Biosolids Processing
Hydrothermal Processing (HTP) – An Opportunity for Biofuel Production from Wet Biosolids Wastes
Background
Over the past year Earth Systems has been actively investigating the use of high temperature pyrolysis (~700ºC) in order to increase eco-efficiency, reduce carbon footprints, and add value to solid waste streams.
Large volumes of biosolids are generated at sewage treatment plants and through both adsorption and absorption can contain complex mixtures of chemicals as well as pathogens and viruses. This can present a disposal problem – their usage as a furnace fuel being a particular problem on account of heavy metal content. The present solution of stockpiling these wastes for sun-drying can consume a large amount of space.
HTP presents a solution to these problems with a range of beneficial outcomes:
- Generation of a crude bio-oil from which biodiesel could be produced;
- Production of fuel gas to assist in driving the pyrolysis process and/or running other facilities;
- The production of stable biochar for carbon sequestration and/or soil improvement;
- Physical compaction of its on-site stockpiles; and
- Pyrolysis of biosolids represents a tangible means for a water authority to offset its carbon budget.
What is Hydrothermal Processing (HTP)?
Hydrothermal Processing is a thermo-chemical process for converting wet biomass feedstocks to crude biofuel products. Feedstock moisture content is typically high (50% – 80% moisture is common). The HTP reaction involves moderate temperatures and pressures (T = 300 to 370 ºC, P = 120 to 190 atm) and residence times of 5 to 20 minutes. Water is present as a liquid under these conditions. In simple terms, HTP can be described as a de-oxygenating process – removing oxygen from the biomass materials as water and CO2. The products from HTP are crude bio-oils (suitable for upgrading to biodiesel), gases (predominantly CO2 with some CO), water, biochar and ash. The aid of a suitable catalyst may enhance methane (CH4) yield in the gas stream. Yields depend on the exact processing conditions, residence times, feedstock characteristics and catalytic effects. The thermal efficiency of the HTP system is high (70 to 90%).
The bio-crude produced is not miscible with water and can be separated into light and heavy crude by extraction. The light crude can be upgraded to a diesel substitute whereas the heavy crude can be co-fired with solid fuels (e.g. for power generation). Importantly, the process dramatically reduces the overall mass of biosolids and stabilises it to prevent ongoing GHG emissions.
The process is ideally suited to wet biomass feedstocks and organic wastes and essentially upgrades these wastes to second-generation biofuels.
Although the majority of activity in HTP development is pre-commercial R&D, there are significant moves afoot with demonstration / commercial-scale process development, the key economic outputs being gas and liquid fuels.
Application to Biosolids
There has been little reported in terms of HTP application to biosolids treatment, though the problem of sludge disposal is of increasing concern in many places (both in Australia and overseas).
Key issues to be considered when investigating the possibility of converting biosolids to biofuels via HTP-based technology are:
- Sludge inorganics (ash) content, including heavy-metals – behaviour in the process and where/how these inorganics report in the final product streams;
- Nitrogen content – What form does nitrogen take in the final products;
- Bio-oil characterisation – is the bio-oil produced from sludge similar to that produced from other feedstocks – i.e. does it form an acceptable feedstock for upgrading to liquid fuels; and
- Economics – is there sufficient economic merit in the use of sewage sludge as a feedstock for a bio-oil production process.
Experience
Earth Systems is currently undertaking a scoping study to:
- Evaluate the characteristics of particular biosolid feedstocks (originating in Australia and overseas), their behaviour in the HTP environment, and the relevant characteristics of the products generated (in particular, the bio-oil component);
- Perform a feasibility study with Melbourne Water in using their biosolids for beneficial reuse, in particular for energy production (especially gas with high methane content) that is suitable for their current and future biosolids production and is able to accept feedstock with toxic metals.
- Carry out a basic financial analysis considering the value of the biosolid disposal service and the value of the biofuel product. This includes an estimation of the rough capital and operating costs of a HTP plant as a first assessment of financial viability for such a process.
