LRC drying and gasification conditions.
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Open access peer-reviewed chapter
By Lukman Adi Prananto and Muhammad Aziz
Submitted: May 26th 2017Reviewed: October 20th 2017Published: December 20th 2017
The growth of economic and living standard leads to more electricity demand. Unfortunately, due to more limitation of power station area and electricity grid development, energy delivery issue is rising up; hence, new method of delivering the power by different energy carrier is necessary to investigate. Hydrogen has the promising potential as an energy carrier due to its high gravimetric energy density and cleanliness to the environment. For comfortable storage and transportation, hydrogen is covalently bonded to methylcyclohexane (MCH) and liquid organic hydrogen carrier (LOHC). In this chapter, novel integrated gasification systems for coproduction of electricity and MCH from low-rank coal and microalgae have been proposed. The total energy efficiency is improved by applying enhanced process integration (EPI) technology to minimize exergy losses throughout the integrated system. The integrated system for microalgae is capable to provide more than 60% of total energy efficiency, while the integrated system for low-rank coal delivers the total energy efficiency of 84%.
Owing to the impact of economic growth and living standard improvement, the requirement for electricity is always climbing up, and therefore more power generators have to be developed to supply the demand . Also, due to limitation of potential area for the plant and the high investment cost of both power station and electrical grid, further approach to deliver the energy by different energy carriers is favorable to meet the energy demand . By far, fossil fuels act as both primary energy sources and energy carriers for electricity generation. However, despite the proven and relatively highly energy-efficient technologies, the adoption of the fossil energy promotes high environmental impacts and drawback of sustainability . Therefore, together with the concern of energy scarcity and environmental issue, further investigation to discover alternative energy carrier is required not only to complement the fossil energy but also for primary utilization by achieving highly efficient energy systems.
Several parameters need to be accomplished for the future energy carrier, including the simple production method, high energy density, and environmentally benign performance. To this, the hydrogen (H2) has the promising potential as an energy carrier . Under the ambient condition, H2 owns high gravimetric energy density (120 MJ kg−1), much higher than gasoline or natural gas, which leads to the extensive heat capacity . However, against the potentials as the future energy carrier, H2 has severe characteristics in the extremely low volumetric energy density (10.8 MJ m−3) and boiling temperature (−252.9°C), which leads to the difficult transportation and storage methods . Thus, instead of dispatching H2 individually, some pretreatment to bond the H2 into the more convenient form of carrier is very beneficial.
Currently, transportation and storage of H2 are carried out by the various schemes, including liquefaction, compression, chemical and physical storages. An alternative solution for H2 storage with high storage capacity and low-risk level can be approached by employing the liquid organic hydrogen carriers (LOHCs) . Covalently bonding the H2 through the hydrogenation process, the LOHCs have the capability to carry out 5–8 wt% of the H2 content . Due to the characteristics of high reversibility (hydrogenation and dehydrogenation processes) in the moderate temperature, low GHG emissions, simple storage, and comfortable transportation method by using vessel or pipeline, LOHCs own the potential for the application of long-distance and large-scale H2 transportation . Moreover, the infrastructure of LOHCs is relatively compatible with current method of fuel transportation, due to the liquid phase of the LOHCs in the room temperature and standard pressure condition .
The LOHCs cycle involves hydrogenation and dehydrogenation processes. In this process, the generated H2 is exothermically reacted with the particular compounds in the catalytic hydrogenation . Recently available LOHCs include cyclohexane-benzene, decalin-naphthalene, and toluene-methylcyclohexane (MCH) cycles . In this chapter, toluene (C7H8)–MCH (C7H14) cycle is investigated as the LOHC cycle for the transportation and storage of H2. Both toluene and MCH are low cost, stable compound, and high flexibility of transportation as the liquid phase is very reliable in the wide temperature range, which is favorable for long-distance transport and long-term storage. In addition to this understanding, Chiyoda Corporation, a well-established process engineering company in Japan, has testified the applicability of the toluene-MCH cycle in a relatively large-scale facility .
