Simulations and process analysis of the carbonation–calcination reaction process with intermediate hydration

Several technologies are currently being developed to separate carbon dioxide from large point sources, such as coal-fired power plants. An emerging technology that shows great potential is a calcium oxide–calcium carbonate cycle. A major drawback is the calcium carbonate decreases in reactivity over multiple cycles. The Ohio State University demonstrated in 2008 the first carbonation–calcination reaction (CCR) process that includes intermediate hydration for sorbent regeneration and its feasibility over multiple cycles at the 120 kW_th scale with actual flue gas from coal combustion. The CCR Process utilizes a calcium-based sorbent to react with the carbon dioxide and sulfur dioxide in a flue gas stream to form calcium carbonate and calcium sulfate, respectively. The carbon dioxide is subsequently released from the calcium carbonate to produce a high-purity, sequestration-ready carbon dioxide stream while regenerating the calcium oxide sorbent. The sulfur dioxide is fixated as calcium sulfate and removed through a purge stream. An intermediate hydration step restores reactivity to the calcium oxide sorbent. Process analysis from computer simulations shows the CCR Process to be highly effective and efficient in removing both carbon dioxide and sulfur dioxide at low energy penalties under realistic conditions. A 20–22% decrease in electricity generation efficiency with the CCR Process is expected, compared with amine scrubbing around 27% and oxy-combustion around 25% energy penalty. A 25–28% increase in thermal energy with the CCR Process is expected to maintain a constant electrical output. Further, the CCR Process consumes half the oxygen necessary for an oxy-combustion plant and 25% less steam necessary for amine scrubbing.     

Novel sorbents for mercury emissions control from coal-fired power plants

The authors have successfully developed novel efficient and cost-effective sorbents for mercury removal from coal combustion flue gases. These sorbents were evaluated in a fixed-bed system with a typical PRB subbituminous/lignite simulated flue gas, and in an entrained-flow system with air simulating in-flight mercury capture by sorbent injection in the ductwork of coal-fired utility plants. In both systems, one of the novel sorbents showed promising results for Hg⁰ removal. In particular, this sorbent demonstrated slightly higher efficiencies in Hg⁰ removal than Darco Hg-LH (commercially available brominated activated carbon) at the similar injection rates in the entrained-flow system. The other novel sorbent showed excellent Hg⁰ oxidation capability, and may enable coal-fired power plants equipped with wet scrubbers to simultaneously control their mercury and sulfur oxides emissions. In addition, fixed-bed results for this sorbent showed that co-injection of a very small amount (∼10%) of raw activated carbon could eliminate almost all of the mercury generated by reactions of Hg⁰ with the sorbent.     

Comparative assessment of coal fired IGCC systems with CO2 capture using physical absorption,membrane reactors and chemical looping

The integrated gasification combined cycle (IGCC) as an efficient power generation technology with lowest specific carbon dioxide emissions among coal power plants is a very good candidate for CO₂ capture resulting in low energy penalties and minimised CO₂ avoidance costs. In this paper, the techno-economic characteristics of four different capture technologies, which are built upon a conventional reference case, are studied using the chemical process simulation package “ECLIPSE”. The technology options considered are: physical absorption, water gas shift reactor membranes and two chemical looping combustion cycles (CLC), which employ single and double stage reactors. The latter system was devised to achieve a more balanced distribution of temperatures across the reactors and to counteract hot spots which lead to the agglomeration and the sintering of oxygen carriers. Despite the lowest efficiency loss among the studied systems, the economic performance of the double stage CLC was outperformed by systems employing physical absorption and water gas shift reactor membranes. Slightly higher efficiencies and lower costs were associated with systems with integrated air separation units. The estimation of the overall capital costs was carried out using a bottom-up approach. Finally, the CO₂ avoidance costs of individual technologies were calculated based on the techno-economic data.     

