In addition to the direct injection of biomass charcoal into BF, producing bio-coke is another feasible way to utilize biomass in BF-

Table 3 Comparison of charcoal and coal with respect to chemical analyses and reactivity [37]. Sample

Charcoal Coal 1 Coal 2 a b c

Reaction characteristics (%/min; min)a

Proximate analysis (% daf)

Ultimate analysis (% daf)

VM

FC

Ash

C

H

N

S

O

MRTb

TRMc

38.7 21.9 18.5

61.3 78.2 81.5

15.7 4.6 10.3

74.7 83.7 89.8

5.0 3.1 4.3

1.4 1.1 2.0

1.0 0.1 0.5

17.9 12.0 3.5

22 16 6

2.67 2.33 5.33

Gasification reaction with CO2. MRT=Maximum reaction rate. TRM=Time to reach the maximum reaction rate.

514

Calorific value (kcal/kg)

6110 7165 7635

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Table 4 Key operating parameters for blast furnaces [41]. Fuel injectant

Coke ratio (kg/t)

Slag ratio (kg/t)

Coke replacement ratioa (kgcoke/kg-injectant)

Top gas energy (GJ/t)

Oxy enrich. (%)

Si (%)

S (%)

P (%)

Alkali oxide loadb (kg/t-HM)

Lignite (briquettes) Sub-bituminous High-vol coal C High-vol coal A High-vol coal B Semi-Anthracite Anthracite

385 370 343 352 353 335 334

234 245 236 246 249 239 242

0.57 0.68 0.87 0.81 0.80 0.92 0.94

5.41 5.14 4.97 5.13 5.11 4.92 4.79

5.4 2.9 0.9 3.2 3.0 1.9 1.0

0.39 0.41 0.37 0.41 0.42 0.37 0.38

0.019 0.023 0.021 0.017 0.020 0.020 0.019

0.076 0.080 0.069 0.070 0.070 0.078 0.069

0.34 0.35 0.44 0.41 0.73 0.37 0.86

Torrefied softwood Charcoal (softwood) Semi-charcoal (h′wood) Charcoal (hardwood) Charcoal (Mallee)

409 321 355

236 223 228

0.39 1.06 0.78

5.57 4.84 4.98

5.6 0.7c 0.8c

0.41 0.32 0.35

0.016 0.014 0.015

0.072 0.069 0.078

0.43 0.40 0.44

331

225

1.02

4.92

0.7c

0.33

0.015

0.081

0.58

331

226

1.03

4.83

0.7c

0.33

0.014

0.067

1.03

a Calculated at 140 kg/t-HM or the specified injection ratio by the tangent method, with the adiabatic flame temperature of the raceway (RAFT) maintained constant by oxygen enrichment. b Shown for reference only. c Requiring additional steam (cooling) to maintain RAFT.

BOF processes [44]. Biomass or charcoal from biomass pyrolysis can be added into a coking coal for coke production aiming to reduce the greenhouse gas emissions [45]. The resulted coke product containing biomass/charcoal is normally called bio-coke. In addition to solid biomass and charcoal, other pyrolysis products, e.g., liquids (oils and tars) and gases have also been applied in coke production [46–49]. 3.2.1. Influence of biomass on coke property Fluidity of coking coal is an important parameter for initial performance evaluation of the coking blend [50–52], e.g., it directly influences the carbon anisotropic texture and the porous structure of the resulted high-temperature coke, which consequently effects the coke mechanical strength and reactivity with carbon dioxide [53,54], although there is some debate on how fluidity influences the important coke properties. Generally, the thermoplastic properties of the coal are examined in a constant-torque Gieseler plastometer. The rotation of a stirrer placed inside a compacted sample was measured, while the sample was heated. The fluidity was recorded in dial divisions per minute (ddpm) as a function of the temperature and the maximum fluidity value at a specific temperature is defined as Gieseler fluidity. The addition of biomass in the coking coal mixture can effectively reduce the fluidity of the coking coal, mainly owning to the greater volatile contents in biomass materials [44,55,56]. The difference between the last two temperatures is taken to be the fluid or plastic interval. It was found that adding raw biomass could improve the performance of the coking process, much better than adding charcoal or a single biomass component (such as lignin or cellulose), as shown in Fig. 2. The effects of biomass addition on coking coal thermoplastic properties, follow the order of: sawdust > cellulose > hydrophobic tar > lignin > charcoal > water-soluble tar > xylan [50]. In another study, however, it was reported that removing some volatile component from biomass before addition into a coking coal was beneficial for improving the quality of the bio-coke [56], thus there is some debate on how fluidity influences the coke properties. As an adverse effect, adding biomass into coking coal blend decreased the bulk density and the cold mechanical strength of coke, due to the increased porosity of the coke [57]. Matsumura et al. [58] investigated the relation between the cold mechanical strength of biocoke and the particle size of biomass. When the particle size of biomass was less than 2 mm, the cold mechanical strength of bio-coke was much less than that of coke without biomass addition, while with biomass of a particle size more than 2 mm, the strength of the former

