Integrating torrefaction of pulp and paper industry sludge with microbial conversion: A new approach to produce bioenergy carriers and biochemicals in a view of bio and circular economy.

Project Information

TOPIS-BioCirc

Grant agreement ID: 843723

DOI

10.3030/843723

Project closed

EC signature date 23 April 2019

Start date 1 September 2020

End date 31 August 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 154 193,28

EU contribution

€ 154 193,28

Coordinated by

EESTI MAAULIKOOL
Estonia

Periodic Reporting for period 1 - TOPIS-BioCirc (Integrating torrefaction of pulp and paper industry sludge with microbial conversion: A new approach to produce bioenergy carriers and biochemicals in a view of bio and circular economy.)

Reporting period: 2020-09-01 to 2022-08-31

Summary of the context and overall objectives of the project

In order to achieve the ambitious targets of EU’s climate action plan, there is a need to decarbonize the energy and chemical industries, which are the driving forces for global economy and heavily depended on fossil resources. Torrefied biomass pellets could be an alternative to coal. However, lower economic competitiveness compared with coal in general market conditions is the main challenge with torrefied pellets. As a common practice, the volatiles released during torrefaction are combusted together with utility fuel to produce the heat energy. These volatiles are rich in organic acids and water, and can be used in applications other than energy production.

The main challenges for the commercial scale production of bio-based chemicals are higher feedstock prices, continuous availability of feedstock, higher operational costs, lower environment feasibility. Thus, there is a need to produce bio-based chemicals from low-cost or no-cost feedstock. Sharing the resources between different industries under the concept of industrial symbiosis could help to reduce the operational expenses and can help to improve the overall feasibility of bio-based products. Because of the reluctant nature of lignocellulosic biomass, hydrolysis is a rate limiting step and requires costly and complex pretreatment to improve the solubility of the biomass components. Thus there is a need to develop a disruptive approach to conventional pretreatments.

On the other hand, pulp industries are known to produce large quantities of sludge during the treatment of wastewater from pulping process, which is rich in organic fraction. Today, pulp industries are handling the sludge through landfilling, anaerobic digestion, incineration, composting and land applications in agricultural activities. All these processes have either policy and/or technical challenges. Thus, there is a need to find advanced valorization strategies to handle the pulp industry sludge.

The aim of this project was to develop an integrated approach to produce bio-coal, biomethane and volatile fatty acids using pulp sludge as a low cost alternative feedstock to forestry biomass by integrating torrefaction with anaerobic digestion. The overall objective of this project was to improve the technical, economical and environmental feasibility of bio-based products compared with fossil resources.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Initially the dewatered pulp sludge was torrefied using batch and continuous torrefaction reactors. The torrefaction volatiles were condensed to produce torrefaction condensate. The physicochemical properties and composition of torrefied pulp sludge and torrefaction condensate was established. Later, the condensate was used as a substrate in anaerobic digestion to produce biomethane both in batch and continuous experiments at mesophilic and thermophilic conditions. The anaerobic digestion of pulp sludge derived condensate was also carried out to produce volatile fatty acids. In order to produce VFA, the methanogensis was inhibited. Finally, the experimental results were simulated to industrial scale in order to understand the feasibility of the process. Finally, the techno-economic and environmental feasibility was established for a commercial scale process.

The torrefied pulp sludge showed superior fuel characteristics compared with some of the agricultural wastes. The ash content of the pulp sludge was increased to maximum of 1.7 times. A theoretical evaluation using different indices based on ash composition showed that the slagging and fouling tendency of the ash is lower compared with agricultural wastes The fiber analysis showed that higher degradation of lignin for pulp sludge compared with woody biomass was mainly because of the catalytic effect of ash. The torrefied pulp sludge can be compared with low rank coals in terms of fuel characteristics.

The composition of pulp sludge derived torrefaction condensate varied significantly compared with biomass derived condensate. This could be mainly because of the catalytic effect of the inorganics present in the ash. Interestingly, the acetic acid content in the condensate reduced with increasing torrefaction temperature for pulp sludge, which is contrary to the biomass. The compounds such as furfural, 5-HMF, and levoglucosan were not identified and higher concentration of 2-furamethanol was observed. Additionally, higher concentrations of lignin-derived compounds, especially the concentration of syringol was much higher.

The biomethane potential of pulp sludge derived condensate is 2 to 3 times higher than conventional substrates used in anaerobic digestion. The rate of methane production is also significantly higher. The reasons for higher methane yield could be the higher solubility and availability of organic fractions (i.e. organic acids) in readily digestible form. The condensate loading at higher concentration however, showed microbial inhibition. The continuous studies showed that, the concentration of condensate higher than 10 wt.% (VS) in the digester is highly inhibitory to the AD process. The sequential batch in semi-continuous approach could be a better strategy for AD of torrefaction condensate. The specific VFA yield in case of methanogenesis inhibition is also 2 to 3 times higher than conventional substrates. Acetic acid is the major compound in the VFA. Compared with raw sludge the VFA yield for condensate is 2 times higher. Following the biogas production, VFA yield also reduced with increasing condensate loading.

The economic feasibility study showed that the torrefied pellets selling price can be reduced from 406 €/t for wood chips to 185 €/t in case of pulp sludge. Considering today’s market price of woodchips and/or coal, the selling price of biomethane and VFA can be reduced by 90 and 64%, respectively compared with their market price. The environmental feasibility of pulp sludge torrefaction is lower compared with forestry biomass torrefaction mainly because of higher energy input for drying the sludge and VFA recovery. Integrating the pulp sludge torrefaction with existing pulp mill in terms of heat energy supply, showed better economic performance. The overall feasibility of pulp sludge torrefaction depended on its moisture content and also the gate fee charged from the pulp mills.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This project proposed an alternative approach for resource recovery from pulp industry sludge and also presented an integrated approach to produce bioenergy carriers and biochemical with improved economic competitiveness. By addressing the major challenges such as 1) solubility of organic fractions 2) mass transfer limitations 3) the inevitable loading of un-digestible fraction such as lignin and ash 4) lower product yield 4) digestate handling this project advances the biomass and/or organic waste valorization and further improves the process feasibility of anaerobic digestion.

By developing a strategy to produce the energy and chemicals from organic residue this project helps to reduce CO2 emissions and directly supports EU’s climate action plan. As several countries have agreed to reduce their dependence on coal under COP26 in a timely manner, this project’s results could support the production of torrefied pellets at competitive price. By proposing an alternative strategy to landfill and/or composting of organic residue, this project supports the recirculation of resources back into the economy and thereby, improves the resource efficiency.

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