The growing atmospheric carbon dioxide (CO₂) concentration exceeded 415 ppm in 2019 from 280 ppm before the industrial revolution because of anthropogenic activities and has become one of the most pressing issues associated with climate change.(1) To meet the Paris Agreement objectives to keep global warming below 2 °C, carbon capture and utilization (CCU) and/or reduction of CO₂ emission of at least 30 gigatons of CO₂ (Gt_CO2)/yr is needed.(2−4) To achieve a meaningful impact on both the economy and the environment, carbon dioxide utilization (CDU) must be conducted for profitable industrial applications in addition to net zero CO₂ emissions.(5,6) The importance of CDU in carbon management has long been recognized, and CO₂ is projected to play an essential role as the future C1 raw material in the post fossil fuels era.(7) However, at the present time, even if the carbon footprint of the energy consumption for CDU, with carbon in the most thermodynamically stable oxidation state of +4, is not considered,(8) the scale of its impact is limited. The current CO₂ market potential for industrial use can reach merely 0.3–0.8 Gt_CO2/yr in the best case scenario,(9,10) with the largest consumer of exhaust CO₂ emission being the fertilizer industry (∼0.12–0.14 Gt_CO2/yr used for urea manufacturing) followed by the oil sector (∼0.07–0.08 Gt_CO2/yr consumed for enhanced oil and/or gas recovery).(11) Alden and co-workers have also recently predicted CDU potential for CO₂ into chemicals to be around 0.3–0.6 Gt_CO2/yr in 2050, and rationalized that CO₂ to fuel has the highest estimated potential for CDU in the range of 1–4.2 Gt_CO2/yr.(6) In this context, dry reforming of methane (DRM, eq 1) has been proposed and marketed as a potential solution that may achieve decarbonization at the scale of multi-Gt_CO2/yr.(12,13) Such an argument has led to a significant increase in R&D activities on DRM in the past two decades,(14−17) as reflected in the increasing number of recent publications. Analyses by Scopus and Web of Science databases indicate that approximately 50% of the total publications in the field were reported in the last five years (Figure 1).(18,19) Unfortunately, claiming DRM to fuels to meet the goal of CO₂ emission relief is likely misleading and inappropriate, based on the impact of DRM integrated with fuel production evaluated according to the data acquired from various reforming processes in industry and literature reports. Because methane (CH₄) is the primary energy source and CO₂ only serves as a carrier, fuels from DRM will create the same amount of net CO₂ emission per energy unit, similar to those of CH₄ combustion and steam methane reforming (SMR, eq 2).(1)(2) Figure 1. Annual publications on DRM by Scopus and Web of Science. Search word: dry reforming of methane (accessed 2020-08-01). DRM was first developed for the utilization of CO₂ contained in methane fields and biogas because of the benefits in converting waste for chemicals without the need of CO₂ separation and purification.