Abstract
Various catastrophes related to extreme weather events such as floods, hurricanes, droughts and heat waves occurring on the Earth in the recent times are definitely a clear warning sign from nature questioning our ability to protect the environment and ultimately the Earth itself. Progressive release of greenhouse gases (GHG) such as CO2 and CH4 from development of various energy-intensive industries has ultimately caused human civilization to pay its debt. Realizing the urgency of reducing emissions and yet simultaneously catering to needs of industries, researches and scientists conclude that renewable energy is the perfect candidate to fulfill both parties requirement. Renewable energy provides an effective option for the provision of energy services from the technical point of view. In this context, biomass appears as one important renewable source of energy. Biomass has been a major source of energy in the world until before industrialization when fossil fuels become dominant and researches have proven from time to time its viability for large-scale production. Although there has been some successful industrial-scale production of renewable energy from biomass, generally this industry still faces a lot of challenges including the availability of economically viable technology, sophisticated and sustainable natural resources management, and proper market strategies under competitive energy markets. Amidst these challenges, the development and implementation of suitable policies by the local policy-makers is still the single and most important factor that can determine a successful utilization of renewable energy in a particular country. Ultimately, the race to the end line must begin with the proof of biomass ability to sustain in a long run as a sustainable and reliable source of renewable energy. Thus, the aim of this paper is to present the potential availability of oil palm biomass that can be converted to hydrogen (leading candidate positioned as the energy of the millennium) through gasification reaction in supercritical water, as a source of renewable energy to policy-makers. Oil palm topped the ranking as number 1 fruit crops in terms of production for the year 2007 with 36.90 million tonnes produced or 35.90% of the total edible oil in the world. Its potentiality is further enhanced by the fact that oil constitutes only about 10% of the palm production, while the rest 90% is biomass. With a world oil palm biomass production annually of about 184.6 million tons, the maximum theoretical yield of hydrogen potentially produced by oil palm biomass via this method is 2.16 1010 kg H2 year-1 with an energy content of 2.59 EJ year-1, meeting almost 50% of the current worldwide hydrogen demand.
1. Introduction
Experts and decision makers widely agree that alleviation of climate change is mankind’s greatest threat and challenge for the 21st century and beyond. Recently in 2007, the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC) concluded that ‘‘Most of the observed increase in globally averaged temperature since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’’ (IPCC, 2007). Progressive emission of greenhouse gases (GHG) has been identified as the main cause of global warming and the target is to limit global temperature rise to a maximum of 2℃. Power- generating plants running on fossil fuels have been identified as the main source of GHG. Approximately 80% of the world primary energy consumption is still dependent on fossil fuel (Goldemberg, 2006); thus, the substitution by renewable energy sources, in conjunction with other clean energy sources, appears to be the best and necessary alternative. There are many other sources of renewable energy such as solar, wind, and geothermal. But biomass seems to have been receiving a lot of attention lately. Nevertheless, biomass has been a major source of energy in the world until before industrialization when fossil fuels become dominant. For example, countries with extreme conditions found in many poor regions of the world such as Ethiopia and Tanzania derive more than 90% of their energy from biomass (Silveira, 2005). In fact, International Energy Agency (IEA) in its 2007 report stated that over 630 million people in the sub-Saharan Africa are using sources from biomass such as wood and crops residues as its primary energy provider (IEA, 2007). Biomass has gained increased attention in the past decade because it not only provides an effective option for the provision of energy services from a technical point of view but is also based on resources that can be utilized on a sustainable basis all around the globe.
