Elsevier

Applied Energy

Volume 169, 1 May 2016, Pages 642-651
Applied Energy

Simulation of the calcium looping process (CLP) for hydrogen, carbon monoxide and acetylene poly-generation with CO2 capture and COS reduction

https://doi.org/10.1016/j.apenergy.2016.02.077Get rights and content

Highlights

  • A novel system is proposed for H2, C2H2, and CO poly-generation.

  • Cycled CaO-based sorbent is reused for CaC2 production.

  • The discarded carbide slag is modified and recycled for coal gasification.

  • CO2 and COS are captured through calcium looping process.

  • The total efficiency of the system reaches 81.62%.

Abstract

In this paper, a novel poly-generation system is proposed for hydrogen, carbon monoxide, and acetylene production with inherent CO2 capture and COS reduction. It is based on calcium looping coal gasification and oxygen-thermal CaC2 preparation. The system is comprised of calcium looping gasification reactors, preheaters, oxygen-thermal CaC2 furnace, and carbide slag modifier. Modified carbide slag and fresh CaCO3 are used as raw materials for sorption enhanced hydrogen production (SEHP). The cycled CaO-based sorbent is fed into oxygen-thermal CaC2 furnace for CaC2 production after calcination. CO-rich gas is generated from the furnace simultaneously. A portion of CO-rich gas which includes COS is fed into calcium looping gasification system to generate extra hydrogen. In addition, CO2 and COS are captured through the CaO-based sorbent. To maintain the heat balance of the system, a portion of CO-rich gas is fed into the regenerator for combustion. The entire process is modeled and simulated using Aspen Plus software. The proposed system efficiency could reach 81.62%. Parameters that affect the target products (H2, CO, C2H2, and CaC2) are investigated in the sensitive analysis, including steam/CO/CaO ratio, riser temperature, coke/CaO/O2 ratio, and coal species.

Introduction

Hydrogen has attracted more and more attentions for its cleaning, efficiency, and high calorific value which can be utilized in various forms [1], [2], [3]. Currently, hydrogen is mainly produced from natural gas steam reforming and gasification of solid fuels [4], [5], [6], [7], [8], [9]. Coal gasification for hydrogen production is expected to be a major supply for hydrogen demand in the future [10], [11], [12], [13], [14], [15]. However, CO2 is a by-product of the coal gasification process whose emission is a contributor to global warming [16]. So it is necessary to conduct CO2 capture and storage from coal gasification. Among all the potential methods for hydrogen production from coal with inherent CO2 capture, sorption enhanced hydrogen production (SEHP) using a calcium looping process is a promising option [17], [18]. The principle of the process is shown in Fig. 1.

The system consists of two reactors: one is the gasifier and the other is the oxygen-fired regenerator. In the gasifier, the coal is gasified. CaO-based sorbent which is regarded as a suitable candidate due to high CO2 capture capacity and low cost [19], [20] is added into the gasifier to capture CO2 and enhance H2 concentration. The primary reactions occurred in the gasifier are described in reactions (1), (2), (3), (4), which includes coke gasification, water gas shift, steam methane reforming, and carbon dioxide absorption. In the regenerator, CaCO3 decomposes into CaO and CO2 as shown in reaction (5). The energy required for calcination is provided by the combustion of unburned coke in the regenerator, shown in reaction (6). When O2 was employed as the calcination medium, pure CO2 can be obtained from the regenerator. The calcium looping gasification system has several advantages: (I) a high concentration of hydrogen can be obtained; (II) a simpler route compared to conventional methods. The conventional method of hydrogen production includes a series of steps such as coal gasification, water gas shift reaction, CO2 removal process, and pressure swing adsorption (PSA) [21], [22]. The calcium looping gasification process accomplishes the above processes in a single step; (III) carbonation as an exothermic reaction compensates the heat needed in gasification process; (IV) CO2 is captured through CaCO3 de-carbonation and coke burning.

