Utilisation of lignin-based biocarbon in pyrometallurgical applications

Abstract Pyrometallurgical processes in the iron and steel industry are energy intensive. Therefore, sustainability in terms of CO2 emissions is highly dependent on the choice of fuel. Large quantities of carbon are used in pyrometallurgical processes, for example in slag foaming, iron oxide reduction or as an alloying element. Currently, the majority of the used carbon originates from fossil sources. The most important properties that are generally required from carbonaceous materials in pyrometallurgical applications are mechanical strength, sufficient apparent density and suitable reactivity. The properties of industrially produced metallurgical coke works as a great reference for evaluation of these properties, since metallurgical coke is used in multiple pyrometallurgical applications in different forms: coke dust is used as a foaming agent and carburiser in the electric arc furnace (EAF) process, and coke lumps are used as a reducing agent and structural bed material in the blast furnace (BF) process and as a reducing agent in submerged arc furnace (SAF) process. This thesis focuses on the utilisation of hydrolysis lignin as a raw material for the production of biocarbon that could be utilised as a carbonaceous material in pyrometallurgical applications to substitute fossil-based carbon. Based on the results of this thesis, it was discovered that the structure of lignin-based biocarbon can be modified using the chosen treatment methods, briquetting and high pyrolysis temperature. With these treatment methods, the important properties (mechanical strength, apparent density and reactivity) were improved and modified, with the compressive strength property of biocarbon even surpassing that of metallurgical coke.Tiivistelmä Terästeollisuuden pyrometallurgiset prosessit ovat energiaintensiivisiä. Sen vuoksi terästeollisuuden ekologisuus CO2 päästöjen osalta on erittäin riippuvainen polttoainevalinnasta. Suuria määriä hiiltä käytetään pyrometallurgisissa prosesseissa, esimerkiksi kuonan kuohutuksessa, raudan oksidien pelkistyksessä tai seosaineena. Nykyään suurin osa käytetystä hiilestä on lähtöisin fossiilisista lähteistä. Pyrometallurgisissa prosesseissa käytettäviltä hiilimateriaaleilta vaadittuja tärkeimpiä ominaisuuksia ovat mekaaninen lujuus, riittävä näennäistiheys ja sopiva reaktiivisuus. Teollisesti valmistetun metallurgisen koksin ominaisuudet sopivat hyvin näiden ominaisuuksien vertailukohdaksi, sillä koksia käytetään useissa eri sovelluksissa eri muodoissa: koksipölyä käytetään kuonan kuohutusaineena ja hiilen tuojana valokaariuuniprosessissa, palakoksia pelkistimenä ja rakenteellisena petimateriaalina masuunissa sekä pelkistimenä uppokaariuunissa. Tämä työ keskittyy hydrolyysiligniinin hyötykäyttöön raaka-aineena biohiilen ja biokoksin valmistuksessa, joita voidaan käyttää hiilen tuojana pyrometallurgisissa sovelluksissa korvaamaan fossiilista lähteistä tuotettua hiiltä. Tämän työn tulosten perusteella hydrolyysiligniini-pohjaisen biohiilen rakennetta pystyttiin muokkaamaan valituilla käsittelymenetelmillä, briketoinnilla ja korkealla pyrolyysilämpötilalla. Näillä menetelmillä biohiilen tärkeitä ominaisuuksia (mekaaninen lujuus, näennäistiheys ja reaktiivisuus) saatiin parannettua siten, että biohiilen puristuslujuus oli jopa suurempi kuin metallurgisella koksilla

Interaction between coal and lignin briquettes in co-carbonization

Abstract The utilization of bio-based side streams in metallurgical coke making promotes two major factors in the mitigation of climate impact in the steel industry. Circular economy as the waste material from biorefinery industry is utilized as a raw material in the steel industry, and mitigation of the production of fossil-based CO2 emissions. In this work, lignin from the hydrolysis process was used in a briquetted form as part of the raw material blend in metallurgical coke making. For the experiments and analyses, lignin briquettes were pyrolyzed at 450, 600 and 1200 °C, while one sample was left non-pyrolyzed. In the co-carbonization of briquetted lignin, lignin chars and bituminous coal, the focus was to evaluate the interaction between char and coal in the carbonization. This was studied by thermogravimetric analysis (TGA), optical dilatometry, and light optical microscopy. The results suggested that the interaction between the coal and lignin reduced when the pyrolysis temperature of the briquettes, prior to co-carbonization, was elevated. This was due to the decrease of overlapping of the pyrolysis rates of chars and coking coal. Combined with the dilation and shrinking behaviour of the chars, presented in this paper, separate char and coke structures were formed in the final coke in co-carbonization

