ABSTRACT

CO₂ adsorption enhanced gasification process is a prospective technology to be used in hydrogen production with nearly zero CO₂ emission. In this paper, a gasification kinetics steady state model is developed based on the chemical kinetics mechanism with the condition of fluidization. It is used to predict the parameters of gas phase and solid phase for a continuous flow reactor with steady state. The variation of components distribution with gasification temperature and pressure are determined, from which the optimal operation parameters of gasifier can be obtained.

1. Introduction

CO₂ adsorption enhanced gasification processes [1,2] apply the concept of one step hydrogen production with in situ CO₂ capture. Coal is gasified with steam and calcium based compounds in the absence of O₂. The gasification reaction is coupled with carbonation reaction by mass and heat exchange. In these processes, hydrogen molar fraction in syngas can be higher than 80%. The remaining part is mainly methane and the content of CO₂ is very low.

Above benefits enable CO₂ adsorption enhanced gasification processes as one prospective technology to be used in hydrogen production with nearly zero CO₂ emission. One of the key point of its research is how to predict the performance of gasification process. Experiment and unit theoretical model are two important ways to make it. Unit theoretical model is especially necessary in the systematic study of coal to energy process. In references [2-8], chemical equilibrium composition of mixture of C/coal with H₂O and CaO is calculated by theoretical model based on Gibbs free energy minimization. The influence of gasification temperature, pressure, ratio of CaO/C and steam/C is investigated. The thermodynamic equilibrium model offers greater flexibility for complex reaction system with limited given conditions for which the reaction pathways are unclear. Usually some correction factors, such as temperature approach, carbon conversion ratio, are applied to improve predicttion precisions [8-11]. But in realistic condition, product distributions and conversion ratios of carbon and CaO depend on chemical kinetics and hydrokinetics, which is determined by reaction temperature, pressure and other factors. As a supplement of previous study, it is necessary to investigate gasification performance from the point of view of chemical kinetics and fluid hydrodynamic. Kinetics models have the advantage to directly illustrate the effect of pressure, temperature, reactants compositions and other parameters on product distributions and reactants conversion ratios. A dynamic simulation model of CO₂ adsorption enhanced gasification processes is established in reference [12]. It is used to study the influences of reaction conditions on the proportion of the reactants. However, there are some drawbacks of kinetics model. Prediction precision is highly dependent on the kinetics parameters, which are usually measured and deduced from experiments that conducted under some specific conditions (e.g. coal rank, reactor type and operation conditions, etc.). So deviation of simulation assumptions from experiment conditions will lead to prediction errors. Furthermore, more inputs and more detailed feed streams characteristics are required when kinetics models are applied.

Comparing with previous work, some changes about gasifier modeling can be made as follows. 1) Because the variation of solid compositions and flow rate are not taken into account in the dynamic model developed in reference, it is necessary to set up mass and energy conservation equations of solid phase, which is required when analyzing the mass and heat interaction between syngas production section with regenerator and other parts; 2) The results calculated from dynamic model vary with reaction time, which make it difficult to be used in steady state systematic performance analysis. A steady state gasifier model based on the kinetics mechanism and with the condition of fluidization should be set up.

Summarily, in this paper, a steady state gasifier model is developed based on kinetics mechanism and with the condition of fluidization. It is used to predict parameters of gas phase and solid phase of a continuous flow reactor at steady state. The variations of components distributions with gasification temperature and pressure are determined, from which the optimal operation parameters of gasifier can be obtained.

2. H₂ Rich Syngas Production with CO₂ Sorption Enhanced Coal Gasification

As shown in Figure 1, CO₂ adsorption enhanced coal gasification is divided into two reactors, namely a syngas production reactor (SPR) with CO₂ sorption and a absorbent regeneration reactor (RER). The mass and heat flux are transferred by recycling solid particles. In SPR, carbon contained feedstock reacts with steam and calcium compound. Pyrolysis reaction, gasification reaction and carbonation reaction are integrated into SPR. Carbon is converted to syngas and meanwhile, CO₂ released from gasification reactions is absorbed by calcium based absorbent. Syngas is composed of H₂ with 60% - 95% molar fraction and methane with 5% - 40% molar fraction. CO₂ and CO content can be lower than 1%. Calcium carbonate produced together with ash, unconverted carbon and CaO are discharged to the regenerator to be calcined in the atmosphere of H₂O and CO₂. CaO regenerated is recycled to SPR. CO₂ from calcinations is released at nearly pure state, which make its possible to capture with relatively lower cost. In the above process, high temperature particles feeding to SPR are not only act as absorbent, but also as heat carrier to provide heat for endothermal reactions.

The production of syngas in SPR can be explained by the following reactions:

C(s) + H₂O(g) ↔ CO(g) + H₂(g) + Q

C(s) + CO₂(g) ↔ 2CO(g) + Q

C(s) + 2H₂(g) ↔ CH₄(g) – Q

CO(g) + H₂O(g) ↔ CO₂(g) + H₂(g) – Q

CH₄(g) + H₂O(g) ↔ CO(g) + 3H₂(g) + Q

CaO(s) + CO₂(g) ↔ CaCO₃(s) – Q 

CaO(s) +H₂O (g) ↔ Ca(OH)₂(s) – Q

3. Performance Prediction Model of CO₂ Adsorption Enhanced Coal Gasifier

3.1. Coal Properties and Model Assumptions

Shen-mu coal is used as the feedstock of systems analyzed in this paper. It contains 65.2% C, 4.63% H (ultimate analysis, daf basis) and 54.8% fixed carbon, 28.15% volatile (proximate analysis, daf basis).

