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Biofuels: Classification, Conversion Technologies, Optimization Techniques and Applications

Sakthivel R1*, Abbhijith H1, Harshini G V1, Musunuri Shanmukha Vardhan1 and Krushna Prasad Shadangi2

1Department of Mechanical Engineering,Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Coimbatore, India

2Department of Chemical Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India

Abstract

To combat climate change, many researchers over the past few decades have focused their attention towards adopting and studying biofuels that could not just cause fewer toxic emissions compared to conventional petroleum-based fuel, but are also being generated from a myriad of plant waste, animal waste, etc. In this chapter, the primary study is focused on the classification of biofuels: 1st gen, 2nd gen, and 3rd gen biofuels. Then, different conversion technologies, such as pyrolysis, gasification, hydrothermal processes, and transesterification, which are commonly used in industries and labs to obtain biofuels from a plethora of biomass are studied in detail. After obtaining the biofuel from any one of the aforementioned conversion technologies, they could be used as an alternative power source and studied for emission and performance characteristics. However, to determine the optimal results, a suitable optimization technique would have to be employed. In this chapter, two such optimization techniques, namely Response Surface Methodology and Genetic Algorithm, are described in the view of optimizing engine parameters. Finally, a clear view is given into the application of biofuels in the transportation sector, particularly in the automotive and aviation sectors.

Keywords: Biofuels, pyrolysis, gasification, hydrothermal processes, transesterification, response surface methodology, genetic algorithm

1.1 Introduction

Back in 2005, the petrol price in India was ? 59.29 per liter and in 2020 petrol prices are at an all-time high of ? 95.53 per liter. The hike in the gap of 15 years is about 62% and it has been concluded that there will be a hike in all fossil fuel-based products in forthcoming years. This trend can be seen all around the world since fossil fuels are being depleting at an alarming rate. Externality means a consequence of an industrial or commercial activity that affects other parties without being reflected in market prices. It is a well-known fact that the usage of fossil fuels causes pollution, but this factor of energy consumption and production of fossil fuels is not considered, hence, making this factor an externality. Under research done by the International Monetary Fund, there is an indication that economic and environmental costs due to fossil fuels add up to $5 trillion [1]. As of now, there is an unprecedented worldwide interest to reduce carbon dioxide emissions which is central to reducing greenhouse gas effects and improving the air quality of the metropolitan cities. In many countries around the globe, the quality of air has been extremely bad in spite of the preventive measures taken by the government. This can be accounted to the over usage of fossil fuels and emissions due to automobiles.

The report submitted by INERA (International Renewable Energy Agency) at the G20 summit to meet the central goals of the Paris climate change agreement showcased that clean energy can achieve up to 90% of energy-related carbon emissions. As of now, 24% of world power generation comes from renewable energy. The primary energy supply stands for energy production plus energy imports, minus energy exports, minus international bunkers, then plus or minus stock exchanges. Right now, in the 24% of available clean energy, 16% accounts for primary energy supply. To achieve desired decarbonization by the year 2050, renewable energy should account for up to 80% and 65% of the primary energy supply. Bringing down the carbon levels in the air can be achieved by only shifting to a clean energy source, or, in other terms, renewable energy. It is important and high time that we move on to adopt and advance renewable energy. Renewable energy is derived from a replenishable source such as the Sun (Solar energy), wind (Wind power), rivers (Hydroelectric power), hot springs (Geothermal power), tides (Tidal power), and biomass (Biofuels). Sunlight is the most abundant form of energy available to us and solar energy is currently constantly seeing a surge in its usage to generate electricity around the world. Solar Photovoltaic (PV) technology can proselytize sunlight to electricity with the use of PV materials. The prices of renewable energy technologies for electricity production in the solar and wind sectors have come down by 70% and 30%, respectively. Denmark has taken up the challenge of transition to 100% clean energy in which wind power is going to play a major role. INERA suggests that biofuel production should increase 10 times by 2050.

