Ibrahim, RM; (2017) Waste Biomass Densification for Thermochemical Conversion. Doctoral thesis , UCL (University College London).

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Abstract

Waste biomass densification into briquettes and pellets improves the characteristics of loose biomass residue for efficient transport, storage and thermochemical conversion into advanced fuels (e.g., syngas, for electricity, liquid fuels and chemicals). Briquettes of good and consistent quality are required but often difficult to achieve as more work is still required to understand how the chemical and physical properties of different biomass types, along with process variables, affect their quality. Also, the additional energy and cost associated with biomass briquetting has raised the issue of the sustainability of briquetting loose biomass before its conversion. This research focuses on the use of novel approaches to improve the quality of fuel briquettes for thermal applications, and further evaluates the sustainability of fuel briquetting, using life cycle assessment (LCA). Experiments were conducted to investigate the potential benefits of blending waste rice husks, corn cobs and bagasse, and with novel binders including enhanced treated biosolids, and microalgae (Chlorella sorokiniana), on fuel briquette properties, using factorial design methods. The new binders were also compared with existing starch binder. The range of briquettes produced in this study had unit densities of up to 3.3 times the loose biomass bulk density, and were stronger than briquettes from the individual biomass materials. Considering average values from two biomass sources, an unconfined compressive strength of 176 kPa was achieved at a compaction pressure of 31 MPa for a 3:7 blend of rice husks to corn cobs with 10% binder (starch + water). These briquettes were durable, with only 4% mass loss during abrasion, and 10% mass loss during shattering, tests. They absorbed 36% less water than loose corn cobs. An unconfined compressive strength of 175 kPa was also achieved for a 2:4:1 blend of rice husks, corn cobs and bagasse with 17% binder (microalgae), also at a compaction pressure of 31 MPa. The statistical analysis of the above results showed that the source of the biomass had a significant effect on densification, which emphasises the need to understand factors underlying biomass variability. Of all the briquettes produced with the three binders, those containing the microalgae binder were found to be most durable, with a higher energy density, slower mass loss during briquette combustion, and a higher afterglow time. Since microalgae may be grown using CO2 from biomass combustion, discovery of their advantages as a binder in briquetting is particularly welcome. To evaluate the sustainability of fuel briquetting, a detailed review of the existing LCA studies on fuel briquetting was carried out. These were found to provide insufficient and inconsistent information, due to different choices in system boundary, data sources, functional unit, allocation procedure, briquetting technology and biomass/briquette properties. An LCA model of biomass briquetting was therefore developed to enable transparent comparison of life cycle environmental impacts of briquetting with individual or blends of biomass feeds with a variety of technological options. The main model components include materials and process inventory databases derived from standard sources, main process calculations, user inputs and results sections. The model is open-access in a user accessible format (Microsoft Excel). A representative case study with mixed rice husks and corn cobs showed that the briquetting unit itself made the largest contribution, 42%, to the total life cycle operational energy of the briquetting system. For all the blends of rice husks and corn cobs explored in this study, the total life cycle energy of briquetting was in the range 0.2 to 0.3 MJ per MJ of fuel briquette energy content. Variation of the LCA input parameters in a sensitivity test for the same blend ratios, gave a range of total life cycle energy of briquetting from 0.2 to 1.7 MJ per MJ of fuel briquette energy content. This indicates that energy use in briquetting is not necessarily recovered, highlighting the need for continuous process optimisation and high quality LCA data. An increase in rice husks content of the blend increased the environmental impact of briquetting including the global warming potential (kg CO2-eq), acidification potential (kg SO2-eq), human toxicity (kg 1,4-DB-eq), ozone layer depletion (kg CFC-11-eq), and terrestrial ecotoxicity (kg 1,4-DB-eq) per MJ briquette energy content, as it was associated with a lower briquette density, which increased the energy required for handling.

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