Compression strength of coke after gasification

Summary In this work gasification of different types of coke (higher quality and lower quality) were subjected with two different gas-atmosphere profiles (H₂-H₂O-CO-CO₂-N₂ and CO-CO2-N2) simulating Blast Furnace circumstances. It was found out that coke was more reactive towards H₂ and H₂O containing gas atmosphere. After gasification, coke samples were subjected to compression strength tests. It was found out that although higher quality coke reacted more intensively in H₂ and H₂O containing atmosphere, the compression strength properties were not harmed by the reaction of a higher extent. The compression strength properties were improved after gasification in H₂ and H₂O containing atmosphere for the higher quality coke. This could be due to increase in small-scale pores which make the porous structure more uniform and therefore decrease the heterogeneity of elasticity throughout the coke structure. Image analysis showed a linear correlation between strength, strain and area percentage of large pores in the coke structure

Evolution of biocarbon strength and structure during gasification in CO₂ containing gas atmosphere

Abstract This work focuses on the properties of hydrolysis lignin biocarbons with a perspective on utilizing the biocarbons in pyrometallurgical processes. Even if the blast furnace and basic oxygen furnace (BF-BOF) process route was replaced by emerging technologies with lower CO₂ emissions in the future, the need for carbonaceous materials in the iron and steel making industry will still exist. Most of these applications do not require as high standards for the properties of carbonaceous materials as BF but the requirements are still similar to those for BF. The most important properties of carbonaceous materials are the mechanical strength and suitable reactivity. In the case of biocarbon, the apparent density is also considered important. The reactivity and strength properties are investigated with isothermal reactivity tests and compression strength tests for the non-gasified and pre-gasified biocarbon and reference coke samples. The mass loss rate of coke gasification (-0.069%/min) was considerably lower than that of least reactive biocarbon L1200 (-0.18%/min). Regarding the compression strength of the samples, the strength of coke dropped by 56.44% for the samples of pre-gasification level of 50% compared to non-gasified samples while the drop was only 40.68% for the L1200 biocarbon samples. The level of gasification was found to have direct correlation with pore area percentage with R² value 0.92 in case of L1200 and 0.98 in case of coke. Further, the pore area percentage correlated with the compression strength with R²of 0.93 in case of L1200 and 0.98 in case of coke

A review of pyrolysis technologies and the effect of process parameters on biocarbon properties

Abstract Biomass-based solutions have been discussed as having the potential to replace fossil-based solutions in the iron and steel industry. To produce the biocarbon required in these processes, thermochemical treatment, pyrolysis, typically takes place. There are various ways to produce biocarbon, alongside other products, which are called pyrolysis oil and pyrolysis gas. These conversion methods can be divided into conventional and non-conventional methods. In this paper, those techniques and technologies to produce biocarbon are summarized and reviewed. Additionally, the effect of different process parameters and their effect on biocarbon yield and properties are summarized. The process parameters considered were final pyrolysis temperature, heating rate, reaction atmosphere, pressure, catalyst, use of binders, and particle size. Finally, the effect of different reactor configurations is discussed. Understanding the combination of these methods, technology parameters, and reactor configurations will help to produce biocarbon with the desired quality and highest yield possible

Effects of briquetting and high pyrolysis temperature on hydrolysis lignin char properties and reactivity in CO-CO₂-N₂ conditions

Abstract Carbonaceous reductants for pyrometallurgical applications are usually obtained from fossil-based sources. The most important properties of the reductants greatly depend on the application and the feeding of the reductant into the process. However, the mechanical strength, calorific value, fixed carbon content, and reactivity of the reductant are the properties that usually define the applicability of the reductant for different processes. The reactivity of the biochars is usually high in comparison to metallurgical coke, which may restrict the applicability of the biochar in reduction processes. One cause of the higher reactivity is the higher surface area of the biochars, which can be suppressed with agglomeration treatment, e.g., briquetting. In this work, hydrolysis lignin was used for slow pyrolysis experiments to produce biochars. The biochars were pyrolyzed in briquetted form and in as-received form at various temperatures. The reactivity values of the biochars were tested in dynamic reactivity tests in a CO-CO₂-N₂ gas atmosphere at temperatures of up to 1350 °C. It was found that the yield of the hydrolysis lignin char only decreased by 3.36 wt% when the pyrolysis temperature was elevated from 600 to 1200 °C, while a decrease in yield of 4.88 wt% occurred when the pyrolysis temperature was elevated from 450 to 600 °C. The mass loss of hydrolysis lignin biochar in the reactivity experiment in CO-CO₂-N₂ atmosphere was significantly decreased from 79.41 wt% to 56.80 wt% when the hydrolysis lignin was briquetted before the slow pyrolysis process and the temperature of the pyrolysis process was elevated from 600 to 1200 °C. This means that the mass loss of the material was suppressed by 22.61 wt% due to the higher pyrolysis temperature and briquetting process

Lignin from bioethanol production as a part of a raw material blend of a metallurgical coke

Abstract Replacement of part of the coal in the coking blend with lignin would be an attractive solution to reduce greenhouse gas emissions from blast furnace (BF) iron making and for obtaining additional value for lignin utilization. In this research, both non-pyrolyzed and pyrolyzed lignin was used in a powdered form in a coking blend for replacing 5-, 10- and 15 m-% of coal in the raw material bulk. Graphite powder was used as a comparative replacement material for lignin with corresponding replacement ratios. Thermogravimetric analysis was performed for all the raw materials to obtaining valuable data about the raw material behavior in the coking process. In addition, chemical analysis was performed for dried lignin, pyrolyzed lignin and coal that were used in the experiments. Produced bio cokes were tested in a compression strength experiment, in reactivity tests in a simulating blast furnace shaft gas profile and temperature. Also, an image analysis of the porosity and pore shapes was performed with a custom made MatLab-based image analysis software. The tests revealed that the pyrolysis of lignin before the coking process has an increasing impact on the bio coke strength, while the reactivity of the bio-cokes did not significantly change. However, after certain level of lignin addition the effect of lignin pyrolysis before the coking lost its significance. According to results of this research, the structure of bio cokes changes significantly when replacement of coal with lignin in the raw material bulk is at a level of 10 m-% or more, causing less uniform structure thus leading to a less strong structure for bio cokes

Effect of coal and coke ash on blast furnace slag formation:comparison between PCI, charcoal, fossil-based coke and bio-coke

Abstract Blast furnace is the most used process for production of pig iron in the world. It is charged mainly with metallurgical coke and ferrous materials such as pellets and/or sinter. When descending inside the blast furnace, iron bearing materials start to reduce and with other burden material start to melt, which leads into formation of so-called primary slags from which the final slag is formed as materials descend inside the furnace. Every primary slag originates from one charge material and has unique effect on the total composition of the blast furnace slag. This work focuses on the primary slags of coke and pulverized coal injection (PCI). The global trend is to decrease the use of fossil-based carbon by replacing it with bio-based carbon. The primary slags of the coke and PCI originate from coke ash and pulverized coal ash. The purpose of this work is to evaluate how blast furnace slag composition is changed when fossil-based coke is replaced with bio-coke and PCI is replaced with charcoal. The effect differs case-by-case as presented in this work, but it was found out that replacing fossil-based coke with bio-coke and PCI with charcoal the solidus and liquidus temperatures as well as CaO/SiO₂ - and MgO/Al₂O₃ -ratios are increased. This comparison is based on mass balance calculations

Effect of coal and coke ash on blast furnace slag properties:a comparison between pulverized coal, charcoal, fossil-based coke, and biocoke

Abstract Blast furnace (BF) is the most used equipment for production of iron in the world. It is charged mainly with metallurgical coke and ferrous materials. When descending inside the BF, iron-bearing materials start reducing and melting with other burden materials. This melting leads to the formation of so-called primary slags from which the final slag is formed as materials descend inside the furnace. Each charge material has a unique effect on the total composition of the BF slag. Herein, the parts of the slag, which originate from ash of metallurgical coke and pulverized coal, and changes in the final slag compositions and properties are focused on. The global trend is to decrease the use of fossil-based carbon by replacing it with bio-based coal. Ash from coke and pulverized coal eventually dissolve in the final slag, affecting its properties. The purpose herein is to evaluate how BF slag composition changes when fossil-based coke is replaced with biocoke and pulverized coal is replaced with charcoal. Based on mass balance calculations, these replacements have both increasing and decreasing effects on solidus and liquidus temperatures, viscosity, and CaO/SiO₂ and MgO/Al₂O₃ ratios depending on the used replacement materials

Effect of charcoal and Kraft-lignin addition on coke compression strength and reactivity

Abstract The aim of this research was to investigate the effects of charcoal and Kraft-lignin additions on the structure, cold compression strength, and reactivity of bio-cokes produced at the laboratory scale. Bio-cokes were prepared by adding charcoal and Kraft-lignin (2.5, 5.0, 7.5, and 10.0 wt %) to medium-volatile coal and coking the mixture with controlled heating rate (3.5 °C/min) up to 1200 °C. In addition, four particle sizes of charcoal were added with a 5 wt % addition rate to investigate the effect of particle size on the compression strength and reactivity. Thermogravimetric analysis was used to evaluate the pyrolysis behavior of coal and biomasses. Optical microscopy was used to investigate the interaction of coal and biomass components. It was found that by controlling the amount of charcoal and Kraft-lignin in the coal blend, the compression strength of the bio-cokes remains at an acceptable level compared to the reference coke without biomass addition. The cold compression strength of the charcoal bio-cokes was higher compared to Kraft-lignin bio-cokes. The reactivity of the bio-cokes with charcoal addition was markedly higher compared to reference coke and Kraft-lignin bio-cokes, mainly due to the differences in the physical properties of the parental biomass. By increasing the bulk density of the coal/biomass charge, the cold compression strength of the bio-cokes can be improved substantially