A few years ago, we referred to the large potential for the production of bioproducts and next-generation biofuels from the waste biomass that accumulates at palm oil plantations and mills. The palm oil tree is one of the most productive plants on the planet. Currently only the oil in its fruit and kernels is used for commercial purposes. However, this resource constitutes only a tiny fraction (less than 10%) of the total amount biomass that is generated on a plantation - the rest is burned or dumped into the environment as waste.
A group of researchers from the Universiti Sains Malaysia now finds that this vast stream of waste biomass holds a considerable potential for the efficient and cost-competitive production of renewable biohydrogen via a process known as supercritical water gasification (SCWG) - of growing interest to bioenergy researchers. The process yields hydrogen twenty times less costly than H2 from electrolysis of water when the primary energy comes from renewables like wind or solar, and one fifth less costly than H2 obtained from steam reforming natural gas - the most likely candidate for large scale hydrogen production in the future. The chemical and energetic properties of the residual palm biomass, especially its high moisture content, make it a ‘perfect’ feedstock for the novel gasification process. The energy balance (’EROEI’) of the biohydrogen was found to be 9.9, indicating a highly efficient use of the resource. The researchers discuss their findings in a recent issue of the scientific journal Energy Policy.
There is no denying that today’s palm oil based biofuels come with their share of problems. They could drive deforestation and when only the oil is used, the fuels could in fact generate more GHG emissions than conventional fossil fuels, because of the emissions resulting from land use change (palm oil biofuel produced from plantations that were established on non-forest land do cut emissions, though). The coproduction of biohydrogen would improve both the greenhouse gas profile of these first generation biofuels as well as their energy balance. Profitable utilization of the residues would also limit the need for further expansion of the palm oil acreage.
Environmental sustainability criteria such as those proposed by the European Commission must ensure that negative land-use effects are minimized. One way of doing so is by converting residual biomass into green energy and thus getting more out of a hectare of land. Depending on which energy product is coproduced, this practise can considerably improve the emissions profile of (first generation) palm oil based biofuels. Both Indonesia and Malaysia, the world’s largest palm oil producers, have understood this message and are concentrating on finding ways to use the large amount of residual biomass efficiently and profitably.
Residues and utilization pathways
Besides a small amount of palm oil (around 5 tonnes per hectare), a plantation produces fronds, leaves, trunks, press fibers, empty fruit bunches (EFB), kernel shells and processing waste such as palm oil mill effluent (POME). This biomass generally consists of cellulose, hemicellulose and lignin, but composition varies according to plant species. The composition of some of the most common residues, as well as their tonnage per hectare, is outlined in table 1 (click to enlarge).
Several utilization pathways for these residues have been analysed, with some being used increasingly by plantations and mills. One of the most straightforward ones consists of using the residual biomass as a fuel source to power palm oil processing plants - the fuel replaces coal or natural gas, and because of its abundance a palm oil plant would feed excess green electricity into the grid - a practise similar to that found in Brazil’s sugar and ethanol processing plants which use bagasse to power their own operations as well as nearby towns. Several palm oil plants have opted for this pathway. However, the high moisture content of the biomass makes alternative uses more energy efficient.
Bioproducts such as bioplastics and fibre products can be produced from several types of non-oil palm biomass. The utilization of lignocellulosic biomass for the production of liquid fuels - via gasification and Fischer-Tropsch synthesis (biomass-to-liquids), pyrolysis or biochemical transformation - is another possibility. One particularly environmentally damaging waste stream - Palm Oil Mill Effluent (POME) - is now being transformed into biogas more and more often. Several of these projects are part of the Clean Development Mechanism.
Tau Len Kelly-Yong, Keat Teong Lee, Abdul Rahman Mohamed and Subhash Bhatia from the School of Chemical Engineering at the Universiti Sains Malaysia now suggest a more futuristic and efficient pathway: using the large biomass resource as a feedstock for the production of renewable biohydrogen.