膜分离捕集燃煤电厂烟气CO2过程优化设计

全球CO₂的排放量不断升高,导致气候问题频发。“双碳”目标下,如何高效、低成本地捕集燃煤电厂烟气CO₂已经成为迫在眉睫的问题。传统的化学吸收法由于能耗高、成本高、溶剂易挥发等问题严重制约了其发展,而膜法碳捕集因为其操作简单、能耗低、环境污染小等优势被认为是最有前景的捕集方式。本文以PI中空纤维膜为分离膜,建立和求解了气体分离膜模型。并以燃煤电厂烟气CO₂为捕集目标,利用多岛遗传算法求解了膜分离捕集CO₂工艺的不同配置,并优化了分离过程中的关键参数(膜面积、操作压力)。结果显示：在二级膜分离工艺中,二级一段膜分离工艺的第一级膜和第二级膜操作压力分别为5.8 bar和7.1 bar,第一级膜和第二级膜的面积分别为448000 m²和180000 m²时,单位捕集成本为27.36 USD/t CO₂。与二级二段膜分离以及其他几种传统的CO₂捕集方法(MEA法、相变吸收法)相比,二级一段膜分离捕集CO₂的捕集成本和能耗均最小。本研究将为CO₂捕集实现低能耗和低成本化提供依据。     

Modeling and analysis of circulation variables of continuous sorbent loop cycling for CO<Subscript>2</Subscript> capture

Carbon capture and storage (CCS) technologies are a cornerstone for reducing CO₂ emissions from energy and energy-intensive industries. Among the various CCS technologies, solid sorbent looping systems are considered to be potentially promising solutions for reducing CO₂ capture energy penalty. We present an evaluation module for a carbonator with sorbent looping cycle to calculate the carbonation efficiency. The module incorporates a simple sorbent activity model, and the solid/gas balances are constructed by assuming simple reactor mixing quality. By conducting simulations, we examine the variation in the carbonation efficiencies as a function of the sorbent looping operation factors and discuss an optimum operating strategy.     

Recent progress of amine modified sorbents for capturing CO2 from flue gas

Under the Paris agreement, China has committed to reducing CO₂ emissions by 60%–65% per unit of GDP by 2030. Since CO₂ emissions from coal-fired power plants currently account for over 30% of the total carbon emissions in China, it will be necessary to mitigate at least some of these emissions to achieve this goal. Studies by the International Energy Agency (IEA) indicate CCS technology has the potential to contribute 14% of global emission reductions, followed by 40% of higher energy efficiency and 35% of renewable energy, which is considered as the most promising technology to significantly reduce carbon emissions for current coal-fired power plants. Moreover, the announcement of a Chinese national carbon trading market in late 2017 signals an opportunity for the commercial deployment of CO₂ capture technologies.Currently, the only commercially demonstrated technology for post-combustion CO₂ capture technology from power plants is solvent-based absorption. While commercially viable, the costs of deploying this technology are high. This has motivated efforts to develop more affordable alternatives, including advanced solvents, membranes, and sorbent capture systems. Of these approaches, advanced solvents have received the most attention in terms of research and demonstration. In contrast, sorbent capture technology has less attention, despite its potential for much lower energy consumption due to the absence of water in the sorbent. This paper reviews recent progress in the development of sorbent materials modified by amine functionalities with an emphasis on material characterization methods and the effects of operating conditions on performance. The main problems and challenges that need to be overcome to improve the competitiveness of sorbent-based capture technologies are discussed.     

煤化工生产和消费过程的碳利用分析

分析了我国煤化工主要生产路线的碳流向和碳利用情况,计算了煤化工生产过程的碳排放。通过分析计算可知,原煤中碳的1/5~1/3进入产品;按转化单位煤炭计,煤化工碳排放强度2.1 t/tce^2.5 t/tce,比燃煤发电低19%~32%;按生产单位热值能源产品计,煤制油气路线碳排放强度比燃煤发电有所降低;煤制燃料和肥料在使用时将碳释放,不再具有留碳功能;煤制化学品的碳可以多次利用,具有更强的留碳能力。以煤制化学品计,2018年我国煤化工行业节碳能力约1.15亿t,实际节碳量约9700万t。     

Syngas combustion in a chemical-looping combustion system using an impregnated Ni-based oxygen carrier

