CO 2 gasification of microalgae ( N . Oculata ) – A thermodynamic study

A new model of CO2 gasification has been developed in the Aspen Plus. The potential of microalgae (N. oculata) for CO2 gasification also has been investigated. The present gasification process utilizes the CO2 at atmospheric pressure as the gasifying agent. The steam is also injected to the gasification to enhance the H2 production. The composition of the producer gas and gasification system efficiency (GSE) are used for performance evaluation. It is found that the CO2 gasification of microalgae produces a producer gas with a high concentration of CO and H2. The GSE indicates that the process works at high performance.


Introduction
The increase of the fossil fuel consumption during the last decades causes a global warming.The renewable fuel has been considered as a potential solution of the corresponding problem.Among the available renewable resources, biomass is considered as a potential alternative resources with minimum negative impact due to its ability to reduce the CO2 from the atmosphere and its low sulfur content [1].
Gasification is a potential biomass conversion process since it produces a producer gas which mainly consists of H2, CO, CO2 and CH4 [2].The gasifying agent has a considerable effect on the producer gas composition.[3], [4] and [5,6] reported the injection of steam to the gasifier enhanced the H2 production in the producer gas.The use of CO2 as a gasifying agent attracts the attention of some researchers.For instance, [7] investigated the gasification of pine sawdust using CO2 over Ni/Al precipitation catalyst.[8] studied the gasification of beech sawdust in the entrained flow reactor.[9] developed a model to investigate the performance of biomass gasification with CO2 using fluidized bed gasifier.Recently, some research have focused on microalgae cultivation and processing because it offers positive environmental impacts [10][11][12].To the best of our knowledge, only one literature discussing the CO2 gasification of algae.[13] carried out CO2 gasification of microalgae char using highpressure thermogravimetric analyzer (HP-TGA).

Methodology
The minimization of Gibbs free energy is used for thermodynamic modeling of gasification process which involves solid, liquid and gas phases [14].The process flow diagram of the gasification system can be seen in Fig. 1.The biomass and the gasifying agents (CO2 and H2O) are fed to the gasifier separately.It is worth noting that the gasifier is modelled using two blocks: the DECOM block (RYield) and the Reduction block (RGibbs).A Fortran code is embedded in the DECOM block to convert the microalgae feed stock (non-conventional component) into conventional components.While the gasification reactions occur based on the method of Minimization of Gibbs Free Energy in the Reduction block.It is worth mentioning that during gasification process the following consecutive reactions occur in the gasifier: The gasification products flow to the cyclone to separate the gaseous products (H2, CO, H2O, and CO2) and solid products (ash and unconverted char).The gaseous products are fed to the reformer.The reformer products is sent to the CO2 absorber to capture the CO2 in the reformer products.The pure CO2 from the absorber is then cooled and compressed to 523 K and 80 bar, respectively.The producer gas is expanded and cooled to 3 bar and 298 K, respectively.The ultimate and proximate analysis of the biomass, including the heating value of biomass are summarized in Table 1.The operating conditions of the gasification process are summarized in Table 2.The performance of the gasification is evaluated in term of the dry composition of producer gas and gasification system efficiency (GSE) (Eq.( 7)).One should note that the energy required for CO2 absorption using amine is 3 MJ/kg CO2 absorbed [15].
Where m and LHV are mass flowrate and low heating value, respectively, while subscript pg and ma refer to the producer gas and the microalgae, respectively.The Egen represents the total energy generated from the process while the Ereq represents the total energy consumed by the process.It is worth noticing that the model validation of the gasification block is conducted using the same feedstock and operating conditions with the ones employed by [16].The composition of the producer gas from the gasifier is used as the baseline for model validation.It is clearly shown in Table 3 that the results of our simulation agree closely with the results reported by other author [16].Indeed the relative error for H2 and CO as the major producer gas component was less than 10%.

Results and discussion
A microalgae feedstock with mass flow rate equal to 100 kg/h is fed to the gasifier.In addition, a CO2 and a steam with mass flow rate of 151 kg/h, and 62 kg/h, respectively, are selected as the gasifying agents.It is worth noticing that the gasification system consists of the gasifier, the reformer and the CO2 absorber, as shown in Fig. 1.
It is clearly shown in Fig. 2a that the microalgae disappears while the amount of H2, CO, CO2, and CH4 significantly increase in the product stream of the gasifier.This indicates that the main task of the gasifier is the conversion of the solid feed stock (e.g., microalgae) into the gaseous products (H2, CO, CO2, CH4 and H2O) with the help of the gasifying agents (H2O and CO2).The decrease of the amount of microalgae and H2O indicates that the microalgae is decomposed into gaseous products through steam reforming reaction (Eq.( 2)).This is also confirmed by the increase of the CO and H2 in the gasifier product.One should notice that the methane formation reaction (Eq.( 3)) also takes place during CO2 gasification of microalgae since the CH4 presents in the gasifier product.When one look at Fig. 2a, the amount of CO2 in the gasifier products is higher than that of in the gasifier feed.This indicates that water-gas shift reaction (Eq.( 4)) occurs in the gasifier.Indeed, this also can be attributed to the decrease of the H2O amount in the gasifier product.
In the reformer unit, the gasifier products is further reacted to produce higher concentration of combustible gases (CO and H2).Fig. 2b indicates that the amount of CO2 and CH4 in the reformer feed stream are higher than their counterparts in the reformer product stream.This can be attributed to the methane reforming reaction (Eq.( 5)) and CO2 reforming reaction (Eq.( 6)).This is confirmed by the increase of the amount of CO and H2 in the stream of the reformer product.
The amounts of the components in the feed of CO2 absorber and in the producer gas stream are depicted in Fig. 2c.The amount of the CO2 in the producer gas stream is lower than its counterpart in the stream of CO2 absorber feed.This indicates that the CO2 absorber significantly reduces the amount of noncombustible gas in the producer gas stream.It is worth mentioning that the producer gas consists of 45% H2, 45% CO, 6% CO2 and 4% CH4 (dry basis), while the GSE of the process is 68%.

Conclusion
The production of producer gas with a high concentration of CO and H2 via the CO2 gasification of microalgae is simulated using Aspen Plus.The results show that the CO2 gasification of microalgae can provide the producer gas with a high concentration of H2 and CO.It is also reported that the GSE of the process is relatively high.In addition, high purity of CO2 is also produced as the side-product of the process.Although the present model is developed only for the purpose of the preliminary study of the CO2 gasification of algae.The results indicate that the CO2 gasification of algae can potentially be a promising alternative of green technology with further optimization.
The research team acknowledges the financial support provided by King Abdul Aziz City for Science and Technology (KACST) to this research under KACST-TIC for CCS project no 03.The team also thanks the facilities and support provided by KFUPM.

Fig. 2 .
Fig. 2. Mole flow of the feed and product of (a) the gasifier, (b) the reformer, and (c) the CO2 absorber.(feed: black; product: grey)

Table 1 .
Proximate and ultimate analysis of microalgae.

Table 2 .
Operating conditions in the simulation.