**5. Feasibility study**

Steam explosion pretreatment feasibility study was reported in several studies, Shafei et al. [84] was reported the feasibility of the economic factor from biomass waste as feedstock for biogas by simulating the paper tube residual and wheat straw using steam explosion pretreatment. The result from the simulation was concluded the application of steam explosion pretreatment was increased 13% of the investment cost, however that application was decreased the production cost of methane production by 36% efficiency which brings about 80% total energy efficiency with costing 0.36 and 0.48 Euro/m3 from paper tube residual and wheat straw, respectively. In this simulation, the feedstock is unloaded from the transporter and continued to chopping process to reduce the feedstock size and collected into storage piles. The crushed feedstock continues to pretreatment process through horizontal conveyor belt which continuously processes low-pressure pre-steamer, removing non-condensable gas,

#### *Steam Explosion Pretreatment: Biomass Waste Utilization for Methane Production DOI: http://dx.doi.org/10.5772/intechopen.102850*

high pressure with a horizontal extruder that uses steam as the driving force. The steam exploded feedstock continues to digestion process which simulated using established solid organic reactor which has 3150 m3 in total volume with 2–4 days retention time and about 20 days of residence time for fully digested by the circulated system by 5:1 ration between the residence feedstock and new feedstock. The final process is dewatering the slurry which fully digested from the digester. Kral et al. [85] was described the life cycle assessment (LCA) from a hypothetical local biogas system by adapting and integrating the steam explosion pretreatment to use unused grassland biomass as co-substrate the existing biogas reactor of Austrian alpine municipality. They used a comparation case study from the status quo of heating oil, wood chips, and grid electricity as reference scenarios for municipal energy resources; and hypothetical local biogas that is also used for municipal energy sources with 500-kWel biogas plant using unused grassland with a steam explosion as the pretreatment. The result was described that the LCA from biogas from biomass and status quo energy resources have significant differences with *ρ* <0.05 from six categories, where the biogas electricity from steam-exploded grassland has a lower impact than the status quo energy with climate change contribution in 0.367 CO2-eq kWhe-1 from and 0.501 CO2-eq kWhe-1, respectively.

The steam explosion pretreatment was reported to enhance the full-scale biogas plant production which used a wheat straw as co-substrate for pig manure [86]. The result from the study stated that the addition of pretreated wheat straw using liquid hot water-steam explosion produced 24–34% higher methane, this condition was obtained from pretreatment at 165°C and 2.33 MPa for 10 minutes steaming time which break the LCC into low-mass polysaccharides, and at this severity factor (SF) did not generate the HMF and furfural that could inhibit the fermentation process. The steam explosion apparatus that used in this study could daily continuous process 2.300–3800 kg of wheat straw that could use 100-160 m3 recycled water from the biogas plant with ration 20:1 and 23:1 between wheat straw and recycled water. Maroušek et al. [87] was used combination pretreatment for sunflower stalks in existing large-scale biogas reactor by maceration under 75 to 95°C for 20 to 200 seconds and continue to steam explosion pretreatment under 0.8 to 2.2 MPa for 2 to 20 minutes of steaming time, where the pretreatment was used the sole heat waste from the existing system. The optimum production was 99 m3 methane VSt−1 from feedstock that macerated at 95°C for 100 seconds and continue to steam-exploded at 2 MPa for 17 minutes, where the steam explosion pretreatment higher than 2 MPa was impacted to the decreasing of methane production due to the formation of inhibitors such as furan and HMF. Pérez-Elvira et al. [88] were reported the pilotscale feasibility study which demonstrated the hydrolysis process using steam explosion, anaerobic digestion with an energy output of cogeneration unit. This study was used an automatic continuous steam explosion of 10 L which connected to a 200 L mesophilic anaerobic digestion reactor and directly connected to the power generation where the engine exhaust gas was utilized to heat the boiler unit for steamed the hydrolysis reactor (steam explosion). The result from this study was described that the combination of steam explosion as thermal hydrolysis and anaerobic digestion which resulted considered for full-scale application. The residence time was only 40% compared to the conventional digestion and proved that this system was fully self-sufficient energy without additional energy input for all the processes. Those systems were generated 1 MW green electricity which is a 246 kW surplus compared to the conventional system, with could generate 58% less volume of bio-waste from the process.