The investigation is emphasized on the effort of integrating the corresponding processes, including the drying, gasification, chemical looping, power generation, and the hydrogenation process to achieve optimum energy circulation based on enhanced process integration (EPI) to obtain the excellent energy efficiency of the total system. EPI is a technology to minimize thoroughly the heat loss of the system by applying the combination of exergy recovery and process integration . With EPI, instead of the optimization of each process individually, the entire energy management of the system is observed to develop a high-efficient integrated plant with minimum waste of energy .
In this chapter, integrated gasification systems which hold capabilities for coproduction of electricity and MCH from different feedstocks are discussed. The investigation of the systems is emphasized on the overall design of the process scheme and evaluation of some operating parameters for system optimization. Low-rank coal  and microalgae  are chosen as the feedstocks for the integrated system due to the opposite characteristics from each feedstock. However, since both feedstocks are far away from the standard quality of fuel due to high moisture content, a pretreatment stage, including the drying process is discussed also in this chapter. To obtain a high overall efficiency of the integrated system, EPI is applied to implement the optimum benefits of heat energy and reduce the exergy destruction.
The theory of EPI has been introduced and applied to several raw materials, including algae , coal [17, 18], biomass wastes [19, 20], and black liquor . EPI is established from two core technologies such as the process integration and the exergy recovery. The latter relates to the concept to circulate the heat throughout a single process. By applying the EPI, overall exergy loss throughout the integrated system can be reduced as the total energy efficiency of the system is improved.
Figure 1 illustrates the principle of heat circulation employed in the integrated system, including an example of the application of the term of exergy elevation of the process stream. Here, the dotted and solid lines represent the cold and hot streams, respectively.
In contrast to conventional process integration technology, the intensification of the process regarding energy efficiency is carried out in EPI through heat circulation to minimize the exergy losses in each process module before performing the overall process integration. Hence, the energy/heat associated with the process is recovered efficiently by employing heat circulation that promotes exergy recovery.
Based on the current status of H2 production, fossil fuel occupies dominant portion as the primary substance with more than 90% share . In terms of sustainability, fossil fuel has a drawback in the issue of energy reserve. Hence, fossil fuel with abundant reserves is favorable for this system. One of the fossil fuel fit with this condition is low-rank coal (LRC). Besides the long-term reserves, LRC exhibits other advantages, including high concentration of volatiles, high reactivity, low sulfur content, and relatively low mining costs . However, due to high inherent moisture content and readsorption behavior of the humidity, the drying of LRC is very challenging due to the energy intensive process . Thus, an investigation based on EPI technology to effectively manage the heat circulation is carried out to develop an integrated system which can accommodate least expensive LRC for large-scale MCH production with optimum energy efficiency .
Another issue for the utilization of LRC is the large amounts of CO2 emission, in which the plant has to be coupled with the CO2 separation (capture) and sequestration facility. For the CO2 capture, several technologies are available, including membrane, algae-based uptake, cryogenic, and chemical looping . The latter is considered as the most potential method for the sequestration of CO2 due to high capability of CO2 capturing and high conversion efficiency.
Some investigations have been carried out to study the production of H2 from coal. An integrated system consisting of hydrogasification, electrolysis, and electricity generation has been carried out by Minutillo and Perna  to produce synthetic natural gas. However, as the conventional process integration was adopted to develop the proposed system, the result of their study obtained relatively low energy efficiency due to significant losses of exergy. Another integrated system consisting of shell-type gasification, chemical looping, and electricity generation has been carried out by Xiang et al. In their proposed system , overall heat circulation was excluded, as the system adopted the pinch technology for heat recovery. Therefore, the system exhibits low energy efficiency. Moreover, Cleeton et al. carried out an integrated system with the combination of chemical looping and steam-coal gasification . After the parameter evaluation, the delivered system showed energy efficiency up to 58%. However, if the effort of exergy optimization was applied, the energy efficiency of the system can be improved significantly.
