1.5. Methods of Processing Biomass
1.5.3. Thermo-chemical conversion technologies
Introduction Chapter-1 Figure 1.7: Anaerobic digestion system (bio-chemical method) for processing biomass feed stock to bio oil. (Source: Energy systems research unit- University of Strathclyde;
www.esru.strath.ac.uk/EandE/Web_sites/03-04/biomass/background info8.html).
This allows controlling the flow to further processing steps. It also allows excess storage in case of any unforeseen amount of feed stream offered. Then feedstock is allowed to pass to pasteurizer where the temperature is maintained at 70oC for at least one hour. This temperature is enough to destroy the pathogens in the feedstock which prevents the bacterial competition in the digestion stage. To allow a constant supply of feed to digestion stage three pasteurizers are used.
Further the material is transferred to digestion tank where the majority of bio gas is produced.
The anaerobic bacteria convert a quantity of organic matter into biogas in a sealed container.
This is continuously stirred and heated to 35oC and the average retention time is approximately 18 days. This allows a significant percentage of organic solids to be converted to biogas. The outflows of biogas are in two forms, the biogas and the liquor/fiber, known as digestate. The biogas generated from the digestion tank finally will be sent to gas storage.
Introduction Chapter-1 referred to devolatization. The thermal decomposition takes place in less than two seconds; is the primary chemical reaction that is precursor for both the combustion and gasification processes.
This thermal decomposition of biomass through pyrolysis leads to the formation of bio-oil, bio char, methane, hydrogen, carbon-dioxide and carbon-monoxide. This process involves simultaneous and successive reactions when organic material is heated in an inert atmosphere. In Pyrolysis, the thermal decomposition of organic components of biomass starts at 350oC – 450oC and goes up to 700oC – 800oC in the absence of air/oxygen.
Figure 1.8: Pyrolysis process (thermo-chemical method) for producing bio-oil from biomass feedstock.
(Source:www.altenergymag.com/content.php?issue_number=09.02.01&article=pyrolysis)
The long chains of carbon bonded with hydrogen and oxygen breaks into smaller ones in the form of gases, condensable vapors, and solid charcoal under pyrolytic conditions. The extent of decomposition and rate of reaction is dependent on the process parameters and reaction
Introduction Chapter-1 temperatures, pressure, reactor configuration and feedstock. The pyrolysis process under lower temperatures (450oC) and when the heating rate is quite slow, the biomass yields bio-char;
whereas at higher temperatures i.e., above 800oC, pyrolysis of biomass yields gases with rapid heating rates. At intermediate temperatures and relatively higher heating rates pyrolysis of biomass yields bio-oil. Pyrolysis process can be categorized as slow or fast pyrolysis. Slow pyrolysis takes several hours to complete the process and results in bio-char as the main product.
On the other hand, fast pyrolysis yields 60% bio-oil, and takes seconds for complete pyrolysis. In addition, it gives 20% bio char and 20% syngas. Thus, the fast pyrolysis is currently used most widely.
Further in this method, there are three main components, namely, combustion zone, reaction zone and pyrolysis zone. In the combustion zone, the combustible solid substance which is the feed stock is made out of elements carbon, hydrogen and oxygen. From the combustion chamber the carbon dioxide is obtained from carbon and water is obtained from hydrogen in the form of water vapor. The reactions that are taking place inside a combustion chamber are exothermic and yield a theoretical oxidation temperature of 1450oC. The products obtained from partial combustion in combustion zone will now pass through a reaction zone consisting of a red hot bed of charcoal where the reduction reaction takes place. These reactions are endothermic and the temperature may be in the range of 800oC -1000oC. Further, the pyrolysis zone is an intricate process that is not completely out broken. The product formation is dependent upon temperature, pressure, residence time, and heat losses. In this pyrolysis zone the temperature is maintained at 700oC. Up to 200oC only water is driven off. Between 200oC -280oC carbon dioxide, acetic acid and water are given off. The real pyrolysis, which takes place between 280oC
Introduction Chapter-1 -500oC, producing large quantities of tar and gases containing carbon dioxide. Besides large tars, some methyl alcohol is also formed.
Advantages of biomass pyrolysis
Biomass pyrolysis can be carried out at small scale and at remote areas which improve energy density of the biomass asset and diminish transport.
This methodology offers an alluring method for changing over organic matter into energy products which is utilized for the generation of high heat, power and electricity.
This process has been accumulating much consideration because of its high productivity and great natural execution qualities.
The char delivered from pyrolysis can be sequestered which has an enormous effect in the fossil fuel outflows overall and goes about as a significant player in the worldwide carbon market (trading of carbon emissions specifically targeted for CO2) with its vigorous, clean and straightforward engineering.
1.5.3.2. Gasification
Biomass gasification includes the partial combustion of solid fuel at the temperature of 1000oC inside a reactor termed as gasifier and the procedure is known as low temperature gasification. In contrast, high temperature results the elevated amounts of hydrocarbons, flammable and combustible gases like CO, H2, tar, dust, and traces of methane also known as producer gas. The producer gas is utilized in internal combustion engines, as a substitute for furnace oil in immediate heat applications and to deliver in a financially feasible way.
Different types of gasifiers for the gasification of solid fuels are shown in Figure 1.9.
Introduction Chapter-1
Figure 1.9: Types of gasifier a) Downdraft gasifier b) updraft gasifier c) cross draft gasifier (Source: www.soi.wide.ad.jp/class/20070041/slides/05/index_42.html)
The generation of these gasses is by response of water vapor and carbon dioxide through a shining layer of charcoal. Along these lines the way to gasifier outline is to make conditions such that a) biomass is lessened to charcoal and b) charcoal is converted to CO and H2. Since there is an association of air, oxygen and biomass in the gasifier, they are characterized by the way air and oxygen is presented in it.