Although a plethora of pretreatment methods have been developed in the recent years, very few could be applied in pre-commercial stage. It is therefore difficult to ascertain which method is the best in terms of its efficiency. The term ‘effi­ciency’ includes several criteria, and for a pretreatment method to be an efficient one, it should suffice all or partly. The key criteria for an efficient pretreatment technology and process are as follows (IEA 2008; Kumar et al. 2009):

• must reduce the crystallinity of cellulose and increase porosity of the material;

• increase the yields of both hexoses and pentoses in downstream processing;

• avoid the loss/degradation of sugars;

• recover lignin for further combustion;

• minimize the inhibitors of enzymatic action and fermentation;

• fungibility with different feedstocks;

• avoid expensive capital cost on biomass comminution;

• minimize waste products and use low-cost chemicals; and

• should have low overall capital cost with low energy requirement.

Currently, none of the pretreatment method is suitable for a range of biomass feed­stocks owing to their different degree of action and varying strengths and weak­nesses. Different feedstocks respond to each pretreatment method in a varying way, and it is difficult to find a single method for all feedstocks type. Presently, the dilute and concentrated acid, and steam explosion are very near to commercializa­tion, in spite of their high capital cost. As described in Table 3, each of the methods has its own limitations, which are inherent to the process and difficult to overcome. Therefore, a single pretreatment method cannot have the potential to commerciali­zation unless integrated/combined with other methods. It is imperative to conduct research on combined pretreatment methods to minimize the limitations and overall reduce the capital cost and improve the efficiency of hydrolysis. In fact, all the pre­treatment methods described in Table 3 are in the varying stages of R&D and require extensive trials before any of them reach the commercial viability.

Catalyst loading is another important parameter that can significantly affect the yields of degradation products. In this study, three different levels of catalyst load­ing, i. e., 5, 10, and 15 % were investigated for effects on the degradation of HDPE. In this study, the reaction time was fixed at 3 h for HZSM-5 (Si/Al = 80) catalyst. Meanwhile, in case of AlSBA-15 catalyst, initially, the reaction was carried out at a catalyst loading of 5 % for 4 h due to slower reaction to achieve significant level of degradation. As shown in Table 5, a catalyst loading at 10 % yielded the highest amount of light liquid while it produced the least amount of volatile gaseous product as compared to the other two catalysts loadings. The solid coke formation was found to be about 7 % for catalyst loadings of 5 and 10 %. Its yield was slightly higher with 15 % catalyst loading as 7.7 wt. % of the overall weight of residue was obtained.

The trace for waxy compound with 5 % catalyst loading amount was detected, but it was negligible and difficult to be accurately measured. The possible reasons of low conversion of degradation product to liquid were due to the concentration of 5 wt. % catalyst which was considered low to provide sufficient number of active sites for the degradation of large- to moderate-sized polymer chains. However, the optimum catalyst loading was found to be at 10 wt. %. It was further noted that larger amount of catalyst loading might promote faster reaction with higher forma­tion of coke in the residue of degradation products. This might be the reason for the catalyst inactivity throughout the degradation process (Ochoa et al. 1996).

In Fig. 9, the highest amount of liquid degradation product was obtained at 15 wt. % of catalyst loading (23.5 wt. % of the feed polyethylene). It also pro­duced moderate amount of volatile gaseous degradation products (52.2 wt. %) as compared to that obtained using HZSM-5(80). Furthermore, it could prove the theory that zeolitic catalysts such as HZSM-5 promote the production of

gaseous products during degradation of polyethylene. At the same time, it was not considered to be an ideal catalyst for the purpose of maximizing the degradation into liquid yield. However, one of the major disadvantages of using higher cata­lyst loading to increase the liquid product yield was that the formation of higher amount of solid coke in the residue might lead to the poor overall catalyst activity.

It was also observed that at 5 % of catalyst loading, no liquid or gaseous prod­ucts were successfully collected due to the formation of large amount of solid waxy compounds leading to a blockage at the reactor outlet. This was due to the rapid solidification of melted polyethylene degradation products when they were exposed to lower temperature at the reactor outlet. The small amount of catalyst loading was also unable to reduce the activation energy of the degradation process to enable the degradation mechanism to take place rapidly. However, significant formation of solid waxy compound was successfully inhibited at 10 wt. % of cata­lyst loading. Consequently, sufficient amount of liquid and gaseous degradation product was successfully collected. As illustrated in Fig. 9, the maximum liquid yield could be obtained at 15 % of catalyst loading. At the same time, less amount of solid waxy compound might be formed.

