Hydrothermal Liquefaction: Technical Risks and Mitigation

Objective Metro Vancouver is building a continuous flow hydrothermal liquefaction (HTL) demonstration facility using wastewater sludge as its feedstock. HTL operates at subcritical temperature and pressure to convert wet biomass into biocrude oil in minutes. Biocrude is then refined to produce low-carbon transportation fuels. As this will be the first HTL system in North America at an operating wastewater treatment plant (WWTP), it is paramount to address risks including:

*Technical safety: the process operates at high temperature and high pressure not typical for municipal wastewater treatment.

*Design risks: odour control and dispersion, fire protection, and design integration risks.

*Regulatory risks and permitting: oil and gas technical regulations typically do not apply to biocrude as it is an emerging product.

*Operational and business risks: viability of HTL at a WWTP depends on sludge feedstock variability and resulting biocrude yield and quality, and low-cost options for byproduct beneficial use or disposal. Status The project team consists of an HTL technology licensee, HTL equipment supplier (Inside-Battery Limits designer/fabricator) and an engineering consultant serving as Owner's Engineer and Outside-Battery-Limits designer. A risk assessment consultant undertook a Qualitative Risk Analysis (QRA). Design is complete with fabrication scheduled to start Q1 2024, on-site construction in Q4 2024, and operation in Q4 2025. Project partner Parkland Refinery will co-process the biocrude. Methodology The project used a continuous improvement approach that assesses and mitigates risk throughout the design process, via:

*3 stages of Hazard and Operability (HAZOP) reviews in preliminary and detailed designs

*2 stages of QRA of toxicity and flammability dispersion

*Detailed design risk assessments of odour control and dispersion, and fire protection

*Intensive project-wide risk assessment workshops and continuous review and update of risk log

*Detailed characterization of biocrude and aqueous and solid byproducts from processing of sludge in bench-top and mobile HTL units

*Developing beneficial use and disposal options for biocrude/byproducts

*Third-party operation by specifically skilled operators Findings The risk assessment workshops highlighted these top-ranked risks:

*High temperature and high pressure process: need to develop equipment applications, operational procedures, PPE, training, and emergency response plans for safe operation.

*Concurrent engineering: multiple interfaces being designed in parallel that can create risk of interface issues if not carefully managed.

*Experimental technology: HTL of wastewater sludge has not been proven at this scale. Limited information is available on process stream characterization and equipment testing in similar conditions.

*Impacts to WWTP: the HTL plant will return streams to the WWTP, which is located beside the WWTP and warehouses, and produces flammable and toxic substances. Abnormal releases or spills could present risks to WWTP operation and human health. The second stage of QRA took into consideration the volume of products and lower potential for run-away reactions based on more accurate product characterization in order to optimize the results obtained by the first stage QRA that has used more conservative assumptions and inputs. The updated QRA showed a significant reduction in dispersion radius for both toxicity and flammability impacts, with all risk contours (location specific individual risk) contained within site and a probability of 0.3 fatalities in 1 million years. This is considered extremely low according to MIACC guidelines as shown in Figure 2. Mitigation of risks during design will be discussed in this paper:

*Design changes to feedstock preparation and delivery (pressure regulation, heat exchanger technology)

*Corrosion assessment plan was developed to use tokens and targeted sampling during operation.

*Bench-scale HTL by academic partner produced biocrude and byproducts for characterization.

*analogous sludge was sourced for processing in mobile HTL unit by technology licensee, providing biocrude for evaluation by refinery partner.

*Feedstock, biocrude and byproduct characterization methods were developed.

*Laboratories that conduct testing across both municipal wastewater treatment and hydrocarbon sectors were identified.

*Aqueous byproduct characterization identified potential issues with loading (Table 1) and toxicity (Figure 3) that could prevent return of the stream to the WWTP.

*UBC researchers screened physical, aerobic, and anaerobic methods for treatment of aqueous byproduct.

*Due to high disposal cost for the aqueous byproduct, pretreatment may be favored to remove visible oils, odour, nitrogen (ammonia) and carbon to allow the stream to be returned to the WWTP.

*Identifying disposal and beneficial use options for solid byproduct prompted changes to solid byproduct handling, required further byproduct characterization and navigating permits at local landfill and waste-to-energy facility (Table 2).

*Biocrude and byproduct management plans influenced storage and transportation design. Significance HTL is a promising solution for wastewater utilities seeking alternatives for biosolids management. Lessons learned from design and operation of this demonstration will inform scale-up of HTL with wastewater sludge, improving the probability of success in future