Biohydrogen is a fully decarbonised energy carrier, it contains no carbon. This implies that the CO2 released during its production can be captured and sequestered. When such carbon capture and storage (CCS) technologies are coupled to biohydrogen production, a carbon-negative fuel is obtained. The net emissions from first generation biofuel (e.g. palm oil biodiesel) and second-generation biofuels (e.g. hydrotreated palm oil biodiesel) would be offset by this coproduced carbon-negative biohydrogen.
This is why Kelly-Yong’s research is so interesting. It points to a possible future in which palm oil plantations - provided they are not based on deforested land - would generate “negative emissions”. That is, the energy they generate would actively remove historic CO2 from the atmosphere (unlike other renewables, which are merely carbon-neutral and do not add new CO2 to the atmosphere). ‘Carbon-neutralised’ liquid fuels would be available for export, whereas decarbonized, carbon-negative biohydrogen would be used locally either in electricity production or as a transport fuel. The overall carbon emissions balance of all energy thus generated would be negative.
The Malaysian team looked at the availability of palm residues on a global scale. They found that the destination of this huge amount of biomass is raising concerns. The supply of oil palm biomass and its processing byproducts were found to be no less than 7 times the availability of natural timber globally. Every year, the oil palm industry generates more than one 184.6 million tonnes of residues worldwide (graph, click to enlarge):
After outlining the complications of current bioconversion pathways used on this biomass, Kelly-Yong and collegues say the urgent need for transforming this residue into a more-valuable end product can be met by converting it into biohydrogen via gasification using supercritical water reaction technology. Oil palm biomass is “the perfect candidate as feedstock for the gasification process”, they write.
The feedstock has a high energy and moisture content (450%), which is an integral requirement for reactions in SCW reaction and for the generation of renewable energy. The insignificant amount of trace minerals in the biomass composition is another advantage for the reaction. Furthermore, the availability of oil palm biomass all over the year allows continuous operation of the process.
Supercritical water gasification
Supercritical water gasification (SCWG) is a relatively novel gasification method, in which biomass is transformed into a hydrogen-rich gas by introducing it in supercritical water (SCW) (schematic, click to enlarge). SCW is obtained at pressure above 221 bar and temperatures above 374 �C. By treatment of biomass in supercritical water - but in the absence of added oxidants - organics are converted into fuel gases and are easily separated from the water phase by cooling to ambient temperature. The produced high pressure gas is very rich in hydrogen.
Characteristic of the SCW-organics interactions is a gradually changing involvement of water with the temperature. With temperature increasing to 600 �C water becomes a strong oxidant and results in complete desintegration of the substrate structure by transfer of oxygen from water to the carbon atoms of the substrate. As a result of the high density carbon is preferentially oxidized into CO2 but also low concentrations of CO are formed. The hydrogen atoms of water and of the substrate are set free and form H2.
The SCW process consists of a number of unit operation as feed pumping, heat exchanging, reactor, gas-liquid separators and if desired product upgrading. The reactor operating temperature is typically between 600 and 650 oC; the operating pressure is around 300 bar. A residence time of � up to 2 minutes is required to achieve complete carbon conversion depending on the feedstock. Heat exchange between the inlet and outlet streams from the reactor is essential for the process to achieve high thermal efficiencies. process overview of biomass gasification in supercritical water The two-phase product stream is separated in a high-pressure gas-liquid separator (T = 25 - 300 �C).
Due to these conditions significant part of the CO2 remains in the water phase. Possible contaminants like H2S, NH3 and HCl are even more likely to be captured in the water phase due to their higher solubility, and in fact in-situ gas cleaning is obtained. The gas stream from the HP separator contains mainly the H2, CO and CH4 and part of the CO2. In a low pressure separator a second gas stream is produced containing relative large amounts of CO2, but also some combustibles. This gas can e.g. be used for internal heating purposes.
The SCW process is in particular suitable for the conversion of wet organic materials (moisture content 70 - 95%) which can be renewable or non-renewable.