Fig. 10 Composition of gas products at 673 K using HZSM-5(80) catalyst over different catalyst loadings

Fig. 11 Composition of gas degradation products using various loadings of AlSBA-15 catalyst at 673 K

Figure 10 shows the data on the gaseous products using HZSM-5(80) catalyst. The highest composition was reported by C4 for 5 and 15 % of catalyst load­ings (27.8 and 33.6 %, respectively). Meanwhile for 10 % of catalyst loading, the highest proportion of C3 carbon chain was 38.4 %. At 5 % of catalyst loading, the amount of catalyst active site was deemed insufficient to cause significant deg­radation of polyethylene. Thus, higher amount of longer carbon chain molecules were produced as they could undergo further cracking reactions into smaller mol­ecules (Pierella et al. 2005). The results also indicated that increasing amount of catalyst would create better uniformity in the products distribution as seen in the case of 15 % of catalyst loading.

Likewise, data regarding catalyst loading and its effects on the degradation of HDPE to gaseous products using AlSBA-15 catalyst at 400 °C are shown in Fig. 11. The highest yield was recorded by the carbon chain of C4 with 15 % of catalyst loading (38.2 %). At 10 % of catalyst loading, its C3 carbon chain composition in the product was 42.1 %. It was also confirmed in earlier findings that the gas prod­uct was more concentrated in the middle of the carbon chain range such as C3 and

Fig. 12 Composition of liquid degradation products by various loadings of HZSM-5 (80) catalyst at 673 K

Fig. 13 Composition of liquid degradation products using various loadings of AlSBA-15 catalyst at 673 K

C4 (Hua et al. 2001). However, no product was detected for Ci products. Similar to earlier case, the use of higher catalyst loading produced more uniform products dis­tribution as in case of HZSM-5 catalyst. By comparing the degradation results using HZSM-5(80) and AlSBA-15 catalysts, it could be concluded that microporous cata­lyst HZSM-5(80) produced more shorter-chain carbon products. At the same time, mesoporous catalyst AlSBA-15 yielded more longer-chain carbon products.

Figure 12 shows the effect of catalyst loading on the distribution of degradation liquid products using HZSM-5(80) as catalyst at 400 °C. The highest composition was recorded with 5 % of catalyst loading to give a C21-C24 carbon chain range of 25.1 %. For the use of 10 and 15 % of catalyst loadings, the compositions of C8—C12 carbon chain range were 34.2 and 30.4 %, respectively. The longest carbon chain range C25+ was the minor product leftover after the degradation process. For this catalyst, it could also be seen that highest product yield was accu­mulated at the lighter end of the C8-C12 and C13-C16 carbon chain ranges.

By comparing to the earlier findings made using HZSM-5(80) catalyst (Koc and Bilgesu 2007), AlSBA-15 mesoporous catalyst showed a more non-uniform distribution (Fig. 13). The highest composition was recorded by a carbon chain

range of C8-C12, i. e., at 10 % catalyst loading (34.2 % yield) while at 15 % catalyst loading, 31.1 % yield of C13-C16 substances was obtained. Similarly, the heaviest carbon chain range, i. e., C25+ again recorded the smallest composition. The difference between the higher end and lower ends was bigger compare to the findings made using HZSM-5(80) microporous catalyst.

Commercial algae production facilities employ both open and closed cultivation systems. Each of these has advantages and disadvantages, but both require high capital input (Pienkos and Damns 2009). Open ponds are much more cheaper than closed systems because it demands relatively high capital and O&M costs associated with installation and operation of PBRs (Benemann 2012; Buford et al. 2012).

Lower costs and the possibility to scale up to several hectares make open ponds the main choice for algae commercial production (Benemann 2012). However, open pond cultures suffer from many limitations that can disrupt algal productiv­ity during unexpected environmental events. Another challenge for this system includes having access to an adequate supply of water for growth due to continu­ing loss of water through evaporation. Therefore, open ponds must be in a geo­graphic setting that has a fairly near source of water and a relatively flat terrain to avoid costly earthworks (Buford et al. 2012). Moreover, the open systems are susceptible to wind-born biological agents that can affect the cultivation, such as grazers, infectious fungi, lytic bacteria, viruses, other algae, and also lower tem­peratures in colder climates (Benemann 2012).