Fig. 2. Variation of Gieseler fluidity of coal with increasing addition amount of sawdust (SD), its carbonization products: charcoal (CH), water-soluble tar (ST) and hydrophobic tar (IT) and biomass constituents: cellulose (CEL), xylan (XYL) and lignin (LIG) [50].

was similar or even higher than the latter, as shown in Fig. 3. As commonly observed, biomass addition at a too high ratio would decrease the coke quality due to the increased porosity of the bio-coke. In order to increase the biomass addition ratio, Kudo et al. [59] demonstrated positive effects of hydrothermal treatment of biomass on the properties of bio-coke. The hydrothermal treatment at the temperature 200–300 °C increased the tensile strength of bio-coke by 8–11 times, from 5 MPa up to 50 MPa, as shown in Fig. 4. The hydrothermal treatment was believed to transform the volatile components in the biomass (i.e., cellulose and hemicellulose) into a solid that contributes to improving the bio-coke properties as effectively as lignin. The hydrothermal treatment also enhanced the plasticity of the biomass by degradation of the lignin to some extent, promoting particles' coalescence/fusion and densification of the briquettes. Note worthily, Das et al. [60] developed a bio-coke using a non-coking coal and biomass. However, different results were also reported in some studies [50], where the impacts of lignin were found to be less significant than those of cellulose on coke quality. Thus, the impacts of lignin and cellulose on bio-coke quality require further study.

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Fig. 3. Relationship between average particle size of cedar biomass and I-type strength of bio-coke [58]. Fig. 5. Dependency of the CSR and CRI of bio-cokes on the addition amount of charcoal in the coking coal blends [44].

particle size of 6.3–9.5 mm, the influence of charcoal addition was much less significant than that of the other Blends using charcoal of a smaller particle size. Thus, using large-size charcoal can increase the charcoal addition ratio without reducing coke quality significantly. Similar conclusions were drawn from the study by Montiano et al. [63] examining the properties of bio-cokes made of a coking coal and various amounts of wood chips of different particle sizes. In summary, adding biomass or charcoal can reduce the fluidity of coking coal, increase the viscosity of coking coal and improve the quality and the reactivity of coke, but decrease the strength of coke after reaction. The addition ratio of biomass to coking coal blends shall not be more than 5 wt%, in order not to sacrifice the coke strength too much. However, the biomass/charcoal addition ratio may be increased by some approaches such as hydrothermal treatment of the raw biomass and using charcoals of a larger particle size. 3.3. Utilization of biomass in iron ore sintering Fig. 4. Combined effect of hydrothermal treatment on bio-coke briquetting [59].

Sintering ore is still the dominating raw materials for blast furnaces, in particular in the iron making industry in China. In a sinter plant, iron ore, fluxes and coke breeze are mixed and loaded to the sintering moving bed. And then the combustion process is initiated when the bed passes through the ignition hood equipped with several burners. Air is drawn through the bed by an induced draught fan to sustain the combustion. The heat generated by the combustion of the coke breeze causes the iron ore granules to agglomerate into lump materials suitable for the blast furnace operation. The iron ore sintering process consumes 9 ~12% of the total energy of the iron and steel processes, ranking second only to the blast furnace process [64]. In addition, iron ore sintering process is also a major source of air emissions, producing large amounts of CO, CO2, SO2, NOx, and organic pollutants [65–68]. Application of biomass as an alternative fuel to coal and coke for iron ore sintering process could effectively reduce fossil fuel consumption and the pollutant emissions.

3.2.2. Influence of biomass on CSR and CRI As well known, coke plays three major roles in the blast furnace operation, i.e., a reducing agent, a heating agent and a skeleton for the charged ore and flux materials [61]. Coke quality is mainly measured by the values of coke reaction index (CRI) and coke strength after reaction (CSR). Reduction of iron ore by cokes in the blast furnace is mainly indirect reduction, i.e., iron ore is reduced by CO produced by the gasification of coke with CO2. The presence of porous structure resulted from biomass addition in a bio-coke up to a certain level could greatly promote the reactivity of the bio-coke with CO2 [45], hence generally adding biomass or charcoal into bio-coke improves its CRI. However, the content of biomass/charcoal in a bio-coke cannot be too high, as the bio-coke with a high bio-content generally decreases CSR [62]. Macphee et al. [44] studied bio-coke samples with charcoal addition ratio from 2 to 10 wt%. As shown in Fig. 5, CRI increases but CSR decreases with increasing the addition ratio of charcoal in the coking coal blends. With Blend 1, its CSR dropped from 57% markedly to 17% while increasing the charcoal ratio to 10 wt%. As given in the Figure, it was also found that charcoal size had an impact on the performance of the produced bio-coke. For Blend 3 using charcoal of a