Chemical-looping combustion (CLC) has emerged as a promising option for CO₂ capture because this gas is inherently separated from the other flue gas components and thus no energy is expended for the separation. This technology would have some advantages if it could be adapted for its use with coal as fuel. In this sense, a process integrated by coal gasification and CLC could be used in power plants with low energy penalty for CO₂ capture. This work presents the results obtained in the combustion of syngas as fuel with a Ni-based oxygen carrier prepared by impregnation in a CLC plant under continuous operation. The effect on the oxygen carrier behaviour and the combustion efficiency of several operating conditions was determined in the continuous CLC plant. High combustion efficiencies (～99%), close to the values limited by thermodynamics, were reached at oxygen carrier-to-fuel ratios higher than 5. The temperature in the FR had a significant influence, although high efficiencies were obtained even at 1073 K. The syngas composition had small effect on the combustion, obtaining high and similar efficiencies with syngas fuels of different composition, even in the presence of high CO concentrations. The low reactivity of the oxygen carrier with CO seemed to indicate that the water gas shift reaction acts as an intermediate step in the global reaction of the syngas in a continuous CLC plant. Neither agglomeration nor carbon deposition problems were detected during 50 h of continuous operation in the prototype. The obtained results showed that the impregnated Ni-based oxygen carrier could be used in a CLC plant for the combustion of syngas produced in an integrated gasification combined cycle (IGCC).     

一种煤基多联产碳循环系统的设计及评价

将高密度三塔式循环流化床(TBCFB)应用于串并联综合型多联产系统,提出一种基于碳循环的流程与参数共优化的煤基多联产系统,促进低阶煤资源的高质高效转化。碳循环体现在两方面,一是系统以热解煤气循环作为热解气氛,提高了焦油产率,实现低阶煤高质化转化;二是在TBCFB使用富氧燃烧,提高了烟气中二氧化碳浓度,将烟气替代氮气直接用于燃气轮机发电工质,减少了氮气消耗。利用Aspen Plus对全系统进行模拟,对多联产系统进行物料、能量和?衡算,研究未反应合成气循环比和烟气注入量对过程的影响;以能量利用效率为优化目标,对煤基多联产碳循环系统的操作条件寻优。结果表明,动力单元注入气体使用烟气时,煤基多联产碳循环系统的能量利用效率达49.7%,高于用氮气作为热解气氛的传统煤基多联产系统,相比传统的单产系统,煤基多联产系统的能量可节约13%,对于年处理30万吨煤的系统,折合减少二氧化碳排放量为14.9万吨/年。     

氧化钙-二氧化碳循环系统脱碳的分析研究

采用燃料氧燃烧直接供热的氧化钙-二氧化碳双床循环系统已经被研究开发,用于从电厂尾气中脱碳。本研究基于实验和化学反应工程原理建立了分析模型,用于系统地研究循环特征,氧化钙活性衰减,燃料及其炉内热利用率的影响。基于模型推导获得了重要参数:最小循环热损失和最小热需求量,以及对应的固体循环比。它们都受供热燃料灰分和含硫量的影响,也受脱碳率的影响。显然最佳固体循环比介于二者之间。另一个重要参数是燃料在炉内的热利用率。高的热利用率不仅降低燃料需求量,降低其灰分和含硫的影响,降低氧的需求量及其辅助功,而且提高蒸汽循环的发电效率。一个发现是热需求量在临界固体循环比接近无穷大,这就限制了固体循环比的可操作范围, 以及燃料的灰分和含硫量。建立的分析模型和推导直接提供了这些变量之间的关系和范围。     