The primary gas produced by the SCW process differs significantly from most other biomass gasifiers: gas is produced at very high pressure, hydrogen content is high, no dilution by nitrogen.
The produced gas is clean (no tar, or other contaminants in high pressure gas even if produced in the process) and it always contains high amounts of hydrogen; the amounts of CO and CH4 depend on the operating conditions. Complete carbon conversion is achieved after relative short residence time, and significant amounts of CO are found, whereas methane content is still low. For long residence times gas equilibrium has been established and CO is almost completely absent, but methane content is significantly increased.
Water plays various roles in facilitating the gasification reaction, due to its unique ability and properties. The hot compressed water molecules can participate in various elementary reaction steps as reactant, catalyst and medium.
In the gasification reaction, the biomass under severe conditions is instantaneously decomposed into small molecules of gases in few minutes, at a high efficiency rate. A gaseous mixture of hydrogen, carbon dioxide, carbon monoxide, methane and other compounds is obtained from the reaction (Ni et al., 2006). The chemistry of the reaction during the gasification under the influence of SCW and pressure is often cited as complicated and complex as it involves multiple reactions that occur simultaneously to produce the gaseous and liquid mixture.
However, 3 main reactions are identified: (1) steam reforming, (2) methanation and (3) water-gas shift reactions (Hao et al., 2003). The reactions are identified as follows :
Biomass + H2O -> H2 + CO; (1)
CO + H2O -> CO2 + H2; (2)
CO + 3H2 -> CH4 + H2O: (3)
In reaction (1), the biomass reacts with water at its supercritical condition in the steam-reforming reaction to produce gaseous mixtures of hydrogen and carbon monoxide. Subsequently, the carbon monoxide produced from the first reaction will undergo an inorganic chemical reaction termed as water-gas shift reaction with water to produce more carbon dioxide and hydrogen as shown in reaction (2). It is possible that the carbon monoxide produced from reaction (1) between water and biomass caused the equilibrium of the water-gas shift reaction to shift to the right, ultimately producing more hydrogen in the end product. In the last reaction, methanation will occur where the carbon monoxide will react with hydrogen in the earlier reaction to obtain methane and water as its end product.
The utilization of SCW medium in biomass gasification has several advantages. It can directly deal with high moisture content biomass. Therefore, preliminary treatment such as biomass drying can be avoided, advantageously preventing the high cost related to that process.
Hydrogen production via SCW technology represents a potential source of renewable energy for the future. It is estimated that the cost of hydrogen production via SCW gasification ranges between US $3-7 per GJ or US$ 0.35 per kg, as compared with the most obvious current method - stream reforming of natural gas - the cost of which averages between US $5-8/GJ.
However, the exact costs are expected to differ slightly for different kinds of biomass depending on its origins. In comparison with other conventional and alternative processes for hydrogen production, SCW gasification of biomass is by far the most cost-efficient method to produce hydrogen (figure, click to enlarge). Comprehensive studies have been carried out with great success on this technology, utilizing biomass sources such as corn starch, clover grass, wood dust, organic waste, industrial waste, etc. The results show a high percentage of hydrogen in the end product and very little
production of residues.
Efficiency and energy balance
Kelly-Yong and his collegues analysed the energy efficiency of the gasification reaction when based on palm oil biomass, the efficiency of pure hydrogen production, and the energy balance taking into account all energy inputs for a palm plantation.
In order to calculate the energy efficiency of the gasification reaction, researchers have taken the following definition: the sum of external energy of the desired products divided by the total process inputs. However, for such an analysis often only hydrogen is taken into account as the desired output, without considering other end products.
For their part, the Malaysian researchers defined the desired end product as a mixture of hydrogen, carbon monoxide, carbon dioxide and also methane. Besides the chemical energy of the mixture gases, it is also vital to include heat recovery into the calculation since it contributes significantly to the efficiency of the reaction.