3.3.1. Effects of biomass on sintering Compared with coke for sintering different features, e.g., low density, content. Replacing coke with charcoal 516

process process, biomass as a fuel has high porosity and high water to replace coke in the sintering

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process generally increases the water addition ratio in the sintering mixture, in particular the water ratio increases rapidly when the replacement ratio exceeds 40 wt% [69]. Owing to the higher volatile content and better reactivity of biomass (mainly charcoal), the combustion rate of biomass is faster than that of coal or coke [40,70]. When bio-char is added in the sintering process to substitute a portion of coke or coal, the combustion rate of the sintering bed increases with increasing the addition ratio of charcoal, depending on the types of biomass precursor for the charcoal. For instance, the effects of charcoal addition decreased in the following order: straw charcoal > wood charcoal > peat charcoal [70]. In addition, adding biomass to the sintering process also decreased the maximum temperature of the sintered material layer and the time for reaching the maximum sintering temperature, and widened the sintering temperature range [70,71]. However, the negative impact of substituting coal/coke with biomass in a sintering process is that the CO content increased in the sintering flue gases, due to much higher gasification reactivity of biomass than coal or coke [36,70,72,73]. For instance, the CO peak concentration in sintering flue gas increased from 2.07 vol% to 2.85−3.11 vol% when using various kinds of biomass [70], and increased from 3.0 to 5.0 vol% by using some biomass fuel in a study by Kawaguchi and Hara [74].

Table 6 Effects of biomass use in iron ore sintering process on air pollutant emissions. Replacing coke or coal with biomass in sintering

Reduction percent of air pollutants CO2

SO2

NOx

Dioxin and PAHs

Replacing 40 wt% coke with charcoal Replacing 20 wt% coke with charred-straw Replacing 15 wt% coke with sawdust Replacing 100% anthracite coal with biomass carbonized char Replacing 10 wt% coke with sunflower husk Replacing 20 wt% coke with charcoal

Not reported but shall be proportional to the replacement ratio

38%

27%

n.a.a

32%

18%

n.a.

43%

31%

n.a.

90%

69%

Slightly higher

[74]

n.a.

n.a.

~10%

[78]

n.a.

n.a.

33%

[79]

a

3.3.2. Effects of biomass on yield and quality of sintering ore When biomass is added in an iron ore sintering process, irrespective of the addition amount or biomass type, however, the yield and tumble index (measuring the ability resisting to impact and friction, defined by the mass percentage of sintering ore whose diameter more than 6.3 mm after rotating for 200 r in the tumbler) of the sintering ore have been found to decline to various degrees of extent. The results of three research teams are summarized in Table 5. As listed in the Table, commonly the yield, the tumbler strength and the utilization factor of sintering ore decrease with increasing the replacement ratio of biomass. The deteriorating effects on sintering ore are greater when adding raw biomass than those of adding charcoal [69,70]. Nevertheless, Kawaguchi and Hara [74] reported that the sintering ore produced by using biomass only as the fuel, although at a yield and tumbler strength slightly lower than those from sintering using coke or coal as a fuel, could meet the BF ironmaking requirement. Adjusting the sintering process, such as changing the particle size of sintering raw material, the ratio of sintering raw material to the amount of wind, can be effective way to control the quality of sintering ore produced from biomass-fueled sintering process. In a conventional iron ore sintering process using coke or coal, the strength of sintering ore can be improved by the formation of a large amount of liquid calcium ferrite becoming crystal or vitreous in the condensation process. When biomass is added into a sintering process,

Replacement ratio of coke (wt%)

Yield (%)

Tumble index (%)

Ref.

