Supercritical Water and Wet Air Oxidation


Bark and twiggy material from the forestry and wood industry have a low economic value and much of is is left in the forest or burnt. Bark can be processed by catalytic hydrolysis and hydrogenolysis, which results in carbohydrates, fatty acids and lignin-related by-products. By controlled partial and complete wet air oxidation (WAO) and/or supercritical water oxidation (SCWO) of the lignin residue functionalised phenols can be produced.

Both WAO (subcritical conditions) and SCWO (supercritical conditions) use water and oxygen as a medium to promote the solubility of materials to facilitate the oxidation of organic compounds. SCWO results in total oxidation to carbon dioxide, water and energy, while WAO is incomplete and leaves small organics. These processes take place under relatively mild conditions. 

Pressure-enthalpy diagram for water


This project has the potential to valorise a waste biomass by-product of the forestry and wood industry that currently has low economic value. Wood bark contains varying amounts of lignin, suberin and carbohydrates.  Lignin is a potential source of aromatic fine chemicals for the chemical industry. Suberin is a potential source of ω-functionalised fatty acids among others.

Suberin: a natural aliphatic- aromatic cross linked polymer



The aim of the pilot is threefold:
• The economic viability of the production of value-added organic products from waste bark needs to be demonstrated.
• An economic and efficient recovery of catalyst metals from the two-stage process needs to be developed.
• To explore the possibilities of treating bark with WAO and/or SCWO directly or integrating the chemical catalytic process with WAO/SCWO into a single process.
• To explore optimal operating conditions (pressure, temperature and supercritical oxidation media other than water, such as CO2).

Difference to BAU technology/approach:

Currently wood bark has a low or partial valorisation as low-grade renewable fuel or a limited market as a horticultural mulch. Much of this woody material, however, is disposed by burning or rotting at the site of operation. The difference in this proposed valorisation of wood waste is the sequential and partial processing allowing value-added products to be isolated in controlled and defined steps that will also facilitate the removal of the corrosive components.

Input stream requirements:

Bark is a pure organic waste stream. It contains several components of potential value for processing to chemical intermediaties: suberin, lignin, carbohydrates.

Tree bark has several biological functions, but is a waste stream from forestry and wood industry.


Available quantity:

Up to 60% of felled timber is left in the forest to rot, and nearly 50% of the harvested timber is wasted as sawdust and offcuts, particularly bark, twigs and small branches. Ca. 50 Mt lignin/lignosulfonate is produced globally as a byproduct of wood pulping.

Drivers for treating this stream:

Value-added products from an underutilised waste source. An increasing amount of the waste is being pelletised for biofuel

but still low in percentage terms. Much of this lignin is discarded or used for low tech applications

such as low-grade fuel and cement binders.

Potential environmental risks related to this waste stream:

No significant environmental risk from the waste.

Economic/technical barriers to collect this stream:

The waste is reasonably concentrated at the sites of the wood processing operations but there will be a cost associated with transport of the material to the new process site or capital implications for building plant at existing forestry sites.

Output stream:

Renewable chemicals such as ω-functionalised fatty acids and aromatics can be obtained.

Potential uses of this output stream:

ω-functionalised fatty acids and aromatics are basic organic chemical components that can be used for various direct applications such as anti-oxidants, nutraceuticals, food additives, surfactants, polymers and also indirect applications as fine chemical intermediates.

Potential environmental risks/benefits related to this output stream:

Benefits: replacement of fossil fuel based intermediates with renewable alternatives. Risks: unknown nature of end product residues at this point. Disposal or destruction may be required.

Economic barriers/drivers for market introduction of this output stream:

Currently bark waste is only used in low tech and low value applications such as low-grade fuel and cement binders. The output streams generated by this process are high-value, but also the associated process costs are higher. A balance still needs to be established.

Technical barriers for market introduction of this output stream:

Separation and purification of products are poorly defined at this stage. The process has difficult operating conditions (temperature, pressure). 

Legislative barriers for market introduction of this output stream:

No direct barriers envisaged.

Pilot description:

The pilot is based on a two-step process. First, bark is processed by catalytic hydrolysis and hydrogenolysis, which results in carbohydrates, fatty acids and lignin-related by-products. Second, controlled partial and complete wet air oxidation (WAO) and/or supercritical water oxidation (SCWO) of the lignin residue produces functionalised phenols. Bark and intermediate compounds between bark and lignocellulosic materials may also be processed directly by WAO and SCWO.