Comprehensive heat recovery unit can increase the percentage of efficiency of about 10-25% higher compared to those without a recovery unit. In the gasification reaction, heat can be recovered from the energy released from product, and from the the heat of the reaction.
Therefore, Kelly-Yong and collegues define the energy efficiency as the ratio of total chemical energy from products (hydrogen, carbon monoxide, carbon dioxide and methane) plus the heat released (product and reaction) to the overall chemical
energy contained in the feedstock (biomass and water) plus the energy required for heating of the biomass, in the reaction. For this reaction, it is assumed that process heat is provided by wood combustion with an efficiency of 75%.
With these parameters, the theoretical energy efficiency of the gasification reaction of oil palm biomass, without heat recovery, is around 46.54%. With heat recovery, the energy efficiency is about 72.91%. The real energy efficiency percentages are estimated to be about 10-25% lower than these thermodynamic values.
Pure hydrogen efficiency
The pure hydrogen production efficiency in the gasification reaction is an important parameter that must be accurately studied, the researchers say. There are several methods to determine the magnitude of this efficiency. They chose to consider the lower heating value of input and outputs. Hydrogen efficiency is then defined as the ratio of hydrogen output to the biomass input plus external energy minus energy recovered, as presented.
The maximum theoretical pure hydrogen production efficiency was found to be 34.93% without heat recovery and 57.96% with heat recovery. Real values about 10-25% lower than the thermodynamic values.
The evaluation of the final energy balance (energy inputs versus energy outputs, ‘EROEI’) for oil palm biomass is also an important parameter. The total energy input needed to obtain the biomass feedstock is estimated (by others) to be 19.2 GJ per hectare per year for an oil palm plantation (seed to processing plant). The gasification of oil palm biomass produces a total energy output of 190.96 GJ per hectare per year. Thus, an EROEI of 9.9 is found.
For the researchers This high ratio is another evidence of the viability of the reaction in transforming the high-energy biomass into higher energy end product.
Hydrogen production potential
After these analyses, the researchers calculated the total potential of biomass waste streams used in large-scale applications of the SCWG technology. Even though this technology requires improvements in energy recovery and the optimization of various parameters to ensure that the reaction is well controlled and is able to reach its maximum conversion, it is already capable of yielding positive energy efficiencies.
To calculate the real potential, the hydrogen percentage in the end product gas mixtures is taken to be 61.29%, and the theoretical maximum yield of hydrogen is about 0.117 kgH2 per kg of biomass. With world oil palm biomass production annually standing at about 184.6 million tons, and taking a 100 and 50% conversion efficiency, between 21.6 and 10.8 million tonnes of hydrogen can be produced every year, respectively.
Currently in 2006, world hydrogen production is estimated to be at about 50 million tonnes and growing at 10% per year. With the inclusion of hydrogen produced from oil palm biomass, world hydrogen production can be increased by up to 43.2% yearly. The increasing expansion of the oil palm plantation acreage in most of the countries where it is cultivated may provide a large source of biomass for hydrogen production.
Picture: empty fruit bunches, one of the residues of palm oil processing.
Biopact wishes to thank co-author Keat Teong Lee for additional information.
Tau Len Kelly-Yong, Keat Teong Lee, Abdul Rahman Mohamed, Subhash Bhatia, “Potential of hydrogen from oil palm biomass as a source of renewable energy worldwide”, Energy Policy 35 (2007) 5692-5701, 7 August 2007
K.T. Tan, K.T. Lee, A.R. Mohamed, S. Bhatia, “Palm oil: Addressing issues and towards sustainable development”, Renewable and Sustainable Energy Reviews, in press.
Biopact: And the world’s most productive ethanol crop is… oil palm - June 21, 2006
On Supercritical Water Gasification, see:
EU sponsored project: SuperH2: Biomass and Waste Conversion in Supercritical Water for the Production of Renewable Hydrogen.
Biomass Technology Group: Biomass gasification in supercritical water.
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