Biogas from Waste and Renewable Resources: Amount of space

2. 1 Amount of space

The amount of space in Table 2.4 is defined as the land area plus the surface area of the water, because algae or water plants in general are biomass and may have potential in the future.

The right hand columns in the table show the amount of space that is available for cultivation of biomass and may have potential.

In theory all the amount of space A_D, including the surface of the water, can be used to produce biomass.

Technically, biomass can be cultivated on all areas except the settlement area, mining lands or badlands. This is an amount of space of A_Dtechn = 0,88 • A_D of the total surface of Germany.

As soon as the micro algae production is developed, then technically an even larger surface, means 95% of the entire available space, could be exploited.

Economically, the cultivation of energy plants competes with the cultivation of other agricultural products. The market will probably equilibrate itself. But overall about 50% of the agricultural area is considered to be available for profitable production of biomass. Some other surfaces will never be agriculturally usable in a profitably way. So the total area for profitable agricultural use for biomass is estimated to be A_Dleclm = 0.56 • A_D.

2.2 Potential yield from biomass

2.2.1 Theoretical potential

Biogas results from the microbial degradation of biomass, formed by photosyn­thesis by solar power E_s.

6CO₂ + 6H₂O+ Es → QH₁₂O₆ + 6O₂

Carbon dioxide + Water + Solar energy -» Sugar (Glucose) + Oxygen

Metabolic processes in the plants, transform the following compounds into sec­ondary products.

Carbohydrates:        Starch, inulin, cellulose, sugar, pectin
Fat:                    Fat, fatty acids, oil, phosphatides, waxes, carotene

Protein:               Protein, nucleoproteid, phosphoproteid

Others:               Vitamins, enzymes, resins, toxins, essential oils.

During the metabolism of the sugar, the plant releases energy, when necessary, to the environment, so that the possible energy yield from plants may vary greatly.

Multiplying the proportion of the main plant components (see Table 2.5) by the entire vegetation, an averaged elementary composition of plants dry matter results:

C₃₈H₆₀O₂₆

With the help of an approximate equation from Buswell (1930), the theoretical maximum yield of methane can be estimated taking the elementary composition as a base:

C_rH_h0₀N_nS_s + yH₂O→xCH₄ + (c-x)CO₂ + nNH₃ + sH₂S

Table 2.5 Main components of plants without nitrogen N and sulfur S.

Carbohydrate                                                       C₆H₁₂O₆,

Fat                                                                          C₁₆H₃₂O₂

Protein                                                                   C₆H₁₀O₂

 

x = 0.125 (4c + h-2o-3n + 2s)
y = 0.250 (4c-h-2o + 3n + 2s)

or, simplified

The hectare yield of methane can hence be calculated from the hectare yield of the dry matter. This again depends on the planting, which should be as productive as possible.

The maximum theoretical possible yield is estimatedat when applying two harvests per year and cultivating C4 plants with an average elementary composition of M_E = 932kg/kmol. Based on the simplified equation from Buswell the yield of CH₄ is E_M = 20kmolCH₄/kg biomass and the energy yield P_theor is calculated by the formula:

to give 144.200 kWh/(ha ^. a). If one multiplies the hectare yield by the entire surface of Germany (35 703 099 hectares), the following equation

results in a primary energy quantity from biomass. Theoretically

the entire amount of primary energy supply in Germany could be covered by biomass alone.

Assuming that the yield of the available cultivable area on earth is proportionally the same as in Germany, an area of 7420Mioha, half of the available area of 14900Mioha on earth, would theoretically be enough to cover the total world primary energy consumption of

If a precondition is that the maximum yield should be guaranteed on a long-term basis, this could be facilitated by

Accurate and targeted addition of fertilizer

Water and fertilizer can be added very accurately by using hoses which are directly led to the roots. The accuracy depends on the characteristics of the local soil, but the overall yield per hectare of conventional agriculture could perhaps be doubled, particularly, when some missing nutrients are supplied with the water.