Besides direct combustion, biomass can be converted through other processes to generate energy, like gasification to produce hydrogen as discussed in this paper. Hydrogen is often cited as the unlimited clean energy resources. It is colorless, odorless and most importantly is a non-poisonous gas. It has long been acknowledged of its capability and advantages from environment and economic standpoint to replace the conventional fossil fuels. The use of hydrogen in fuel cells is a promising technology to supply heat and power for various applications. Vehicles powered by hydrogen fuel cell technology are three times more efficient than a gasoline-powered engine (Momirlan and Veziroglu, 2005). This technology is already used by several major car producers, which include BMW, American Honda Company and also Toyota Motors. These vehicles are powered by a fuel cell in combination with a nickel metal hydride battery (Momirlan and Veziroglu, 2005). This environmental-friendly technology is certainly in line with the Kyoto Protocol expected to be taken into effect in 2007, which demands the industry to reduce GHG emissions through reduced diesel use (Nath and Das, 2003).
An observation of the past 200 years shows a relation- ship between the level of industrialization and the dependence of fossil fuels of a particular country. Many countries have thus realized the need to harness local resources to increase the security of energy supply and reverse fossil fuel dependency. As a result, there is a general trend to search for alternative energy involving locally renewable resources. Countries have chosen different paths to move toward sustainable energy systems. For example, the UK Government has set out its ambition of securing 20% of electricity from renewable sources by 2020 (Gross, 2004), while the Ministry of Economic Affairs of Nether- lands stated its goal of 10% renewable energy by 2020 (Agterbosch et al., 2004). Nevertheless, the accomplishments of utilizing renewable energy vary significantly depending heavily on characteristics like government policy and the attitudes and behavior of relevant policy makers (Voogt et al., 2001).
The energy crisis faced by the Brazilian Government during the 1970s due to drastic increase in fuel prices, which led to the formation of Brazilian National Alcohol Policy (PROALCOOL), is a classic example. Perhaps one of the world’s most ambitious efforts to produce renewable biomass fuels (Puppim de Oliveira, 2002), the main objective of PROALCOOL, is to substitute gasoline, the country’s primary energy supply with ethanol obtained from biomass sources such as sugarcane, cassava and sorghum (Rosillo-Calle and Cortez Luis, 1998). Various measures are implemented by the Brazilian Government, which includes transforming the existing agricultural and industrial policies. Realizing the need for cooperation from the private sector, incentives are given to them to encourage innovation and increase investment for related activities. Ultimately, car owners are given further incentives to entice them to shift to alcohol-fueled cars in order to increase the demand. The outcome is promising in the beginning of the implementation with reports stated in 1984, 94.4% passenger cars in Brazil are fueled by ethanol (Rosillo-Calle and Cortez Luis, 1998). However, there is a significant drop in the percentages in the following years due to several contributing factors, which includes the increase in ethanol-fuelled passenger cars, stagnation of the ethanol production, uncertainty by the policy makers toward the program and also global issues that affect the outcome locally (Rosillo-Calle and Cortez Luis, 1998). In the end, this episode shows the importance of policy- making decisions and correct approach to ensure its effectiveness. This is also proven in the case of generation of wind energy in Germany. Within a 10-year period, the generation of wind energy in Germany has increased from about 200 MW in 1993 to about 14,500 MW in 2003. Apart from that, Germany also has the most successful industrial development of renewable energy. This accomplishment is mainly due to the laws by former red–green parliament since 1999: tax exemption for biofuel; strengthen research budget; subsidies and active promotion by people, companies, non-governmental organization (Sawin, 2004). In short, the dynamic growth of renewable energy in Germany has been driven largely due to the adoption ofa mix of strong policies to create markets for renewable energy. Nevertheless, before any strong policies on renew- able energy from biomass can be drafted and imple- mented, convincing data must be available to prove the availability of biomass to sustain as a sustainable and reliable source of renewable energy in the long run. Thus, the aim of this paper is to present the potential availability of oil palm biomass that can be converted to hydrogen (leading candidate positioned as the energy of the millennium) through gasification reaction in supercritical water (SCW), as a source of renewable energy to policy makers.