Schematic process overview of the pilot installation


The reactor design / porting and therefore its installation and interaction to the test cell control system are still in the iterative system design process and yet to be “frozen”. This will happen after completion of the functional / non functional specification and its theoretical validation and completion of the Process and Design FMEA’s.


The QUB Batch tester has a volume of 2 litres.

Scale of the equipment:

Large lab scale

Main technological barriers for market introduction:

The current prototype process is a biphasic heterogeneous reaction in aqueous dioxane. It still needs to be investigated if better co-solvent can be found, such as methanol or ethanol. Water is important to the process both as a reactive solvent (hydrolysis) and as an efficient heat sink to provide a controllable process. This aqueous phase can also contain valuable compounds which could be extracted: this needs to be investigated. 

The current process leaves a significant fraction (>50%) of the biomass as a waste by-product and the primary objective will be to employ more aggressive processing conditions to valorise this waste using WAO/SCWO. The conversion of the residue from the first stage of the valorisation has yet to be demonstrated to deliver high value-added products: partially hydrolysed and oxidised carbohydrates (furfural, hydromethylfurfural, levulinic acid etc) and specialised hydrothermal chars, which are functionalised charcoals with specialist properties and applications. It is still unclear whether it will be possible to develop an integrated process combining the chemical catalytic steps with the WAO/SCWO. Alternatively, the raw biomass can be directly converted into value-added organic products in a one-stage process using WAO/SCWO.

Main economic barriers/drivers for market introduction:

Wet Air Oxidation and hydrolysis under sub- and super-critical conditions has been commercialised by several companies for the treatment of waste and the valorisation of biomass but capital plant costs and technical barriers such as corrosion, blockage and energy balance have been prohibitive. The value of the products needs to determined more accurately if the cost of the process is to be balanced economically. Energy input costs will be considerable and the process might require final oxidation of the organic residues to help balance these costs.
A significant barrier to the development of this process to full commercialisation will be the high initial investment by a stakeholder to fund the necessary research and development of a process that still carries a high risk because of the technical barriers that still need to be solved and the uncertainty around the final value of the product streams. A pilot plant to investigate a continuous or semi-continuous process is likely to be in the region of Euro 1 million, based on a similar type of pilot plant designed and implemented in Ireland in 2012.
The process is based on the premise that the target waste substrate, bark and twiggy offcuts from the forestry industry, is currently of low to zero economic value providing an opportunity for a valorisation process. However, the size of the potential market for this material is not certain and will require input from interested stakeholders in this area. Although the main products from this process have been identified (see Output section) the potential value and size of the market for these products is still uncertain and will require thorough market research to establish the economic viability of the process.
Further, the process will be producing other streams of by-products, such as aqueous carbohydrates, which have an intrinsic value but will require collaboration to determine their optimum end use and intrinsic value. Early steps with one of the ReNEW partners, University of Limerick, have already started in a collaborative study.
Life cycle analysis and calculation of the carbon footprint of the process will also need to be built in to the next stage of the development and we are seeking collaboration in this area. Apart from the energy balance of the process itself, the logisitics of transporting raw materials and location of the plant will need to be factored in to the calculations.

Research steps:

• Definition of process and key viable products;
• Separation technology;
• Design and implementation of reactor.

Economic steps:

A full economic costing of the process and equipment and market evaluation of the products will be required.

Legislative steps:



Expected time to market: minimum 10 years to full commercialisation. Demonstration of principle by the end of ReNEW.

Contact details:
Organisation: Queen's University Belfast

Department: School of Chemistry and Chemical Engineering

Leading researcher: Dr. Gary Sheldrake & Dr. Stephen Glover

Phone number:

E-mail: &



Presentation: Valorisation of Lignocellulosic Waste and the potential for Supercritical Water and Wet Air Oxidation – part 1, G. Sheldrake, Queen’s University Belfast, RENEW Technology Foresight Conference 24/04/2013

Presentation: Valorisation of Lignocellulosic Waste and the potential for Supercritical Water and Wet Air Oxidation – part 2, S. Glover, Queen’s University Belfast, RENEW Technology Foresight Conference 24/04/2013