A technique to control mercury from flue gas: The Thief Process

The Thief Process is a mercury removal process that may be applicable to a broad range of pulverized coal-fired combustion systems. This is one of several sorbent injection technologies under development by the U.S. Department of Energy for capturing mercury from coal-fired electric utility boilers. A unique feature of the Thief Process involves the production of a thermally activated sorbent in situ at the power plant. The sorbent is obtained by inserting a lance, or thief, into the combustor, in or near the flame, and extracting a mixture of partially combusted coal and gas. The partially combusted coal or sorbent has adsorptive properties suitable for the removal of vapor-phase mercury at flue gas temperatures that are typical downstream of a power plant preheater. One proposed scenario, similar to activated carbon injection (ACI), involves injecting the extracted sorbent into the downstream ductwork between the air preheater and the particulate collection device of the power plant. Initial laboratory-scale and pilot-scale testing, using an eastern bituminous coal, focused on the concept validation. Subsequent pilot-scale testing, using a Powder River Basin (PRB) coal, focused on the process development and optimization. The results of the experimental studies, as well as an independent experimental assessment, are detailed. In addition, the results of a preliminary economic analysis that documents the costs and the potential economic advantages of the Thief Process for mercury control are discussed.     

Process design and energy requirements for the capture of carbon dioxide from air

A process to capture carbon dioxide from air to reduce its atmospheric concentration and to mitigate climate change is studied. It is based on the absorption of carbon dioxide in a sodium hydroxide solution, its precipitation as calcium carbonate, and its release as pure gas stream through oxy-fuel calcination. The process utilizes existing commercial technologies wherever possible, particularly in the case of the absorber, whose design is carried out in detail. The analysis allows deriving material and energy balances for the whole process and determining energy demands that can be used for a technical, economical, and environmental feasibility evaluation of the technology. In particular, it indicates that the real specific energy demand is larger than the heat released to emit the same amount of CO₂ by the combustion of coal, and smaller than that of methane.     

燃煤sCO2布雷顿循环及其工质传热特性研究进展

超临界二氧化碳(supercritical carbon dioxide,sCO2)布雷顿循环作为动力循环的主要优势是效率高、结构简单、系统紧凑、热源适应性广,有望在下一代核反应堆、燃煤电站、余热回收及可再生能源(太阳能、地热能等)领域得到大规模应用。作为新型动力循环工质的sCO2具有温和的临界点条件(31.1℃/7.38 MPa),同时在临界点附近物性变化剧烈。鉴于我国以煤为主的能源结构及严峻气候挑战,sCO2动力循环与富氧燃烧、流化床锅炉、煤气化等技术结合为实现煤炭的清洁高效低碳利用提供了新的思路。笔者分析了sCO2工质的性质,介绍了间接加热式和直接加热式两类sCO2布雷顿循环的基本原理,总结了sCO2动力循环应用于燃煤电站的研究进展。sCO2循环燃煤电站的发展可分为以下2条路径:①间接加热式sCO2循环取代蒸汽朗肯循环应用于燃煤电站,可与煤粉锅炉、循环流化床锅炉、富氧燃烧等技术相结合;②发展更加高效且固有碳捕捉能力的直接加热式sCO2循环燃煤电站技术,与带有碳捕捉(carbon capture and storage,CCS)的整体煤气化联合循环(IGCC)电站竞争。分析了sCO2动力循环与燃煤电站结合的多种技术方案,讨论不同方案的优势、技术挑战与发展方向。在此基础上,重点阐述了sCO2作为工质在常规管径圆管、细管道圆管、微细管道圆管及印刷电路板式换热器(printed circuit heat exchanger,PCHE)中的传热试验研究和传热特性,总结了sCO2工质在圆管内和PCHE内流动传热经验关联式并进行分析比较,同时介绍了sCO2工质流动传热的数值模拟研究。最后,从基础理论、系统设计、设备研发层面指出了现有研究的不足和对未来研究的展望。CO2减排在未来几十年将是燃煤发电的主要研究方向,具有更大效率优势和固有碳捕捉能力的直接加热式sCO2循环燃煤发电技术将引起更多关注。在我国将sCO2布雷顿循环应用于燃煤电站更具现实意义,目前我国关于sCO2循环发电技术的研究与国外仍存在相当差距,应依托超超临界燃煤发电机组和IGCC电站的技术积累,快速推动燃煤sCO2循环发电技术的研发进展。     

Computational modeling of furnace sorbent injection for SO2 removal from coal-fired utility boilers