Coke-1# Charcoal-1# Charcoal-1# Charcoal-1# Charcoal-2# Sawdust Coke-2# Anthracite Biomass A Charcoal-3# Coke-3# Charcoal-4# Charcoal-4#

0 40 60 100 40 40 0 100 100 100 0 50 100

73 65 55 41 62 54 76 73 72 71 – – –

65 63 54 23 58 50 72 70 73 72 68 63 60

[69]

[70,76]

Not analyzed.

the CO concentration on the iron ore surface increases but the highest sintering temperature decreases due to the lower heating value of biomass and its high combustion and gasification rates, not favorable for the formation of liquid calcium ferrite phase. The decrease of calcium ferrite phase in mineralogical structure of a sintering ore would lower the tumbler strength of the sintering ore using biomass as a fuel [69,70, 75]. The porous structure of charcoal determines its combustion characteristics, and hence its effects on the quality of the sintering ore. Some possible measures may be taken to improve the quality and yield of sintering ore, via improving carbonization process to reduce the pore volume of the resulted charcoal, and reduce the combustion rate of charcoal using passivating agent [70]. 3.3.3. Effects of biomass use in iron ore sintering process on air pollutant emissions Since the sulfur and nitrogen contents of biomass is generally much lower compared with those of fossil fuels, the use of biomass can effectively reduce SO2 and NOx emissions from the sintering process, as summarized in Table 6. As was reported in some previous studies [70,76], replacing coke with charcoal, charred-straw and sawdust at 40 wt%, 20 wt% and 15 wt%, respectively, decreased SO2 emission by 38%, 32% and 43%, respectively, and NOx emission by 27%, 18% and 31%, respectively. Kawaguchi and Hara [70] conducted sinter pot tests using raw biomass and biomass carbonized char. It was found that using raw biomass should be avoided as it resulted in poor yield of sintering ore yield and less influence on exhaust gas emission. However, replacing anthracite coal with biomass carbonized char could not only increase the productivity but effectively decrease SO2 and NOx emissions by approx. 90% and 69%, respectively. As also indicated by this study [70], using biomass char for the sintering, it is necessary to optimize operation (size control and moisture control of the biomass char), because combustion rate of the biomass char is much higher than that of anthracite or coke. Dioxins and polycyclic aromatic hydrocarbons (PAHs) are two common organic pollutants, both carcinogenic substances, generated in substantial amount from iron ore sintering process [68,77]. Kawaguchi and Hara [74] found the dioxin emission of a sintering process using biomass char was lower than that using coke, but slightly higher than that using anthracite coal. Ooi et al. [78] studied the

Table 5 Effects of biomass types on yield and quality of sintering ore. Type of fuel

Ref.

[74]

[75]

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Fig. 6. Biomass utilizations in blast furnace iron making processes.

charcoal replaces 50 wt% of coke in the sintering process, substitutes 5 wt% of coking coal in bio-coke production, and is the only fuel for blast furnace injection. Other calculation conditions are described in the literature work [38]. The results are shown in Fig. 7. The exergy loss of charcoal (sintering) system is much higher, reaching 6.20 GJ/t under the same sintering and blast furnace operation conditions. The exergy loss of charcoal and coal system is 4.91 GJ/t, which is also higher than the conventional system (4.29 GJ/t). From the results given in Fig. 7, the overall energy consumption of an iron making system using biomass is higher than the conventional system, likely due to the high energy cost in biomass carbonization treatment for charcoal production. In addition, the heating value of charcoal is lower than fossil fuels (coal and coke), leading to the decrease of hot air temperature and needing more energy in order to improve the air temperature [38].

organic pollutants emission of sintering using sunflower husk to replace 10 wt% of coke. Adding sunflower shells to the sintering process decreased the total emissions of 2,3,7,8 -PCDD/Fs slightly by approx. 10% from 1.0 ng/Nm3 to 0.91 ng/Nm3. The same group by Ooi et al. [79] observed reduction of dioxin emission by approx. 33% when replacing 20% coke with charcoal (Table 6). The influence of adding biomass on the emission of PAHs has also been studied [68,79,80]. When sunflower shells were used in the sintering, the emission of tricyclic aromatic hydrocarbons such as fluorine, phenanthrene and anthracene increased, but the emission of naphthalene decreased significantly. As tricyclic aromatic hydrocarbons are less carcinogenic than naphthalene, using biomass in sintering lowers the overall emission of carcinogenic compounds. 3.4. Effects of biomass application in the BF-BOF route on energy consumption and CO2 emission

3.4.2. Analysis of CO2 emission One of the most remarkable advantages of using biomass – a carbon neutral renewable energy source is its potential in reduction of CO2 emission. Fig. 8 illustrates the CO2 emission from the iron making processes using charcoal in comparison with that from the conventional iron making system. The CO and the HCs in the coke oven gas

From the above discussion, biomass can be used in the BF – BOF route, mainly in ironmaking processes, i.e., the BF injection, bio-coke production and as an alternative fuel for iron ore sintering, as illustrated in Fig. 6. Firstly, it should be noted that raw biomass cannot be used directly for blast furnaces, and biomass treatment via carbonization/pyrolysis is needed, e.g., the conventional pyrolysis of biomass to produce charcoal, and the coking process to obtain biocoke. Charcoal can also be utilized in the bio-coke production process. In addition, raw biomass or charcoal can be also directly used in iron ore sintering process. Finally, biomass can be used in a BF by directly injection from blast tuyere as an injection fuel (charcoal powder, biomass synthesis gas, tar, etc.) or charging as a bio-coke from the BF top.