2. Availability of oil palm biomass
Oil palm, Elaeis guineensis, is a tree whose fruits are used for extraction of an edible oil. Originated from South Africa, it is cultivated in all tropical areas of the world and it has become one of the main industrial crops. The reddish colored fruit grows in large bunches, each weighing at about 10–40 kg. Inside each fruit is a single seed, also known as the palm kernel, surrounded by the soft pulp. The oil extracted from the pulp is an edible oil used as cooking, while that extracted from the kernel is used mainly in soap-manufacturing industries. Oil palm fruit is usually harvested after 3 years from planting. Maximum yield is achieved in the 12–13th year, and then continuously declines until the end of the 25th year (Abdullah, 2003). Replanting usually occurs after the 25th year. Oil palm topped the ranking as number 1 fruit crops in terms of production for the year 2007 with 36.90 million tonnes produced or 35.90% of the total edible oil in the world (MPOC, 2007). Oil palm is a multipurpose plantation and also a prolific producer of biomass as raw materials for value-added industries (Basiron and Simeh, 2005). For example, fresh fruit bunch contains only 21% palm oil, while the rest 6–7% palm kernel, 14–15% fiber, 6–7% shell and 23% empty fruit bunch (EFB) are left as biomass (Umikalsom et al., 1997).
Oil palm is now one of the major economic crops in a large number of countries, which triggered the expansion of plantation area around the world (Yusoff, 2006). Its availability is a strength, since oil palm tree can be cultivated at any tropical country and it is one of the main oil crop in the Asian and African region. The data of estimation on the oil crops production and area harvested for the year 2004 in Asia and Africa are shown in Table 1. Overall, oil palm accounts for about 29.04% of the total oil crops production in Asia and 21.16% for Africa (FAO, 2007). Currently, Malaysia is the largest producer and exporter of palm oil, producing about 47% of the total world supply. Its total mature areas of oil palm plantation represent 56% of total agricultural land and 11.75% of the country’s total land area. The evolution of world plantation area of oil palm from 1980 to 2005 is shown in Fig. 1 (Abdullah, 2003). In 2005, productive oil palm plantations, in million hectares, were Malaysia (3.410), Indonesia (3.320), Colombia (0.160), Ivory Coast (0.152), Papua New Guinea (0.088), others (1.406), with a grand total of 8.536 million hectares worldwide (Basiron and Simeh, 2005).
With the projected growth in the cultivation of oil palm, the destination of the huge amount of residues raises concerns. The supply of oil palm biomass and its processing byproducts are found to be 7 times the availability of natural timber (Basiron and Chan, 2004). Every year, the oil palm industry produces more than one hundred million tonnes of residues worldwide. One hectare of oil palm plantation generates about 21.625 tonnes per year of biomass residues. Fronds and EFB are almost 50.31% and 20.44%, respectively, as shown in Table 2 (Saka, 2005; Singh et al., 1999; Goyal et al., 2006). The amount of residues produced from oil palm plantation is much larger in comparison with other types of biomass produced in Malaysia. It is estimated that oil palm plantation generates 73.74 million tonnes of biomass per year. In 2000, paddy residues were 1.327 million tonnes, sugarcane residues were 0.356 million tonnes, wood industries residues were 2.177 million tonnes and municipal solid waste 5.05 million tonnes (Pusat Tenaga Malaysia, 2006). World annual production of oil palm residues amounts to 184.6 million tonnes and world agricultural amounts to 9.10 billion tonnes. The percentage of biomass produced from oil palm has increased tremendously since 1980 until recently, contributed by the expansion of the crop plantation due to the high demand for palm oil as shown in Fig. 2 (Nath and Das, 2003; Malaysia Palm Oil Council, 2007). This abundant biomass can be converted through gasification via supercritical water process to produce a highly valuable end product, i.e. hydrogen.