Furnace sorbent injection (FSI) is used to remove SO₂ formed during coal combustion by injecting sorbent into the high temperature zone of a furnace above the fireball. FSI is cost effective for older coal-fired boilers, especially when space or capital budgets are limited. To optimize the design and performance of FSI, an SO₂/sorbent modeling scheme that simultaneously considers calcination (or dehydration), sintering, and sulfation has been developed and implemented. It is coupled with a three-dimensional combustion model based on computational fluid dynamics to determine the most desirable locations for sorbent injection and to optimize the amount of sorbent needed to achieve a targeted SO₂ removal efficiency. A sensitivity analysis was conducted to determine the effect of flue gas temperature, particle diameter, and SO₂ concentration on the extent of sulfation. This SO₂/sorbent sub-model was applied to a 126-MW front-wall fired boiler firing eastern bituminous coal. The SO₂ removal efficiencies predicted by the model agreed well with those measured in the field. The modeling results indicated that sorbent injected directly into the furnace through boosted over-fired air ports is more effective at removing SO₂, due to longer residence time and better mixing, relative to ports higher in the furnace with poor mixing. This modeling approach is optimized for full-furnace application to facilitate the design process.     

A technical and environmental analysis of co-combustion of coal and biomass in fluidised bed technologies

The use of biomass, which is considered to produce no net CO₂ emissions in its life cycle, can reduce the effective CO₂ emissions of a coal-fired power generation system, when co-fired with the coal, but may also reduce system efficiency.The technical and environmental analysis of fluidised bed technologies, using the ECLIPSE suite of process simulation software, is the subject of this study. System efficiencies for generating electricity are evaluated and compared for the different technologies and system scales.Several technologies could be applied to the co-combustion of biomass or waste and coal. The assessment studies here examine the potential for co-combustion of (a) a 600 MWe pulverised fuel (PF) power plant (as a reference system), (i) co-firing coal with straw and sewage sludge and (ii) using straw derived fuel gas as return fuel; (b) a 350 MWe pressurised fluidised bed combustion (PFBC) system co-firing coal with sewage sludge; (c) 250 MWe and 125 MWe circulating fluidised bed combustion (CFBC) plants co-firing coal with straw and sewage sludge; (d) 25 MWe CFBC systems co-firing low and high sulphur content coal with straw, wood and woody matter pressed from olive stones (WPOS); (e) 12 MWe CFBC co-firing low and high sulphur content coal with straw or wood; and (f) 12 MWe bubbling fluidised bed combustion (BFBC), also co-firing low and high sulphur content coal with straw or wood.In the large systems the use of both straw and sewage sludge resulted in a small reduction in efficiency (compared with systems using only coal as fuel).In the small-scale systems the high moisture content of the wood chips chosen caused a significant efficiency reduction.Net CO₂ emissions are reduced when biomass is used, and these are compared for the different types and scales of fluidised bed technologies. NO_x emissions were affected by a number of factors, such as bed temperature, amount of sorbent used for SO₂ capture and HCl emitted.     

Mechanisms governing the fate of mercury in coal-fired power systems

This paper summarizes the results of a significant research and development investment to understand the fundamental mechanisms governing the speciation of mercury in coal-fired power systems. An extensive experimental search was conducted through all the parameter variations characterizing the majority of full-scale U.S. coal-fired power plants, in an effort to quantify the impact each parameter variation had on mercury oxidation and removal from the gas phase. As a result of this extensive investigation, the mechanisms responsible for mercury speciation differences observed for different coal types, power plants, and pollution control devices were elucidated. Specifically, unburned carbon was found to catalytically enhance mercury oxidation, while the actual differences in chlorine concentration from plant to plant were found to be of secondary importance to carbon. In addition, synergistic enhancement of mercury capture by carbon and calcium in flyash was quantitatively described for various coal-fired configurations, such as coal blending and sorbent injection, including development of contour plots of mercury removal at an ESP inlet and exit as a function of carbon and calcium concentration at the ESP inlet.     