The schematic flow of energy and material of the integrated system is shown in Figure 2. Here, the dotted and solid lines represent electricity and material/heat flows, respectively. At the beginning of the process, the moisture content of raw LRC particles is extracted by the drying module. The product of this module is the high calorific value of LRC as the result of low moisture content. Next, the dried LRC particles are converted to syngas by the gasification module. The produced syngas is thus fed to the chemical looping module to generate H2, CO2, and thermal energy for the power generation. The by-product of CO2 generated in the chemical looping is sequestered to keep the clean energy of the integrated system, while the desired products of chemical looping are discharged to the combined cycle module and the hydrogenation module for the generation of electricity and MCH, respectively. The MCH is prepared in the liquid phase. Hence, the compound can be easily transported to the specified place. The generated electricity from the combined cycle is partially consumed for the house load operation for the internal processes, but the remainder can be sold to utilities via a connection to a power grid.
The drying process is carried out to meet the target of the moisture content of the LRC particle so that the calorific value is increasing and the high gasification temperature can be achieved. In the drying process, equilibrium moisture content significantly affects the immediate environment because the particle of LRC will reach a water concentration equal to the ambient environment. Hence, the temperature, pressure, and relative humidity of the environment, as well as the partial vapor pressure, determine the moisture content. Among any methods used for high moisture content drying process, the superheated steam exhibits numerous advantages, including the energy efficiency, high capacity, and has been widely investigated for drying scheme . Thus, by employing the superheated steam as the drying method, the drying and gasification modules were developed, as shown in Figure 3. The relationship between the relative vapor pressure, , and the equilibrium moisture content, M, can be approximated by Eq. (1) .
Initially, the discharged heat produced in the chemical looping module and the high energy compressed steam is employed to preheat the high moisture content LRC particles (D1) in HX1 and HX2, respectively. Subsequently, the preheated LRC particles undergo drying process to omit the water content inside the particles. The type of dryer applied in this system is the fluidized bed owing to the benefit in the uniform temperature distribution and heat transfers, an extensive area of the contact surface, and proper particle mixing . The immersed heat exchanger (HX3) is furnished inside the fluidized-bed dryer for the superheated steam process.
The next step is the gasification process, which produces combustible gases, including H2, CO, and CH4. Series of reactions are involved in this process, including the water-gas shift, Boudouard, and oxidation, as well as methanation. Due to the necessity of high gasification temperatures, dry-feeding gasification is employed in the system instead of slurry-feeding gasification . Here, a dual-circulating fluidized bed (gasifier and combustor) is furnished to achieve higher carbon conversion efficiency and conversion rate .
After the gasification, both of the raw syngas and flue gas are fed into SEP1 and SEP2 for removal of ash and slag by the ceramic particulate removal which exhibits high efficiency under high-temperature conditions . Table 1 summarizes the conditions assumed for LRC drying and gasification study.
|Coal flow rate (Mg h−1)|
Initial moisture content (wt% wb)
Target moisture content (wt% wb)
Fluidization velocity Udry/Umf,dry
Mean particle diameter (mm)
Bulk density (kg m−3)
Heating value (MJ kg−1 HHV)
1, 2, 3, 4
|At moisture content of 18 wt% wb|
|LRC ultimate analysis|
|C (wt% db)|
H (wt% db)
N (wt% db)
O (wt% db)
S (wt% db)
Cl (wt% db)
Ash (wt% db)
|Combustion temperature (K)|
Gasification temperature (K)
LRC mean particle diameter (mm)
Mean bed material diameter (mm)
The effect of steam-to-fuel ratio during gasification is evaluated to define the optimum combination of steam and fuel. Two different steam-to-fuel ratios (0.9 and 1.4) are investigated to confirm the optimum ratio for the performance of the chemical looping system. Table 2 shows the composition of the syngas resulting from different steam-to-fuel ratios.
|Produced syngas||Steam-to-fuel ratio|
|H2 (Nm3 kg-fuel−1 daf)|
CH4 (Nm3 kg-fuel−1 daf)
CO (Nm3 kg-fuel−1 daf)
CO2 (Nm3 kg-fuel−1 daf)
Steam content in produced gas (vol%)
Among any methods, direct chemical looping and syngas chemical looping are the typical methods for the chemical looping. However, due to the utilization of the metal oxide for oxygen carrier in the system, the syngas chemical looping is employed due to the beneficial in the solids handling and energy efficiency. The oxygen carrier exhibits no direct contact between atmospheric oxygen and the fuel during the combustion process; hence, the high-purity CO2 can be immediately separated without any further handling, which thus promotes an efficient and clean energy conversion. Thus, due to the excellent mechanical properties, large capacity content of oxygen carrier, and the high conversion of syngas and steam, the iron-based materials are applied as the recirculated oxygen carriers .
