Production of Biochar from Pyrolysis Blends: Biochar Sequestration Potential, Simulation, Techno-Economic Analysis and Life Cycle Assessment

Like other growing nations, Qatar has significant waste management issues in the wake of its rapid economic growth, improved lifestyle, and urbanisation. The two primary types of trash in Qatar are animal and municipal solid wastes. These two sources can be used to create fuels and goods with additional value, especially biochar, because they are rich in energy and chemicals. Biochar has grown to be one of the most sought-after commodities as Qatar's arable land continues to expand.

This research starts with a literature review where multiple sections were covered to identify temperature, heating rate, reactor bed height and type, residence time, pressure, feedstock type and blending ratio of feedstock as the determinants that had the highest influence on the yield, stability, and carbon content of biochar. Most importantly, the technical readiness level (TRL) of biomass pyrolysis, plastic pyrolysis and co-pyrolysis of biomass and plastic to identify the technical maturity.

Following that, the thesis is divided into 3 main sections each to serve a different objective. Section 1 of this study performs a co-pyrolysis study of four main types of feedstocks, such as camel manure (CM), date pits (DP), low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Additionally, different mixing scenarios of those feedstocks are also studied. For this purpose, a simulated equilibrium model for the pyrolysis process has been developed using Aspen Plus^®, which also investigates the impact of waste types, blends, and temperature on pyrolysis product distribution. Pyrchar and pyrgas production are major products for camel manure and date pits and to a higher extent for plastic wastes. In biomass feedstock, the production of pyrgas is highest at 623.78 kg/hr for camel manure and 555.69 kg/hr for date pits at 600oC. The maximum production of pyrchar 485.43 kg/hr for LDPE and 618.46 kg/hr for HDPE is for plastic feedstock. As for mixing scenarios, the production of char is maximum at a ratio of 0.2CM: 0.2DP: 0.3LDPE: 0.3HDPE (S6) and 0CM: 0DP: 0.5LDPE: 0.5HDPE (S7) at a temperature of 600oC resulting in 448.15 kg/hr and 614.69 kg/hr respectively. The pyrgas production at mixing ratios of 0.25CM: 0.25DP: 0.25LDPE: 0.25HDPE (S5) and 0.5CM: 0.5DP: 0LDPE: 0HDPE (S8) is maximum with 405.21kg/hr and 589.24 kg/hr respectively at a temperature of 600oC. Pyroil production is highest at a temperature of 300oC for all scenarios resulting in a maximum production of date pits at 309.92 kg/hr.

Section 2 of this study considers the potential for carbon sequestration through biochar obtained from the pyrolysis of the same proposed feedstock mixture, whilst enhancing water and food security. The study conducted for a Qatar case study indicates that the best waste blending scenario for maximum carbon sequestration potential was 20.4% Camel manure: 27% date pits: 26.3% LDPE: 26.4% HDPE. Furthermore, the optimum char blend for maximum carbon sequestration corresponding to the minimum cost of char mix is computed. The optimal biochar mixing percentage for highest net emission was obtained at a mixing ratio of 96.8% of date pits, 1.5% of LDPE and 1.7% of HDPE with 0% of date pits with an optimal cost of 313.55 $/kg biochar. Solar PV was the best fuel source in this pyrolysis study due to its reduced carbon emissions in comparison to other sources studied such as natural gas, coal and diesel. However, when studying the optimal solution in terms of cost solar PV is not the best option due to its high construction, operation, and maintenance cost. Moreover, the optimal water source was further investigated including wastewater treatment, multi-stage flash and reverse osmosis.

Section 3 was performed to analyze the techno-economic aspect and the life cycle assessment of pyrolysis plant. Results show that the total CAPEX and OPEX was estimated to be 280 and 68.4 M US. $ respectively. In addition, the annual sales are estimated to be 107.6 M US. $ and the total annual profit was 39.2 M US.$. Moreover, the levelized cost of products such as biochar, bio-oil and syngas were high for biochar and estimated to be 0.166, 0.293, and 0.022 M US.$. Furthermore, energy and carbon footprints were high during pyrolysis process followed by the drying and grinding processes due to energy usage emissions, while the water footprint was estimated to be 3,360 m³/year and a land footprint for plant construction was around 32.4 ha/lifespan.