Multiple harvests per year

Yields of 25-30 Mg DM/ha.a can be obtained if the field crops shown in Table 2.6

are cultivated immediately after each other during one year.

Today the most frequently cultivated crop rotation consists of the following three crops:

1. The domestic cold-compatible C3 plants: winter rape or
winter rye

2. The southern C4 plants: corn (mass-producing species),
as main crop during summer

3. The cold-resistant C3 plants: GPS.

In order to generate energy, all the plants are harvested as soon as they finish their growth without leaving them time to fully develop. The costs of cultivation are 61-84 US$/Mg for the cultivation of winter wheat, winter barley, and triticale a crossing of wheat and rye in Germany.²¹⁾

Overall the cultivation of energy plants has just started. Besides maize, some other C4 plants like sorghum, sugar cane, or Chinese reed seem to be efficient when used as biomass.²²⁾ Their yield, though, still needs to be improved. Also, certain C3 plants such as grain, grasses, hemp, rape, beet, sunflower, or winter peas seem to have good potential as energy sources with a yield still to be increased, too. In future this broader range of energy plants will allow interesting new com­binations and an increased level of flexibility in deciding on the crop rotation system.

2.2.1.1 C3 plants (energy plants)

The enzyme most important for the production of energy is RuBisCo (Rubilose 1.5-diphosphate carboxylation-oxygenase). It is the most frequently produced enzyme of all organisms and can be found in the chloroplasts of the plants in the form of proteins. Their level in the proteins amounts to 15%.

RuBisCo catalyzes photosynthesis and photorespiration. It binds oxygen as well as C0₂ and acts as oxygenase. For photorespiration to occur, the chloroplasts, mitochondria, and glyoxisomes, cell components around the mitochondria, need to be involved.

The ratio of photosynthesis to photorespiration is defined by the ratio of CO₂ and 0₂ in the air. With a higher concentration of CO₂, the output of the photosyn­thesis increases.

In moderate zones, e.g., in Central Europe, photorespiration in plants plays a subordinate role. Predominantly C3 plants occur, which use the light-independent reaction, the Calvin cycle (Figure 2.4), to bind CO₂. They are called C3 plants, because the first stable product in the Calvin cycle after the CO₂ fixing 3PGS (Phosphoglycerate) has 3 C-atoms. Also the molecule which is reduced from 3PGS with NADPH+H+ to 3PGA (Phosphoglycerin aldehyde) in the following phase of the Calvin cycle contains 3 C-atoms.

The leaf structure of C3 plants is layer-like. In warm summer weather the transpi­ration and the evaporation at the surface of the sheets increases. In order to minimize the water loss, the plants close their pores. CO₂ cannot be absorbed by the pores any longer. Thus the photosynthesis is stopped and the biomass yield is limited.

In addition, the biomass yield depends on the soil as well as the entire climatic conditions: in some regions of the world the yield can be up to five times higher than in Germany. It is not possible, however, to obtain the theoretically projected yields just by cultivating C3 plants (Table 2.7).

Other typical representatives of C3 plants are onions, wheat, bean, tobacco.

Most C3 plants are well adapted to the moderate climatic zones but not to arid, saline areas with hot and dry air. Under such climatic conditions the ratio of photo­synthesis to photorespiration increases from 2:1 and negatively impacts the yield.

2.2.1.2  C4 plants and CAM plants

There is a large group of 1700 variants of C4 plants and/or CAM plants which are all well adapted to hot and dry climates and do grow in arid, saline areas. This is possible since the CO₂ fixing occurs in C4 plants spatially separated from where the Calvin cycle occurs. In CAM plants the CO₂ fixing happens at a different time of the day from that when the Calvin cycle occurs (Figure 2.5). Such plants can utilize even the smallest CO₂ concentrations.

The separation of the CO₂ fixing occurs with the help of the enzyme PEP carbox­ylase (PEP = phosphoenolpyruvate), which possesses a substantially higher affinity to CO₂ than RuBisCo. The first product of the photosynthesis which is stable is oxalacetate (see Figure 2.7), a C4 product. This characterizes the so-called C4 plant.