7000

Exergy loss˄MJ/t˅

6000

3.4.1. Analysis of exergy consumption Nogami et al. [38] analyzed the overall exergy loss of the blast furnace ironmaking system where charcoal replaced all coal and coke. The exergy loss of the conventional system including coking, sintering and blast furnace processes, is 4.29 GJ/t. The exergy loss of the charcoal-lump ore system using charcoal and lump ore as raw materials is 4.53 GJ/t, slightly higher than that of the conventional system. However, modern blast furnace uses sintering ore as the main raw material, rather than lump ore. Moreover, charcoal cannot replace coke in blast furnace stock column, so the exergy losses of the following two systems are calculated in this paper, i.e., (a) charcoal (sintering) system: only charcoal and sintering ore are used, without using any coal and coke in the sintering process; (b) charcoal and coal system:

5000

Coke oven Carbonization kiln Sintering Blast furnace Hot stove

4000 3000 2000 1000 0

Fig. 7. Comparison of exergy loss in four ironmaking systems.

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As shown in the Figure, in a process of biomass gasification and iron ore gas reduction, CO/ H2-rich reducing gas is first produced in biomass convention processes, and then supplied into an iron ore reduction reactor to reduce iron ore pellets or iron ore powder, producing DRI. Obviously this process is same as the conventional gas-based direct iron ore reduction processes, except for different feedstocks used for gas production. Thus, it can operate in a shaft furnace [91] or a fluidized bed reactor [92], etc. As for the other process, i.e., the iron ore-biomass pellets reduction, firstly fine powders of iron ore and biomass (charcoal) are pelletized, then the pellets are reduced in reduction reactors for the production of DRI. Iron ore-biomass pellets have the similar characteristics as iron ore-coal pellets, so they both can be reduced in a shaft furnace [93], a rotary hearth furnace [94], or a rotary kiln [95], etc. 4.1. Iron ore and biomass pellets Non-blast furnace ironmaking technologies, mainly including direct reduction and smelting reduction, have been developed to address the challenges of both environmental performance and resources depletion. Iron ore pellets containing carbon are used for various direct reduction processes such as shaft furnace, fluidized bed reactor and rotary hearth furnace [81,82]. Iron ore pellets containing carbon can be reduced by itself at the high temperature and then produce direct reduced iron fed into the blast furnace or electric arc furnace for iron/ steel steelmaking [83]. In recent years, direct reduction iron can also be produced by using iron ore – biomass pellets as raw materials [81,82]. Iron ore – biomass pellets refers to a kind of new ironmaking raw materials, consisting of biomass powder, iron ore powder and a small amount of binder materials, made on a pelletizer or briquetting press. The biomass used in the pellets may be raw biomass or charcoal. At a high temperature, iron ore can be reduced by biomass within the pellets via the following reactions [84–89].

Fig. 8. Comparison of carbon dioxide emission from two ironmaking systems.

(COG) and the blast furnace gas (BFG) are counted as carbon dioxide emission because they are eventually converted into CO2 by combustion in other processes. The gross CO2 emission of the charcoal system is about 787 kg-C/t vs. about 528 kg-C/t for the conventional system. It shall however be noted that about 737 kg of carbon is sourced from charcoal- a renewable fuel, thus the net CO2 emission from the charcoal system is only about 50 kg-C/t [38], which is mainly from power generation and the limestone fed into the BF process. A similar conclusion was also drawn in another study [62], where a charcoal system released 62.8 kg-C/t net CO2, remarkably lower than that in a conventional system (about 1552 kg-C/t). Thus, the role of biomass in reducing CO2 emissions in various ironmaking process is obvious. 4. Biomass applications in DR – EAF processes Utilization of biomass for the production of direct reduced iron (DRI) has been studied although mostly limited in laboratory scale. No industrial or semi-industrial experiments have been reported. There are two types of iron ore direct reduction processes to which biomass can contribute as alternative reactant and fuel, i.e., (1) biomass gasification & iron ore gas reduction, and (2) iron ore-biomass pellet reduction, as illustrated in Fig. 9.