Oil palm biomass generally consists of cellulose, hemicellulose and lignin, and composition varies according to plant species. Cellulose with a molecular weight of about 100,000 is essentially a polymer with linear chains of glucopyranose units linked to each other by its 1, 4 in the 𝛂 configuration. Hemicellulose is a complex mixture of several polysaccharides such as mannose, glucose, xylose, arabinose, methylglucoronic and galaturonic acids. Its average molecular weight is of about 30,000, and it is a component of the cell wall. Lignin is a mononuclear aromatic polymer also found in the cell wall. Due to the near position of hemicellulose and lignin in the cell wall, adjacent to each other, both these compounds can form a complex termed as lignocellulose (Goyal et al., 2006).
Components of the oil palm biomass residues that can be used for gasification are EFBs, mesocarp fibers, palm kernel shells, palm tree trunks and fronds (Saka, 2005). Table 2 identifies the chemical composition of each type of biomass residue, which concludes that cellulose and hemicellulose are the main components of oil palm biomass, especially for EFB, fronds, mesocarp fibers and palm tree trunks. The only exception is for palm kernel shells where lignin is the largest constituent.
Currently, oil palm biomass is converted into various types of value-added products via several conversion technologies that are readily available. For example, fibers from empty fruit bunches are found to be an ideal material for the making of mattresses, seats, insulations, etc. (Basiron and Simeh, 2005). Paper-making industry has long utilized paper pulp from oil palm biomass for its various end-use purposes. Ashes produced from incinerat- ing the empty fruit bunches are used as fertilizer/soil conditioner due to its high organic and nutrient content beneficial to crops. Nevertheless, the current utilization of oil palm biomass has its limitations. In the paper-making industry, the presence of even a small quantity of oil can cause fouling to the end product, therefore affecting its quality. On the other hand, the volume of oil palm biomass produced annually is much larger than the amount used in these conversion processes. Therefore, surplus will occur, ultimately causing the biomass to be discarded. Fiber, shells and empty fruit bunches that form a large quantity of biomass are generally dumped in open areas or disposed off in open burning, generating pollutant gases (Yusoff, 2006). In other cases, fiber and shells are used as the source of energy for the processing mill itself to generate heat and electricity via combustion (Yusoff, 2006). However, this is not practical due to the high moisture content in the biomass and the huge amount of energy required for complete combustion, thus reducing the energy efficiency. Realizing the above complications, there is an urgent need for transforming this residue into a more-valuable end product. A promising option is by converting it into hydrogen via gasification using SCW technology. Oil palm biomass is the perfect candidate as feedstock for the gasification process. It has high energy and moisture content (>50%), 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 an advantage for the reaction. The availability of oil palm biomass all over the year allows continuous operation of the process.
3. Oil palm biomass gasification in supercritical water (SCW)
The properties of water displayed beyond critical point plays a significant role for chemical reactions especially in the gasification process. Below the critical point, both the liquid and gas phases exhibit different properties, although it is apparent that these properties become increasingly alike as the temperature arises. Ultimately, when it reaches the critical point (temperature >374℃, pressure >22 MPa), the properties of both liquid and gas become identical. Over the critical point, the properties of this SCW vary in between liquid-like or gas-like conditions (Kruse and Dinjus, 2007). Liquid water, well below the critical point, could not be utilized in reaction with biomass feedstock, since it is not miscible with organic substances. On the other hand, SCW is completely miscible with organic substance as well as with gases. Other roles of water for chemical reaction in the supercritical state have been reported in detail in the literature (Lu et al., 2006). 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 (Aurand, 2001):
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. It has been discovered in previous researches that the methanation reaction can be suppressed by using water in the liquid form instead of steam with the addition of nickel catalyst (Minowa and Inoue, 1999).
