Use of plants and microbes for production of fuels and chemicals

Introduction

Currently, the major feed stocks for chemical and transportation fuel production are derived from petroleum. The petroleum refining and manufacturing processes involved result in significant CO2 (carbon dioxide) emissions. These emissions could be reduced if the gas itself served as feedstock for chemical and fuel manufacture.

In terms of chemical structure, this requires the joining up of the carbon atoms from CO2 molecules into the larger molecules that comprise the chemicals and fuels in question. The problem is that this requires energy. However, there are organisms that are able to utilise a suitable form of energy to build up large, complex molecules from simple ones. Such organisms are called autotrophs. Plants and algae are prominent autotrophs. In the process of photosynthesis, they absorb light energy from the sun to build up complex organic (carbon-containing) chemicals. (Organisms like ourselves that obtain their energy for life from the breakdown of complex molecules such as the carbohydrates synthesised by plants are called heterotrophs.

Plants and microalgae are at the forefront of the drive to replace petroleum-derived products for both chemical and fuel manufacture. Plants have provided humans with biofuel for thousands of years in the form of wood for warmth and cooking. But in industrialised countries this traditional source has been replaced by fossil fuel (coal, petroleum and natural gas). Oil crises and pressure to reduce CO2 emissions have stimulated research into developing plant-derived products that can serve as alternatives to fossil fuel. The two major biofuels so obtained are bioethanol and biodiesel, both of which are used mainly as transportation fuels.

Globally in 2012, about 83 billion litres of bioethanol and 22.5 billion litres of biodiesel were produced. These products fuel a very small proportion of the world’s transport; about 3% in 2012 (see figure 8, page 30 and page 31 of report by the Renewable Energy Policy Network for the 21st century REN21).

This article outlines the chemical nature of some of the products (especially biofuels) made by plants and microalgae as well as the synthetic processes involved. Biofuels are the only viable fuel to replace petroleum-derived fuels for road, sea and aviation transport. Electric vehicles may be appropriate for short-distance road travel, but long-distance travel awaits development of suitable batteries. The article briefly mentions some hurdles that need to be overcome before the technologies are deployed commercially. Brief reference is also made to developments in which other (non-photosynthetic) autotrophic microbes are being exploited for their ability to synthesise biofuels and chemicals from mixtures of CO (carbon monoxide), CO2 and hydrogen (H2) (See article on Coal Gasification).

First generation biofuels

First-generation or conventional biofuels are biofuels made from sugar, starch, and vegetable oil. Sources for production of this first cohort of biofuels are food crops. Those high in carbohydrates, i.e. sugar or starch (sugar beet, sugar cane and maize (corn)) were used to produce bioethanol. It has been mandatory in Brazil since 1976 to use bioethanol/petrol blends that contain at least 10% bioethanol. These remain major sources today. It has been projected that approximately 40% of the corn grown in the US in 2011 will be used for bioethanol production. Later, plant oils from palm, soybean, rapeseed and other plant oils were sourced to produce biodiesel. Using plant oils has the advantage over carbohydrate in that oil has twice as much energy gram for gram. The first use of biodiesel blended with petroleum-derived diesel (5:95) was in 2005. While Europe tends to concentrate more on biodiesel (plant oils contribute about 5% of diesel in Europe), the US and Brazil concentrate more on bioethanol, which contributes about 6% of the petrol volume.

Figure 1 (page 2 below) shows the route from CO2 (absorbed by plant leaves from the air) and H2O (absorbed through roots) through crop growth (photosynthesis) and ethanol production to the products of combustion of ethanol in the internal combustion engine. That the combustion products are the same as the starting materials (CO2 and H2O) shows that the materials have been recycled. The figure also illustrates qualitatively the energy relationships between the intermediates from starting materials to end-products. The further up the diagram the higher the amount of energy locked up in the intermediate. For further detail on the chemistry and chemical structures involved click here.