Figure 4 represents a process flow diagram for the chemical looping and combined cycle modules. The chemical looping module is developed from three connected reactors: reducer (RED), oxidizer (OXD), and combustor (COM2). In the reducer and oxidizer, a counter current moving bed reactor is employed, while an entrained fluidized bed is furnished for the combustor.
In the RED, the compressed syngas is fed as the fluidizing gas. After leaving the RED (C3), the high pressure fluidizing gas is thus recovered by the expander (GT1) for electricity generation. The reactions in the RED assumed to occur during reduction are as follows :
The formation of by-products, including Fe3C and carbon soot due to Boudouard reaction has been noticed in the continuous operation in RED. However, efforts to diminish the formation of Fe3C have been performed in previous studies, including the modification of iron-based oxygen carrier with CeO2 and exhaustive selection of the used iron-based oxygen carriers .
CO2 and steam are generated during the reduction step and then leave the reducer for the cooling process in preparation for separation (CD1). The separated CO2 (C7) is then compressed and ready for the sequestration purposes. Other product from the reduction step, the iron particles (C9), is fed to the OXD in which the oxidation takes place with the steam as reactant to produce highly pure H2 (C16). The reactions inside the OXD in the presence of excess steam can be written as follows:
The generated H2 is discharged to the hydrogenation module for further process, while the metals move to the COM2 for recirculation. The reaction inside the COM2 is shown below.
The reduction process is performed by the discharged heat from the combustion process implanted inside the metal oxide. The high energy from compressed flue gas is expanded for electricity generation in GT3 and ST1 by employing combined cycle. The conditions of chemical looping and combined cycle modules are explained in Table 3.
Syngas conversion (%)
Iron particle diameter (mm)
Produced CO2 purity (%)
Produced H2 purity (%)
Excess steam at outlet (%)
Air excess at outlet (%)
|Gas turbine (F-class)|
|Turbine polytrophic efficiency (%)|
Maximum turbine inlet temperature (K)
|HRSG and steam turbine|
|Turbine polytrophic efficiency (%)|
Inlet pressure (MPa)
Maximum turbine inlet temperature (K)
Minimum outlet vapor quality
The conditions during toluene hydrogenation and the schematic flow diagram are shown in Table 4 and Figure 5, respectively. The heat generated from the exothermic reaction of hydrogenation is applied as the heat source for electricity generation in the steam turbine (ST1). The theoretical gravimetric and volumetric hydrogen concentrations in MCH under ambient conditions are 6.2 and 47%, respectively . The hydrogenation reaction is as follows:
|Reaction temperature (K)||473|
|Operating pressure (kPa)||130|
|Catalyst particle size (mm)||0.3|
Figure 6 indicates the effect of drying fluidization velocities to the compressor and blower duties, net generated power, and power efficiency. These values are carried out using a steam-to-fuel ratio during gasification and a basic chemical looping pressure of 0.9 and 3 MPa, respectively. A compressor (CP4) is prepared after the gasification module to pressurize the syngas into the pressure of chemical looping.
Based on the calculation results, there is barely significant shift in the compressor duty as the compression work is almost constant at 1.8 MW. On the other hand, the duty of blower is increasing as the fluidization velocities during drying are uprising. Here, the blower duty at the lowest fluidization velocity (Umf,dry, 1.28 m s−1) is 0.7 MW and rise to 2.7 MW when the fluidization velocity is increased to 4 Umf,dry. As high amount of energy is required for the high duty of the blower, a rapid fluidization velocity during drying results in a lower total efficiency.
