Elsevier

Renewable Energy

Volume 146, February 2020, Pages 1717-1724
Renewable Energy

Mechanical strength of bio-coke from briquettes

https://doi.org/10.1016/j.renene.2019.07.139Get rights and content

Highlights

  • The strength of cokes prepared from biomass, coal and binder has been tested.

  • The combination of lignin and a bituminous binder produces high strength coke.

  • The density of the briquettes is related to the porous texture of the biomass.

  • High percentage of large inerts in the coke is detrimental for coke strength.

Abstract

Cokes made from briquettes composed of a high volatile bituminous coal combined with four different biomasses and four different binders were analyzed in order to evaluate the influence of these materials on their mechanical strength. The results presented in this work are part of a more extensive research plan aimed at widening the range of alternative raw materials that can be included in coking blends. The briquettes were studied by means of proximate and elemental analyses and density evaluation, whereas the cokes were subjected to micro-strength, compression strength, porous characterization and quantitative evaluation of the textural composition by means of polarized light microscopy (PLM). Various parameters derived from these different techniques were used to explain the effects of biomass and binder on the strength of the coke prepared with the briquettes. Bituminous binders are the most effective because they increase Gieseler fluidity and have a lower volatile matter content than molasses and paraffin. The biomasses that gave rise to the most resistant bio-cokes were lignin and a bio-coal, derived from hydrothermally treated waste lignocellulosic biomass.

Introduction

Nowadays, environmental pollution is one of the main challenges facing the steel industry. 26% of global CO2 industrial emissions comes from the iron and steel industry [1]. Therefore, to ensure the growth of the steel sector, greater attention must be devoted to developing environmentally friendly processes. Another major challenge for this industry is the huge amount of raw material it consumes. The inclusion of biomass in coking blends could be a possible solution to both of these problems. Biomass has been increasingly investigated in recent years with a view to widening the range of alternative raw materials that can be included in coking blends [1,2]. One of the reasons for using biomass is its renewability and carbon-neutrality which translates into zero global emissions of CO2 to the atmosphere. At the same time, the addition of biomass to a coking blend has been shown to decrease coke quality when the level of addition is higher than 2 wt% [2]. Densification of raw materials by thermal treatment and briquetting have shown to mitigate the worsening in the quality of coke produced by blends of coal and biomass [3]. Torrefaction and hydrothermal carbonization (HTC) have been studied for its use in combustion and in the preparation of carbonaceous materials but there are hardly any published works related to its use in the steel industry. There are some studies on the direct addition of charcoal [3,4] but the use of pre-treated biomass in the form of briquettes is very limited [5]. Bituminous binders commonly used to make briquettes generate emissions of polyaromatic hydrocarbons (PAHs) [6]. In this work, non-bituminous binders have been used in the briquettes to minimize the emissions of PAHs. These type of compounds are considered to be carcinogenic and therefore it is better to avoid them [6].

The coke performs three functions inside the blast furnace: acts as an iron ore reducing agent, provides heat and acts as a physical support. Coke is the most important raw material fed in the blast furnace, because it supports the burden as it descends down the furnace, enhance gas distribution and gas permeability in the shaft and facilitate the percolation of liquid iron and adsorption of dust. The use of a high quality coke in the blast furnace will result in higher productivity and lower hot metal costs. For these reasons, coke mechanical strength is one of the most important parameters for evaluating coke quality [7].

Coke strength is important throughout all phases of the steelmaking process. Before reaching the blast furnace, the coke undergoes physical deterioration during handling and transportation. In the blast furnace, the coke is subjected to slow abrasion as it travels down the furnace. Compression strength is also important because the coke forms a porous bed in the lower part of the blast furnace which needs to withstand the high loads without disintegrating. Low coke strength is one of the factors that can produce an inactive deadman that will deteriorate hot metal quality and eventually reduce campaign life [8].

Coke strength has been related to many different parameters. Mochizuki et al. found a positive correlation between the indirect tensile strength of coke prepared from pelletized samples and its maximum fluidity values [9]. In agreement with their observation, a recent study by Montiano et al. showed that the addition of increasing percentages of biomass to metallurgical coking blends reduces the maximum Gieseler fluidity in accordance with a logarithmic equation. These percentages of biomass addition also produce an impairment of coke strength as determined by means of the Japanese Industrial Standard JIS K2151(DI150/15 index: material remaining +15 mm square hole after 150 revolutions) [2]. On the other hand, Ueki et al. attribute this impairment to the release of biomass volatile matter, arguing that connectivity between the coal particles during carbonization is inhibited by the release of the volatile matter from woody biomass [10]. Bulk density has also been proved to be an important parameter. Montiano et al. compared the strengths of cokes obtained by direct addition and partial briquetting, and concluded that the strength is greater when briquettes are used due to the increase in bulk density [11]. Qiu et al. found a linear relationship between MICUM cold mechanical strength indices (M25 i.e. percentage of coke with particle size >25 mm after rotating 50 kg of coke 100 times, M10 percentage of coke with particle size >10 mm after rotating 50 kg of coke 100 times) and the relative content of macropores, indicating that this parameter has a large influence on coke mechanical strength [12]. Optical textural components, which represent the degree of structural order of coke pore walls, have also been related to coke quality [13]. With respect to the use of binders in briquettes, Sharma et al. have reported that a C/H atomic ratio of the binders greater than 1 yields strong carbonized briquettes [14].

To the best of our knowledge, the present study is the first that aims at finding the best combination binder/biomass including four binders and four biomasses to produce high strength briquettes. Physical and chemical characterization is used to scientifically explain the mechanical strength of the bio-cokes. Finding a binder, less pollutant than bituminous binders, with good performance in relation with the strength of the briquettes is seen as an additional aim.

Section snippets

Materials and methods

Blends of biomass, coal and binder were briquetted in a hydraulic press in order to conform 15 briquettes of different composition. The briquettes contained 70 wt% of a bituminous coal, biomass (15 wt%, except when molasses were employed as binder in which case 20 wt% was used) and binder (15 wt%, except for molasses where 10 wt% was used). Four different biomasses were employed i.e. lignin (Lg), pine sawdust (SP), torrified pine sawdust (SPT) and a solid from the hydrothermal carbonization of

Strength of biocokes

The micro-strengths of the cokes from the briquettes with and without biomass are shown in Table 3 and Fig. 1. The cokes present differences depending on the type of biomass used. The results of this study, in the case of ternary blends, show that the lowest R3 indices correspond to the briquettes prepared with lignin and T/CTS as binder (R3 = 54 wt%). On the other hand, the worst strength corresponds to the briquettes containing pine sawdust with molasses and paraffin as binders.

The following

Conclusions

The effect of four binders and four biomasses on the strength of cokes prepared from briquettes has been studied. The following order has been established for the binders: T/CTS > T > Molasses > Paraffin. Bituminous binders are the most effective because they increase Gieseler fluidity and have a lower volatile matter content than molasses and paraffin. Briquettes prepared with paraffin produce cokes with a lower strength than molasses. The most likely reason for this is their high volatile

Acknowledgements

The research leading to these results has received funding from the European Union's Research Fund for Coal and Steel (RFCS) research program under grant agreements No. [RFCR-CT-2014-00006] and No [RFCS-CT-2010-00006].

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