The utilization of SCW medium in biomass gasification has several advantages. It can directly deal with high moisture content biomass (>50%). Therefore, preliminary treatment such as biomass drying can be avoided, advantageously preventing the high cost related to that process (Calzavara et al., 2005). With this flexibility, other biomass sources with high water content can be used in this particular reaction. For instance, in the year 1993–2000, a comprehensive experimental investigation was carried out by Hawaii Natural Energy Institute (HNEI). The gasification temperature is 650℃ and pressure was above the critical pressure of water (22 MPa) utilizing various kinds of biomass feedstocks such as wood sawdust and sewage sludge (Antal et al., 2000). Positive results were achieved with near 100% gasification efficiency and high content of hydrogen (57 mol%). Minowa and Ogi (1998) from the National Institute for Resources and Environment (NIRE) of Japan conducted a thorough study using cellulose and wood from Japanese oak. With a temperature of 350℃ and a pressure of 17 MPa inside a batch reactor, the end product obtained is a gas composed of hydrogen and carbon dioxide. It is found that the percentage of char and tar formation is greatly reduced and gas yields reached up to 94 wt% when using cellulose as the raw material and 55 wt% for wood. In 2003, Kruse and Gawlik (2003), using both batch and continuous reactor, treated 2 different starting materials of model compounds and real waste in SCW with a temperature of 600℃ and 250 bar with the addition of KOH salts. It is observed that the raw material is 100% transformed into hydrogen-rich gas without any formation of tar and chars.
The SCW medium allows the optimization of the strongly pressure-dependent properties. The hydrogen is produced at high pressure, therefore a small volume reactor and low energy for pressurization in the storage tank are required. Minimum production of organic compounds and solid residue are added advantages, since tars and chars can cause plugging in the reactor if they are not constantly removed.
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 GJ-1 or US $0.35 kg-1 (Ni et al., 2006) as compared with the current method, stream reforming of natural gas, whose cost averages between US $5–8 GJ-1 (Watkiss and Hill, 2002). 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 as shown in Fig. 3 (Watkiss and Hill, 2002; Ni et al., 2006). Comprehensive study has been carried out with great success on this technology, utilizing biomass such as corn starch, clover grass, wood dust, organic waste, industrial waste, etc. (Saka and Ueno, 1999; Van de Beld et al., 2001; Matsumura, 2002; Hao et al., 2003; Yoshida et al., 2004; Jesus et al., 2006). The results report high percentage of hydrogen in the end product and very little production of residues.
4. Thermodynamic analysis of hydrogen production from oil palm biomass
4.1. Energy efficiency of the gasification reaction
In order to calculate the energy efficiency (Ee) of the gasification reaction, Prins et al. (2003) define it as the sum of external energy of the desired products divided by the total process inputs. However, in their studies, only hydrogen is taken into account as the desired output, without considering other end products. In this paper, for a more complete analysis of the reaction and the energy efficiency, we define the desired end product as a mixture of hydrogen, carbon monoxide, carbon dioxide and also methane. As shown in Table 4 for comparison between different fuels, hydrogen has the highest energy content, 120 MJ kg-1, compared with others such as automotive diesel, which is about 45.6 MJ kg-1.
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. As shown in previous studies by Calzavara et al. (2005), which compares the energy yield with and without heat recovery in the SCW gasification for corn starch and sawdust determine that a 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, ΔHp, and the heat of reaction, ΔHr. Therefore, the energy efficiency defined in this paper is 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 ΔHf, in the reaction. For this reaction, it is assumed that process heat is provided by wood combustion with an efficiency of 75%.