These open pond limitations stimulate PBRs development; however, only a few commercial plants use closed PBRs systems, mainly due to high costs as men­tioned before. Nowadays, according to Benemann (2012), microalgae cannot be grown in PBRs for commodities and are not even successful for high-value prod­ucts. However, PBRs can be used for seed culture production, though only for ~0.1 % of the biomass. Closed PBRs are significantly more expensive to construct, but have not been engineered to the extent of other reactors in commercial prac­tice, and so there may be opportunities for significant cost reductions.

Neither open ponds nor closed PBRs are mature technologies. Therefore, until large-scale systems are built and operated over a number of years, many uncer­tainties will remain. Cultivation issues for both open and closed systems, such as reactor construction materials, mixing, optimal cultivation scale, heating/cool — ing, evaporation, O2 build-up, and CO2 administration, have been considered and explored to some degree, but more definitive answers await detailed and expansive scale-up evaluations (Pienkos and Darzins 2009).

Concerning the various algal species and strains, they vary from study to study, depending on location and culture techniques. For that reason, it is not yet possible to predict what species or strain will be the best suited for commercial biofuel pro­duction, but it is most likely that it will differ from case to case, depending on the location, cultivation techniques chosen, processing technologies available, nutri­ents source, local climacteric conditions, and among other potential factors.

Armenta et al. (2008) established the creation of the term green analytical chem­istry based on: (1) sample treatment; (2) oriented scanning methodologies; (3) alternatives to toxic reagents; (4) waste minimization; (5) recovery of reagents; (6) online decontamination of wastes; and (7) reagent-free methodologies. Thus, it should be considered that the analysis of biomass should be based on the 12 principles of green chemistry proposed by Anastas and Warner (1998), since the context of its use is reflected in the sustainability of feedstock and processes.

Some of the 12 principles of green chemistry are closely related to the imple­mentation of green analytical methodology, which are as follows: (1) atomic and

energy economy; (2) use of catalytic reactions instead of stoichiometric reactions; (3) decreasing solvent use; and (4) a decrease in residues (Anastas and Warner 1998). The application of these principles will contribute to achieve a more sus­tainable analytical methodology, as can be seen in Fig. 9.

In some cases, it is very difficult to apply all of those principles presented in Fig. 9, because each analytical method has its particularities and limitations. Then, we need to seek other principles as waste prevention, design for energy efficiency, use of real-time analysis for pollution control, and inherently safer chemistry for accident prevention; this strategy will ensure a greener chemical analysis and ana­lytical chemistry.

3 Conclusions

Chemical analysis of biomass is an important branch of analytical chemistry because it can provide information about the constitution of feedstocks, processes, products and by-products, and residues. Analytical techniques are at the core of the analytical laboratory, and the understanding of its principles is necessary for real-world applications. Then, this can be applied on a whole biofuel chain to solve many technical and scientific problems, as: best uses for a biomass, improvement of conversion processes, increase in the quality of biofuel, and control of residues.

Nowadays, green chemistry and sustainability of processes and products are themes that passed from academic discussion to practical use. Then, analytical chemistry as part of chemical sciences should follow this current trend, which can contribute to a bioeconomy based on biomass use instead of non-renewable raw sources, as the oil, and an advance in biomass knowledge to develop their best uses.

Ahmad Zuhairi Abdullah, Shazia Sultana, Steven Lim and Mushtaq Ahmad

Abstract The potential application of acidic HZSM-5 and AlSBA-15 materials for catalytic degradation of high-density polyethylene (HDPE) into liquid hydrocarbon fuels was investigated using a tubular batch reactor. The reaction was carried out at various catalyst loadings between 5 and 15 % with 1:1, 2:1, and 3:1 HZSM-5 to AlSBA-15 ratios. The catalysts exhibited remarkable catalytic activity with conver­sions into liquid light hydrocarbons of up to 25 %. The gaseous product distribution showed a wide spectrum of hydrocarbons (Ci-C5) while the most predominant prod­ucts were C3 and C4 (47 and 40 %, respectively). Meanwhile, the liquid products were mostly in the range of C8-C25 depending on the reaction parameters and the amount produced decreased with increasing carbon number. Thus, catalytic degrada­tion of HDPE was a promising route for obtaining valuable fuels and petrochemicals from waste polymers. It required relatively low degradation temperatures to obtain liquid hydrocarbons with boiling points within that of gasoline range. At the same time, it could reduce the environmental problems caused by waste polymers.

