Systematic comparison of hydrogen production from fossil fuels and biomass resources

: Fossil fuels are the main energy source to satisfy the worldwide energy demands. However, the energy demands are increasing and the supply of fossil fuels is decreasing, thus many countries are looking for other fuel sources. Differing from the traditional fuels, hydrogen is considered as one of the most promising energy sources due to its intrinsic features such as clean, efficient, safe and sustainable. Developing novel technologies for hydrogen production from renewable sources (such as biomass) becomes a core area for the investigation of hydrogen industry. Within this work, different pathways for hydrogen production including steam reforming, electrolysis, and biomass gasification have been systematically compared in terms of yield and cost. This comparison is unique since the systematic evaluation was conducted from many aspects for all the hydrogen production pathways, especially those by involving the biomass gasification that still lack of available literatures. The assessment methods involved energy analysis, exergy analysis and economic analysis. It was concluded that steam reforming remains the cheapest method of hydrogen production at 1.748 $/kg, however, steam reforming is not an ideal process currently or for the future, gasification and electrolysis remains competitive with high yield but requires relatively high initial and annual expenditure. For biomass gasification, though its energy efficiency is lower than steam reforming, it has relatively higher mass yield, demonstrating the feasibility of this process for hydrogen production. Further for biomass gasification, the selection of correct feedstock is a key to maximize its yield, i.e. a yield of 82.47% is possible with corn stover fed gasification.

Hydrogen is seen as one of the leading energy alternatives since its combustion only produces water which is much cleaner than carbon dioxide that being produced from fossil fuel. However, many technologies of hydrogen production encounter problems due to not being economically viable or the complexity of the technology involved. Currently, hydrogen is being produced in industry, for example, hydrogen production is firmly established in the USA [1] , however, the production is primarily from fossil fuels, specifically coal [2,3] , via a traditional technology called steam reforming.
For this technology, the environmental benefits of the downstream hydrogen utilization are diminished. Therefore, to be commercially viable, hydrogen needs to be produced in large quantities at a relatively low cost and environmental-friendly way, one possible solution to this is to use a renewable fuel source [4] , such as biomass as a feedstock, instead of fossil fuels.
Currently hydrogen demand is met by the steam reforming of coal or natural gas/methane. Steam reforming process has been favoured for large scale production [5] , particularly in petroleum refineries. The methane is fed into the steam reformer, where it reacts with high temperature steam (700°C -1100°C ) in an endothermic reaction to produce synthetic gas (syngas).
Syngas is a mixture of carbon monoxide and hydrogen.
To increase hydrogen production, after steam reforming, water gas shift reaction is performed at 360°C [6] .
During the process different catalysts are used to maximize hydrogen yield and limit other products being formed [7] . Whilst steam reforming is an industrial proven process, the associated CO 2 production is high due to the utilization of fossil fuels. It is possible to reduce the carbon released into the atmosphere by retrofitting/implementing carbon capture and storage equipment.
There are some potential ways to produce hydrogen from biomass, and pyrolysis is one of them. Pyrolysis involves the heating of biomass feedstock, at temperature of 377°C -527°C and pressure of 0.1-0.5 MPa in the absence of air. For hydrogen production, high temperature, relatively higher heating rate and longer volatile phase residence time are required [8] . To maximise hydrogen yield a water-gas shift reaction can also be performed [9] . As the pyrolysis pathway has not been fully developed for hydrogen production in industry, however, numerous experiments have been conducted allowing estimates and projections to be established.
Padró [10] estimated hydrogen production cost of biomass pyrolysis to be in range of 7.26 $/GJ to 12.68 $/GJ depending on facility size and biomass type. The findings state that the use of biomass should be feasible when compared to existing methods on cost grounds.
Gasification is another potential technology for production of hydrogen from biomass. Gasification takes place above 850°C in low oxygen conditions. Similar to pyrolysis, the gaseous mixture produced from gasification can also be steam reformed to increase hydrogen yield [11,12] . The yield can be further improved by conducting a water-gas shift reaction. The gasification process is applicable to biomass having moisture content less than 35% [13] . This could make gasification less attractive to companies who do not have facilities to remove moisture of feedstock onsite [14] .
Estimated cost of hydrogen production from biomass [15] gasification are seen to be similar to natural gas reforming, this means that there is likely to be no negative profit detriment for companies to switch from traditional methods of production during normal machine replacement/ modernisation.
Another technology for hydrogen production is electrolysis. It involves the use of electricity, to split H 2 O into hydrogen and oxygen, using an electrolyser.
Due to broader operating conditions, peak efficiency is slightly reduced and the equipment cost is high [16] . By considering the environmental benefits for electrolysis, it is possible to use biomass as the source for electricity production. In this case, without considering the energy used to produce the equipment, electrolysis is a zero emission process [17] when the electricity is supplied by a renewable biomass source.
As a summary, there is a potential to produce hydrogen from renewable source at an industrial level via the aforementioned pathways. However, few studies have systematically evaluated those pathways from both technical and economic aspects. This research focuses on hydrogen production from lignocellulosic biomass feedstock, especially on those pathways which are emerging as viable ways of mass production in terms of cost and yield. Through analysis, a comparison will be made on pathways fuelled by biomass and those currently fuelled by fossil fuels. The comparison of biomass feedstock is also carried to assess pathways of hydrogen production. Exergy analysis will be conducted, producing a techno-economic assessment for industrial sized applications compared with current methods and determining the suitability of each pathway for hydrogen production.