In order to determine the thermodynamic values of the reaction, it is vital to determine the stoichiometry of the reaction first. In the reaction, oil palm biomass is assumed to be fully converted to gases (hydrogen, carbon monoxide, carbon dioxide and also methane) and the reaction occurs at a temperature of 1000 K (727℃) and at a pressure of 30 MPa. The stoichiometry of the gasification reaction between oil palm biomass (cellulose) and water is expressed as below:
From Eq. (5), in terms of mole percent, the gas product consists of 61.29% hydrogen, 32.25% carbon dioxide, 3.23% methane and 3.23% carbon monoxide. This percentage of hydrogen and other end products acquired from the stoichiometry equation is consistent with the experimental and theoretical results reported in previous studies using different biomasses with similar constituents (Feng et al., 2004; Antal et al., 2000). The chemical external energy of a compound can be defined as the total work that can be obtained when a particular compound is brought from its reference state to its dead state (Prins et al., 2003). For the gaseous compounds, the chemical external energy can be calculated from Gibbs free energy of formation and fugacity where information such as enthalpy, entropy and heat capacities can be easily obtained from the literature. In the case of oil palm biomass, the thermodynamic properties are not available. Therefore, a specific correlation must be used. The chemical external energy for biomass can be calculated from a correlation of Szargut and Styrylska (1964) (in Saletes et al., 2004) as given below in:
where
The weight fractions of each component in the biomass are determined from the ultimate analysis of the oil palm residues and are presented in Table 3 (Umikalsom et al., 1997; Saletes et al., 2004). The chemical external energy of oil palm biomass calculated is 21.21 MJ kg-1 biomass. As mentioned previously, the chemical energy of the end gas product is represented in terms of Gibbs free energy of formation and fugacity (Tang and Kitagawa, 2005). It can be calculated from Eq. (8) below where DH and DS are the enthalpy and entropy of the respective gases (Table 4).
ΔG = ΔH - TΔS (8)
Enthalpy, entropy and heat capacity data for each component at its reference state are presented in Table 5 (Smith et al., 2001; Tang and Kitagawa, 2005). All the data are obtained from established database utilized elsewhere for the same purpose of calculation. Utilizing Eq. (7) and data from Table 5, we can now calculate the energy efficiency of the reaction as summarized in Table 6. Substituting into Eq. (4) above, the theoretical energy efficiency of the gasification reaction of oil palm biomass, with heat recovery, is about 72.91%. Without heat recovery, the energy efficiency is only around 46.54%. The large difference in values proves that heat recovery plays an integral role in the reaction. The percentages are overestimated since it is based on ideal case with no energy losses to the surrounding and heat recovery unit functioning at 100% efficiency without heat losses. The real energy efficiency percentages are about 10–25% lower than the thermodynamic values (Calzavara et al., 2005).
1 LHV = lower heating value; equation is valid only for ZO2 / ZC < 2.67; ZO2, ZC, ZH2 and ZN2 are the weight fractions of oxygen, carbon, hydrogen and nitrogen, respectively in the biomass
4.2. Pure hydrogen production efficiency
Pure hydrogen production efficiency (Eh) in the gasifica- tion reaction is an important parameter that must be accurately studied. There are several methods to determine the magnitude of this efficiency. The method developed by Calzavara et al. (2005), and used in this paper, considers the LHV—lower heating value of input and outputs. Therefore, the hydrogen efficiency is the ratio of hydrogen output to the biomass input plus external energy minus energy recovered, as presented in Eq. (9). Similar reaction conditions (temperature, pressure and source of heating) as mentioned in the previous section are applied. The equation used to calculate the Eh is shown in Eq. (9) below and the calculation results are shown in Table 7.
From Table 7, the maximum theoretical pure hydrogen production efficiency was found to be 34.93% without heat recovery and 57.96% with heat recovery. It is assumed to be an ideal case when there is no energy loss, therefore the values are overestimated to about 10–25%. Large-scale application of this technology requires improvements in energy recovery and the optimization of various para- meters to ensure that the reaction is well controlled and is able to reach its maximum conversion. In conclusion, the positive energy efficiency obtained in the laboratory scale points out a viability for hydrogen production from oil palm biomass gasification.
