Abstract
In recognition of the possibility of utilising biomass waste streams as sustainable carbon resources and thus reduce society’s dependence on non-renewable carbon sources, the application of the biorefinery concept has been proposed and extensively investigated in this research. In conjunction with notable meat processing experts in New Zealand, the waste streams generated by meat processing plants were initially assessed and the major waste streams, that may constitute significant environmental concerns, identified. The meat processing dissolved air flotation sludge and the stockyard waste streams were subsequently specified as major waste streams currently posing significant management challenges in New Zealand’s meat processing industry. These meat processing waste streams were therefore selected as biomass sources that could sustainably serve as carbon resources in the proposed biorefinery system.
To assess the current status of waste management in the meat processing industry, a review of the existing practises employed in meat processing waste stream management was undertaken, with the unsustainability of the largely utilised landfill disposal and incineration practices firmly established. For an improved understanding of the proposed alternative meat waste biorefinery system, the need for a comprehensive examination of existing biomass conversion technologies was identified as crucial. A review of these existing biomass conversion technologies was therefore undertaken. In the course of the investigative review undertaken, the biomass conversion technologies were broadly categorised into thermochemical, physicochemical and biological conversion technologies and the associated sub-classifications also assessed for their respective advantages and limitations. A comprehensive understanding of the functionalities of the biomass conversion technologies provided the conceptual basis for proper screening of possible biorefinery system configurations while also ensuring that value extraction from the organic waste streams was enhanced. A biorefinery system that integrated hydrolysis, esterification, anaerobic digestion and hydrothermal liquefaction biomass conversion technologies was subsequently proposed as the preferred system that will facilitate a drying-free conversion of high moisture content meat processing waste streams to useful products while also ensuring that downstream secondary waste contamination concerns are eliminated.
Based on information acquired during several site visits, the meat processing dissolved air flotation (DAF) sludge waste stream was identified as a possible source of low grade lipids for biodiesel production. Experimental investigations into the physicochemical properties of the intrinsic lipids in the sludge were therefore initially undertaken. An assessment of the physicochemical properties of DAF sludge lipid extract facilitated an improved understanding of its nature and physical state with special emphasises placed on its fluid deformation behaviour. Experimental investigations were therefore able to show that the lipid extract existed as a highly viscous liquid that behaved like a non-Newtonian fluid. The methylation of the lipids present was also undertaken for fatty acid methyl ester (FAME) formation via an integrated hydrolysis and esterification pathway. This integrated pathway was utilised due to its established sufficiency in processing low grade lipids, characterised by high free fatty acid content. Assessing the properties of the FAME product generated from the DAF sludge feedstock, this research was able to demonstrate that the fuel properties of the DAF sludge lipid FAME product, namely density, kinematic viscosity, oxidative stability, higher heating value and cetane number satisfied biodiesel fuel requirements as specified by European (EN 14214) and American (ASTM D6751) biodiesel standards. Having demonstrated the feasibility of FAME production from DAF sludge lipids and also assessed its fuel properties, concerns were raised with respect to the energy required for the initial drying operation undertaken for moisture removal from wet DAF sludge prior to lipid extraction. This is because the high moisture content of the sludge implies that there will be a significant energetic penalty when high moisture content DAF sludge is dried. This energetic penalty will be expected to escalate as the mass of the high moisture content DAF sludge being utilised as a biodiesel feedstock increases. This research therefore proposed a novel in-situ hydrolysis process that is catalysed using an environmentally benign polystyrene resin, as a sufficient intensification approach that will circumvent the need for a preliminary drying operation to aid lipid extraction. Using the central composite response surface experimental design method, it was demonstrated that an almost complete lipid hydrolysis via the catalysed in-situ pathway was feasible under conditions of moderate temperature of 92.5 oC, pressure of 1 atm, reaction time of 1 h, and catalyst load of 0.09216 kg resin catalyst/kg wet wet DAF sludge. Initial studies also suggested that resin recovery and regeneration will lead to a further reduction in unit biodiesel production cost. For clarity, the economic implication of employing the proposed alternative in-situ hydrolysis step in the biodiesel production process was also initially investigated. A comparative assessment of the unit cost of biodiesel production from processes incorporating the in-situ hydrolysed hydrolysis step and the unit cost of biodiesel produced from processes incorporating conventional lipid extraction technologies was undertaken. Using approximate calculation techniques, it was shown that biodiesel production processes that employ the in-situ hydrolysis step are capable of reducing the unit production cost of biodiesel by almost 40 % relative to the unit production cost of biodiesel production processes that incorporated conventional lipid extraction steps. It was also shown that biodiesel production from DAF sludge constituted a cheaper process compared to existing biodiesel production from soybean oil only when the preliminary drying operation was avoided via the application of the in-situ hydrolysis technology.
To further demonstrate the economic and environmental performance of biodiesel production from the freely available DAF sludge as an unconventional feedstock, a comprehensive steady state simulation study of the production process was undertaken. The simulation study was able to show that the net energy ratio of the biodiesel production process using sludge as feedstock ranged from 1.76 to 3.32 when the electrical duty requirement was satisfied using fossil energy sources and ranged from 1.82 to 3.36 when electrical duty requirement was satisfied using renewable energy sources. This range was incorporated to reflect uncertainties in the underlying assumptions (i.e. mass of DAF sludge available) that constituted the basis of the simulation study. The simulation study was also able to show that the unit production cost of of the biodiesel product from DAF sludge ranged from $US0.41 per kg to $US0.71 per kg. The results obtained reinforced the validity of the proposed utilisation of the DAF sludge as a feedstock more so as the production process was determined to be renewable for all the production scenarios considered. The calculated mean unit biodiesel production cost was also shown to be cheaper than the unit biodiesel production cost reported for existing biodiesel production processes. Crucially however the integrated in-situ hydrolysis and esterification pathway proposed was also shown to present a possible environmental challenge since large masses of residual wet hydrolysed DAF sludge are generated as an in-situ hydrolysis by-product and must therefore be managed. In line with the proposed biorefinery system, the anaerobic co-digestion of the wet hydrolysed DAF sludge residue and meat processing stock yard waste was therefore investigated.
