(630d) Design and Eco-Technoeconomic Comparison of Biomass Pyrolysis Byproduct Utilization Methods in Steelmaking

Steelmaking currently accounts for about 8% of annual global anthropogenic carbon emissions [1]. To reduce these emissions, certain fossil fuels in the steelmaking process can be replaced with biomass-derived alternatives, including biochar, a solid carbon product made from the pyrolysis of biomass [2]. Although it has been shown that using biochar in place of coal in ironmaking and steelmaking reduces net process emissions [3], there are several major obstacles which prevent biochar from being more commonly used. This work focuses on remedying two of these. The first is that biochar is far more expensive than coal on a per mass basis[4], [5]. The second is that, in addition to the solid carbon biochar product, there are two groups of by-products made by the pyrolysis process which are difficult to deal with [6]. These by-products are generally sorted into either light gases, which include hydrogen, carbon monoxide, carbon dioxide, methane, and other light hydrocarbons [7]; and bio-oil or tar, which is consists of heavier hydrocarbons, such as acids, furans, and alcohols [8]. Bio-oil in particular poses a large challenge in the adoption of biochar, as some bio-oil compounds begin to condense to a liquid phase at temperatures as high as 450 °C, which can cause damage and fouling in pipes and other equipment, and do not have much value as a product without further processing [9].

To simultaneously address these two problems, this work focuses on designing processes which utilize the hot pyrolysis by-products before they condense. Compared to the status quo of coal, the normalized basis of comparison for this study, biochar on its own has a much lower value than its cost. However, by introducing methods to utilize the by-products, the gap between the total value of the pyrolysis products and the cost of the coal substitute biochar can be greatly reduced. This allows for the transition to biomass-based carbon sources to have a net-zero or even net-positive effect on the overall profit of processes which utilize these carbon sources, especially in regions where there is a carbon tax.

To enhance the value of by-products, this project focuses on the design and eco-technoeconomic comparison of several by-product utilization process designs. The base case is the current status quo, where the by-products are either treated as hazardous waste or combusted in a flare for no value; a process in which the by-products are combusted in an enclosed flare to recover heat; a process in which the by-products enter an autothermal steam reformer followed by an amine-based carbon dioxide removal process to create pure syngas for use in a direct reduced iron shaft furnace; and combustion of the by-products for the production of electricity.

To date, there have been several papers which have investigated aspects of processes similar to these, such as the value of waste tire pyrolysis by-products [10], [11], the use of biomass pyrolysis by-products for self-sustaining pyrolysis [12], [13], techno-economic analyses of plants which focus on making bio-oil specifically [14], [15], experimental design of catalysts for tar reforming [16], [17], and general information on the heating value of pyrolysis products [18]. This work is unique as it focuses on designing processes to utilize pyrolysis bio-oils before they condense for the purposes of enhancing their value for steelmaking applications. The method is comprehensive as it rigorously compares several different designs on an eco-technoeconomic basis. This comparison is enhanced by utilizing real pyrolysis data from detailed biomass pyrolysis experiments performed at Natural Resources Canada, with further collaboration and design feedback from ArcelorMittal Dofasco, one of the largest steel companies in Canada, and CHAR Technologies, a local company which specializes in the production of pyrolysis products from various forms of biomass.

The processes are mainly modelled in AspenTech’s software suite, ProMax, and Microsoft Excel, which provide information necessary for the eco-technoeconomic analyses. Results are presented on a basis of costs, revenue, and carbon emissions reductions from the utilization of by-products (by offsetting the use of fossil fuels such as natural gas) from a normalized basis of a single metric ton of biochar as produced by a 40 metric kiloton per year biochar production facility. In-depth design and results discussion for some of the methods have been presented at the PSE 2021+ conference held in Kyoto and the 2022 Canadian Chemical Engineering Conference (now also called CSChE). Results published [19] and presented at these conferences showed that utilizing the by-products that come from producing one metric ton of biochar can offset enough fossil fuels to save anywhere from one to four metric tons of carbon dioxide emissions while also generating a profit of $100 USD or more relative to the status quo. This work extends the analysis by considering additional designs, including combusting the by-products for electricity generation. A detailed eco-technoeconomic analysis is then performed between all designs, which considers sensitivity analyses on key economic and simulation parameters, such as carbon tax costs and the price of natural gas. Results show which method is best to use dependent on the value of the underlying parameters, which is useful to account for differing market conditions around the world.

References:

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