The following are some assumptions and limitations of the model: 1) It is assumed that the coal conversion process can be divided into a pyrolysis zone and a gasification zone. Devolatilization is completed instantaneously and then products from coal pyrolysis including volatile and char react with steam and CaO in gasification section; 2) Solid phase is treated as continuous stirred flow; 3) Solid components including char, ash, calcium compound, are distributed homogenously in the reactor; 4) Gas phase is composed of H₂, CH₄, CO₂, CO, H₂O, H₂S, N₂ and treated as plug flow; 5) Gas components are assumed to be functions of the height of gasification section; 6) Temperature is homogeneous in the syngas production reactor (SPR). Ca(OH)₂ is not included in reaction system; 7) Surplus heat generated in reactor is removed by steam generation.

3.2. Mass Balance of Syngas Production Reactor

During pyrolysis process, coal is converted into volatile and char instantaneously after it is fed to the furnace. Volatiles are assumed to be composed of CH₄, CO, CO₂, H₂, H₂O, N₂ and H₂S. According to the prediction method in reference [13], mass fractions of coal pyrolysis products are calculated.

The input parameters for gasification section include: flow rates, compositions of pyrolysis products, appointed gasification temperature (or Q_ST: appointed heat exchanged with cooling agent), appointed gasification pressure, appointed steam parameters (flow rate, inlet temperature and pressure), appointed absorbent parameters (inlet temperature, pressure, compositions and flow rate). Output parameters include: conversion ratios of char (X_char) and CaO (X_CaO), coal to H₂ efficiency, cold gas efficiency, gases distributions along height of reactor, calculated compositions and flow rates of bed materials and discharged solid, calculated yield and composition of hydrogen rich syngas, calculated Q_ST (or calculated operation temperature).

3.2.1. Mass Balance of Solid Phase in Gasification Zone

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(2)

(3)

(4)

(5)

(6)

(7)

Equations about solids phase in gasification section are composed of definitions of conversion ratio of char and CaO, components (char, ash, CaCO₃, CaO) mass balance, mass balance of bed materials. Where, Symbol X_char and X_CaO means conversion ratio of char and CaO respective. Symbols G means mass flow rate. Symbol W_i,j means mass fraction of component i contained in control volume j, j = bed, ent, slag. Symbols bed, ent and slag mean bed material, entrained solid in raw hydrogen rich syngas and slag respectively. Symbol R means chemical reaction rate. It is assumed that compositions of solid entrained in raw syngas stream and discharged as slag are of the same compositions with that of bed material.

3.2.2. Mass Balance of Gas Phase in Gasification Zone

Along the direction of gasification reactor height L, reaction zone is divided into numbers of cells in which mass balance for each gas components is set up. In each cell, the mass balance of component i is expressed as:

One specific gas flow rate is determined by its flow rate and chemical reaction rates with other gases and solids, where symbols P, T are the total pressure and temperature of SPR section. is the molar fraction vector of gases. R_i-j means reaction rate of reaction between component j and i. Symbols g and s mean gas phase and solid phase respectively.

3.2.3. Chemical Kinetics Rates in Gasification Zone

Chemical kinetics rates of gas-solid reactions, gas-gas reactions are determined by reaction temperature, total pressure, molar fractions of gases, solid contents, and hydrodynamic parameters, where f is a function of several variables. It stands for reaction rate and the definition can be found in references [14-16].

3.3. Energy Balance of Syngas Production Process (Pyrolysis Zone + Gasification Zone)

Pyrolysis sub-reactor and gasification sub-reactor are put together to model energy balance. At given input parameters (steam and absorbent flow rates, gasification pressure, reactor heat loss, and appointed gasification temperature/or appointed heat Q_ST transferred with heating/cooling medium), the result of heat Q_ST or gasification temperature can be calculated. Enthalpies of components are defined according to reference [17].

3.4. Model Solution Methodology

Figure 2 shows the simulation procedure of SPR. Its main inputs are coal properties, ratio of CaO to coal, ratio of steam to coal, reaction pressure, reaction temperature (or heat exchanged with medium), reactor parameters (height, superficial velocity, bed materials loading). Outputs include gas compositions and yields, solid phase compositions, discharged solid, solid entrained by gas, heat exchanged with medium (or temperature). Carbon conversion ratio, CaO conversion ratio and gas compositions distributions along height of reactor are estimated and determined by iteration procedure.

3.5. Comparison with Experimental Results

In Tables 2(a)-(b), calculation results based on model described above and data of experiment and demonstration plant are compared. In Table 1(a), running results of commercial Winkler gasifier and U-gas demonstration fluidized bed are collected from reference [10]. In Table 2(b), experimental data are got from a continuous experiment high pressure reactor [18] fed by Taiheiyo coal. Reaction temperature is set at 650˚C. The description of test rigs and more detailed experiment results can be found in references.

Table 2(a) shows the percentage deviations of H₂ and CO compositions between computed and experimental gas compositions are less than 20%. In Table 2(b), it can be seen that the variation tendencies of gas molar fraction with temperature are coincident with that of experiment. The maximal prediction error of H₂ molar fraction is 6.6%. But prediction errors of CO₂, CO and CH4 molar fractions are quite high. In Table 2(b), it is found that the