The Brilliance of Bioenergy

In Business and In Practice

Ralph E H Sims

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781134072330
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

The time for modern biomass has come. It has long been overshadowed by other, more widely-publicized renewable energy technologies such as wind, solar and hydro, and still retains an outmoded image in comparison to its apparently more attractive cousins.The potential for biomass to act as a store of solar energy, and yet to be converted efficiently when required into heat, power, transport fuels and even substitutes for plastics and petrochemicals, is not widely appreciated. The increasing abundance of well-designed, successful bioenergy projects around the world is creating new interest in this renewable, sustainable and low-emission-producing source of energy.The Brilliance of Bioenergy covers all the main resources and technologies, principles, practice, social and environmental issues as well as the economics involved. The book also presents valuable, practical experiences - both 'how to' and 'how not to' - in the form of case studies of both small and large scale projects in both developed and developing countries.The Brilliance of Bioenergy is for those wishing to learn more about biomass, the technologies and the business potential. It will be welcomed by all involved in biomass production, bioenergy utilization, planning and development, and in renewable energies in general, as well as students, academics and researchers in the subject.

Chapter 1

What is biomass and what is bioenergy?

The extremely varied nature of biomass, and the many routes possible for converting the biomass resource to bioenergy in the form of useful energy products and services, make this whole topic a complex subject. For other renewable energy sources, such as wind, solar and hydro, the energy conversion technology employed is the key component. For biomass it is the whole system that is critical, and this entails gaining an understanding of:

• the nature of biomass and its greenhouse gas mitigation potential (Chapter 1)
• the range of diverse biomass resources, including wastes (Chapters 2 and 3)
• the processing and delivery of these resources to the energy conversion plant (Chapter 4)
• the thermochemical conversion of dry biomass fuels (Chapters 5 and 6)
• the biochemical conversion of wet biomass fuels (Chapter 7).

Biomass can be transformed into both heat and electricity simultaneously through cogeneration (Chapter 8), into transport fuels (Chapter 9), and even into petrochemical substitutes. At the smaller domestic and village community scale, biomass also has good potential as a fuel for micro-turbines and fuel cells (Chapter 10). In all cases environmental issues are key factors, and can be dealt with by developing good practice guidelines and understanding life cycle analysis. A range of socio-economic benefits will also result, particularly at the smaller scale. There seems little doubt that biomass will provide an increasing share of the global primary energy supply, and that in the future new and more efficient bioenergy technologies will continue to be developed (Chapter 11).

In the words of Professor David Hall, one of the pioneers of modern bioenergy who sadly died in late 1999 prior to his vision being fulfilled: ‘Biomass is forever.’

So what is biomass?

From a renewable energy perspective, biomass can be defined as:

recent organic matter originally derived from plants as a result of the photosynthetic conversion process, or from animals, and which is destined to be utilized as a store of chemical energy to provide heat, electricity, or transport fuels.

Biomass resources include wood from sustainably grown plantation forests, residues from agricultural or forest production and organic waste by-products from food and fibre industries, domesticated animals and human activities.

The chemical energy contained in the biomass is derived from solar energy using the process of photosynthesis (based on the Greek photo, implying to do with light, and synthesis, the linking together of several parts). Photosynthesis is the process by which plants take in carbon dioxide and water from their surroundings and, using energy from sunlight, convert them into sugars, starches, cellulose, lignin etc. These components of vegetable matter are loosely termed carbohydrates, shown chemically for simplicity as [CH₂O]. Oxygen is produced and emitted into the atmosphere during the process:

It is of course not that simple, and many specialist books exist on the very complex photosynthetic process for anyone wanting more information.

All plant material on Earth, both terrestrial and marine, is formed using this process. Further down the food chain animals that graze plant material as well as carnivorous species all indirectly depend on photosynthesis. Animal products and organic wastes can therefore be classified as forms of biomass when used for energy purposes.

Only a small proportion (0.02%) of the solar energy reaching the Earth each year is fixed and stored by terrestrial biomass, and more by the algae, plankton, aquatic plants, etc. found in the oceans and waterways. The small amount of solar energy captured is equivalent to perhaps seven or eight times the global anthropogenic primary energy consumption, which currently exceeds 400 EJ/year (details of energy units are given later in the chapter).¹

Depending on the time of year and specific location, the global irradiance is around 600–1000 W/m² of the Earth’s surface on a clear sky summer day, reducing to 200–500 W/m² on a cloudy day to perhaps 50–100 W/m² on an overcast winter day. Since the sky conditions vary day by day, season by season, and with latitude, the solar energy reaching the Earth’s surface is very variable but can be assumed to average around 1000 kWh (or 3600 MJ)/m² per year.

