Your choices:
1 Transport; 2 Biomass (fermentable sludge); Fuel: liquid
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| 2 Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | 1 Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Alcohol fermentation by common yeast is limited by the fact that yeast fungi basically can ferment only such sugars with 6 carbon atoms in them (hexose's) while many of the sugars present in biomass are pentose's, i.e. they contain 5 carbon atoms, or pentose's are formed as intermediate products. This process is a mature technology that is carried out industrially. When lignocellulose is the substrate for ethanol production, there may be some drawbacks, however. These drawbacks relate to the complexity of the lignocellulosic cell wall components that make up the biomass.
With natural yeast the fermentation is actually limited only to glucose but starch as well as some other types of sugar can be enzymatically transformed into glucose and hence qualify as fermentable. Thus, only some materials, those where fermentable sugars are readily available for the enzymes and the yeast, are naturally suitable for alcohol fermentation while most are not. However, two of the main constituents in plant cell walls – cellulose and hemicellulose – are both basically built up by fermentable sugars. The problem is that they are partly crystalline and are embedded in lignin, so that the sugars are not accessible. Different pre-treatments including dilute acids, hot water or ammonia make it possible to break this structure, de-crystallize the cellulose and make the sugars in cellulose and hemicellulose accessible for subsequent fermentation. These processes are still under development and so far they suffer from high costs as well as of some of the by-products being inhibitors to the fermentation process. There is also research on-going to genetically modify micro-organisms so as to render pre-treatment unnecessary.
Hence the dominating process is fermentation followed by distillation and further removal of water in a dehydration process.
Thus, only some materials, those where fermentable sugars are readily available for the enzymes and the yeast, are naturally suitable for alcohol fermentation while most are not.
Todays' production of ethanol is to a significant extent based on dedicated energy crops. This raises the ethical question of large-scale use of agricultural land for energy purposes.
The simplest starting point is to view our planet as a sphere with a circumference of 40000 km. From that assumption, it takes only the simplest math from secondary school to compute the total surface area – which becomes approximately 500 million square km. Now again return to school and remind yourself that approximately 70% of the earths' surface is covered with water – seas and major lakes. Hence the land area is approximately 150 million square km. Now divide this number by the number of people on this planet – about 7 000 million people, and you will find that the land area available to each of us is about 21 000 square meters which is just about 3 football fields. One football field is assumed as 100×70 m2. So three football fields is what every human being has as her "rightful share" of the land of this planet.
Of this land, about 6 000 m2 is infertile like deserts, high mountains and ice-covered etc. Another 6 000 m2 is forested. Almost 6 000 m2 is savannahs, steppes and such meagre land. Of the 3 000 m2 left, 2 000 are rich grassland. Only 1 000 m2 is well suited for agricultural production so that is where to grow food and to grow fibre for clothing.
These are hard facts and cannot be denied. The change of the global climate will affect the distribution and may increase – or decrease – the amount of each biome – but it will not change the size of the planet. Rather, the three football fields are rapidly shrinking because of the population growth.
Imagine making your living only from "your own land" and then make a choice if you want to grow dedicated energy crops on "your" agricultural land – or if you would prefer to grow edible crops. In a sustainable energy system dedicated energy crops are banned unless they are part of a sustainable crop rotational scheme as green fertilizers, nitrogen fixation crops or alike.
This view of the world is the core of sustainable thinking and is of course extremely simplified – but it serves it purpose to illustrate in a simple way what "sustainability" is about: The first step towards a sustainable development is to adopt our use of resources and our use of natural resources to what our planet really has to offer. This puts a strict upper limit to how much biofuel is available. But it also puts focus on the qualities of biomass resources – unless we prefer to compete with agriculture about the 0.15 football fields.
Rather than using dedicated energy crops for alcohol fermentation, the energy planner should look for fermentable industrial sludge and for fermentable agricultural by-products. Mainly, such sludge will originate from food processing industry but to some extent also pulp mills may be contributors.