مروری بر منابع تولید، مزایا و معایب سوخت‌های زیستی به عنوان یک منبع انرژی تجدید پذیر

Today, the world is facing a huge crisis due to the increase in global energy demand and the greenhouse gases (GHGs) emissions caused by the use of fossil fuels. The consumption of energy carriers produces large amounts of carbon dioxide and other pollutants that pollute the environment. According to the reports of the World Energy Organization in 2019, the amount of carbon dioxide emissions has increased by 0.5% and primary energy consumption by 1.3% worldwide, which can be a warning for humans and the environment. On the other hand, most of the world's energy needs are provided by oil, coal and natural gas resources, all of which are limited and will be exhausted in a short time with the current consumption. The severe decrease in fossil fuel resources, the industrialization of most countries, their dependence on energy, and as a result the increase in the global price of fuel, on the one hand, has caused an energy crisis, and on the other hand, the increase in fuel consumption and inaccuracy in the way it is used has caused It has become a global environmental crisis. Therefore, the energy crisis, pollution and environmental effects caused by the use of fossil fuels have caused humankind to look for new sources of renewable energy, which is a type of renewable energy, biofuels. Biofuels are classified into four categories of first, second, third and fourth generation biofuels based on the raw materials used in their production. The first generation biofuels include biomass related to food products, while the second generation includes lignocellulosic biomass. The third generation has potentially renewable resources in the form of algal biomass, while the fourth generation includes genetically modified (GM) algal biomass. The main purpose of the article is to review the various sources of biofuels, compare different generations of biofuels and discuss the advantages and disadvantages of different types of biofuels.

Introduction
Dramatic industrial progress, innovation in style, and the increase in the number of vehicles in the world have led to a dramatic increase in living off oil. Currently, more than 80% of the primary energy consumption in the whole world is fueled by oil, of which 60% is the share of energy and consumption. The continuous exploitation of oil fuel reserves to meet the available energy is the rapid use of these energy sources. The continuous growth and fluctuations of crude oil prices, along with the major contribution to the emission of greenhouse gases (GHGs) by their consumption, cause negative effects on human health along with the earth's ecology. Therefore, finding alternative energy sources is a new need, which should be variable, sustainable, environmentally friendly, efficient and economical.
The report of the International Energy Agency (2015) predicted that oil and gas reserves and crude oil supply shortages are likely to bring a severe energy security emergency to the world. It is estimated that the growing demand for energy will exceed the limited supply of oil globally from 2020. Among many alternative energies, biofuel has attracted more attention worldwide because biofuel is considered the most stable and compatible energy source, which can be used in three forms: solid, liquid, and gas. The energy content of biofuel is derived from biological sources and organic matter that make up the body of living organisms. In fact, biofuel is a type of fuel that comes from biomass sources. This means that the nature of biofuel goes back to plants, and this makes it renewable. Recently, the popularity of biofuels has increased due to the increase in oil prices and the need to ensure energy security.
Biofuels have shown great potential for future energy supply and achieving sustainable energy security. The use of biofuels as renewable energy reduces the emission of air pollutants, including greenhouse gases, especially CO2 during the combustion process. Therefore, the overall pollution load and other environmental effects are minimized. It is predicted that about 10-50% of the world's energy consumption will be produced from biomass by 2050. This estimate shows that biomass will be one of the largest sources of sustainable energy worldwide. Different types of biofuel production technologies are used around the worldwide, generally known as first, second, third, and fourth generation biofuel production technologies. Various studies have reported that the biofuel produced from the first generation technology has certain limitations. However, second and third generation biofuels can produce more biofuels. The main purpose of this article is to review and discuss the types of biofuels, their production sources, advantages, disadvantages and finally compare different types of biofuels with each other.

Methodology
The current research is a descriptive-review study whose data were obtained through library studies and different sources were used to process the materials. Considering the importance of the use of biofuels as a renewable energy source, we tried using the most relevant and up-to-date sources containing valuable points regarding biofuels as much as possible. The main purpose of this article is to review and discuss the types of biofuels, their production sources, advantages, disadvantages and finally compare different types of biofuels with each other.
Although first-generation biofuels are biodegradable and have many environmental and social benefits, food versus fuel and the need for extensive land and time to grow edible raw materials are some of their disadvantages. Also, the high cost of raw materials, which consumes more than 70% of the production cost, is also a limiting factor.
With increasing criticism of the sustainability of first generation biofuels, attention was directed toward the potential of second generation biofuels. In fact, second generation biofuels have been developed as a solution to some disadvantages associated with first generation biofuels and can be made from non-edible raw materials such as waste cooking oils, animal fats, recycled oils and bio Produce lignocellulosic biomass, such as grass, wood, sugarcane bagasse, agricultural residues, forest residues, and municipal solid waste.
The challenges associated with first and second generation biofuels, especially regarding the selection of raw materials, led to the development of third generation biofuels. Algae, which are the main feedstock for third-generation biofuels, do not interfere with the food chain and do not require land or fresh water for cultivation, either naturally or artificially. Other raw materials for the production of third generation biofuels include yeast, fungi and cyanobacteria. Examples of third -generation biofuels include bioethanol, vegetable oil, biodiesel, bioethanol, and jet fuel. Fourth generation biofuels are processed using genetically modified (GM) algae and photobiological solar fuels and electrical fuels. Genetically modified algae biomass is effective in producing biofuels, improving the photosynthetic efficiency and increasing light penetration. Solar raw materials for fourth -generation fuels are widely available, economically cheaper and inexhaustible.

Conclusion
Due to the limitation of oil resources and non-renewability of these resources, the world will undergo many changes in the not too distant future. One of the most important and practical methods is to replace fossil fuels with biofuels. Reducing the environmental damage caused by the emission of greenhouse gases, preventing global warming, is one of the important achievements of the use of biofuels. In this study, the types of biofuels were examined based on the raw materials of production, and the advantages and disadvantages of different generations of biofuels were briefly stated.
First generation biofuels represent a move toward cleaner and renewable energy. However, they have limitations because they are derived from food biomass, which is a food source in the world. This becomes a problem when there is not enough food to feed everyone, and millions of people around the world are having hunger. It is true that second-generation biofuels solve many problems associated with first-generation biofuels. Second generation biofuels do not compete between fuel and food products because they are obtained from non-edible biomass. . However, some biomasses still compete with land use for second-generation biofuels because some biomasses are grown in the same climatic conditions as food crops. Additionally, the production process of second generation fuels is more complicated than first generation biofuels.
Third-generation biofuels meet the highest expectations and offer the best chance of gaining an alternative fuel opportunity, as they do not compete with food and have a relatively higher energy efficiency than first- and second-generation biofuels. However, a lot of research needs to be done to reduce production costs and make this type of fuel commercially viable and cheap. Fourth - generation biofuels are more advanced than third - generation biofuels. Metabolic engineering of algae forms the basis of the fourth -generation biofuel production. Compared to other biofuels, this fuel can absorb CO2 and has a high production rate. However, there are still challenges in providing them in terms of economics and feasibility. The production technology is in the research and development phase, and the conversion of raw materials into the final product of biofuel requires new technologies.