Taking an example, the total annual solar energy reaching a one hectare (1 ha = 10,000 m²) wheat field in say the Canadian Prairies, a temperate climate in a location receiving about the average solar irradiation of 1000 kWh/m² per year, is therefore

• annual energy received = 36,000 GJ
• but only one third arrives during the growing period of the plants = 12,000 GJ
• of which only 20% reaches the leaves of the growing plants = 2400 GJ
• 20% loss occurs from reflection of some of the light = 2000 GJ
• 50% is lost as photosynthetically active heat radiation = 1000 GJ
• 30% of the remaining energy is converted into stored energy = 300 GJ
• of which 40% is consumed in sustaining the plant = 180 GJ.

So, typically, 1 ha of land can store approximately 180 GJ/year in the growing crops.

If in the example the field is planted in say Miscanthus, a vegetative grass grown for energy purposes in Europe which yields around 9 t dry matter/ha per year, then, since biomass dry matter contains around 20 GJ/t, this equates to around 180 GJ of available energy contained in the above-ground harvestable biomass. This is approximately only 0.5% of the 36,000 GJ solar input received, which is therefore the conversion efficiency percentage of solar energy to stored chemical energy in the crop. It results from a range of factors and therefore varies widely.

A plant grows by photosynthesis. It also continually respires, during which process the carbohydrates are oxidized to produce carbon dioxide and water. When the plant, or components of it, die (as in the leaf fall of deciduous trees), the material decays. Oxygen is used and heat is released, as is the case for combustion, although the latter occurs over a far shorter period. Thus combustion of biomass can be considered to be simply a more rapid process of either of the natural processes of decay or respiration. In essence the interception of biomass from the natural or agricultural cyclical process and use for energy purposes is the reversal of photosynthesis.

Globally around 55 EJ/year of biomass is currently used for energy purposes, mainly for cooking and heating in developing countries, but also for running a growing number of large-scale modern biomass energy plants. By comparison the world population consumes around 10 EJ/year of energy in the form of food, which of course is a biomass energy resource in itself. (One bowl of cornflakes costing say 10 cents contains enough energy for the consumer of it to cycle 10 km as opposed to driving a car the same distance and consuming around 1 litre of petrol costing around $1 – never mind the additional externality costs involved from the greenhouse gas emissions!)

When the biomass, with its store of chemical energy, can be used usefully it becomes a fuel. In simple terms, the atoms making up the molecules of fuel are linked by electrical forces. Combustion can be considered as a process that converts this stored electrical energy into heat, since the energy in the carbon dioxide and water molecules remaining at the end is much lower. Provided that biomass consumption does not exceed the natural level of plant growth occurring to replace the volume of biomass consumed, then, when converting biomass into useful bioenergy, no more total heat is generated or carbon dioxide created than would have been produced by the natural decomposition processes. So in theory biomass is an energy source that, when managed and used sustainably, has few if any adverse effects on the environment.

In addition to the aesthetic value of the planet’s flora and its wonderful biodiversity, biomass represents a useful and valuable resource. The value of biomass is related to the chemical and physical properties of the large molecules of which it is made. Humans have long exploited biomass by burning it for heat and eating it for the nutritional energy from its oil, sugar and starch content. More recently, particularly in the last 150 years, humans have exploited fossilized biomass in the form of coal, oil and gas. This fossil fuel is the result of very slow chemical transformations that converted the sugar polymer fraction of the biomass into a chemical composition that resembles the lignin fraction. The additional chemical bonds formed in the fossil fuels represent a more concentrated source of energy. Since it takes millions of years to convert biomass into fossil fuels, they are not considered to be renewable, based on the short time frame over which we are consuming them.

The chemical composition of biomass varies among plant species, but it generally consists of approximately 50% carbon, 44% oxygen and 6% hydrogen (plus water), in the form of 25% lignin and 75% cellulose and hemicellulose (carbohydrates). Most species also contain about 5% of smaller molecular fragments such as resins, collectively known as extractives. Typically 1 t of biomass contains around 5–20 GJ of energy depending on the type of material and moisture content. The amount of energy contained in any given source of biomass is valuable information to have in order to enable the size of conversion plant to be designed around the amount of fuel available.

When a crop is to be grown to provide a fuel for bioenergy, it is also useful to know in advance the amount of biomass energy that can be obtained from a particular area of land in terms of GJ/ha per year. In practice this depends upon the climate, weather, location, soil type, soil nutrient levels, water supplies and of course the species grown and crop yield. For examp...