(20,21) In DRM, greenhouse gases, CH₄ and CO₂ in a 1:1 ratio, react to afford an equimolar mixture of CO and H₂, known as synthesis gas (syngas). This process facilitates access to CO-rich syngas that offers potential for certain downstream processes. Although the obtained CO-rich syngas (H₂/CO = 1:1) has a limited number of direct applications, such as acetic acid synthesis (eq 3), it is more valuable when integrated with SMR for producing syngas suitable for Fischer–Tropsch (FT) synthesis (H₂/CO = 1.7–2.4). Additionally, it can be used to synthesize syngas with high CO content (H₂/CO = 0.4) by encouraging reverse-water gas shift (RWGS, eq 4) reaction at high temperatures, and the obtained CO can be exported as a valuable feedstock.(3)(4)Use of syngas for direct dimethyl ether (DME) synthesis invented around the early 1980s has renewed the interest in this field as a possible CDU pathway, because DME was identified as an alternative transportation fuel for diesel engines.(22−24) With potentially low capital investments, this process is envisaged as an efficient approach for DME synthesis over indirect synthesis through methanol dehydration (eq 5).(25) However, practical applications of DRM are limited because of the high carbon deposition propensity of its synthesis mixtures (low O/C and H/C ratios) which leads to severe coking and rapid catalyst deactivation.(26,27) Numerous efforts have been made to overcome these limitations and to develop DRM as a valuable technology.(15) Although some DRM pilot plants were established, they are still in their infancy and additionally employ steam/H₂ to avoid coke formation and improve H₂ content.(28,29)(5)Among various CH₄ reforming processes, SMR and DRM are both significantly endothermic (Table 1 and Figure 2). While the high CO content in DRM-syngas gives a higher overall calorific value (1050 kJ/mol_CH4) (lower heating value (LHV) of CO = 283 kJ/mol and H₂ = 242 kJ/mol) compared to that of SMR-syngas (1009 kJ/mol_CH4) with a higher H₂ content, DRM requires more energy input. As a result, the thermal efficiencies of DRM and SMR processes are similar. SMR consumes ∼295 kJ/mol_CH4(30−32) and thus emits 0.33 mol of CO₂ equivalent per mole of CH₄ (CO₂e/mol_CH4) (1 GJ = 50.5 kg CO₂e as per US EIA)(33−36) as the thermal energy is mainly provided by external CH₄ combustion. On the other hand, while DRM may appear to be a simpler process than SMR, as it does not require the use of steam, its energy demand (ΔH_298K = 247 kJ/mol_CH4) is higher because of the exceptional thermodynamic stability (standard enthalpy of formation, ΔH_f° = −394 kJ/mol) and chemically inert nature of CO₂.(32−37) Hence, DRM is operated at high temperatures in order to overcome the high activation barriers and needs at least 340 kJ/mol_CH4, resulting in 0.38 CO₂e/mol_CH4 (Table 1). Thus, DRM consumes 1.38 mol of CH₄ for the utilization of 0.62 mol of CO₂ in the best case scenario. While DRM can be used for FT synthesis, it requires additional H₂ for which it should be integrated with other reforming processes.(38) In contrast, DME synthesis is more straightforward for the application of DRM to fuels (eq 5). Although DRM utilizes 1 mol of CO₂/mol_CH4 in the primary reforming process, it re-emits 0.67 mol of CO₂/mol_CH4 during the production of DME. Therefore, considering the energy-intensive nature and low CDU potential of DRM to DME pathway, the overall transformation of natural gas to the final products will be analogous to those of the modern CH₄ reforming technologies.