Syngas Production from Agriculture Residues: Sudan

The study is aimed at evaluating the availability of agriculture residues for syngas production, a case study for Sudan. 10 types of biomass are investigated: sugarcane (bagasse), cotton stalks, sesame straw, groundnut shells, maize straw, sorghum straw, millet straw, sun ﬂ ower husks, wheat straw, and banana leaves. The available biomass is about 11Mt/year (3.68 Mtoe). Aspen plus software is applied to simulate the gasi ﬁ cation process. The study covered a wide range of operating conditions of steam to biomass ratio ( 0 < SB < 2 ) and equivalent ratio ( 0 > ER > 0 : 5 ). For all types of syngas characteristics, H 2 is 0.32-0.42 (mole fraction), CO is 0.13 to 0.16 (mole fraction), LHV is 5.0 to 8.0MJ/kg, and the yield is ≥ 1.5. Wheat, groundnut, and sun ﬂ ower have the best characteristics, while millet and bagasse yield the poorest characteristics. In addition, all types of syngas have H 2 / CO > 2 except Millet. These characteristics make all types of syngas except millet suitable for both energy and industry applications. The potential syngas production is 14.17Mt/year.


Introduction
In Sudan, agricultural residues are poorly managed. They are left in the field to decompose or burn as a part of land preparation. Burning results in significant CO 2 emissions. Table 1 shows Sudan's available biomass from agricultural residue, excluding grass and forestry. The available biomass for energy also excludes that goes as animal feed, construction, and other industries. The available biomass for energy is about 11 Mt/year, with an energy content of 154 PJ/year (154 × 10 15 J), equivalent to 3.68 Mtoe (1 TJ = 2:388 × 10 −5 Mtoe) [1][2][3][4]. The potential electric generation from biomass is about 1 GW. Sudan biomass potential could increase four times the current amount. Sudan cultivable land is estimated at 74 million hectares, only 25% of which is currently used [5].
Due to the growing world population, the food, energy, and environmental conservation demands are high. One of the mitigation measures to sustain energy production in the event of fossil oil depletion is to cut fossil fuel consumption. Renewable energy, in particular, biomass is a promising candidate that can ensure sustainable energy production and environmental conservation [8]. Many studies on the perfor-mance of biomass in cogeneration (combined heat and power (CHP)) have been reported in the literature [8][9][10][11].
The previous literature on the biomass understudy indicated that all biomass listed in Table 1 have been studied except sesame [11][12][13][14]. The studies covered chemical and physical properties (the ultimate and approximate analysis and calorific values), the performance in CHP, and syngas production. However, search on sesame stalks meets no hits on syngas and CHP; only hits on chemical and physical properties are met.
The chemical energy in biomass is converted into energy via thermochemical processes. Syngas production is an example of biomass gasification. Syngas is a gas mixture of H 2 , CO, CO 2 , and CH 4 . It is utilized in power generation, fuel synthesis using the Fischer-Tropsch process, and ammonia production. Gasification advantages over direct combustion are low NO x emissions [15]; direct combustion occurs at a high temperature > 1300°C, which is favorable to NO x formation. The net CO 2 emission of biomass in gasification is zero [16,17]. It can be concluded that biomass is a sustainable energy source and a solution to the CO 2 emissions.
The key gasification steps are drying, pyrolysis, combustion (oxidation reactions), gasification (reduction reactions), and gas cleaning. The first four processes occur inside the reactor, while gas cleaning occurs outside. Drying or moisture removal occurs at 100°C-200°C. Drying heat demand could be supplied from recovered heat. The dried biomass undergoes thermal decomposition through the pyrolysis process at a temperature between 350°C and 650°C according to the following chemical reaction (equation (2) The char converts into syngas in the oxidation and reduction reactions as shown in Table 2. The key operating parameters are reaction temperature, steam to biomass ratio (SB), and air equivalent ratio (ER). The literature review shows the operating conditions are in the following range: the gasification temperature (500°C-1000°C), SB (0 to 2), and ER (<1) [12,16,17,20,21]. Table 3 shows proximate and ultimate analyses of different biomass. The chemical compositions are in the following range: 30-53% C, 38-55% O 2 , 3.0-6.3% H 2 , <2% N 2 , <1% S, and 2-11% ash. Except bagasse, the moisture content is in the range of 3-11.5% while for bagasse it is about 50%. C/N of all biomass is greater than 30. The data are reproduced the way presented in the literature although the proximate compo-sition (FC, VM, and ash) does not add to 100%, similarly the ultimate analysis.

