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Hydrothermal Liquefaction and Gasification of Biomass
Hydrothermal Liquefaction (HTL)
Hydrothermal liquefaction is only one of the several ways to harvest energy from the biomass like algae, wood, etc. This process uses water as a solvent under extreme temperatures and pressures. Here the biomass or fossil fuel based carbon rich material is converted into carbon monoxide, hydrogen and carbon dioxide gases at high temperatures without combustion and with a controlled oxygen supply.
Here the conversion of the wet biomass into crude oil occurs which also a natural process undergoing beneath the Earth’s surface. The oil obtained through this process is named as Bio oil or Bio crude.
The process occurs at a moderate temperature and high pressure for several minutes. In this process organic material like wet biomass which may be garbage, wood, crops, landfill gas, alcohol fuels etc., coals, sewage sludge, animal waste etc. are mixed with the water in the absence of the oxygen where the temperatures are between 250Oc – 550OC and high pressures between 5 – 25 MPA applied on this mixture and keep it for 20-60 minutes.
During this retention time chemical reaction takes place although higher and lower temperatures are used to optimize the gas or liquids. At these temperatures water becomes either sub or super critical depending on the conditions and acts as solvent, reactant and catalyst to facilitate the reaction of biomass to bio-oil.
Here the biomass hydrolyzes and decomposes initially to form low molecular weight compounds which then re-polymerize to form a viscous crude bio-oil. The process is then cool down and pressure decreased and gets separated into 3 phases, CO2, water and bio-crude oil.
This bio-crude oil has high density with lower heating value. Depending on the process this fuel can also be used in heavy engines like marine and railways by upgrading it to diesel, gasoline and jet fuels.
Factors Affecting the Crude Oil Quality
Some of the factors that affect the quality and the yield of the crude oil produced through the HTL process includes the following.
The components like cellulose, hemicellulose and lignin in the feedstock or biomass obtained from the different sources affect the yield and quality of bio-crude oil.
Ideal temperatures usually depend on the biomass used and any temperature above this ideal values increases the char and the gas formation. And if the temperatures are lower than the ideal value reduces the de-polymerization and yield.
Pressure is another factor which determines the overall reaction kinetics.
As HTL is a fast process, there will be lo retention time for de-polymerization to occur. This RT is again dependent on the feedstock, solvent ratio and temperature. Hence optimization of RT is necessary.
Hydrothermal Gasification of Biomass
Gasification of biomass is the main innovative thermo-chemical process by which a combustible gas is formed as an end product by burning of the solid biomass in a limited air supply. The poorly valued biomass can be utilized to transform the renewable gas. This process is particularly well suitable for the effluents which contains about 5-50% of the dry matter like effluents from industries, liquid residues from the methanation units, effluents from the livestock manure, sludge from sewage treatment plants.
This process takes place at a temperature of about 1000OC and the reactor in which the combustion occurs is known as gasifier. Here in gasifier the wet biomass is compressed to about 300 bar. A salt separator then discharges all the non-organic solid residues like mineral salts which are used in agriculture.
Gasification process includes the high conversion of carbon which is about 90% into a methane gas, hydrogen and CO2. The remaining liquid after the formation of gases is the useful nitrogen rich water.
This process which has to be carried out for a whole day is done within 2 minutes of time. The obtained product has high energy efficiency that is about 60-70% due to heat recycling process. As the output gas has usually high pressure, it is purified and injected as a low carbon and renewable gas as a substitute to natural gas.
Following are the steps involved in the production of Hydrogen gas.
Drying is done to reduce moisture content in the biomass.
Decomposition and pyrolysis occurs at high temperatures.
Gasification of biomass occurs where the reactions involve in conversion of the biomass into hydrogen gas.
Later the produced gas is separated from the other bye products.
Conclusion
As world is facing an increased environmental problem with wet wastes like sewage sludge, many disposable methods are becoming costly. So in order to convert this waste biomass into renewable energy many of the thermo-chemical conversions are really helpful.
These includes hydrothermal liquefaction and gasification to produce liquid crude oil and syngas respectively. Here in Hydrothermal Liquefaction, long chain carbon molecules and biomass are thermally broken into water and carbon dioxide and the catalyst is used to improve the yield and product quality.
Carbon and hydrogen atoms in the biomass is thermo-chemically converted into the crude oil which is a hydrophobic compound. Hydrothermal gasification is based on the thermochemical conversion of the biomass from the different sources to the renewable gas called methane gas. Here the oxygen amount and the steam supplied for the process shows effects on the yield of hydrogen.
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