Natural Calcium‐Based Sorbents Doped with Sea Salt for Cyclic CO2 Capture

The calcium‐looping process for post‐combustion carbon dioxide capture, an economically and technically feasible method suitable for large‐scale use, has recently gained much attention. However, the capture capacity of calcium‐based sorbents rapidly decreases after only a few cycles. Herein, calcium‐based sorbents with enhanced cyclic CO₂‐capture capacity have been derived from cheap, natural raw materials by using a simple impregnation method. Limestone and shells were used as the calcium‐based raw materials, with sea salt as dopant. Modified limestone had the highest CO₂‐capture capacity after multiple carbonation‐calcination cycles. Sea‐salt‐doped sorbent showed a relatively stable porous surface during cycles, which resulted in a higher CO₂‐capture capacity.     

Mathematical modeling of a moving bed reactor for post‐combustion CO2 capture

A mathematical model for a moving bed reactor with embedded heat exchanger has been developed for application to solid sorbent‐based capture of carbon dioxide from flue gas emitted by coal‐fired power plants. The reactor model is one‐dimensional, non‐isothermal, and pressure‐driven. The two‐phase (gas and solids) model includes rigorous kinetics and heat and mass transfer between the two phases. Flow characteristics of the gas and solids in the moving bed are obtained by analogy with correlations for fixed and fluidized bed systems. From the steady‐state perspective, this work presents the impact of key design variables that can be used for optimization. From the dynamic perspective, the article shows transient profiles of key outputs that should be taken into account while designing an effective control system. In addition, the article also presents performance of a model predictive controller for the moving bed regenerator under process constraints. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3899–3914, 2016     

Carbon capture from stationary power generation sources: A review of the current status of the technologies

The world will need greatly increased energy supply in the future for sustained economic growth, but the related CO₂ emissions and the resulting climate changes are becoming major concerns. CO₂ is one of the most important greenhouse gases that is said to be responsible for approximately 60% of the global warming. Along with improvement of energy efficiency and increased use of renewable energy sources, carbon capture and sequestration (CCS) is expected to play a major role in curbing the greenhouse gas emissions on a global scale. This article reviews the various options and technologies for CO₂ capture, specifically for stationary power generation sources. Many options exist for carbon dioxide capture from such sources, which vary with power plant types, and include post-combustion capture, pre-combustion capture, oxy fuel combustion capture, and chemical looping combustion capture. Various carbon dioxide separation technologies can be utilized with these options, such as chemical absorption, physical absorption, adsorption, and membrane separation. Most of these capture technologies are still at early stages of development. Recent progress and remaining challenges for the various CO₂ capture options and technologies are reviewed in terms of capacity, selectivity, stability, energy requirements, etc. Hybrid and modified systems hold huge future potentials, but significant progress is required in materials synthesis and stability, and implementations of these systems on demonstration plants are needed. Improvements and progress made through applications of process systems engineering concepts and tools are highlighted and current gaps in the knowledge are also mentioned. Finally, some recommendations are made for future research directions.     

STAGED CASCADED FLUIDIZED BED COMBUSTOR (SCFBC) EVALUATION

The SCFBC is basically a countercurrent contacting unit of downward moving solids with the hot combustion products moving up the stages. Each contacting stage is a shallow fluidized bed providing intimate contact between gas and solids. The design of SCFBC's requires rigorous determination of temperatures, stream flow rates, and heat transfer rates at each stage. The determination is made by solving material balance, energy balance, and appropriate rate equations for each stage.

In this work a technical and economic evaluation of the SCFBC is presented. Analysis of an SCFBC producing 75,000 Ib/hr steam at 200psig is presented with respect to its thermal efficiency, carbon combustion efficiency, sulfur capture efficiency, and sorbent utilization. Effects of the Ca/S ratio on the percent sulfur retention, and carbon burnout efficiency, and effects of stack gas temperature on plant thermal efficiency, total capital costs, and unit cost of steam, and effects of boiler size on the total capital cost and on the unit cost of steam are presented. Effects of variations in cost of operating parameters, e.g., water, sorbent, fuel, etc., on the steam cost are discussed.