Figure 7(a) presents the effect of different steam-to-fuel ratios to the amount of H2 generated, H2 production efficiency, net generated power, and the total efficiency, while Figure 7(b) presents the effect of different steam-to-fuel ratios to the amounts of produced H2 and MCH. These values are carried out using a specific fluidization velocity during drying and a basic chemical looping pressure of 2 Umf,dry and 3 MPa, respectively. Generally, the higher H2 production efficiency and power generation are both achieved in the steam-to-fuel ratio of 0.9 instead of 1.4. The produced H2 amount and net generated power at a steam-to-fuel ratio of 1.4 are 4.3 t h−1 (H2 production efficiency of 66.5%) and 24.1 MW (power generation efficiency of 11.2%), respectively. These values rise up to 4.6 t h−1 (with a H2 production efficiency of 71.9%) and 26.3 MW (with a power generation efficiency of 12.2%), when the steam-to-fuel ratio is set at 0.9.
Figure 8 Shows the effect of basic chemical looping pressure to the net generated power and power generation efficiency. These values are carried out by applying a fluidization velocity during drying and a steam-to-fuel ratio during gasification of 2 Umf,dry and 0.9, respectively. Generally, the increase of chemical looping process pressures leads to increase in both the net generated power and the power generation efficiency.
Besides the high potential in the pharmaceuticals, industrial materials, and food production, microalgae own a high potential for the energy source . Among other biomasses, microalgae are very exceptional due to its excellent growth rate, ability to grow in a harsh environment, and highly efficient solar energy conversion . Currently, many products of fuel are derived from the microalgae, including bio-oil, biohydrogen, and biodiesel . However, as the microalgae grow in an aqueous environment far from the industrial or residential area, it has to be planted remotely and transported to the designated area for the utilization of microalgae in the large scale. To this, a process chain from the microalgae cultivation to the H2-based MCH for LOHC generation can solve the transportation and storage issues of the large-scale utilization of microalgae.
For the investigation, an alga species with ability to grow rapidly in normal condition and rich protein is necessary. To this, Chlorella vulgaris is selected as a sample for system evaluation. . The properties of Chlorella vulgaris, including the results of proximate and ultimate analyses are listed in Table 5.
|Moisture content (wt% wb)||90|
|Dry solid content (wt% wb)||10|
|Protein (wt% db)|
Fat (wt% db)
Fiber (wt% db)
Ash (wt% db)
Carbohydrates (wt% db)
|Carbon (wt% db)|
Hydrogen (wt% db)
Nitrogen (wt% db)
Oxygen (wt% db)
|Calorific value (dried, MJ kg−1)||18.49|
Among any candidates in the thermochemical process, gasification owns the highest rank due to its conversion efficiency . There are two gasification methods widely used, the conventional thermal gasification and supercritical water gasification (SCWG). The former requires drying process to low moisture content before the gasification, while the latter is performed in the aqueous state, in which drying process is avoidable , and more favorable for the gasification of microalgae, owing to its high moisture content (70–90 wt% wb) . However, SCWG is an energy consuming process, which tends to absorb a huge portion of energy and create a lowly energy-efficient system . To this, the EPI technology can tackle the challenge of the high energy issue of the SCWG, and leads to a novel process for the large-scale utilization of microalgae into LOHC.
The basic schematic energy and material flows of the integrated system consists of SCWG, H2 separation, hydrogenation, and the combined cycle. Figure 9 explains the detailed schematic process flow diagram of the proposed integrated system.
Due to the supercritical regime of the reaction, the decreasing water density causes a low static relative dielectric constant, hence, weaken the hydrogen bonds and improve the yield of the syngas . Moreover, syngas cleaning can be disregarded due to no formation of NOx or SOx in the process. In the proposed process, a fluidized-bed reactor was selected due to its beneficial characteristics, including better particle mixing, ability to avoid plugging, uniform temperature distribution, and a high conversion rate .