4.3. Energy and hydrogen production and potential
After evaluating the efficiency of pure hydrogen produc- tion, it is important to determine the potential amount of hydrogen that can be generated from oil palm biomass. In the calculation, the stoichiometry equation (5) is used as the model to obtain the estimation. The hydrogen percentage in the end product gas mixtures is 61.29%, which is consistent with the experimental data recorded on studies for gasification reaction using other biomasses with similar constituents (Feng et al., 2004; Antal et al., 2000). From Eq. (5), the theoretical maximum yield of hydrogen is about 0.117 kg H2 kg-1 biomass. As mentioned before, world oil palm biomass production annually is about 184.6 million tons (1.846 1011 kg biomass yr-1). Therefore, considering these data and both 100% and 50% efficiency, 21.6 and 10.8 million tonnes of hydrogen can be produced every year, respectively. Currently in 2006, the world hydrogen production is estimated to be at about 50 million tonnes with 10% expansion yearly (Momirlan and Veziroglu, 2005). With the inclusion of hydrogen produced from oil palm biomass, the world hydrogen production can increase maximum up to 43.2% yearly. The increasing expansion of oil palm plantation area in most of the countries where it is cultivated may provide a large source of biomass for hydrogen production (4–10%, yearly). This certainly indicates the potential of the above method to increase the hydrogen supply in the world. The evaluation of the input:output (O/I) ratio of energy values for oil palm biomass is also an important parameter. Basiron and Simeh (2005) have estimated the total input energy of 19.2 GJ ha-1 yr-1 for oil palm. The gasification of oil palm biomass produces a total energy output of 190.96 GJ ha-1 yr-1 . Thus, an energy O/I ratio of 9.9 is found. The high ratio is another evidence of the viability of the reaction in transforming the high-energy biomass into higher energy end product.
The capacity and contribution of the energy generated via this method depend on biomass availability. Capability and viability will depend more on efficiency of the process at a large-scale unit. For the past decades, Malaysia has been heavily dependent on fossil fuels as its source of energy, which caused renewable energy to be left behind and Malaysian crude oil reserves are expected to be completely depleted in the year 2010 (Mohamed and Lee, 2006). In 2005, in Malaysia, fossil fuels (coal, coal products, crude oil, natural gas liquids and natural gas) were 87.9% of the total energy supply and renewable sources (hydroelectricity and primary solid biomass) were 12.1%. The total primary energy production in Malaysia, in 2004, was 2.381 EJ with only 4.9% (0.117 EJ) of the total energy coming from combustible renewables and waste (International Energy Agency, 2005). From the estimation based on total oil palm biomass produced in 2006 (73.74 million tons), the nett energy that could potentially be produced in Malaysia via this method would be 0.62 EJ yr-1 with an assumption of 50% production efficiency. If the potential energy produced is then added to the current total energy for Malaysia, then it will see a significant increase from 4.9% to 30.95% in the total energy that comes from renewable source and overall 26.04% increase in the total energy production for Malaysia.
From the world aspect, International Energy Agency (2005) reports total world primary energy supply in 2004 of 11,059 Mtoe (464.48 EJ). Renewable sources were 58.9 EJ (12.7%). Considering world oil palm biomass production and 50% efficiency, 1.55 EJ could be generated. However, it is important to bear in mind that the values estimated are at the very minimum. Higher percentages of production are achievable with a better-equipped comprehensive system.
Hydrogen production via the SCW gasification of oil palm biomass represents a potential source of renewable energy for the future. Oil palm biomass availability increases substantially as demand for palm oil increases and supply energy security is likely to be guaranteed. Hydrogen production is 10.8 million tonnes each year with an O/I energy ratio of 9.9.
5. Conclusions
This paper gives an overview on the potential of oil palm biomass as the raw material in the gasification reaction using SCW technology to produce hydrogen. From the discussion and theoretical calculations carried out, has been proven the feasibility of obtaining hydrogen from biomass as a source of renewable energy has been proved. With an annual world oil palm biomass production of about 184.6 million tons, the maximum theoretical yield of hydrogen potentially produced by oil palm biomass via this method is 2.16 1010 kg H2 yr-1 with an energy content of 2.59 EJ yr-1 , meeting almost 50% of the current worldwide hydrogen demand.
Source: Tau Len Kelly-Yong, Keat Teong Lee, Abdul Rahman Mohamed, Subhash Bhatia - School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan,
14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia
