## 4 Modified Steel slag as Catalysts  
### Catalytic Pyrolysis  
In the process of catalytic pyrolysis, the practical application of bio-oil is limited due to its disadvantages such as high oxygen content, low calorific value, high acidity, highthermal instability, and so on [34]. In order to make the properties of bio-oil closer to fossil fuel to a greater extent, it is necessary to improve the quality of bio-oil. Catalytic cracking is considered to be a more effective method [35], so it is very important to find a suitable catalyst. Metal oxides and non-metal oxides are also commonly used in the catalytic upgrading of bio-oil. For example, [PERSON] et al. [36] carried out catalytic pyrolysis experiments on wood-polypropylene composites using oxides such as CaO, Fe\({}_{2}\)O\({}_{3}\), and MgO. CaO, due to its strong alkalinity, can effectively remove the oxygen content in the produced oil. At the same time, it can reduce the acid compounds in the pyrolysis products and increased the content of cyclopentanone and olefin. It was reported in [37] that the catalytic performance of Fe\({}_{2}\)O\({}_{3}\) and MgO is weaker than that of CaO, but MgO has strong chain-breaking activity, and the increase in olefin is obvious. Fe\({}_{2}\)O\({}_{3}\) can promote the generation of aromatic hydrocarbons. In short, these three oxides can reduce the oxygen content of bio-oil and improve its quality. Oxides such as Al\({}_{2}\)O\({}_{3}\), CaO, MgO, CuO, and Fe\({}_{2}\)O\({}_{3}\) were selected for catalytic pyrolysis experiments in [38]. Due to more decarboxylation or less dehydration, CaO and Al\({}_{2}\)O\({}_{3}\) have the best performance when considering the yield and deoxygenation of bio-oil comprehensively. When considering the mass, water content, and pH value of bio-oil, CaO shows the best performance in the balance of the three conditions. The oxygen in bio-oil is mainly removed by CO and CO\({}_{2}\) through the reaction with oxides, so the amount of gas in catalytic pyrolysis is increased, and these bio-syngases can also be used as gas fuel. Therefore, through the catalysis of these oxides, not only can the quality of bio-oil be improved, but also the gas fuel content can be increased. It can be seen from Table 1 that the main components of steel slag contain these oxides. As long as we remove the active components through some simple modification methods and increase the contact between the pyrolysis products and the active components, the steel slag can show good catalytic performance to the maximum extent. The following will introduce some research achievements in the catalytic pyrolysis of modified steel slag in recent years. [PERSON] et al. [39] used modified steel slag as a catalyst to improve gas production efficiency and the conversion rate of tar when studying the properties of pyrolysis and tar cracking and reforming of materials. For the research, after the steel slag calcined at 900 \({}^{\circ}\)C, relatively stable Fe\({}_{2}\)O\({}_{3}\) and MgFe\({}_{2}\)O\({}_{4}\) crystals formed on the surface of the steel slag to produce a loose porous structure, which was conducive to the contact between the active components in the steel slag and the reactants to improve the catalytic activity of the steel slag. The reduced gas generated in the process of biomass pyrolysis reduced the Fe\({}_{2}\)O\({}_{3}\) in the steel slag to low-valent iron. When the catalytic performance of the steel slag was further improved, the conversion rate of tar and the efficiency of synthesis gas increased accordingly with the maximum conversion rate of 94.1% for tar obtained. Steel slag was effective in long-term applications as a catalyst in the removal of biomass tar. This not only recycled the steel slag, but also improved the efficiency of biomass pyrolysis and tar cracking and reforming, and avoided secondary pollution caused by the production of tar in the process of biomass pyrolysis. [PERSON] et al.