Aspen plus Simulation.
Due to its capability to process solids, Advanced System for Process Engineering (Aspen) Plus software is a widely used package in the modeling and simulation of biomass gasification process [28]. The process flow diagram (PFD) for biomass gasification is almost standard. It consists of four blocks: drying, pyrolysis (decomposition), char gasification, and gas cleaning. RStoic, RYield, and RGibbs reactors are used for drying, pyrolysis, and char gasification, respectively. In gas cleaning, a cyclone and a flash separator facilitate ash and water removal, respectively [20].
The modeling and simulation of the biomass gasification process are performed with Aspen Plus v10 software. Biomass is defined as a nonconventional component through its ultimate and proximate analysis. Sulfate and carbon are defined as solids. The fluid package of Peng-Robinson (PR-ROB) equation of state is selected to evaluate all physical properties of the conventional components. HCOALGEN and DCOALGEN models are selected for the evaluation of the enthalpy and density of both biomass and ash. Figure 1 shows the biomass gasification PFD. The wet biomass is fed into the "RStoic" reactor block (R-101), where moisture is liberated. The RStoic outlet stream (stream 2) goes into the separator (V-101), which separates the stream into moisture (stream 4) and dry biomass (stream 4). The dry biomass passes to the "RYield" reactor (R-102) for pyrolysis. The RYield reactor block calculates the yield distribution of the products without the need to specify reaction stoichiometry and reaction kinetics [31]. The "RYield" reactor converts biomass into conventional components (O 2 , H 2 , N 2 , H 2 O, S, tar, and char). The pyrolysis products (stream 5) are fed to the RGibbs reactor (R-103). The option of "Restrict chemical equilibrium-specify temperature approach or reactions" is chosen for the simulation of the RGibbs reactor. The combustion and oxidation reaction are supplied ( Table 2). The RGibbs reactor outlet (stream 6) moves to the cyclone (S-101), which separates stream 6 into wet syngas (stream 7) and ash (stream 8). The wet syngas (stream 7) is cooled in the heat exchanger (E-101) and the associated water is condensed. The cold wet syngas (stream 9) goes to the flash (V-102), which separates (stream 9) into syngas (stream 11) and water (stream 10). The air (stream 12) and steam (stream 13) as gasifying agents are added to the RGibbs reactor (R-103).

Results and Discussion
3.1. Model Validation. The simulation model is validated using the experimental data of De Filippis et al. [29] and the simulation data of Mavukwana et al. [20]. The proximate     [20] simulation results in the prediction of methane as both works yield no CH 4 formation. No formation of methane is a common problem experienced by many simulations works [30].
Concerning H 2 , the simulation under predicts Mavukwana et al.'s [20] results by about 7% and over predicts De Filippis et al.'s [29] experimental results by about 10%. Deviations on CO and CO 2 relative to both simulation and experimental results are high. One reason is the RGibbs assumption of minimum value of the total Gibbs energy at chemical equilibrium. This assumption may not be satisfied at the experiment. Another reason is that ER and SB values may not be the optimum combination. The optimum ER and SB correspond to the point where CO and CO 2 composition crossover. More light on the optimum ER and SB conditions is shed in the subsequent sections.

Operating
The calculation is made on moisture and ash free basis. The steam to biomass ratio (SB) is Stoichiometric steam to biomass ratio (SB) is 100 : ð7Þ Table 5 shows ultimate and proximate analysis of data presented in Table 3 in a normalized form. The stoichiometric air to fuel ratio (AFR) and theoretical steam to biomass ratio (SB) are calculated using equations (4) and (7), respectively. Stoichiometric AFR varies between 3.18 for millet to 6.01 for groundnut. Theoretical steam to biomass ratio (SB) varies between 0.45 for banana and 0.75 for groundnut. The estimated SB does not include steam required for water gas shift and steam reforming reactions.
Figures 2(a)-4(d) show CO and CO 2 concentration against SB (0 to 2.0) for a wide range of ER (0 to 0.5). CO and CO 2 concentration profiles assume opposite trends and reach a point of crossover. The opposite trend is attributed to the competitive oxidation and reduction reactions. The point of crossover is called the carbon boundary point (CBP). It is the point where carbon is depleted or consumed. It is considered the point of optimum ER and SB combination [16,31]. ER > 0 is needed to provide the energy demand for oxidation reactions. However, ER should not be excessive to shift the reaction towards oxidation: CO 2 production. In most studies, ER is limited to 0:13 > ER > 0:5 [20,21]. An ER of 0.15 is considered in this work. For all biomass understudy, the corresponding SB is determined using Figures 2(a)-4(d). The CBP at ER = 0:15 is indicated by the dotted line. Figure 5 shows the SB for different biomass at the CBPs. Groundnut demands the highest SB of 1.2, and bagasse demands the lowest SB of 0.4. Groundnut has the highest carbon and hydrogen contents among the studied biomass. The steam demand for C-water reaction and Water-gas shift reaction is higher than that required by the other biomasses. The low SB demand for bagasse is attributed to the high moisture content. The moisture is controlled at 10% in the simulation process for those biomasses with higher moisture. The other biomasses have a moisture content of less than 10%.
Another feature that can be seen in Figures 2(a)-4(d) is the point of crossover of all CO 2 curves at one point. The Table 4: Validation of simulation results (T = 850°C, SB = 1.9, ER = 0.38).

Component
This work Ref. [29] Ref. [20] (Table 3) while the C/N ratios of bagasse and millet are 361 and 1387, respectively. Hence, C/ N is a limiting factor. However, the author is unaware of such a finding and has no clear explanation of the correlation between C/N and syngas production. Table 6 summarizes the gasification operation conditions (ER, SB, and T); syngas characteristics (composition, mole fraction, H 2 /CO ratio, LHV, density, MW, and syngas to biomass ratio); and the potential production. The potential syngas production is 14.17 Mt/year (11 billion m 3 /year).

Conclusion
Biomass information on 10 agriculture residues is collected. The potential energy is estimated in Mtoe. A simulation program is tailored using Aspen plus software. The program is run for each type of biomass for a wide range of ER and SB. The data is analyzed, and the syngas characteristic for each syngas is established. The work concluded that cotton stalks, sesame straw, groundnut shells, maize straw, sorghum straw, sunflower husks, wheat straw, and banana leaves produce syngas of high quality. Bagasse and millet straw produced syngas of poor quality. The work also concluded that the biomass gasification process is not fully understood. The work identified two points of equilibrium: the carbon boundary point (CBP) and the oxygen boundary point (OBP). However, OBP needs further investigation to be confirmed.

Data Availability
All information used to support the findings of the study is available from the corresponding author upon request.

Conflicts of Interest
The author declares that he has no conflicts of interest.