As a frontier and potential technology, calcium looping coal gasification has been extensively studied. The process is on the basis of CaO as a regenerable sorbent in carbonation/calcination cycles. However, the drawback of CaO is the decreased tendency of the adsorption efficiency over multi-cycles [23], [24]. It is generally acknowledged that the capacity of CaO-based sorbent exhibits a rapid decay due to the pore blockage and sorbent sintering [25], [26]. Currently, different methods have been proposed to raise CO2 capture capacity of the CaO-based sorbent such as hydration treatment [27], [28], thermal pretreatment [29], [30], and utilization of sintering-resistant calcium precursors [31], [32], [33]. However, the tendency of decreased CO2 capture capacity is inevitable. For the continuous H2 production and CO2 gas capture, cycled CaO-based sorbent must be replaced by fresh CaO or CaCO3 to maintain the sorption performance in the system. Ca-looping for post-combustion CO2 capture has been demonstrated in the megawatts-scale circulating fluidized bed combustors by many institutes [34], [35], [36], [37]. Thus, with the development and demonstration of calcium looping coal gasification and especially post-combustion CO2 capture power plants, more and more CaO-based sorbents after cycles are expected to be disposed of. This will lead to CaO waste and other environment issues.C+H2OCO+H2H2O+COCO2+H2CH4+H2OCO+3H2CaO+CO2CaCO3CaCO3CaO+CO2C+O2CO2

Calcium carbide is a basic chemical in the industrial application of acetylene gas production. It is primarily used in acetylene and acetylene-derived products synthesis, such as polyethylene (PE), polyvinyl chloride (PVC) and vinyl acetic acid. Nearly A fraction of 78% of PVC is synthesized by acetylene which is derived from calcium carbide using the reaction (7) in China [38]. 1.5–1.9 tons of carbide slag are disposed of as 1 ton of PVC is produced [39]. Thus, millions of tons of carbide slag are produced each year. The carbide slag is strongly caustic and corrosive. It is harmful to the environment if not properly disposed of. In the process of calcium carbide production, the flue gas from the furnace is at 700–1100 °C. It contains CO, CO2 and other traces such as toxic HCN, COS [40] as well as dust. The CO fraction is around 70–80 vol%. The CO2 composition in the CO-rich gas is generated due to the calcination of CaCO3 in the calcium carbide production [41]. It decreases the heat value of the CO-gas. So this high-temperature CO-rich gas is usually burnt directly with air in a torch without heat recovery, which causes air pollution and energy waste. It is necessary to search alternative routes to solving the carbon dioxide and toxic gasses emission as well as recover the wasted heat.CaC2+2H2OC2H2+Ca(OH)2CaO+3CCaC2+COC+12O2COCO+12O2CO2

A novel system based on calcium looping process is proposed for hydrogen, carbon monoxide, and acetylene poly-generation in this work. CO2 capture is also achieved with COS mitigation in the system. This system combines the calcium looping coal gasification technology and the advanced calcium carbide preparation method. Fig. 2 is the schematic flowsheet of the proposed system. The calcium transformation is CaO  CaCO3  CaO  CaC2  Ca(OH)2  CaO, which occurs in the gasifier, regenerator, furnace, C2H2 reactor and modifier. The features are as follows:

  • (1)

    The autothermic oxygen-fuel heating method for calcium carbide production, which is proposed by Liu et al. is adopted in this study [42], [43]. It could avoid 60% of the energy loss in coal-fired power generation [44]. It is based on the reaction of fine coke and CaO. Oxygen and coke react to form CO-rich gas and supply the energy demanded in the production of CaC2, as seen in reactions (8), (9).

It proves that fine particles are beneficial to reducing temperature, shorten reaction time, and increase thermal efficiency for the oxygen-fuel heating method. Cycled CaO-based sorbent, the solid waste, whose particles are seriously in attrition, shows fine particles exactly. These are not suitable for conventional calcium looping gasification system [45]. The cycled CaO-based sorbent is proposed as a raw material for calcium carbide production in this paper. So, the system can recycle the “disposed” CaO-based sorbent.