This research was able to demonstrate experimentally that the co-digestion of the substrates of wet hydrolysed DAF sludge residue and meat processing stock yard waste will always lead to enhanced biomethane yields due to the introduction of synergising effects. Having investigated different mixtures of the selected substrates, the ‘preferred’ co-digestion substrate mixture, defined as the co-digestion substrate mixture responsible for the highest biomethane yield, was determined. It was shown that the preferred substrate mixture contained stockyard waste and wet hydrolysed DAF sludge residue mixed in 4:1 ratio on a volatile mass basis. The anaerobic co-digestion of this preferred substrate mixture generated a cumulative biomethane yield of 264 L per kg volatile solid of the substrate mixture. The experimentally determined cumulative biomethane yield was significantly greater than the theoretical cumulative biomethane yield, obtainable from the anaerobic digestion of a similar substrate mixture but in the absence of synergising effects and estimated to be 149 L per kg volatile solid of the substrate mixture.
The viability of introducing an additional hydrothermal liquefaction processing of the co-digestion biogas digestate step to enhance resource recovery was also initially assessed. Assessments were undertaken theoretically via the estimation of the yields of useful products obtained from the hydrothermal processing of the biogas digestate residue. The range of energetic performances of the hydrothermal liquefaction process was also predicted. The possibility of satisfactory product yields from the hydrothermal liquefaction processing of digestate was established. The theoretical generated results also provided a compelling justification for undertaking further experimental investigations into the viability of employing the hydrothermal liquefaction technology as a post-anaerobic digestion processing step for enhanced value recovery. The variability of the carbon content of the biochar product was determined to be a major determinant of the extent of energy recovery and the favourability of the energetic performance that will characterise the hydrothermal liquefaction processing step.
Experimental investigations into the application of the hydrothermal liquefaction for enhanced value extraction from digestate were subsequently undertaken. Experimental investigations using the Box-Behnken response surface design method established that optimal co-generation yields of the desired product streams namely, the energy dense biocrude product and the insoluble biochar product of 0.205 kg and 1.377 kg respectively, are feasible when 100 kg of high moisture digestate containing only 3.02 % wt. total solids is utilised as the feedstock. Other hydrothermal liquefaction products, namely soluble solids in the post-HTL water phase and gaseous products were determined to be 0.559 kg and 0.878 kg, respectively. Crucially biocrude products were characterised by impressive higher heating values ranging from 32 to 40 MJ/kg, with their utilisation as a possible liquid fuel initially proposed. The hydrogen-carbon-oxygen distribution of the digestate-sourced biocrude, which is indicative of its level of carbonisation, was subsequently assessed in order to amplify compositional similarities between the biocrude products and liquid fossil sourced fuels. Also the chemical compound composition of the optimally produced biocrude product was investigated by employing proton nuclear magnetic resonance, Fourier transform infrared and gas chromatography-mass spectrometer compositional assessment techniques. The experimentally determined chemical compound composition therefore facilitated an improved understanding of the usefulness of the digestate sourced biocrude as a petroleum crude alternative. On the other hand, digestate-sourced biochar products were characterised by unfavourable heating values ranging from 2.49 to 8.78 MJ/kg. The unfavourable heating values of the biochar products generated suggested that the previously proposed application of biochar product as a solid fuel cannot be justified, with its alternative utilisation as an additive for enhancing the soil properties for improved agricultural product yield, proposed and subsequently investigated. Some crucial agronomic properties of the optimally generated biochar product, such as the thermal stability (thermogravimetric analysis), pH value, electric conductivity, porosity (scanning electron microscopy analysis) and nutrient content (inductively coupled plasma mass spectrometry analysis) were subsequently investigated with its sufficiency as a soil additive demonstrated.
Having demonstrated experimentally the viability of employing the selected biomass conversion technologies for enhanced value extraction from meat processing waste streams, it was also considered crucial to demonstrate the viability of a large-scale integration of the biomass conversion technologies. Large-scale integration of the biomass conversion technologies was achieved via a simulation study, with the simulation results suggesting that the economic performance was always favoured by economics of scale. The simulation results however showed that the environmental performance initially was improved up to an optimum value then gradually dropped with increments in the mass feed rates of the waste streams. Based on these observations the mass feed rates of the waste streams that would be considered as sufficient to produce a compromise trade-off between the economic performance and environmental performance was subsequently determined.
Possible concerns with respect to the high capital investment cost and high operating cost of the proposed biorefinery system were highlighted with preliminary comparative assessments of the biorefinery and the existing meat processing waste management systems therefore undertaken. Comparative assessments were undertaken with respect to environmental performances and economic performances of both the biorefinery system and the existing meat processing waste management systems. It is anticipated that the work presented in this dissertation could provide a basis for further research in the area of biorefinery research using high moisture content food processing waste. Parts of this research have been accepted and published in academic peer reviewed journals, peer reviewed book chapters and peer reviewed conference proceedings as listed in the “list of publications” below. Research outputs yet to be published are also listed and clearly specified.