FeOx + CO → FeOx-1 + CO2

(1)

C + CO2 → 2CO

(2)

FeOx + H2 → FeOx-1 + H2O

(3)

However, when charcoal is used in the pellets, reactions (1) and (2) are the main reactions due to insufficient amount of hydrogen in charcoal [85,88]. Direct reduced iron with a higher metal ratio can be produced, if keeping appropriate proportion of biomass in the pellets. Fu et al. [85] studied different pellets under simulated rotary hearth furnace conditions. Bamboo char, coconut shell char and rice husk char were used in the iron ore-biomass pellets, achieving metallization rate

Fig. 9. Two types of iron ore direct reduction processes using biomass.

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Fig. 10. TG profiles of various biomass materials with and without iron oxide powder (Fe2O3).

5. Biomass integrated gasification combined cycle for scrap – EAF processes

of 95.56%, 95.42% and 49.3%, respectively. The metallization rate of the pellets with bamboo char and coconut shell char are even higher than the metallization rate of iron ore-coke pellets (88.07%). Ueki et al. [28] studied the reduction characteristics of iron ore-wood sawdust pellets and found that iron oxide could be completely reduced to metallic iron at temperatures of 1000–1300 °C. In addition, Srivastava et al. [90] studied the smelting reduction of magnetite using biomass as a reducing agent. Results showed that high quality pig iron could be produced at various temperatures for a short residence time.

Currently, more than 30% of steel produced worldwide is from scrap – EAF processes. [17] These processes consume more than 40% of energy in an iron and steel plant, supplied dominantly from fossil fuels, mainly natural gas and coal. To reduce greenhouse gas emissions and the cost of steelmaking, biomass has been proposed to substitute fossil energy in scrap-EAF processes [17]. Fig. 11 illustrates a novel biomass integrated gasification combined cycle (BIGCC) proposed for co-generation of electricity and thermal energy for EAF process. As shown in the Figure, the cogeneration system consists mainly of a biomass boiler, steam turbine and condenser, and EAF furnace. Through the biomass combustion in the boiler, superheated steam is produced and fed into a steam turbine for electricity generation. The electricity produced is then used to supply the electricity demand of the EAF. The hot exhausted gas from the biomass boiler is also used as a heating source for the EAF or for

4.2. Biomass utilization for iron ore direct reduction processes In our recent study on the reducibility of various biomass materials (organosolv lignin, hydrolysis lignin, sawdust, spent coffee grounds, cellulose and corn stalk), thermogravimetric analysis (TGA) was performed on these materials with and without Fe2O3 powder in N2 atmosphere heated from room temperature to 900 °C (1173 K). The TG results are illustrated in Fig. 10. Based on the TG plots for the mixtures of biomass and Fe2O3 powder, the following three biomass materials: organosolv lignin, hydrolysis lignin and sawdust, showed better reducibility than the rest materials (spent coffee grounds, cellulose and corn stalk). From the TG plots of these materials without Fe2O3, one can estimate (fixed carbon+ash) contents using the residual mass % at 973 K. Assuming negligible ash content for the lignin or woody biomass, the fixed carbon content of organosolv lignin, hydrolysis lignin and sawdust is approximately 38 wt%, 23 wt% and 18 wt%, respectively, which is higher than that of cellulose and corn stalk. The above results suggest that biomass with a higher fix carbon content would show better reducibility in the iron ore-biomass pellets reduction, which is in a good agreement with some reported literature work, demonstrating that biomass with a high fixed carbon content, such as the residue char from biomass refining processes, is more appropriate for the iron ore-biomass pellets application [22,96]. However, the roles of volatile matters (involving some reducing gas such as H2, CO, etc.), fixed carbon and ash components in the iron ore-biomass pellets reduction are not elucidated by far, and hence currently researched by the authors’ group. Luo et al. [97] demonstrated effectiveness of combining the above described processes, i.e., gas-based iron ore reduction and iron orebiomass pellet reduction, for DRI production using biomass, but in a laboratory scale. Thus, more research is needed to prove the feasibility of this gasification-based process.

Fig. 11. Biomass integrated gasification combined cycle for generation of electricity and thermal energy for EAF process [17].

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(or coking), and it was found that PKS had the highest ratio of deposited carbon. The deposited carbon within iron ore showed potential as a highly reactive reducing agent even at low temperatures. The results show promise of applications of biomass tar as an alternative low-energy approach for producing metallic iron from low-grade iron ore. Rozhan et al. [114] studied the influence of biomass pyrolysis rate on carbon deposition, and they found that fast pyrolysis is desirable for carbon deposition. In addition to biomass tar, decomposition (or coking) of coal pyrolysis tar also achieved similar carbon deposition [115–117] applicable for low-grade iron ore reduction.