  • (2)

    Currently, most of the carbide slag have not been reused and is usually land-filled, leading to land occupation and other environmental pollutions. Carbon dioxide capture using carbide slag in the calcium looping process was proposed by Li et al. [46], [47], [48], [49], [50], [51]. Their investigation revealed that the modified carbide slag exhibited a higher CO2 capture capacity than that of untreated CaO. The carbide slag, as the industrial waste, can be reused as a CaO-based sorbent for SEHP after modification. The system we propose achieves the waste carbide slag recycled.

  • (3)

    A modification process suggests that parts of ash component such as SiO2, Al2O3, and Fe2O3 should be freed from the carbide slag to minimize ash content. Many types of research have been performed to synthesize precipitated calcium carbonate (PCC) from calcium extracted from blast furnace (BF) slag and it was possible to synthesize chemically pure PCC (>98% Ca) through the waste carbide slag [52]. The fresh CaCO3 is also added into the regenerator together with modified carbide slag to dilute the ash content in the system. A relatively low content of ash (no more than 5%) such as Al2O3, SiO2, and MgO can be neglected after the modification process and calcium carbonate dilution. The modified carbide slag is then fed into the regenerator as CaO-based sorbent together with fresh CaCO3.

  • (4)

    The CO-rich gas from the furnace at high temperatures is utilized in calcium looping coal gasification system and divided into two parts. One is fed into the gasifier to enhance hydrogen production, the other is sent to the regenerator for combustion. All the CO2 generated through the whole process can be separated in the regenerator block, as seen in the reactions (5), (6), (10). In addition, the heat of the off gas is mostly used in the calcium looping coal gasification process.

  • (5)

    Cycled CaO-based sorbent is fed into the furnace after calcining in the regenerator at the temperature of 900 °C. It can be concluded that most of CaCO3/Ca(OH)2 has been converted into CaO before feeding into the furnace. Thus, the concentration of CO-rich gas generated from the furnace is higher compared to the fresh CaCO3 as raw material fed into the furnace.

In this study, the effects of (1) steam/CO/CaO ratio on H2 yield and concentration; (2) riser temperature on H2 yield and concentration; (3) CaO/coke/O2 ratio on CaC2 yield and mass fraction; (4) CaO/coke/O2 ratio on CO yield and concentration; (5) different coals; (6) COS reduction under different conditions are investigated.

Section snippets

Process configuration

The proposed system for hydrogen, carbon monoxide and acetylene poly-generation based on calcium looping gasification and oxygen-thermal calcium carbide preparation system is shown in Fig. 3. The calcium looping gasification system adopted was proposed by Chen et al. [53]. It consists of two main sections: the gasifier with an extra riser and the regenerator. Details such as operation conditions and exergy analysis of the system were shown in our previous studies [53], [54]. In this system,

Effects of steam/CO/CaO ratio on H2 yield and concentration

Fig. 4(a1), (a2), and (a3) show the effects of steam/CO/CaO ratio on hydrogen concentration under different CaO cycle rate. Fig. 4(b1), (b2), and (b3) present the H2 production distribution. The molar flow rate of steam fed into the gasifier varies from 50 to 180 mol/s. The molar flow rate of the CO-rich gas varies from 10 to 50 mol/s. CO-rich gas is added into the gasifier for hydrogen production. An endothermic water gas shift reaction occurs in the gasifier which absorbs a proportion of heat

Conclusion

A hydrogen, carbon monoxide, and acetylene poly-generation system have been proposed in this paper. The suitable operation region of the system is identified according to the heat balance. The primary influential factors, such as steam/CO/CaO ratio, riser temperature, coke/CaO/O2 ratio, and coal species, are investigated. Among all the coal used, Zhundong coal is more promising for this poly-generation process due to its low ash content.

A typical condition is adopted at a carbon conversion rate

Acknowledgment

The authors gratefully acknowledge the National Natural Science Foundation of China (51576042, 50976116).

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