Biomass feedstock
Biomass feedstocks involve energy crops, agricultural residues, forestry waste and residues, industrial and municipal waste. Within this research, three typical biomass feedstocks are selected: poplar, sugar cane and corn stover. Their compositions [18] including the moisture content and the C, H, O content are listed in Table 1.

Methodology
The aforementioned pathways for hydrogen production need to be systematically assessed to finalize the sustainable industrial scale hydrogen production pathway. As shown in Figure 1, this assessment will be carried out from three aspects: energy analysis, exergy analysis and economic analysis. The energy available for extraction in biomass will determine the reaction yield and ultimate hydrogen production. The energy analysis model developed in this research is versatile to allow different biomass types to be input. The energy contained in biomass will be calculated using ultimate analysis, as previously mentioned. As energy will inevitably be lost through the entire process, for example, large energy losses can occur when reactions are uncontrolled and also result in undesirable by-products being produced, measures were put in place to minimize the lost to increase the efficiency.
For various biomass feedstocks, there will be variation in energy loss depending on the source of supply due to the change in moisture content of the feedstock.
For the heating value of biomass feedstock, two type of value are normally considered: higher heating value (HHV) and lower heating value (LHV). HHV is equal to LHV multiplies by the vaporization of the water content in the feedstock, as shown in Equation (1) [19] .
where, LHV and HHV are in the unit of MJ/kg; H represents the percentage of hydrogen; M represents the percentage of moisture; and Y represents the percentage of oxygen, all in a received basis.
As for the energy efficiency can be calculated using Equation (2) [20] . Where bio m and

Exergy analysis
The exergy of the biomass must be considered to determine the potential hydrogen yield from any proposed industrial site. Exergy is defined as the maximum usable work gained by bringing a system into equilibrium with its environment, the energy that can be used. Exergy analysis brings elements of conservation of mass, conservation of energy and the second law of thermodynamics together to form a complete analysis.
From the definition, it can be seen that exergy must be measured in relation to its immediate environment.
Therefore it is necessary for a specified temperature, pressure and chemical composition of this reference environment to be obtained. The exergy values of chemicals will be obtained, for biomass, average values will be taken.
Similar to energy efficiency, the total exergy efficiency of the process [20] can be defined as: where, Ex bio , Ex agent , Ex H2 indicate the exergy of the biomass, gasifying agent, produced hydrogen, respectively. The exergy in a material stream can be calculated as the sum of its chemical exergy Ex ch and physical exergy Ex ph .
The formula of correlation factor β for biomass is given below, where C, H and O are the molar fractions of C, H and O in biomass, respectively.

Techno-economic analysis
The high cost for hydrogen production is currently the biggest barrier for its industrialization, especially when biomass is considered as a feedstock. The production cost varies by site and location due to different infrastructure requirements. For this reason, a comparison must be made to determine the most costly effective method. It may be the case that using cheap feedstock that produces more hydrogen per dollar, or the case of using expensive feedstock with a high hydrogen total yield. This will have to be considered based on availability of feedstock and desired pathway at any given location. The associated production cost would be an indicator in determining the most economical feedstock after the process inefficiencies have been subtracted.