Keywords High-density polyethylene • HZSM-5 • ALSBA-15 • Catalytic degradation • Hydrocarbon fuel

1 Introduction

In recent decades, plastic materials consumption has undergone a significant growth. According to an estimate, the global production and consumption of plas­tics have increased by about 10 % annually. At the same time, synthetic plastics

A. Z. Abdullah • S. Sultana (*) • S. Lim

School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia e-mail: shaziaflora@hotmail. com

M. Ahmad

Biofuel Laboratory, Quaid-i-Azam University, Islamabad 45320, Pakistan

A. Domingos Padula et al. (eds.), Liquid Biofuels: Emergence, Development and Prospects, Lecture Notes in Energy 27, DOI: 10.1007/978-1-4471-6482-1_13, © Springer-Verlag London 2014

recycling has gained considerable attention all over the world due to the serious environmental problems caused by waste plastics and resources needed for man­ufacturing of such huge quantity of plastic materials. There are several methods practiced currently for degradation and disposal of high-density polyethylene (HDPE) such as landfill, thermal degradation, and incineration. However, all of these methods have not gained social acceptance for disposal of the waste mate­rial (Lee et al. 2002). The disposal is also currently becoming legally restricted because of a decrease in landfill availability, cost increase, and strong pollution concerns such as emissions of various combustion products.

Recycling of plastics should be projected to minimize the pollution. It has lower energy demand to support the process to offer high energy conservation while at the same time enhancing the efficiency of the process. Plastics recycling technolo­gies have been historically divided into four general types. Primary and secondary recycling processes involve the processing of waste/scrap materials into a product with characteristics similar to and different from those of original product, respec­tively. Tertiary recycling involves the production of basic chemicals and fuels from plastics waste/scrap as part of the municipal waste stream or as a segregated waste. Meanwhile, quaternary recycling process retrieves the energy content of waste/ scrap plastics by burning or incineration. Although the primary and secondary recy­cling processes are the most common being applied in the society, tertiary recycling and quaternary recycling have been regarded as more sustainable (Garforth et al. 1997). In contrast to landfill, thermal, and incineration methods used for degrada­tion of HDPE, chemical recycling using catalysts has emerged as a potentially inter­esting alternative. It can achieve degradation of plastic wastes for conversion into a variety of useful products, mainly as liquid fuels and raw chemical feedstock (Lee et al. 2002). In this regards, catalytic degradation using materials such as zeolites is considered to be suitable due to their unique pore diameter. These materials seem to be especially useable as catalyst supports for waste polymer degradation.

Generally, zeolites are aluminosilicate members of the family of microporous solids known as ‘molecular sieves’ and an ‘open’ structure that can accommodate a wide variety of cations. Meanwhile, mesoporous silicates such as MCM-41 and SBA-15 are porous silicates with huge surface areas (normally >1,000 m2/g), large pore sizes (2 nm < size < 20 nm), and ordered arrays of cylindrical mesopores with very regular pore morphology (Garforth et al. 1997).

In recent years, several researchers have reported the synthesis of a new class of porous materials which are supposed to combine the properties of both zeolites and ordered mesoporous aluminosilicates (Trong et al. 2001). Some examples of mesoporous materials that have been investigated in the past include MCM-41, KFS-16, and SAPO-37 (Jalil 2002; Marcilla et al. 2002; Miskolczi et al. 2005; Sakata et al. 2002). In continuing the efforts to degrade the HDPE waste into useful products such as liquid fuel, this study has been initiated. The objective of the research is the use of a laboratory tubular batch reactor to study: (i) the potential application of HZSM-5 and AlSBA-15 as acidic microporous and mesoporous catalysts for degradation of HDPE, (ii) the activ­ity of these catalysts and their effects under various operating conditions on product dis­tribution and selectivity, and (iii) to enhance the potential benefit of catalytic polymer recycling for industrial application in the future.

