VALORIZATION OF WASTE BIOMASS THROUGH CATALYTIC PYROLYSIS TO PRODUCE UPGRADED BIO-OIL

Abstract

Increase in energy demand and depletion of fossil fuels have compelled researchers to search for a new and renewable source of energy. Biomass is a cleaner and economical source of energy with high sustainability (Açıkalın and Karaca, 2017a). There are numerous techniques to obtain energy from biomass such as pyrolysis, gasification, liquefaction, etc. Among these, pyrolysis is the most suitable and widely used technique, which recovers liquid fuel along with value-added chemicals from biomass (Gupta and Mondal, 2019a). Waste biomass with no economic value acts as the most suitable feedstock for pyrolysis process. Pine needles have a gross annual production capacity of 1.9 million tonnes in Central Himalayan regions of Uttarakhand. It creates hassle to the local residents due to its ability to catch fire and thus, their safe disposal is a matter of concern. Disposal of harmful pine needle wastes by converting them into biofuels via pyrolysis can provide a combined sustainable solution to both the problems of local residents due to pine needles accumulation as well as the problem of energy crises due to fossil fuel depletion. Thus, waste pine needles were employed as biomass in the present investigation. It’s proximate analysis (volatile matter: 69.82%, fixed carbon: 24.83%, ash content: 1.95%, and moisture content: 3.40%) and ultimate analysis (carbon: 48.42%, hydrogen: 7.30%, oxygen: 43.81%, nitrogen: 0.42%, and sulphur: 0.05%) data also revealed it as a promising raw material for the pyrolysis process. The present investigation explored the catalytic effect of three different catalysts: conventional γ-Al2O3 (AO) based catalysts, spent aluminium hydroxide adsorbent (AHNP) based catalysts and biochar (BC) based catalysts in the pyrolysis of pine needle biomass. The prepared catalysts were characterized through XRD, FTIR, BET, FESEM and NH3-TPD techniques. The effect of catalysts on pyrolysis behavior was firstly investigated through thermogravimetric (TG) analysis to determine the kinetics and thermodynamics of the process and then laboratory scale semi-batch reactor was used to determine the distribution and composition of the pyrolysis products. Prediction of kinetics and thermodynamics of biomass degradation assists in the designing, modeling and development of pyrolysis process as well as reactors. The kinetic parameters (such as activation energy (Ea) and pre-exponential parameter (A)) were evaluated using isoconversional models: Ozawa Flynn Wall (OFW) and Kissinger Akahira Sunose (KAS). The results showed significant reduction in the activation energy after catalysts incorporation in the process. Thermodynamic parameters (such as Gibbs free energy, enthalpy and entropy) were also determined for all the considered processes. The average activation energy of non-catalytic (thermal) degradation of pine needle biomass was 133.52 and 130.01 kJ/mol, as determined by OFW and KAS method, respectively. Incorporation of γ-Al2O3 catalyst reduced the average activation energy to 112.29 and 108.22 kJ/mol, as predicted through OFW and KAS models, respectively. Ni/γ-Al2O3 made further reduction in the process activation energy to 106.20 and 101.74 kJ/mol, as determined by OFW and KAS model, respectively. Amongst different metal doped AHNP derived catalysts (Al, Ni/Al, Fe/Al, Cu/Al, Zn/Al and Mo/Al), Ni/Al showed the largest reduction in the activation energy to 124.53 kJ/mol, as determined by OFW model, and 120.95 kJ/mol, as determined by KAS model. Likewise, for the case of biochar-based catalysts (BC, Ni/BC Ni/BC-ZnCl2, Ni/BC-H3PO4 and Ni/BC-NaOH), Ni/BC-H3PO4 exemplified the largest reduction in the activation energy to 122.18 and 119.20 kJ/mol, as determined by OFW and KAS models, respectively. The non-catalytic pyrolysis was carried out in semi-batch reactor and multiparameter optimization of process parameters (namely temperature, heating rate and inert gas flow rate) for maximizing the bio-oil yield was performed through face-centered central composite design (FCCD) of response surface methodology (RSM). Besides this, the input-output models were also developed for non-catalytic pyrolysis using RSM and ANN approach. R2 close to 1 and low error demonstrates the viability of the developed models. Temperature had been determined as the most predominant variable influencing the yield of the products. The non-catalytic pyrolysis was optimized using RSM software at process conditions: 552.06 °C temperature, 50 °C/min heating rate and 164.40 mL/min inert flow rate, and the maximum bio-oil yield has been predicted as 51.11 wt.% (by RSM) and 51.70 wt.