Results and discussion
The four pathways being considered in this research are: steam reforming without carbon capture and storage (CCS), steam reforming with CCS, biomass gasification and electrolysis. It should be noted that biomass pyrolysis for hydrogen production has not been presented in the results. The reason is that the process is currently only being trialled in small scale test plant [21] . There is limited data available of expected yields, set chemical processes and infrastructure required for a general pyrolysis process for hydrogen production at a large scale [22] . It has been excluded as a comprehensive model that could not be produced to the same degree as the other processes.
The results within this section are presented based on the analysis methods. First part explains for energy and exergy analysis of all the four selected pathways, second part focuses on the economic analysis of the pathways, the analysis on different feedstock is presented in third part, and the last part provides the suggestions on the opportunity for hydrogen production in the future.

Energy and exergy analysis of different pathways
By quoting data in Table 2 and calculating using forementioned formulas, results of the energy efficiency, exergy efficiency and the mass yield concerning relevant pathways are illustrated in Figure 2. Table 2 Input parameters of the analysis for three hydrogen production processes [23] Hydrogen production Item Value  Selecting high efficient catalyst could improve the gasification conversion process and minimise the char and tar by-products [24] . An increased yield and suitable feedstock supply combined with the minimal operating costs would certainly make the gasification process more attractive.

Economic analysis of different pathways
Calculating based on the data in Table 2 [23] For a pathway to be attractive for a company to invest and build a plant, it must have a good return. This means that the production cost must be lower than the market value. As the market value varies with demand, it is imperative the production cost be as minimal as possible. Figure 4 shows the estimated production cost of each process. The production cost is presented as the cost to produce a quantity of hydrogen per unit mass, $/kg.
Steam reforming can be seen to be the most cost effective per kg of hydrogen produced at 1.748 $/kg, this was expected due to it being the current industrial process.
It has had both time and money spent to develop the process, techniques and equipment to extract hydrogen as efficiently as possible. However, steam reforming is not an ideal process currently or for the future, due to the large quantities of CO 2 that are produced as a by-product.   [20] .

Effects of feedstock
A comparison of feedstock type is presented in Figure   5. It can be seen that both poplar and sugar cane stand out as being of very similar average make up. The poplar feedstock contains 1% more hydrogen and has identical moisture content to sugar cane, however it can be seen that there is a vast difference in the purchasing price. The corn stover is the cheapest biomass analysed at 53.42 $/t, whilst it contains the least hydrogen, it is also the driest on average at 22%. The low moisture and purchase cost make corn stover a promising feedstock for gasification.

Figure 5 Comparison of biomass composition and purchase cost
The corn stover is considered as a suitable option for hydrogen production from gasification can also been reflected in Figure 6. Even though the hydrogen content of corn stover is lower than the other two feedstocks as shown in Table 1, however, the high mass yield and low cost again shows it is an attractive feedstock. generates effects for the environment, such as driving up the CO 2 output of the conversion process [25] . For the corn stover, one concern is that as a market developed, producers would be less focused on the production of food crops and so land could be repurposed and biodiversity lost. The high cost of poplar previously shown in Figure 6 can be explained in a similar way, with the cost increasing with demand.

Opportunity for substitute fossil fuels using biomass
The desire for countries to reduce their carbon output and fossil fuel consumption is increasing faster than the development of low carbon alternative technologies.

Conclusions
Different pathways for hydrogen production including traditional steam reforming, electrolysis, and novel biomass gasification, have been systematically compared within this research. The assessments methods involve energy analysis, exergy analysis and economic analysis.
These assessments lead to a number of conclusions as follows: (1) Steam reforming remains the cheapest method of hydrogen production at 1.748 $/kg. This is due to its highly developed and refined state. However, steam reforming is not an ideal process currently or for the future, due to the large quantities of CO 2 . Due to its high yield, gasification and electrolysis remains competitive but requires relatively high initial and annual expenditure; (2) Both gasification and electrolysis are within range to be developed into alternative hydrogen production pathways. The energy and exergy efficiencies demonstrate the possibility for improvements to be made within the processes, whilst yields are already above that of steam reforming; (3) The use of the correct feedstock is key to maximise yield, a yield of 82.47% is possible with corn stover fed gasification; (4) Carbon capture and storage can be used to reduce CO 2 output without the need to build a new plant and has no negative effect on hydrogen yield, only an increase in production cost; (5) The selection of feedstock should be determined by location, the increase in biomass production needs to be monitored with certain types removing biodiversity for the environment and having the potential to reduce food crop harvests.