The use of enzymes for the pretreatment of biomass feedstocks can significantly reduce the capital cost, under the condition that enzymes are produced by microorgan­isms during the process of fermentation, also known as simultaneous saccharification

and fermentation (SSF) (Olofsson et al. 2008). However, the major drawbacks of SSF are the need to find optimal conditions of temperature and pH for both the enzymatic hydrolysis and the fermentation, and the difficulty to recycle the fermenting organism and the enzymes (Olofsson et al. 2008). Nevertheless, the application of enzymes can facilitate the fast, efficient, and cheap conversion of cellulose to glucose. Enzymatic hydrolysis can give higher yields of sugars in contrary to acid hydrolysis. However, the enzyme system for hydrolyzing lignocellulose is quite complex and involves the cellulase system as shown in Table 4. The efficacy of the enzyme systems depends on various factors that should be overcome to achieve the maximum yields of glucose. The key barriers that impede the action of enzyme system are as follows: (1) unreac­tive nature of crystalline cellulose; (2) the presence of lignin-blocking reactive sites; (3) low substrate surface area; (4) low rates of hydrolysis; (5) substrate and product inhibition; and (6) enzyme denaturation.

In order to develop an effective enzymatic hydrolysis process, it is important that inhibitors that impact the enzyme activity are removed (Taherzadeh and Karimi 2007). Another issue that requires attention is the cost reduction of the enzymes, which can be achieved probably by recycling the enzyme or by producing micro­bial enzymes during the SSF process. Recycling of enzymes can be achieved by repeated batch hydrolysis of feedstocks and immobilization of enzymes on an inert material (Das et al. 2011). The application of immobilized enzyme enables easy post­hydrolysis separation of the enzyme from the reaction mixture (Das et al. 2011). The advantage of enzyme immobilization is that it ensures the enzyme structure and conformation is preserved in addition to imparting improved thermostability. Enzyme modifications and active site mutations could possibly provide much effec­tive enzymes with high rates of hydrolysis, reusability, and resistance to denaturation. Modified/novel enzymes have the potential to reduce the cost of enzymatic hydrolysis.

The combined effects of microporous HZSM-5 (Si/Al = 80) catalyst and mesoporous acidic AlSBA-15 on the degradation of HDPE are presented by data in Table 6. It was observed that a conversion of about 100 % yield toward liquid, gaseous, and residue products without any formation of solid waxy compound was achieved. According to literature, these two catalysts constitute principally Lewis and Bronsted acid sites with variable surface areas and pore sizes (Ooi and Bhatia 2007).

However, results shown here are in accordance with the shape selectivity effect in microporous and mesoporous materials. It was associated with the narrow pores to access active sites. It was noted that for acidic enhancement SBA-15 catalyst, the larger pore size and channels allowed the formation of higher hydrocarbon products such as light liquid products. The high yield of gaseous products shown by HZSM-5 catalyst was mainly due to the slow diffusion of cracked products within the internal pores. The findings made in this work were consistent with those reported by pre­vious researchers. It has been reported that the diffusion rate of hydrocarbon mol­ecules could be retarded using microporous catalysts (Urquieta et al. 2002).

Overall, the combined effects of HZSM-5 (80) and AlSBA-15 catalysts gave rise to positive improvements as compared to the performance of the individual cata­lyst. For all three combinations of catalyst ratio, light liquid degradation product yields were higher than 20 wt. %. The optimum liquid yield was demonstrated by the HZSM-5 (80) to AlSBA-15 catalyst ratio of 1:2 (which was 26.5 wt. %). Although the increment of both catalysts increased the liquid yield as compared to the catalyst ratio of 1:1, further addition of AlSBA-15 could provide higher liquid yield while simultaneously suppressing the yield of gaseous product more effec­tively. Another advantage of applying the 1:2 catalyst loading ratio was that it also inhibited the solid coke formation on the catalyst. This could be explained from the theory that larger composition of HZSM-5 catalyst will contribute to higher coke

Fig. 14 Composition of gas products at 673 K using composite HZSM-5(80) and AlSBA-15 catalysts at different ratios

Fig. 15 Liquid degradation products at 673 K using composite HZSM-5 (80) andAlSBA-15 catalysts at different catalyst ratios

formation and more rapid deactivation. In this respect, the mesoporous catalyst is generally more effective than the microporous catalyst in suppressing the formation of large molecules and thus causing less carbon deposits (Urquieta et al. 2002).

As can be seen in Fig. 14, the gas products distribution was quite uniform. For the catalyst ratio of 1:1, the smallest compositions were the carbon chain of C1 (12.4 %) and C5 (12.7 %) while the highest were the carbon chain of C3 (28.9 %) and C4 (28.6 %). However, all of them were marginally different. For unequal cat­alyst mixing ratios such as 2:1 and 1:2, the uniformity of product distribution was not able to be maintained. For HZSM-5(80) to AlSBA-15 catalyst ratio of 2:1, the smallest compositions were the carbon chain of C1 and C5 (about 13.0 %) while the highest proportion was the carbon chain of C4 (31.7 %). On the other hand, for catalyst ratio of 1:2, increasing amount of AlSBA-15 composition resulted in lower production of C5 carbon chain (8.9 %) while showing higher composition in the carbon chain of C3 (31.4 %).