(39,40) Figure 2. CH₄ reforming pathways (a) and energy changes (b) to produce syngas/mol_CH4. Lower heating value (LHV) of CO = 283 kJ/mol, H₂ = 242 kJ/mol, CH₄ = 803 kJ/mol. Total heat required (energy input). E_Q-total – Q (CH₄). 1 kJ = 0.05 g CO₂e. Environmental concerns due to the continual acquisition of emitted CO₂ from ground transportation have prompted the use of alternative and renewable fuels other than conventional gasoline and/or diesel. In addition to improving vehicle efficiency, the development and use of alternative fuels with proper choices and infrastructure is of utmost importance. Thus, well to wheel (WTW) efficiencies of various fuel-processing technologies have to be considered in order to evaluate their impacts on CO₂ emission relief (Table 2 and Figure 3). For the direct use of CH₄ in a compressed natural gas vehicle (CNGV), the transportation and distribution of fuels are highly efficient (98%). However, because of the low tank to wheel (TTW) efficiency from the CH₄ internal combustion engine (16–20%),(41) the WTW efficiency is only 14.4–18.0%, indicative of an overall CO₂ emission of 312–390 g_CO2/mile.(41) Although the theoretical TTW efficiency for DME in the diesel engine is much higher than that of CNGV (25–30%),(25) the energy efficiency in the DRM to syngas to DME processes can reach only 59–64%.(39,40) As a result, the WTW efficiency (13.4–17.5%) and CO₂ emissions (321–419 g_CO2/mile) of a DME-DRM-based vehicle are approximately the same as those of CNGV, in addition to the conceivable higher capital and operating investments compared to the conventional CNGV process. While DRM to DME was proposed to save 30% CO₂ emissions over conventional SMR by incorporating CO₂ into DME during synthesis,(28) its utilization as fuel offers no significant advantage in terms of CO₂ mitigation. Although the exploitation of renewable energy has also been proposed to make DRM CO₂ neutral,(42,43) such energy can be more efficiently utilized by fuel cell electric vehicles (FCEV) and battery electric vehicles (BEV) with zero tailpipe emissions.(41,44) Drilling, processing, and distribution. Electricity transport and charging. H₂ compression and fueling. No tailpipe emissions. Battery manufacturing emissions not considered. Figure 3. Well to wheel efficiency and CO₂ emissions of vehicles using CNG and DME from DRM. In addition to the existing challenges on the lab-scale conditions, e.g., (i) low gas hour space velocity (GHSV; 10–100 L g_cat^–1h^–1), (ii) low pressures (∼1 bar), (iii) dilute feed (reactants/inert gas ≥4), etc.,(15,27,34,45) DRM catalysts are sensitive to feedstock impurities, and a CO₂ capture and purification system is essential to avoid catalyst poisoning. The capture and purification of imported or recovered CO₂ from industrial sources require additional 2.1–5.7 MJ/kg_CO2 based on the technology employed.(38,46) More importantly, the current stats are based on the lab-scale conditions under dilute conditions. With the scale-up, these limitations can play a significant role, and the amount of CO₂ sequestered may decrease further. Theoretical and experimental studies specify that CH₄ and CO₂ conversions are sensitive to change in temperature and pressure.