(668f) Optimization and Performance Improvement of Bioethanol Biorefinery through the Integration of Thermochemical-Utilized Lignin Residue

In global economy, energy crisis has become a main issue since the global energy demand is increasing exceptionally, whilst natural reserves of crude oil are depleting as the time passes. Therefore, it is necessary to find alternative renewable energy sources. These sources include wind, solar, geothermal, hydro- and bioenergy. Bioenergy (e.g. biofuels) is the most promising renewable energy source and bioethanol is one of the main biofuels that is mostly used to be as a transportation fuel. During the production of bioethanol, a huge amount of lignin is produced as a byproduct. The global market of lignin is expected to exceed USD 6 billion by 2022, growing at 4.9% from 2015 to 2022. Due to that, strong industrial R&D has been exploring the commercialization of lignin downstream applications to produce valuable products to increase its revenue.

In this research, a biorefinery was designed to produce bioethanol by simultaneous saccharification and fermentation (SSF) of softwood (spruce) biomass after the extraction of lignin. The performance of this process is enhanced and optimized using Aspen Plus (V.2006) simulation software by investigating the potential of integrating extracted lignin back into the biorefinery. The simulation results are generally not 100% accurate but it provides a reflective prospect to process design and economic analysis of suggested processes in a limited period of time.

Firstly, the recovery of lignin from the biomass was maximised to ensure maximum production rate of bioethanol. The optimal removal rate of lignin was 20.100 tonne per hour (tph), corresponded to bioethanol production rate of 19.117 tph from a total feed charge of about 160tph.

The primary aim of this research is either to generate combined heat and power (CHP) or activated biochar. The first case was generating CHP in steam turbines that can be integrated back into the biorefinery to meet its energy requirements to minimise operating costs. The optimal turbine power of about 470 MW was produced from steam generated by the gasification of lignin. The flow rate of the steam was 2516.138 tph with turbine mechanical and isentropic efficiencies of 97% and 80% respectively. The optimal power generation was achieved by recycling steam through the gasifier to generate as much as 180 times more power than the unrecycled steam. This increase was achieved even that there was a trade-off with the gasifier power requirement, where more energy was required by the gasifier to raise the LP (low pressure) steam to superheated steam temperature. Also, the continuous recycling of steam had minimised wastewater treatment.

Heat integration was carried out to recover the waste heat from the entire system at a power duty of about 54 MW. Heat integration provided a part of the biorefinery energy requirements; reducing further its operating costs. For example, the power recovered and integrated from cooling the steam leaving the last turbine to the boiler feed water (BFW) temperature of about 105^°C resulted in annual cost savings of about 37 million US dollar.

The second case was producing activated biochar at a flow rate of 4.703 tph from the pyrolysis of lignin. Biochar was firstly produced at a flow rate of 7.236 tph and then thermally treated to produce activated biochar with about 35% weight loss. The activation of biochar was achieved in the presence of CO₂ at a flow rate of 1.266 tph and activation temperature of 850°C. Using steam for biochar activation showed no benefit since it reduces power generation and has more GHG emissions when compared to CO₂ activating agent. The product streams were cooled to atmospheric conditions for heat recovery. It was concluded that utilising steam generated from the pyrolysis reactor to produce activited biochar was not benefical in terms of power, cost and environment as a whole. Producing activated biochar had reduced the power generation of the whole process and minimised the potential of heat integration, which resulted in an increase in total production cost. Also the unused carbon dioxide from lignin pyrolysis was discharged to the atmostphere and more CO was formed, making the pyrolysis process harmful to the enviroment.