Meanwhile, the effect of varying catalyst mixing ratio on the liquid phase degradation products is illustrated in Fig. 15. Mixture of HZSM-5(80) and AlSBA-15
catalysts with catalyst loading ratio of 1:1 produced nearly 89 % of carbon chain range C8-C12, C13-C16, and C17-C20 from the overall liquid composition. Increasing the ratio of either HZSM-5(80) or AlSBA-15 amount in the catalyst mixture appar­ently shifted the product distribution toward heavier carbon chain. The effect of adding HZSM-5(80) was more dominant as it increased the carbon chain range of C17-C20 to 35.7 % while adding AlSBA-15 only increased the carbon chain range of C13—C16 to 28.4 %.

4 Conclusions

The results reported and discussed in the present work demonstrate that micro — and mesoporous materials show promising properties to be used as catalysts in the degradation of HDPE into gaseous and liquid hydrocarbon fuels at 350-500 °C. HZSM-5 (14), HZSM-5 (80), SBA-15, and AlSBA-15 were used under various operating conditions to obtained liquid biofuels. Mixture of HZSM-5 (80) and AlSBA-15 with ratio of 1:2 exhibits higher degradation activity to yield higher liquid biofuels with valuable gas product at 400 °C. In addition, significant HDPE conversions into liquid fuel with lower coke contents were achieved in a batch reactor over the HZSM-5 catalyst as compared to mesoporous silica catalyst SBA-15. The pore shape of zeolites was very important for deter­mining their activities and product selectivity in the degradation of polymeric materials because it influenced the degradation and deactivation rates simulta­neously. SBA-15 containing aluminum catalyst was of a potential interest in the cracking of heavier feedstock such as palm oil waste into biofuels. The weaker acid properties exhibited by the mesostructured catalysts, i. e., SBA-15 and Al-SBA-15 were responsible for their reduced gaseous product production capaci­ties. However, their presence coupled with the combination of HZSM-5 catalysts in the conversion of polyethylene materials promoted a substantial conversion of the original long-chain hydrocarbons into lighter liquid hydrocarbon products (up to 26.5 wt. %). Furthermore, larger pore dimensions exhibited by these sol­ids did not allow for any product selectivity, resulting in the possible formation of a wide range of branched hydrocarbon and alkyl derivative aromatic products. These results suggest that catalytic degradation of HDPE leads into higher liq­uid hydrocarbons yield at lower temperature. It is stated that such type of chemi­cal recycling, i. e., conversion of waste HDPE into hydrocarbon feedstock used as resource for biofuel has been recognized as an ideal approach and could sig­nificantly reduce the net cost of disposal. It is concluded that under appropriate reaction conditions, suitable catalysts such as HZSM-5 (80) and AlSBA-15 have the ability to control both the product yield and product distribution from HDPE degradation, potentially leading to a cheaper process with more valuable products such as biofuel.

The algal biomass production process requires one or more solid-liquid separation steps. Generally, first stage involves a separation of biomass from the bulk sus­pension (including flocculation, flotation, or gravity sedimentation). The second stage (thickening) raises the concentration of the slurry through techniques such as centrifugation, filtration, and ultrasonic aggregation; hence, it is generally a more energy intensive step than bulk harvesting (Brennan and Owende 2010).

The flocculation is the first (preparatory) stage that is intended to aggregate the microalgae cells in order to increase the effective “particle” size. Unlike floccula­tion, flotation methods are based on the trapping of algae cells, using dispersed microair bubbles. Gravity and centrifugation sedimentation methods are based on characteristics of suspended solids and are determined by density and radius of algae cells and sedimentation velocity. It is the most common harvesting technique for algae biomass in wastewater treatment because of the large volumes treated and the low value of the biomass generated. The filtration process is better suited for harvesting relatively large (>70 mm) microalgae such as Coelastrum and Spirulina. The membrane microfiltration and ultrafiltration (hydrostatic pressure) are viable alternatives to recovery of biomass from smaller algae cells (<30 mm), such as Dunaliella and Chlorella (Brennan and Owende 2010). Some species are much easier to harvest, considering algae densities and size. The strain character­istics, cost, and energy efficiency are the main factors to select harvesting technol­ogy (Brennan and Owende 2010).

