Exploring Hydrochars from Lignocellulosic Wastes as Secondary Carbon Fuels for Sustainable Steel Production

Abstract

This study investigates the suitability of different lignocellulosic sources, namely eucalyptus, apple bagasse, and out-of-use wood, for injection into blast furnaces (BFs). While wastes possess carbon potential, their high moisture renders them unsuitable for direct energy utilization. Additionally, the P and K impurities, particularly in apple bagasse, can pose operational and product quality challenges in BF. Thus, different thermochemical processes were performed to convert raw biomass into a more suitable carbon fuel. Low-temperature carbonization was selected for eucalyptus, yielding a biochar with properties closer to the low-rank coal. Hydrothermal carbonization was chosen for apple bagasse and out-of-use wood, resulting in hydrochars with enhanced fuel characteristics and fewer adverse inorganic species but still limiting the amount in binary PCI blends. Thermogravimetry evaluated the cause-effect relationships between coal and coal- and bio-based chars during co-pyrolysis, co-combustion and CO₂-gasification. No synergistic effects for char formation were observed, while biochars benefited ignition and reactivity during combustion at the programmed temperature. From heat-flow data in combustion, the high calorific values of the chars were well predicted. The CO₂-gasification profiles of in situ chars revealed that lignin-rich hydrochars exhibited higher reactivity and conversion than those with a higher carbohydrate content, making them more suitable for gasification applications.

Keywords: PCI blends; biomass; biowaste; blast furnace; charcoal; coal injection; combustion; gasification; hydrochar; pyrolysis.

Figure 1

Classification of raw materials and coal- and bio-based chars in the van Krevelen diagram.

Figure 2

Ash elemental composition of apple bagasse and out-of-use woods and their respective hydrochars by XRF.

Figure 3

Pyrolysis DTG curves of the raw materials and respective chars of coals (a), eucalyptus (b), apple bagasse (c) and out-of-use woods (d).

Figure 4

Experimental pyrolysis DTG profiles of LVC and HVC coal (a) and blends composed of the LVC coal with 20 wt% of HVC coal (b), HVC char (c) and eucalyptus char (d); and blends of the LVC coal with 10 and 20 wt% of AB12 (e), AB22 (f), OW12 (g) and OW22 (h). Profiles include the single components and those calculated by applying the additivity law.

Figure 5

TG−DTG−HF co-combustion profiles of LVC (a), metallurgical coke (b) and blends with HVC at 20 wt% (c), CEU at 20 wt% (d), AB12 at 10 wt% (e), AB22 at 10 wt% (f), AB12 at 20 wt% (g), AB22 at 20 wt% (h), OW12 at 10 wt% (i), OW22 at 10 wt% (j), OW12 at 20 wt% (k) and OW22 at 20 wt% (l).

Figure 6

Relationships between the area under the heat flow curve (A_HF) and the higher heating values experimentally obtained in a bomb calorimetric (HHV_exp) and those estimated by the Channiwala and Parikh equation (HHV_estimated) from elemental composition (C, H, N, S and O) and ash content (A) [27]. HHV_estimated = 0.3491C + 1.1783H + 0.1005S − 0.1034O − 0.0015N − 0.0211A.

Figure 7

Evolution of the conversion degree (X, dashed lines) and reactivity to CO₂ (R, solid lines) of the chars produced in situ in the thermobalance from LVC, single additives and the blends of LVC with: HVC (a), CHV (b), CEU (c), AB12 (d), AB22 (e), OW12 (f) and OW22 (g). Metallurgical coke added for reference.