preheating the scrap [17]. Oliveira et al. [17] also calculated the energy consumption of a cogeneration system, comparing three common biomass types (rice husks, coffee shells and elephant grass) with natural gas as the fuel for the boiler. The results show that the energy consumption of biomassfueled process is higher than that of natural gas-fueled one, which is expected due to the much lower energy content of biomass compared with natural gas fuel. However, compared with natural gas, biomass is a renewable and carbon-neutral fuel, so the biomass-fueled BIGCC process yields much lower CO2 emission, and hence much less impact on the environment. Moreover, the same authors [17] conducted an economic analysis and showed that using elephant grass would lead to the lowest operational costs, for a cost reduction by about 9–15% in relation to that with natural gas, whereas the operational costs of the process are higher for rice husk and coffee shells. Thus, although the heating value of biomass is much lower compared those of natural gas, if choosing a right type of biomass (such as elephant grass), the biomass-fueled BIGCC process for co-generation of electricity and thermal energy could still be of a great promise for steel production via the scrap – EAF processes with respect to operational costs, compared with natural gas-fueled IGCC process.

6.3. Adsorption of metallurgy pollutants using activated charcoal Iron and steel industry, especially the iron ore sintering process, produces large amounts of greenhouse gases and other air pollutants including SO2, NOx, dioxins and polycyclic aromatic hydrocarbons (PAH), etc. [118, 119, 120]. Biomass can be used to produce activated carbon materials as effective adsorbents for removal of these air pollutants from flue gas. For instance, walnut shells were used for production of novel activated carbon adsorbents achieving efficient flue gas desulphurization. [121] It has also been demonstrated by Amstaetter et al. [122] that biomass-derived activated carbon worked better than coal-based activated carbon for removal of PAHs. Similarly, activated carbon proved to be effective for removal of NOx and dioxins at a very high efficiency of > 85% and > 99%, respectively [123,124]. Various pollutants are commonly present simultaneously in the flue gas. Compared with other conventional pollution control technologies such as flue gas desulfurization, flue gas denitrogenation, etc., activated carbon adsorption technology is advantageous as it could remove different pollutants at the same time to various degrees of extent depending on the pollutants and the adsorbents [123,124]. Exhaust gas from iron and steel industry, in particular from the iron ore sintering process, is featured by a very high volume flow rate and low concentrations of various pollutants, which render challenges for flue-gas clean-up using either conventional technologies or adsorption with adsorbents. Developing activated carbon materials from biomass with high adsorption ability for multiple pollutants and designing efficient absorption systems for simultaneous removal of air pollutants in exhaust gas from the iron and steel industry will remain a focus of future research.

6. Biomass applications in other processes 6.1. Applications of biomass in magnetic separation of refractory low grade iron ores Due to the decrease of high-grade iron ores resources, low-grade iron ore began to be employed for iron/steel making, especially in China [98,99]. For enrichment of low-grade non-magnetite iron ores, the iron ores need to be firstly reduced to magnetite generally using pulverized coal or biomass as a reducing agent, followed by magnetic separation [100,101]. Wang et al. [100] studied magnetization of low grade limonite ore by biomass. The results showed that magnetization using biomass as a reducing agent could be achieved at a lower temperature (650 °C), compared with 750 °C when lignite was used. Xu et al. [101] studied the magnetization of high phosphorus hematite by biomass reduction, and found that biomass reduction not only magnetized hematite, but also reduced the phosphorus contents in the hematite. In contrast, Tang et al. [102] reported that the phosphorus content of hematite was unable to be reduced using biomass alone, unless Na2CO3 was added to the pig iron. In addition, biomass can also be used to reduce polymetallic ores. Zhang et al. [103–105] successfully produced manganese oxide and magnetite from low-grade manganese iron ore using biomass as a reducing agent. It was believed that in the biomass reduction process, the main reducing substance is the organic volatile derived from biomass pyrolysis [106,107,108].

7. Concluding remarks – challenges and outlook (1) Charcoal injection into BF can reduce coke and slag ratios, and improve the quality of pig iron. However, the ash content of charcoal is harmful for BF operation due to the operational problems related to corrosion and deposition of alkaline ash compounds inside BF. Low ash charcoals are highly desirable for BF injection. (2) In summary, adding biomass or charcoal can reduce the fluidity of coking coal, increase the viscosity of coking coal and improve the quality and the reactivity of coke, but decrease the strength of coke after reaction. The addition ratio of biomass to coking coal blends shall not be more than 5 wt%, in order not to sacrifice the coke strength too much. However, the biomass/charcoal addition ratio may be increased by some approaches such as hydrothermal treatment of the raw biomass and using charcoals of a larger particle size. (3) Commonly the yield, the tumbler strength and the utilization factor of sintering ore decrease with increasing the replacement ratio of biomass. Adjusting the sintering process, such as changing the particle size of sintering raw material, the ratio of sintering raw material to the amount of wind, can be effective way to control the quality of sintering ore produced from biomass-fueled sintering process. (4) The use of biomass in sintering process can effectively reduce SO2

