The utilization of forest side-streams is associated with the applied bioenergy technology that must be impellent to support the increasing demand for biofuels and resources while lowering greenhouse gas (GHG) emission from the transport sector. This study aimed to estimate potential biofuel production from eutrophic (EL) and mesotrophic (ML) lake bottom biomass and the manufacturing side-streams from the pulp and paper mill (PI – PVIII). Theoretical biogas and bioethanol productions were modeled by Aspen Plus® simulation through 1) saccharification and fermentation, 2) gasification and mixed alcohol synthesis, 3) gasification-syngas fermentation, and (4) anaerobic digestion processes. In addition, the different process stages of the pulp and paper side-streams was studied by ABE fermentation using Clostridium acetobutylicum DSM 1731.
The bioethanol produced from EL and ML biomass from indirect gasification and mixed alcohol synthesis were 244.5 L/t and 57.1 L/t, whereas the yields from saccharification and fermentation were 137 L/t and 40 L/t, respectively. The EL biomass produced the most profitable bioethanol production from the latter process. The ML and EL biomass produced biogas of 38.9 mL/g volatile solid and 136.6 mL/g volatile solid, respectively. The ash from the EL and ML biomass and the dried samples of PI and PIII could be used as fertilizer because the harmful elements for Finnish fertilizer products were below the detectable limit. The primary sludge (PII) sample had found high N and P concentrations and cadmium (Cd) concentration (3 mg/kg), which exceeded the Cd limit for Finnish fertilizer products (1.5 mg/kg). However, wet primary sludge (PII) forming 300,000 tonnes/year (72600 dry tonnes) produced anhydrous ethanol about 3011 kg/h (24,090 tonnes/year) when PII was used for the gasification-syngas fermentation process in the bioethanol plant model.
Three pulp and paper side-streams (PI, PII and PIII) with unwashing and water washing were pretreated with dilute acid (0.2% H2SO4 at 180 °C for 10 min), followed by saccharification and ABE fermentation. The results suggested that water washing did not affect the PII and PIII prehydrolysate sugar recovery, as well as enzyme hydrolysis of the rejects from kraft pulping (PI) did not require prewashing before dilute acid pretreatment. In addition, the unwashed PI side-stream yielded the highest ABE concentration of 12.8 g/L, compared to the unwashed PII and PIII side-streams, 5.2 g/L, and 6.3 g/L, respectively. The side-streams from different process stages in pulp and paper mill were concluded to be high potential feedstocks for biofuels production due to their chemical compositions. The unwashed PI was suitable feedstock for butanol production, while PII could be fully utilized in the integrated gasification-syngas fermentation process. Primary sludge (PII) was found to be a promising feedstock for bioethanol and an internal rate of return (IRR) of 15 % can be obtained by two implementations. One was a cost-competitive ethanol selling price (ESP) of €0.61–0.71/L with an ethanol subsidy of €150/t at different tax rates, and the other was an ESP of €0.60–0.70/L with the imposition of a €20/t gate fee. In the future, the addition of an integrated biofuel production operations unit, installed close to a pulp and paper mill, could utilize the different pulp side-streams and create further revenues to the mill owners.