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Nanocatalysts and Biofuels: Applications and Future Challenges

Desikan Ramesh1, Thangavelu Kiruthika2, Balasubramaniam Prabha2, Maduraimuthu Djanaguiraman3, and Subburamu Karthikeyan2

1 Horticultural College and Research Institute for Women, Tamil Nadu Agricultural University, Tiruchirappalli, Tamil Nadu, India

2 Department of Renewable Energy Engineering, Agricultural Engineering College and Research Institute, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu, India

3 Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu, India

1.1 Introduction

The economy of the developing countries is entirely based on fossil fuels and variation in the price of fossil fuels. On the one hand, the demand for and consumption of fossil fuels are increasing every year because of an increase in population, rapid growth of the automobile sector, and industrialization. Energy consumption, economic growth, and population are interlinked. A recent estimate shows that crude oil, gas, and coal resources will be exhausted in the next five decades if production continues at current resource extraction rates (Behera and Varma 2019). On the other hand, increasing fuel demand, fluctuating fuel prices, uncontrolled population growth, global warming, and ill effects of environmental pollution will force us to search for an alternate ecofriendly fuel to fossil fuels. Among the renewable energy sources, biomass sources-namely plants, oils, and fats-are considered as feedstock to produce a variety of biofuels as future resources (Martini and Schell 2012).

Biomass feedstocks include all types of residues from the agricultural field and processing operations, wood processing industry wastes, forestry residues and branches, lignocellulosic feedstocks, organic fraction of municipal solid waste, and animal wastes, etc. The estimated annual global biomass production is 104.9 billion metric tons of carbon (Field et al. 1998). The annual photosynthesis yield in the world is ca. 720 billion tons of organic raw cellulose materials (Tong 2019) that have potential for conversion to biofuels.

Generally, biomass resources are playing an influential role in supplying food or fuel. Originally, the raw biomass materials were used for the production of heat and other energy requirements, which can make an essential contribution to satisfying the energy needs of society (Ruiz-Altisent 1994). Recently, biofuels production from biomass feedstocks is getting more attraction in developed/developing countries. The reasons for this interest are due to the reduction of foreign currency/crude oil imports, reduced dependence on crude oil, emissions from burning of fossil fuels, and their impact on the environment, i.e. air pollution as well as global warming, etc. To overcome the abovementioned environmental issues, biofuels can be promoted to replace conventional commercial diesel and petrol fuels in the transport sector. There are several biofuel technology pathways of production from various biomass feedstocks. To mitigate the greenhouse gas emissions, we have to start avoiding fossil fuels and/or promote the use of biofuels. Today, the biofuel industries are facing several challenges: specifically, poor supply chain and logistics, more expensive raw materials, higher costs for processing and production compared to petrofuels, low efficiency of the conversion process, and lack of supporting biofuel policies for promotions. Researchers are focused on improving the conversion efficiency of different biomass conversion methods, which can indirectly reduce the process cost and biofuel price. In conclusion, the economically viable biomass conversion technologies will reach commercial scales.

Recently, nanotechnology has been attempted to improve the overall performance of different biomass conversion systems, which, although in the research stage, have the potential to address the problems currently faced by the biofuel industries. In this chapter, the current research on application of nanocatalysts in the field of biofuels production is presented and their impact on product yield is also discussed.

1.2 Biofuels Production

Biofuel is a solid or liquid or gaseous fuel that can be generated from biomass feedstocks, which can replace (partially or wholly) conventional petrofuels. The biofuel production from feedstocks may be produced through biomass conversion methods. The biofuels can be produced in the form of liquid or gaseous or solid (Figure 1.1).

The kind of biofuel mainly depends on the process conditions used in the technology and nature of feedstock materials. The biofuel production technologies for biomass feedstocks have reached the fourth generation, depending on conversion methods and feedstocks used. The first generation deals with the production of biofuels using food crops, and technologies under this category are commercialized for biodiesel and bioethanol production. Feedstocks used for this generation include various carbohydrate and lipid sources for bioethanol and biodiesel production. The second generation deals with non-food crops for biofuel production. This generation's target is to produce bioethanol from all types of lignocellulosic feedstocks. The third generation focusses on production of biofuels (biodiesel/bioethanol) from microalgae. The fourth generation aims to produce biodiesel/bioethanol from genetically modified crops or microbial lipids. Among them, only the first generation for biofuel production from food crops is commercialized. Other generation technologies are still at the research and development stage.

Generally, there are three major biofuel production routes: thermochemical, biochemical, and chemical conversion methods. The thermochemical conversion technologies (TCCTs) deal with the conversion of feedstocks into biofuels using heat with or without air/oxygen, whereas biochemical conversion technologies (BCCTs) use microorganisms under aerobic or anaerobic conditions. The comparison of the BCCT and TCCTs on biofuels is presented in Table 1.1. The chemical conversion technologies (CCTs) are used to produce biodiesel from vegetable oil feedstocks. Biofuels are facing difficulty in selling at a commercial level; conventional fossil fuels are of higher calorific value and cheaper than biofuels. It is very challenging to enhance the calorific value of biofuels and make them on par with fossil fuels. This may result in an increase of the production cost, which is a major challenge for scaling-up to a commercial level. Hence, the production costs should be brought down through technological breakthrough or government policies that provide support in the form of incentives and tax benefits to promote biofuels and protect the environment.

Figure 1.1 Types of biofuels production from various biomass feedstocks via different biomass energy conversion methods.

Table 1.1 Comparison of BCCTs and TCCTs for biofuel production.

1.3 Role of Catalysts in Biomass Conversion

The biomass composition is one determining factor that prescribes the biofuels and biochemicals that can be produced from the biomass feedstocks via TCCTs, BCCTs, or CCTs. The yield of end products is varied when it comes to biomass types and reaction conditions used. The process conditions are determined by the catalyst types and quantity, reaction temperature, reaction pressure, reaction time, biomass compositions, and its properties. The catalyst has a significant influence in speeding up the reactions in the process and thus, the product yield. The catalysts are classified into four categories viz., homogeneous, heterogeneous, biocatalyst, and hetero-homogenized types (Philippot and Serp 2013). The strength and weakness of homogeneous and...