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
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.
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.
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.