% (by ANN) at the optimized conditions. A combined approach was employed to accomplish the individual limitations of both the modeling methods. Comparison of the R2 and mean squared error (MSE) of the developed RSM and ANN models showed relatively higher R2 and lower MSE values of the ANN model, indicating its better accuracy for prediction of the pyrolysis yield from the knowledge of corresponding process parameters. In contrast, less complex model developed by RSM was quite good for prediction of interaction as well as significance and insignificance of process parameters. The study revealed that such a combinational approach has the better ability to model the pine needle pyrolysis process compared to the individual ones. Impact of three distinct catalyst varieties (γ-Al2O3, AHNP and biochar-based catalysts) on the distribution and quality of pyrolysis products were also examined. Inclusion of catalysts increased the hydrocarbon and phenolic content in the bio-oil and reduced the oxygen content Compared to non-catalytic (PN) pyrolysis, addition of catalysts in the process reduced the bio-oil yield with simultaneous increase in the gas yield. This is due to the fact that catalysts addition promoted the formation of lighter molecules at the expense of heavier volatiles by facilitating different series of reforming, cracking and deoxygenation reactions. For the case of γ-Al2O3 (AO) based catalysts, Ni/AO showed more promising results in terms of oxygen reduction potential and production of hydrocarbon and phenolic compounds as compared to unsupported AO catalyst. AO and Ni/AO catalysts increased the carbon content of bio-oil to 52.05 and 62.98 wt.%, respectively, from 48.37 wt.% in case of non-catalytic bio-oil. Similarly, oxygen content of AO and Ni/AO catalytic bio-oil was reduced to 39.83 and 27.55 wt.%, respectively, from 42.98 wt.% in case of non-catalytic bio-oil. Spent AHNP (Al) derived catalysts also showed significant deoxygenation potential towards oxygenated compounds, that was around 7.03, 37.41, 26.52, 14.63, 6.09 and 13.94% for Al, Ni/Al, Fe/Al, Cu/Al, Zn/Al and Mo/Al catalysts, respectively, by converting them into their corresponding hydrocarbon and phenolic derivatives. Among all the AHNP based catalysts, Ni/Al and Fe/Al produced the highest quality bio-oil by enriching the bio-oil’s carbon content to 62.93 and 60.14 wt.%, respectively, and heating value to 31.41 and 26.86 MJ/kg, respectively. Moreover, GC-MS analysis showed significant enhancement in the bio-oil’s hydrocarbon content from 9.15 area % in non-catalytic bio-oil to 36.41 and 36.01 area % in Ni/Al and Fe/Al catalytic bio-oil, respectively. Similarly, phenolic compounds were also increased from 13.32 area % in non-catalytic bio-oil to 46.04 and 41.67 area % in Ni/Al and Fe/Al catalytic bio-oil, respectively. Furthermore, physicochemical characteristics of catalytic bio-oil derived using biochar-based catalysts also indicated the substantial oxygen removal from bio-oil and consequent improvement in its HHV. Catalyst incorporation in the process increased the production of phenols as well as aliphatic and aromatic hydrocarbons. The presence of surface acidic functionalities coupled with metallic sites in Ni/BC-ZnCl2, Ni/BC-H3PO4, and Ni/BC-NaOH catalysts increased the aromatics selectivity to 35.18, 36.47, and 35.64 area %, respectively. Similarly, aliphatics were also enhanced to 9.61, 10.12, and 11.68 area % using Ni/BC-ZnCl2, Ni/BC-H3PO4, and Ni/BC-NaOH catalysts, respectively. Also, chemically activated biochar catalysts (Ni/BC-ZnCl2, Ni/BC-H3PO4, and Ni/BC-NaOH) showed high stability towards deactivation compared to non-activated ones (BC and Ni/BC). Increment in the content of CO and CO2 gases in the evolved non-condensable gases also confirmed the occurrence of deoxygenation reactions during catalytic breakdown. Hydrocarbon and phenol-rich bio-oil can find its application either as a replacement for petroleum fuel or an industrial-grade chemical The environmental impact of catalytic and non-catalytic pyrolysis was determined through life cycle analysis (LCA) and process economics was assessed through operating cost estimation. Inclusion of co-product recycling and market export had significant impact on the environmental load and process economics in different scenarios. The results elucidate that biochar exported for coal replacement to earn credits is more advantageous for reducing environmental impact than the biochar recycled back to process for reducing natural gas usage, while the reverse is true from the economic viewpoint Amongst all the three distinct catalyst varieties, AHNP catalyzed processes have the lowest environmental load and operating cost due to the utilization of waste/spent residue and minimum processing steps involved during catalyst preparation, relative to conventional γ-Al2O3 and biochar catalyzed processes. Conventional γ-Al2O3 catalyzed processes also involved the environmental impact and cost of large number of chemicals employed during the conventional synthesis of γ-Al2O3 support, while biochar-based catalysts required relatively larger number of processing and chemicals (such as ZnCl2, H3PO4 and NaOH). Thus, focusing on the catalyst preparation stage, such as, type of precursor or support employed, preparation technique, etc. can assist in reducing the process environmental impact and cost. Though catalysts bring some increment in the overall cost of the process but, enhancement in bio-oil properties and process efficiency as bought out with catalysts involvement is very essential and cannot be overlooked. Catalytic pyrolysis required less energy (in terms of reduced activation energy) and produced higher quality of bio-oil, which makes it a cleaner option for the sustainable utilization of harmful pine needles. The study will assist in the design, optimization and scale-up of the process at large industrial level.