For high performance of gasification, a catalyst set of Ru/TiO2 which exhibits H2-rich syngas and complete carbon conversion is injected inside the gasifier. Moreover, fluidizing particles (e.g., alumina) can be proposed to increase the particle dynamics inside the reactor and prevent the production of ash layer and char on the reactor wall . In this study, the flow rate and initial moisture content of wet microalgae are fixed at 1000 t h−1 and 90 wt% wb, respectively. Two gasification pressures are evaluated: 25 and 30 MPa. Table 6 gives the gasification conditions and produced syngas composition.
Fluidization velocity U/Umf (−)
Fluidizing material (−)
Density (kg m−3)
Particle diameter (mm)
Voidage at minimum fluidization (−)
Gasification catalyst (−)
Weight ratio of catalyst to wet algae (−)
1, 2, 3, 4
|Carbon conversion efficiency (%)|
H2 (dry mol%)
CO (dry mol%)
CH4 (dry mol%)
C2H6 and C3H8 (dry mol%)
CO2 (dry mol%)
Membrane-based separation is adopted for the H2 extraction from the syngas. This method promotes low energy consumption, simple handling process, and mild conditionings. Among the membrane separation methods, polymeric membrane separation, consists of microporous film acting as a semipermeable barrier, owns the widest commercial application . The assumed conditions during H2 separation and hydrogenation are given in Table 7.
Hydrogen recovery (%)
Operating temperature (°C)
Feed inlet pressure (kPa)
Product outlet pressure (kPa)
Product H2 concentration (mol%)
Product CO concentration (mol%)
Product other gas concentration (mol%)
Catalyst particle size (mm)
After the H2 is separated from the syngas, the remaining syngas is employed as a fuel for combustion (COMB) in the combined cycle. Moreover, due to high temperature of the flue gas from the gas turbine (GT), it is thus used to superheat the mixture of syngas and steam from the SCWG reactor. At last, the remaining heat is recovered in HRSG for the steam turbine (ST). The conditions of the combined cycle modules, including combustor, gas and steam turbines are presented in Table 8.
|Combustor and gas turbine|
|Compressor outlet pressure (MPa)|
Compressor polytrophic efficiency (%)
Combustor pressure drop (%)
Gas turbine inlet temperature (°C)
Gas turbine adiabatic efficiency (%)
Air to fuel ratio (−)
|HRSG and steam turbine|
|HRSG outlet pressure (MPa)|
Heat exchanger temperature difference (°C)
HRSG pressure drop (%)
Steam turbine inlet temperature (°C)
Steam turbine polytrophic efficiency (%)
Minimum outlet vapor quality (%)
Figure 10 shows the relationship between total energy efficiency, ηtot, and fluidization velocity for different gasification pressures of 25 and 30 MPa. Subsequently, about 3.5 t-H2 h−1 can be converted to MCH by hydrogenation process with toluene. The increase of fluidization velocity leads to lower total energy efficiency. Therefore, gasification carried out under a lower gasification pressure at 25 MPa has better total energy efficiency than that at 30 MPa.
The increasing in fluidization velocity leads to more water for the fluidizing steam which thus increases the flow rate of steam exhausted from the SCWG reactor. As a result, the heat available as hot stream in the HRSG decreases, leading to a decrease in actual work obtained from the steam turbine. On the other hand, the increasing of gasification pressure leads to more flow rate of the fluidizing steam under same fluidization velocity. Thus, the increase in gasification pressure finally leads to lower actual work by the steam turbine.
Novel integrated gasification systems for coproduction of electricity and MCH from low-rank coal and microalgae have been proposed. Enhanced process integration technology is applied for both systems to minimize exergy losses throughout the integrated system, thus improving the total energy efficiency. However, the models are carried out in the condition of ideal zero heat loss. Therefore, approximately 5–10% of heat losses are necessary to consider for the investigation in the actual case of the systems. For the case of microalgae, the effects of the fluidization velocity and gasification pressure on the total energy and electricity generation efficiencies were evaluated for optimum integrated system, while for the case of low-rank coal, the optimization is subjected to the investigation of fluidization velocity, steam-to-fuel ratio, and the chemical looping pressure. Finally, the integrated system for microalgae is capable to provide more than 60% of total energy efficiency, while the integrated system for low-rank coal delivers the total energy efficiency of 84%.
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