6.2. Utilization of bio-tar in iron and steel production processes In addition to charcoal and bio-gas, bio-tar can also be used in iron and steel production processes via carbon deposition in the pores of iron ore to form carbon-iron oxide mixture [109,110,111]. At a high temperature, the carbon-iron oxide mixtures will be reduced to produce ferrous oxide or metallic iron [112]. Compared with conventional reduction of iron oxide by the tar-carbon, the tar reduction usually occurs at a lower reduction temperature and a high reaction rate due to the high dispersion of the tar-deposited carbon [109]. Cahyono et al. [113] investigated an innovative process to reduce pisolite ore (low-grade ore) with deposited carbon derived from tar vapors decomposition (or coking). Tars were produced from pyrolysis of various solid fuels, including a high-grade bituminous coal, a lowgrade lignite coal, and a biomass – palm kernel shell (PKS). Carbon was deposited within the pores of the iron ore by tar vapors decomposition 521

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(5)

(6)

(7)

(8)

(9)

Graduate Creative Talent Project: Ph.D Students Short-term Visiting Abroad, and from the State Key Program of National Natural Science Foundation of China (Grant No. 51234008).

and NOx emissions, and lower the overall emission of carcinogenic compounds, e.g., dioxins and polycyclic aromatic hydrocarbons (PAHs). Moreover, the role of biomass in reducing CO2 emissions in various ironmaking process is obvious. There are two types of direct reduced iron (DRI) production processes to which biomass can contribute as alternative reactant and fuel, i.e., 1) biomass gasification & iron ore gas reduction, and 2) iron ore-biomass pellet reduction. Some preliminary tests showed that biomass with a higher fix carbon content would show better reducibility in the iron ore-biomass pellets reduction, while the roles of volatile matters (involving some reducing gas such as H2, CO, etc.), fixed carbon and ash components in the iron orebiomass pellets reduction are not elucidated, hence more research is needed. Moreover, utilization of biomass for the production of direct reduction iron is mostly limited in laboratory scale. For better demonstration of the potential of utilizing biomass for DRI production, future studies on a pilot/large scale are needed. Biomass can be utilized for steel production via the scrap – EAF processes through biomass integrated gasification combined cycle (BIGCC), although the energy consumption of a biomass-fueled process is higher than that of natural gas-fueled one. However, compared with natural gas, biomass is a renewable and carbon nature fuel, so the biomass-fueled BIGCC process yields much lower CO2 emission, hence much less impact on the environment. Although the heating value of biomass is much lower compared those of natural gas, if choosing a right type of biomass (such as elephant grass), the biomass-fueled BIGCC process for co-generation of electricity and thermal energy could still be of a great promise for steel production via the scrap – EAF processes with respect to operational costs, compared with natural gas-fueled IGCC process. Biomass can be used as reducing agent for reducing low-grade non-magnetite iron ores to magnetite, followed by enrichment of the iron ores by magnetic separation. It was found that biomass reduction could not only magnetize hematite, but also reduce the phosphorus contents in the hematite, as well as polymetallic ores. It was believed that in the biomass reduction process, the main reducing substance is the organic volatile derived from biomass pyrolysis. In addition to charcoal and bio-gas, bio-tar can also be used in iron and steel production processes via carbon deposition in the pores of iron ore to form carbon-iron oxide mixture. At a high temperature, the carbon-iron oxide mixtures will be reduced to produce ferrous oxide or metallic iron. Compared with conventional reduction of iron oxide by the tar-carbon, the tar reduction usually occurs at a lower reduction temperature and a high reaction rate due to the high dispersion of the tar-deposited carbon. Exhaust gas from iron and steel industry, in particular from the iron ore sintering process, is featured by a very high volume flow rate and low concentrations of various pollutants, which render challenges for flue-gas clean-up using either conventional technologies or adsorption with adsorbents. Compared with other conventional pollution control technologies, activated carbon adsorption technology is advantageous as it removes all pollutants at the same time. Developing activated carbon materials from biomass with high adsorption ability for multiple pollutants and designing efficient absorption systems for simultaneous removal of air pollutants in exhaust gas from the iron and steel industry will remain a focus of future research.

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Acknowledgements The authors would like to acknowledge the funding from BioFuelNet Canada, a Network of Centres of Excellence, NSERC through the Discovery Grants and the NSERC/FPInnovations Industrial Research Chair in Forest Biorefinery, USTB through 522

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