Production of Bio-oil from Biomass by Fast Pyrolysis Technology
It is widely accepted that the problems related to environmental pollutions, global warming, domestic energy shortage, economic crisis and dependency of crude oil import are rooted from the unbalanced utilisation of fossil fuels for electricity generation, transportation, industries and agricultures. The use of fossil fuels leads to the increase of pollutants and greenhouse gases such as sulphur oxides (SOX), nitrogen oxides (NOX) and carbon oxides (COX) in the atmosphere. One of the potential solutions to solve or mitigate these problems is to employ renewable resources such as biomass in place of fossil fuels. There are several types of biomass available. The appropriate ones should be non-food. Otherwise, other problems related to the shortage of food supply would occur. Non-food biomass such as agricultural and industrial residues is considered as potential raw material for alternative energy production.
Fast pyrolysis products
Biomass can be directly used as fuel or converted to other forms of fuels. It can be converted by a technology called “fast pyrolysis” for liquid fuel production. The liquid product is known as “bio-oil”. Bio-oil can be directly used as a burner fuel or further upgraded to transportation fuels. Fast pyrolysis technology has been investigated and developed for more than 30 years. Currently, many countries around the world are involved in the development of industrial scale fast pyrolysis plants. These include Canada, Netherlands, China, Germany, Finland, United Kingdom, United States of America, and Spain. The largest fast pyrolysis plant at the present time is in Canada by Dynamotive Energy System Corporation with the biomass throughput of 200 ton/day. Many other countries such as Brazil, Ukraine, France, Australia, Belgium, Denmark, Greece and Italy have also investigated fast pyrolysis technology at research level.
Pyrolysis research and development in Thailand is still in its infancy. Some examples of organisations involved in this technology are Mahasarakham University (MSU), Thailand Institute of Scientific and Technological Research (TISTR), National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), Chulalongkorn University (CU), King Mongkut’s University of Technology North Bangkok (KMUTNB) and PTT Public Company Limited.
In fast pyrolysis process, biomass is dried to a moisture content of below 10wt%, ground to a particle size range of less than 5 mm depending on the reactor type and put in the biomass hopper as shown in Figure 1. Biomass particles are continuously fed in a reactor. For a fluidized – bed type, heat transfer medium such as sand is put in the reactor. Since pyrolysis is an endothermic reaction taking place in the absence of oxygen, the supply of external heat and the removal of oxygen are required. Heat can be supply by combustion of non-condensable gas and char products. The removal of oxy gen or air can be performed initially by purging the system with an inert gas such as nitrogen. After the pyrolysis system starts, part of the non-condensable gases can be recycled and used as the carrier gas.
In fast pyrolysis of biomass, the internal reactor or heat transfer medium temperature should be around 400-600°C. It is also beneficial if the carrier gas is pre-heated before entering the reaction zone. This is to avoid the cooling of the fluidised bed. Once a biomass particle gets into the reactor, heat is transferred from the sand and carrier gas to the particle leading to the rapid thermal decomposition. The biomass devolatiles giving off the so-called “pyrolysis vapour” and leaving solid char as residues. The pyrolysis vapour and char are quickly entrained out of the reactor by carrier gas, passing one or more cyclone separators where char particles are removed from the pyrolysis vapour. Depending on the system design, the pyrolysis vapour may be further filtered by a hot vapour filtration unit in order to remove char fines of particle size less than 10µm. The pyrolysis vapour is then quickly cooled to room temperature by a series of condensers or quench towers. The system may include an electrostatic precipitator (ESP) for capturing the pyrolysis vapour. The liquid product is collected and stored in a bio-oil tank. After the condensation unit the non-condensable gases exit the system including carbon dioxide (CO2), carbon monoxide (CO) and combustible gases such as methane (CH4), propane (C3H8), propylene (C3H6), ethane (C2H6), ethylene (C2H4) and hydrogen (H2).
There are several types of fast pyrolysis reactors developed so far such as bubbling fluidised bed, circulating fluidised bed, rotating cone, augur, ablative, vacuum and entrained flow reactors. Figure 2 shows the fast pyrolysis units developed in Thailand at Mahasarakham University. Typical bio-oil shown in Figure 3 can be used as a fuel for boilers, engines or gas turbines for generation of heat and electricity. Bio-oil can be upgraded either in the process or after production by, for example, the application of catalysts. An example of bio-oil produced from the catalytic pyrolysis is also shown in Figure 3. As can be seen, the appearance of the catalytic bio-oil is different from the typical one. This may be used as transportation fuel. However, future research and development of the catalytic fast pyrolysis is to be carried out in order to prove the feasibility of the process in large scale.
In conclusion, fast pyrolysis is a promising technology for biofuel production. It requires careful considerations in design, construction, commissioning and operation. Current studies in Thailand have shown the high potential of the technology as it can apply virtually all types of biomass available. The further research and development of this process in Thailand should be directed to a pilot plant for production of both catalytic and non-catalytic bio-oil. Since biomass such as agricultural residues are not gathered in one place, shipping of the bulky solids to a large scale fast pyrolysis plant would not be economically feasible. Therefore, the development of a mobile fast pyrolysis plant in Thailand would be more appropriate provided that there are available applications accepting the bio-oil product as their fuels.
Bio-Energy and Renewable Resources Research Unit, Faculty of Engineering, Mahasarakham University
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| date/time : 2017-09-11 | read : 297