PRODUCTION OF BIOETHANOL FROM WASTE CARTONS

ABSTRACT

Recent interest into the production of biofuels such as bioethanol and biogas has increased due to a concern of global climatic change and the depletion of fossil fuels.  Sustainable feedstock for the production of these biofuels need to be found.  One such source is a waste stream from the pulp and paper industry.  Paper sludge, emanating from the primary clarifier in the waste water treatment area of the paper mill, consists of high amounts of cellulose and water which makes it an ideal substrate for biological conversions, such as fermentation and anaerobic digestion.  The extensive mechanical and chemical processing during paper making acts as a pre-treatment step by disrupting the biomass structure, making it amenable for enzymatic hydrolysis, the first stage of the biological conversion process.  Another advantage is the continuous, localised supply of paper sludge.

Two possible biological conversion processes, fermentation and anaerobic digestion, were investigated for the energy yields and economic benefits.  The feasibility of these processes has been proven at laboratory scale, with the working volume of experiments conducted in the range of 250 ml, however the scale up to pilot scale still needs to be investigated.  The impact of compositional variability in waste cartons from different mills was investigated by comparing process yields from a tissue and printed recycling mill, corrugated recycling mill and virgin fibre pulping mill.

Ethanol production through simultaneous saccharification and fermentation was investigated in 20 L reactors, using commercially available enzymes and an industrial Saccharomycescerevisiae strain.  Tissue printed recycle waste cartons yielded a final ethanol concentration and conversion of 27.8 g/L and 70.6%, respectively at a solids loading and enzyme dosage of 33% (w/w) and 15 FPU/g dry substrate, respectively.  Corrugated recycle waste cartons yielded an ethanol concentration of 39.4 g/L and a conversion of only 65.7%, with a solids loading and enzyme dosage of 27% (w/w) and 11 FPU/g dry substrate, respectively.  Virgin pulp waste cartons had the highest ethanol concentration and conversion of 46.8 g/L and 87.4%, respectively at a solids loading and enzyme dosage of 18% (w/w) and 20 FPU/g dry substrate.

Anaerobic digestion for biogas production was tested in 30 L reactors, using mixed inoculum obtained from the waste water treatment facility of a brewery.  The methane production for tissue printed recycle, corrugated recycle and virgin pulp waste cartons was 31.6, 54.4 and 37.2 L/kg paper sludge, respectively.  The achievable solids loading for tissue printed recycle and corrugated recycle waste cartons was 10% (w/v), while virgin pulp waste cartons was 6% (w/v).

The conceptual biofuel plant design for the tissue and printing recycle mills, corrugated recycle mills and virgin pulp mills was based on average waste cartons production rates of 66, 10 and 36 dry tonnes/day respectively.  A cash flow analysis was completed for each processing scenario as well as each waste cartons in order to determine what the minimum fuel selling price would be.  For the fermentation process, the minimum ethanol selling price for viable investments were R8.34/L, R15.68/L and R5.47/L, respectively at a weighed cost of capital of 12% (real terms).  The current market price for ethanol is R8.39/L, showing that virgin pulp waste cartons was the most viable for bioethanol production via fermentation.  For the anaerobic digestion process, the minimum methane selling prices were R99.71/kg, R102.39/kg and R146.46/kg, respectively for tissue printed recycle paper sludge, corrugated recycle waste cartons and virgin pulp paper sludge.  However, the current market price of methane is R27.26/kg, making the anaerobic digestion process unfeasible.  This is due to the low methane yields achieved from waste cartons digestion as well as the high capital investment required to process large volumes of paper sludge.

This study proved the feasibility of value addition to paper sludge, which subsequently reduces the amount of waste sent to landfill and benefits the industry revenue with an additional energy stream.  Future endeavours are aimed at further reduction of landfill volumes through the anaerobic digestion of the residues after fermentation.

 

 

Chapter ONE

Introduction

Background to the study

The production of biofuels gained more traction due to the decrease in the availability of crude oil as well as the pressure to change to more environmentally friendly and renewable biofuels (Deenanath et al. 2012).  Several waste streams from industrial biomass processing contain cellulose and can be considered for biofuel production, with one such stream being the paper waste sludge stream originating from the pulp and paper industry.  The pulp and paper industry produces a substantial amount of waste water, rich in short fibres that have been rejected during the paper making process (Lever 2015; Fan et al. 2003).  Severe restrictions on landfilling have been put in place to prevent ground water pollution and reduce greenhouse gas production, with these restrictions also being applied to waste cartons due to its high moisture content.  Natural degradation of organic waste on landfilling sites are prone to release high amounts of uncaptured gasses into the atmosphere, mainly carbon dioxide and methane (Kamali & Khodaparast 2015).

Waste cartons (PS) has a high moisture content of approximately 50%, which makes it unsuitable for combustion or incineration, resulting in little or no energy benefit.  However, the high cellulose and hemicellulose content makes the stream ideal for energy production via bioprocessing (Alekhina et al. 2015). The environmental benefits of biofuel production from water rich waste are ample: avoided landfilling and subsequent landfill emissions, avoided transportation costs and the reclamation of valuable process water (Bajpai 2015).

The pulp and paper industry is one of the sectors that utilise large amounts of cellulosic biomass as feedstock for their products.  The biofuels of potential interest from waste cartons in this study are bioethanol and biogas, which could be used as energy source within the paper mill to alleviate the dependence on fossil based energy sources (Kamali & Khodaparast 2015).

         Research questions and objectives

The research questions that need to be answered in this study are:

• How do the characteristics and composition of waste cartons affect the fermentation and anaerobic digestion capabilities at pilot scale (i.e. working volumes of between 10L and 20L)?
• What is the economic feasibility of producing bioethanol or biogas from paper sludge, using the pilot scale experimental data as input for the economic model?

The research objectives are to:

• Select and characterise waste cartons from mills that are representative of the various types of Nigerian paper and pulp mills.
• Determine the bioethanol concentration and conversion yields from fed-batch simultaneous saccharification and fermentation (SSF) experiments based on optimal process conditions reported in previous studies.
• Determine the biogas production yields and methane content during anaerobic digestion of paper sludge.
• Construct an Aspen Plus simulation to determine the mass and energy balances for bioethanol and biogas plants.
• Determine the key economic indicators (minimum fuel selling price) by completing a full economic analysis on bioethanol and biogas production, including the required capital investment as well as a cash flow analysis on the two process scenarios

 

PRODUCTION OF BIOETHANOL FROM WASTE CARTONS

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