Compared to C3 plants, the leaves of C4 plants are anatomically different. The spatial separation of the CO₂ fixation takes place in cells at a distance from each other, the bundle sheath cells and the mesophyll cells, both containing chloro-plasts but different types: the mesophyll cells contain normal chloroplasts while the budle sheath consist of chloroplasts with grana. The vascular bundles to trans­port the cell liquid are covered by a layer of thick bundle sheath cells which are surrounded by mesophyll cells.

An intensive mass transfer is continuously happening between the bundle sheath cells and the mesophyll cells. This starts with the formation of oxalacetate (Figure 2.6), a result of the enzymatic reaction of PEP-carboxylase binding CO₂ to PEP = (phosphoenolpyruvate). Oxalacetate is then enzymatically transformed into malate and transferred to the chloroplasts of the bundle sheath cells. In the bundle sheath cells it degrades into pyruvate and C0₂ while forming NADPH+H+ as a by-product. CO₂ is introduced into the Calvin cycle while pyruvate is transported back into the mesophyll cells.

CAM plants actually belong to the group of C4 plants. The name "CAM plants" is derived from the Crassulaceae Acid Metabolism (acid metabolism of the Crassula-ceae), since the metabolism was first observed in the plant species "Crassulaceae".

Because of the high water loss, these plants open their stomata only at night to take up CO₂ which is stored in form of malate. During the day, CO₂ is released and transformed in the Calvin cycle forming ATP as a by-product.

Like C3 plants, the CAM plants have layer-like structured leaves.

Some species of CAM plants are cactuses, pineapple, agave, Kalanchoe, Opuntia, Bryophyllum, and the domestic Sedum spec, or Kalanchoe (Crassulaceae).

C4 and/or CAM plants show the following advantages, compared to C3 plants:

• C4 and/or CAM plants can generate biomass twice as fast if
  conditions are favorable (see Table 2.8).
• The upper leaves of C4 and/or CAM plants are
  perpendicularly directed to the sun, so that the low-hanging
  leaves still get sufficient light even under unfavorable light
  conditions.

 

Figure 2.8 Sorghum (above right), Micro-algae chlorella in glas tuber (below). Energy maize abt. 5 m heigh (above left).

• C4 and/or CAM plants once planted grow again after biomass has been harvested.

2.2.1.3    Micro-algae

By cultivating micro-algae, even the surface of water as well as the area of rooftops can be exploited in a profitable way.

A yield of 15-17 Mg biomass per year seems theoretically be achievable by plant­ing micro-algae and cultivating them in well-lit bioreactors.²⁹⁾

 

Most of the micro-algae naturally grow much better when the light is somewhat diffused rather than in direct clear light. Sun may even limit the growth. In order to control the light in the latest bioreactors, the micro-algae are cultivated in airlift reactors in which a circular flow is caused by changing the direction of the gas bubbles (Figure 2.9). The light reflects at the outer wall of the reactor.

The circular flow is set in such a way that the algae are located mainly in the outer area of the incidence of light. The algae absorb just enough light to keep the Calvin cycle alive for a maximum yield of biomass.

In reactors erected in the sea, the sea water can actually be used to help maintain a moderate temperature inside the reactor. The micro-algae may also serve to clean the water, especially in cases where the reactor is located close to a river mouth and the water is led through the reactor.

Micro-algae can not only be used to produce biogas but also to provide lipids, fatty acids, vitamins, e.g., vitamin E, beta-carotene, or even pigments like phyco-cyanin or carotenoids. Antioxidants like tocopherols or omega fatty acids may also be extracted, which are very interesting from a pharmaceutical point of view.

In 2000 the first farm for micro-algae was inaugurated close to Wolfsburg, in the middle of Germany. Within a fully closed system of bio-reactors (about 6000 m³ in total) chlorella algae are converted into about 150-200 Mg animal food produced annually.³⁰⁾

Print version

up