### **6. Methane conversion: secondary energy**

The methane conversion as secondary energy through the biogas purification to get higher methane content for household, fuel transportation, and the methane conversion into hydrogen. The biogas purification for secondary energy was mandatory to get high content methane and to reduce the carbon dioxide (CO2) to increase the density and the calorific value, and cleaning out the hydrogen sulfide (H2S) due to the corrosivity character for the metal part of in all the system such as gas storage tank, piping system, compressor, engine, and also the toxicity that harmful to the environment [89]. The CO2 removal could be removed through physical absorption by water or organic scrubbing that could be physically bound with CO2 [90, 91]. The absorption using organic solvent could also remover the H2S, ammonia (NH3), hydrogen cyanide (HCN) and also water vapor with low losses of CH4, and included into regeneration system with low temperature waste, however the operation and technology investment is expensive; chemical absorption by using di-methyl ethanol amine (DMEA) or mono ethanol amine (MEA), and solution of alkali such as NaOH, K2CO3, KOH, iron hydroxides (Fe(OH)3), and FeCl2 that could actively absorb the CO2 [92, 93]; pressure swing absorption by sequences process of adsorption, desorption, and pressurization by hiring the synthetic resin, zeolite, activated carbon, silica gel, or activated charcoal which also could separate the N2, H2S and O2 [94]; cryogenic separation which takes advantage of the different boiling points of CO2 and CH4 by condensation process on gas cooling at elevated pressures that could separate the CO2 and also the other gas content such as O2, N2 and siloxanes [95]; membrane separation which base on the properties of the selective permeability of the membrane through two system i.e., gas–liquid separation where the liquid absorbs the CO2 and also the H2S diffusing via the membrane, gas–gas separation by the gas phase from the both side of membranes [96, 97]; hydrate formation which based on the equilibrium partition of the components between gaseous and hydrate phases, clathrate phase equilibrium for the water-phenol-carbon dioxide system [98, 99]. Other than that, CO2 and H2 compounds in biogas also could be utilized via biological conversion by hiring the microbial to convert the CO2 and H2 into methane [100, 101]. The H2S could be removed by physical and chemical absorption by converting H2S to elemental sulfur or metal sulfide utilizing either water or organic solvent in the physical absorption process or aqueous chemical solutions 98. The water adsorption could generate cheap operation as long as the water is available and easy to get, this system also could remove the H2S at the same time, however, this system was included in a not-regenerative system and require high-pressure conditions and complex engineering [102]; activated carbon adsorption that catalyzed the H2S oxidation into metal sulfide or sulfur which usually used impregnated activated carbon and catalyticimpregnated carbon which has highest oxidation rate compare with activated carbon [103, 104]; adsorption by iron oxides (Fe2O3), Fe(OH)3 or zinc oxides (ZnS) that could easily reacted with H2S and forming the FeS and ZnS from the reaction [105, 106]; biological biofiltration and desulfurization using litautotrophic bacteria that can convert H2S into sulfate and sulfur bases using electron donors from H2S and carbon sources from CO2 (**Figure 2**). Moreover, the content of H2S in biogas could be prevented by in-situ prevented via dosing the oxygen in the digester system, where the microbiological oxidation converted the H2S into elemental sulfur [89, 107]. The other in-situ treatment was using iron chloride (FeCl2) dosing into the digester by forming the iron sulfide (FeS), where the FeS could be easily removed through the solid discharge which is a good content fertilizer nutrient [108]. Other than that, the

*Steam Explosion Pretreatment: Biomass Waste Utilization for Methane Production DOI: http://dx.doi.org/10.5772/intechopen.102850*

other compounds such as nitrogen (N2), oxygen (O2), volatile organic compounds (VOCs), carbon monoxide (CO), and NH3 were removed to get the methane purity [109]. Methane as secondary energy was widely applied in several countries.

The hydrogen conversion from the methane commonly through the conversion system such as SRM [37, 110–112], DRM [113, 114], CDM [115, 116], and POM [117, 118]. The SRM was widely used in industrial applications with a high theoretical H2/CO ratio and its efficiency with low operational and production costs. The SRM system could SRM could continuedly one system with water gas shift (WGS) which could convert more hydrogen in the process where the steam and CH4 mixed and produced syngas from hydrocarbon and water reaction [37, 112]:

$$H\_2O + CH\_4 \rightarrow 3H\_2 + CO\_2$$

The WGS process continue to convert the CO by water reaction [37, 112]:

$$H\_2O + CO \rightarrow CO\_2 + H\_2$$

However, the SRM facing the complex system depends on the quality of biogas, high COx emission, water demand, and high investment capital [118]. The conversion through the DRM has a good point with CO2 reduction, however, the still facing with the carbon deposition problem, influenced on CO disproportionate and reverse water gas shift reaction, and carbon deposition problem [119]. The POM was offering high selectivity and conversion rates with short residence time, and is known as a simple system with less desulphurization and not using catalyst during the process [115, 120]. Nevertheless, pure O2 was required for the process with high COx emission and possibility the of producing NOx emission with soot formation during the process [121]. The CDM was the simplest process with only one step with a single reactant, produced H2 with high purity by mild reaction condition and no GHG emission during the process. The CDM also could produce nanocarbon material by carbon sequestration which forms a stable solid. Even though it looks promising, the CDM is still in lab level experiment which is necessary for catalyst deactivation, unreacted methane in out-stream with low purity nano-carbon, and the catalyst regeneration produced the secondary emission [114, 116, 121].