Biswarup Sen

Abstract In the past decades, the ‘food versus fuel’ debate has caused a transition of first-generation biofuels to advanced biofuels. Although the later seems quite promising, due to its sustainability and low GHG emissions qualities, it is still far from deployment. The major hurdles to the deployment of advanced biofuels include technical and economic challenges, which must be overcome in the near future. Extensive R&D is in progress to bridge the gap between the current techno­logical status and commercial venture. To overcome the significant challenges that make the commercialization of advanced liquid biofuels unrealistic, at this moment, is of prime importance. One of the most significant challenges is the technological barriers, which will probably require some more years of extensive R&D efforts to minimize the issues and concerns. This chapter deals with the technological challenges that the liquid biofuels industry is currently facing in the biochemical conversion of second- and third-generation feedstocks to advanced liquid biofuels. A general introduction to the topic includes the types of liquid biofuels categorized under ‘advanced biofuels’ and their common routes of production namely biochem­ical and thermochemical. A detailed description of the current technological issues in the biochemical conversion process is presented mainly under the subcategories: improving feedstocks, pretreatment methods, hydrolytic enzymes efficacy and cost, and process integration. The chapter ends with a review of the current status of R&D in biochemical conversion route for advanced liquid biofuels.

B. Sen (*)

Department of Environment Engineering and Science, Feng Chia University,

Taichung 40724, Taiwan

e-mail: bsen@fcu. edu. tw; bisens@yahoo. com

B. Sen

Master Program of Green Energy Science and Technology, Feng Chia University, Taichung 40724, Taiwan

B. Sen

Green Energy Development Center, Feng Chia University, Taichung 40724, Taiwan

A. Domingos Padula et al. (eds.), Liquid Biofuels: Emergence, Development and Prospects, Lecture Notes in Energy 27, DOI: 10.1007/978-1-4471-6482-1_10, © Springer-Verlag London 2014

1 Introduction

Biofuels can be produced from agricultural or industrial wastes and are renewable with a potential to decrease our society’s dependence on petroleum. Focus on bio­fuels has gained global attention both amidst the general mass and scientific com­munity, due to various compelling factors such as increasing oil prices, low carbon emission of biofuels, and less impact on the environment. Among all biofuels, liq­uid biofuels have attracted attention of the scientific community, as it is the most convenient form of fuel for the automobile industry. Liquid biofuels usually include bioethanol, biodiesel, butanol, and oil from algae (Demirbas 2009). Bioethanol is produced by fermentation of sugars (carbohydrates) usually derived from sugar — rich crops like sugarcane or sugar beet and/or from starch-rich crops like corn (first-generation biofuel). Bioethanol is also produced from cellulosic biomass (non-food sources) and from grasses and trees (second generation). Bioethanol is widely used in Brazil and also in the USA.

Biodiesel, on the other hand, is produced by trans-esterification of oils, and its chemical composition consists of fatty acid methyl esters (FAMEs). Feedstocks from which biodiesel is produced usually include animal fats, vegetable oils, palm oil, soy, jatropha, mustard, flax, sunflower, pongamia, and algae. Biodiesel can be used in blends with petrodiesel, the purest form of which is B100; however, B20 and lower blends are suitable for diesel engines. Recently, biobutanol production is being researched extensively owing to its better properties as a fuel than bioethanol and is usually produced under anaerobic fermentation called ABE (acetone, butanol, and ethanol) fermentation. Starch can be fermented by microorganisms like Clostridium to produce ABE in the ratio of 3:6:1. Ralstonia sp. can be used to produce biobu­tanol in electro-bioreactor using carbon dioxide and electricity. Metabolically engi­neered E. coli have also been shown to produce butanol. DuPont and BP have jointly ventured into the large-scale production of butanol (Anton and Dobson 2008).