It would not be an understatement to say the way forward is renewable energy. Biomass is going to play a major role in the coming future. Biomass is plant or animal materials that can be processed to generate biofuel and such processes could also be used to generate heat and electricity. Biomass was the first method known to man to generate heat as early humans used wood logs to generate heat. There are many types of biomass; if we consider the type of production, it can be chemical or biological, liquid or gas if we consider the type, and heat, current, or transport if we consider the purpose [2]. Biomass is obtained from specific energy crops, agriculture crop residues, forest residue, processed wood residue, algae, municipal waste, etc. Figure 1.1 shows the various forms of biomass. Biofuels are derived from biomass which includes, but is not limited to, animal waste, plants, and algae. Three major benefits from biomass usage are greenhouse gas reduction, low dependence on foreign oil, and an increase of opportunities in forestry and agriculture fields. Ethanol and biodiesel are very well-known biofuels. Biofuels are distinguished into three categories, namely 1st gen. biofuel, 2nd gen. biofuel, and 3rd gen. biofuels. These biofuels can be obtained from different feedstock yields. The given table below lists various biofuel alternative feedstocks currently and long-term yields under the International Energy Agency (IEA).

1st generation biofuels are derived from biomass such as sugar, starch, and vegetable oils. To attain 1st generation biofuels, many well-known methods such as fermentation, distillation, and transesterification are used. 2nd generation biofuels are processed through wood, organic, and food waste along with some specified biomass crops. 2nd generation biofuels biomass go through a pretreatment process in which lignin is broken down. This pretreatment consists of thermochemical or biochemical reactions. After the pretreatment, the process is parallel to the production of 1st generation biofuels. The 2nd generation biofuels generate higher energy yields compared to that of 1st generation biofuels. 3rd generation fuels specifically use algae as the feedstock to make biodiesel. The algae's oils are converted into biodiesel using a similar process as 1st generation biofuels. It is a well-known fact that 3rd generation fuels are highly energy-dense compared to 1st generation and 2nd generation biofuels. Unlike 1st and 2nd generation biofuels, 3rd generation biofuels do not depend on crops, which relieve the stress on water and land. Meanwhile, they can be termed as high-energy, renewable, and low-cost sources of energy. Seaweed is also being reviewed as a possible energy source for 3rd generation biofuels, with the end product being biomethane. The BMP (Biochemical Methane potential) values of seaweed stock vary from 101.7 (L CH4/kg VS-1) to 357.4 (L CH4/kg VS) per the species selected [3]. In the last decade, there has been a lot of research done on fast pyrolysis that can be used to extract high bio-oil yields. In this method, raw biomass is rapidly subjected to heating under inert atmosphere and immediately condensed to obtained liquid product. Pyrolysis is accounted as thermal decomposition of the feedstock with a low level of oxygen [4]. Gasification is also one of the methods to obtain biofuels based on fermentation of the biomass to obtain products like ethanol, butanol, hydrogen, methane, and acetate. After gasification is completed, the obtained syngas is processed into acids and alcohols with the help of specific microorganisms by the fermentation process. Lignocellulosic biomass, after fermentation, goes through size reduction that can be achieved in two methods to obtain biofuels either by pretreatment, hydrolysis, fermentation, and purification or gasification and fermentation. Gasification takes place in a low oxygen environment and fuel abundant conditions with an equivalence ratio of 0.25 (mass of O2/stochiometric mass of O2) [5].

Figure 1.1 Various forms of biomass.

Hydrothermal processes are used for the extraction of third-generation biofuels which must be abstracted from microalgae and macroalgae. One of the main reasons for using hydrothermal processes for 3rd generation biofuels is due to high moisture content in the aquatic biomass. In this method, the biomass is processed wet in hot compressed water. This process is temperature dependent and we get different end products per the operating temperature of the operation. At lower temperatures (less than 200 °C) hydrothermal carbonization (HTC) occurs in which the end product is biochar. At intermediate temperatures (200 - 375...