Pretreatment of Biomass

Process-controlled Enzymatic Hydrolysis (PEH)

The degree and rate of degradation of substrates in the biogas process are strongly dependent upon the chemical structures of the materials involved. Whereas fats, proteins, and starch are very rapidly and almost completely transformed into biogas, the fibrous components, and especially the cellulose fraction, pose a challenge for biogas generation. On the one hand, the structure of the cellulose fibres hinders enzymatic hydrolysis; on the other hand, the conditions typically encountered in a biogas digester (pH value, temperature) are unfavourable for cellulolytic enzymes and significantly reduce their activity. Fibrous plant components are consequently degraded only slowly and incompletely in biogas plants.

Against this backdrop, PFI Biotechnology has developed a special pre-hydrolysis process to substantially increase the degree and rate of degradation of the fibrous components. In process-controlled enzymatic hydrolysis (PEH) the biomass is pretreated in a specially designed hydrolysis vessel before it reaches the biogas digester. Optimum pH and temperature condition for the activity of hydrolytic enzymes are maintained in this controlled and sensor-monitored process. After thorough laboratory and pilot-scale testing, the process is currently undergoing full-scale trials (see also Development).

Thermal Pressure Hydrolysis (TPH)

Use of lignified biomass requires the application of further pretreatment processes, especially if sugar-enriched hydrolysates are to be provided for fermentation.

Here PFI Biotechnology is working hard on a hydrothermal pretreatment process, known as Thermal Pressure Hydrolysis (TPH). The hemicellulose fraction of lignified substrates (such as straw) is extensively hydrolysed in a special batch process and the lignocellulose prepared for subsequent enzymatic hydrolyse (see also Development).

Lignocellulose: is the most widely distributed of all naturally occurring organic materials and is the primary building block of plant cell walls. The three major components of lignocellulose are: cellulose, hemicellulose, and lignin. These three biopolymer structures are together responsible for the stability of plant materials. For example, the pronounced durability of wood is attributable to these substances.

Above all, lignin is particularly stable and protects plant structures against microbial degradation. However, it is precisely this persistence which hinders the biotechnolocal transformation of lignocellulose into chemical raw materials, fuels (2G), energy, etc.

Sustainable conversion of lignocellulose requires mixtures of enzymes which hydrolyse cellulose and hemicellulose into their component sugars. At present these enzymes are very costly to produce. Industrial-scale implementation of the processes is therefore currently uneconomical.

2G Fuels: Biofuels  – i.e. fuels derived from biomass  – can nowadays be divided into two categories: If substrates containing sugar or starch, e.g. maize or sugar cane, are used in their production, they are described as 1st generation (1G) biofuels. Such 1G fuels are comparatively easy to produce and are manufactured on an industrial scale, for example in the USA and Brazil (bioethanol). However, their popularity is steadily declining because the land used for growing fuel crops cannot be used for cultivation of food crops.

Intense research is therefore being conducted on the production of fuels from residual biomass such as straw. Residual biomass consists mainly of lignocellulose and is not suitable for consumption as food or animal feed. Fuels derived from lignocellulose are designated as 2nd generation (2G) fuels. Their industrial production is currently very expensive. The main cost drivers are the enzymes used in their production process (see lignocellulose). 

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