(38) Any significant changes in the pressures can lead to coking (whisker formation), catalyst fractionation, and pressure drop.(27) Industrially, these limitations were realized, and therefore, other technologies, such as partial oxidation (POX), steam-CO₂ reforming (SCR), autothermal reforming (ATR), and trireforming, have been explored in place of DRM for producing CO-rich syngas.(27,44) In summary, from a practical standpoint, even if these arduous issues of DRM are tackled and resolved and the energy penalty in this endothermic process is not considered, CDU to fuels at the scale of multi-Gt_CO2/yr by the DRM approach will, at its best, make negligible contributions to CO₂ emission relief. Broad deployment of DRM for such a purpose will be a politically, financially, and energetically costly distraction. DRM remains a valuable reaction that may serve the business need for value-added manufacturing, but the goal of CO₂ emission relief has to be realized with low-carbon energy.(47) Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. Professor Yu Han is acknowledged for insightful review and discussions. We are grateful for financial support from King Abdullah University of Science and Technology (KAUST). enthalpy of reaction at 298 K standard enthalpy of formation acetic acid methane carbon monoxide carbon dioxide CO₂ equivalent dimethyl ether total energy input energy used in reforming gram of CO₂ emissions per mile gigajoule gigatons of CO₂ hydrogen kilogram kilojoule liter per gram of catalyst per hour megajoule per mole per mole of methane the lower heating value of syngas year autothermal reforming battery electric vehicles carbon capture and utilization carbon dioxide utilization compressed natural gas vehicle dry reforming of methane fuel cell electric vehicles Fischer–Tropsch gas hour space velocity lower heating value partial oxidation research and development reverse-water gas shift steam-CO₂ reforming steam methane reforming synthesis gas tank to wheel United States Energy Information Administration well to wheel This article references 47 other publications.

中文翻译：

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甲烷干重整在CO ₂排放缓解中的作用不大

由于人为活动，2019年不断增长的大气中二氧化碳（CO ₂）浓度从工业革命前的280 ppm超过415 ppm，已成为与气候变化有关的最紧迫的问题之一。（1）实现《巴黎协定》的目标将全球变暖保持在2°C以下，需要碳捕获和利用（CCU）和/或每年至少减少30千兆吨CO ₂（Gt _{CO 2}）的CO ₂排放量。（2-4）对经济和环境都有影响，除了净零CO ₂之外，还必须对有利可图的工业应用进行二氧化碳利用（CDU）（5,6）人们早已认识到CDU在碳管理中的重要性，并且预计CO ₂作为后化石燃料时代的未来C1原材料将发挥至关重要的作用。（7）然而，目前但是，即使CDU能耗的碳足迹（在最热力学上稳定的氧化态为+4的碳）也没有考虑，（8）其影响的规模是有限的。在最佳情况下，当前工业用途的CO ₂市场潜力只能达到0.3–0.8 Gt _{CO 2} /年，（9,10），最大的废气CO ₂排放消费者是化肥行业（〜0.12-0.14 Gt_{一氧化碳2}/ yr用于制造尿素），其次是石油部门（〜0.07-0.08 Gt _{CO 2} / yr，用于提高油气采收率）。（11）Alden及其同事最近还预测了CDU的CO ₂潜力到2050年，化学物质的排放量大约为0.3–0.6 Gt _{CO 2} /年，并合理化为燃料的CO ₂估计的CDU潜力最高，为1-4.2 Gt _{CO 2/} yr。(6）在这种情况下，已经提出了甲烷的干法重整（DRM，等式1）并作为潜在的解决方案进行销售，该解决方案可以在多Gt _{CO 2}的规模上实现脱碳/yr.(12,13）在最近的二十多年中，这种论点导致过去二十年间，针对DRM的R＆D活动显着增加（14-17）。Scopus和Web of Science数据库的分析表明，过去五年中，该领域的出版物总数约占50％（图1）。（18,19）不幸的是，声称使用DRM燃料可以达到CO ₂的目标根据根据行业和文献报告中各种重整过程获得的数据评估的DRM与燃料生产相结合的影响，排放救济措施可能会产生误导和不适当的影响。因为甲烷（CH ₄）是主要能源，而CO _2是仅作为载体，来自DRM的燃料将产生每单位能量相同量的净CO ₂排放，类似于CH ₄燃烧和蒸汽甲烷重整（SMR，eq 2）。（1）（2）图1. Scopus和Web of Science在DRM上的年度出版物。搜索词：甲烷的干重整（访问2020-08-01）。DRM最初是为CO的利用开发的₂包含在甲烷字段和因为在将废化学品，而不需要CO的益处的沼气₂的分离和纯化。