Worldwide biofuel production has reached 105 billion liters in 2010, up by 17 % from 2009; still biofuels just fulfill 2.7 % of the world’s fuel need for transporta­tion. Brazil and USA are currently top producers, accounting for 90 % of total global production of biofuels, while biodiesel production by the EU accounts for 53 % of total biodiesel production as of 2010. The International Energy Agency (IEA) has a mission for biofuels in meeting the demand for global fuel production at least by a quarter by 2050. Global ethanol production for use as bioethanol tripled between the period 2000 and 2007, which amounts to 52 billion liters. In recent years (2011), its production reached 84.6 billion liters; the USA topped with 52.6 billion lit­ers ethanol production, contributing 62.2 % in global production, whereas Brazil with 21.1 billion liters ranked second. Ethanol-based fuel is largely used in Brazil and in the USA, responsible for 87.1 % global ethanol-based fuel production as of 2011. Most cars in the USA run on blends of up to 10 % ethanol. Brazilian gov­ernment has made it mandatory since 1976 to blend ethanol with gasoline; from 2007 onwards, the legal blend is E25. As of December 2011, Brazil had 14.8 mil­lion automobiles and 1.5 million motorcycles that use only pure ethanol fuel (E100).

The USA uses com as a major source to produce bioethanol. Com in general is an energy-intensive crop, consuming a unit of fossil-based fuel energy to create just 0.9-1.3 energy units of bioethanol. General Motors has initiated production of E85 fuel from cellulose ethanol for a possible projected cost of $1 a gallon.

A directive issued in 2010 by the EU has a targeted goal where all members are required to achieve a 5-10 % biofuel usage by 2020. India and China are vastly exploring the usage of both bioethanol and biodiesel. Currently, India is expanding Jatropha plantations to be used in biodiesel production. India is also setting a target of incorporating at least 5 % bioethanol into its transportation fuel. China that is a major bioethanol producer in Asia has a task plan for 15 % bioethanol incorporation into transport fuels. In the developing countries, biomass like cattle dung, wood, and other agricultural wastes are used extensively as fuel for cooking and heating. IEA claims that biomass energy provides for 30 % of energy supply in developing coun­tries for over 2 billion people. In spite of the many advantages of using biofuels for transportation and energy supply, there exits several technical issues that need to be resolved before biofuels can enter into the market with a cost equivalent to gasoline.

There are some common issues related to the use of liquid biofuels. Higher amount of alcohols in petrodiesel fuel blends is reported to cause corrosion of components in aluminum-based designs; this corrosion can be minimized with the addition of water to the blends; tests based on this concept showed that when water content was up to 1 %, there was no evidence of corrosion; only material discolouration was visualized. Biodiesel under low-temperature conditions showed molecular aggregation and formed crystals. Biodiesel usually contains small quan­tities of water, which arise during trans-esterification attributing to the occurrence of mono — and diglycerides because of incomplete reactions. These molecules act as an emulsifying agent making very small quantity of water miscible. Presence of water reduces fuel efficiency causing more smoke, leads to corrosion of fuel system components. Water presence can also interfere with the production process and may also impact the additives used.

On the other hand, butanol is toxic and its production and usage needs to undergo Tier 1 and Tier 2 health effects testing as per the EPA guidelines. As of 2010 food grade algae cost $5,000/tonne, this is attributed to high capital and operating costs, which may impact its contribution as a second-generation biofuel crop. The US Department of Energy estimates that 15,000 square miles of land will be required for algal cultivation if it has to augment replacement of conventional fuel in the USA. The USA alone consumes nearly 1 million barrels/day of conventional biofuels, and the world consumes about 2 million barrels/day. This number will certainly increase twofold to threefold in the next 20-30 years. However, most conventional biofuels (use first-generation feedstock) are highly government subsidized, which mean they are not economically sustainable, except ethanol from Brazil. Therefore, significant technical challenges must be overcome to ensure that biofuels can become economi­cal and affordable at large scale worldwide. The future of biofuels largely depends on the price of biomass and oil-based fuels, which in turn will increase as the demand for biofuels rises. Therefore, technological breakthroughs in the non-food feedstocks development are the most important challenge that needs to be resolved.

1.1 Chemicals and Catalysts

Microporous HZSM-5 (Si/Al = 80), HZSM-5 (Si/Al = 14) and mesoporous SBA-15 and AlSBA-15 catalysts, high-density polyethylene, nitrogen gas, cooling water, hydrochloric acid, N-hexane, triblock copolymer (TCP), tetraethylorthosilicate, and aluminum chloride were the chemicals or reagents, and they were used as received from respective suppliers.

1.2 Preparation of HDPE Sample

The material in pellet form was crushed into powder and then subsequently sieved using a sieving machine to obtain the desired particle size range. Sample of 60-150 mesh size was used in this study.