（20,21）在DRM中的温室气体，CH ₄和CO ₂以1：1的比例反应生成等摩尔的CO和H ₂混合物，称为合成气（syngas）。该过程有助于获得富含CO的合成气，这为某些下游过程提供了潜力。尽管所获得的富含CO的合成气（H ₂ / CO = 1：1）的直接应用数量有限，例如乙酸合成（eq 3），但与SMR集成以生产适合于Fischer– Tropsch（FT）合成（H ₂ / CO = 1.7–2.4）。此外，通过促进高温下的逆水煤气变换（RWGS，eq 4）反应，可用于合成高CO含量（H ₂ / CO = 0.4）的合成气，所得CO可作为有价值的原料输出。（3）（4）在1980年代初发明的合成气用于直接合成二甲醚（DME），使人们对该领域的兴趣重新成为可能的CDU途径，因为DME被确定为柴油发动机的替代运输燃料。（22-24）作为资本投资，该方法被认为是DME合成的有效方法，而不是通过甲醇脱水进行的间接合成（eq 5）。（25）但是，由于DRM的合成混合物的碳沉积倾向高（O较低），因此其实际应用受到限制。 / C和H / C比）会导致严重的焦化和快速的催化剂失活。（26,27）为了克服这些限制并开发了DRM作为有价值的技术，已经做出了许多努力。（15）尽管有些DRM中试装置建立，他们还处于婴儿期，还使用了蒸汽/高温₂，以避免焦炭形成和提高ħ ₂含量。（28,29）（5）在各种CH _4种重整工艺，SMR和DRM都是显著吸热（表1和图2）。尽管DRM合成气中的CO含量较高，但其总热值较高（1050 kJ / mol _{CH 4}）（CO的较低热值（LHV）= 283 kJ / mol和H ₂ = 242 kJ / mol）。H ₂含量较高的SMR合成气（1009 kJ / mol _{CH 4}），DRM需要更多的能量输入。结果，DRM和SMR工艺的热效率相似。SMR消耗约295 kJ / mol _{CH 4}（30-32），因此释放出0.33 mol CO每摩尔CH ₄（CO ₂ e / mol _{CH 4}）₂当量（根据美国EIA，1 GJ = 50.5 kg CO ₂ e）（33-36），因为热能主要由外部CH ₄燃烧提供。另一方面，虽然DRM似乎比SMR更简单，但由于不需要蒸汽，因此其能量需求（ΔH _298K = 247 kJ / mol _{CH 4}）更高，因为其出色的热力学稳定性（标准形成焓，ΔH _f °= -394 kJ / mol）和CO _2的化学惰性（32-37）因此，DRM在高温下运行以克服高活化障碍，并且需要至少340 kJ / mol _{CH 4}，导致0.38 CO ₂ e / mol _{CH 4}（表1）。因此，在最佳情况下，DRM消耗1.38摩尔的CH ₄以便利用0.62摩尔的CO ₂。虽然DRM可以用于FT合成，但它需要额外的H ₂，它应与其他重整过程整合。（38）相比之下，DME合成对于将DRM应用于燃料更为简单（等式5）。尽管DRM使用1 mol CO ₂ / mol _{CH 4}在一次重整过程中，在DME生产过程中，它会重新排放0.67 mol CO ₂ / mol _{CH 4}。因此，考虑到DRM转化为DME途径的能源密集型性质和较低的CDU潜力，天然气向最终产品的整体转化将类似于现代CH ₄重整技术。（39,40）图2. CH _4个重整途径（a）和能量变化（b）以产生合成气/ mol _{CH 4}。CO的较低发热量（LHV）＝ 283kJ / mol，H ₂＝ 242kJ / mol，CH ₄＝ 803kJ / mol。所需总热量（能量输入）。Ë _Q-总- Q（CH ₄）。1 kJ = 0.05克CO ₂ e。由于不断从地面运输中获取排放的CO ₂而引起的环境问题促使人们使用除常规汽油和/或柴油之外的替代燃料和可再生燃料。除了提高车辆的使用效率，开发和使用替代燃料的适当选择和基础设施是非常重要的。因此，必须考虑各种燃料处理技术的轮转效率（WTW），以评估其对释放CO ₂的影响（表2和图3）。直接使用CH ₄在压缩天然气汽车（CNGV）中，燃料的运输和分配非常高效（98％）。但是，由于CH ₄内燃发动机的轮对缸（TTW）效率较低（16–20％），（41）WTW效率仅为14.4–18.0％，表明总CO ₂排放为312– 390 g _{CO 2/} mile。(41）尽管柴油机中DME的理论TTW效率比CNGV的理论TTW效率（25–30％）高得多，（25）DRM中合成气转化为DME工艺的能效可以仅达到59–64％。（39,40）结果，WTW效率（13.4–17.5％）和CO ₂排放量（321–419 g _{CO 2}基于DME-DRM的车辆的每英里行驶里程与CNGV的行驶里程大致相同，此外，与传统的CNGV流程相比，可以想象到更高的资本和运营投资。虽然提出了通过在合成过程中将CO ₂掺入DME的方法，将DRM转化为DME与传统SMR相比可节省30％的CO ₂排放，但将其用作燃料在减少CO ₂方面没有明显优势。尽管还提出了利用可再生能源来使DRM CO ₂中和的方法（42,43），但尾气排放为零的燃料电池电动汽车（FCEV）和电池电动汽车（BEV）可以更有效地利用这种能源。（ 41,44）钻井，加工和分销。电力运输和充电。H₂压缩加油。无尾气排放。不考虑电池制造排放。图3.使用DRM的CNG和DME，车辆的轮毂效率和CO ₂排放量。除了实验室规模条件下的现有挑战外，例如（i）低气时空速（GHSV； 10–100 L g _cat ^–1 h ^–1），（ii）低压（〜1 bar）， （iii）稀进料（反应物/惰性气体≥4）等。（15,27,34,45）DRM催化剂对原料杂质敏感，CO ₂捕集和纯化系统对于避免催化剂中毒至关重要。从工业来源捕获或纯化进口或回收的CO ₂需要额外的2.1–5.7 MJ / kg _CO2基于所采用的技术。（38,46）更重要的是，当前统计数据基于稀薄条件下的实验室规模条件。随着规模的扩大，这些局限性将发挥重要作用，而隔离的CO ₂量可能会进一步减少。理论和实验研究表明，CH ₄和CO _2的转化对温度和压力的变化敏感。（38）压力的任何重大变化都可能导致焦化（晶须形成），催化剂分馏和压降。（27）工业上，这些局限性得以实现，因此其他技术，例如部分氧化（POX），蒸汽CO ₂已经探索了重整（SCR），自热重整（ATR）和三重整形来代替DRM来生产富含CO的合成气。（27,44）总之，即使解决了DRM的这些艰巨问题，从实际的角度出发并解决了这一问题，并且不考虑这种吸热过程中的能量损失，通过DRM方法以多Gt _{CO 2} / yr规模生产燃料的CDU最多只能对CO ₂排放减轻做出微不足道的贡献。为此目的而广泛部署DRM将会在政治，财务和精力上造成巨大的成本分散。DRM仍然是一种有价值的反应，可以满足增值制造的业务需求，但CO ₂的目标必须使用低碳能源来实现减排。（47）该观点表达的是作者的观点，不一定是ACS的观点。作者宣称没有竞争性的经济利益。于涵教授因其深刻的评论和讨论而闻名。我们感谢阿卜杜拉国王科技大学（KAUST）的财政支持。在形成乙酸甲烷一氧化碳二氧化碳CO的298 K标准焓反应焓₂在重整克CO的等效二甲醚的总能量输入能量使用₂每CO的英里焦耳亿吨排放₂氢公斤千焦耳升/克催化剂每小时兆焦耳/摩尔每摩尔甲烷较低的合成气发热量年自动热重整电池电动汽车碳捕获和利用二氧化碳利用压缩天然气汽车甲烷燃料电池干法重整菲舍尔Tropsch气体时空速度较低的热值偏氧化的研究与发展反水煤气变换蒸汽CO ₂重整蒸汽甲烷重整合成气罐装轮美国能源信息署（US Energy Information Administration）装轮本文还参考了其他47种出版物。