Fuel 113 (2013) 287–299

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Fuel

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Review article

Coke oven gas: Availability, properties, purification, and utilizationin China

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.05.070

Abbreviations: COG, coke oven gas; GDP, gross domestic product; BF, blast furnace; Syn-gas, synthesis gas; CV, calorific value; BFG, blast furnace gas; BTX,toluene and xylene; HCs, hydrocarbons; CHP, combined heat and power; SOx, sulfur oxide; DRI, direct reduction iron; EAF, electric arc furnace; PSA, pressuadsorption; WGSR, water–gas shift reaction; PO, partial oxidation; FTS, fischer tropsch synthesis; CPO, catalytic partial oxidation; S/C, steam/carbon; RWGSR, reversgas shift reaction; COx, CO and CO2; GHGs, green house gases; GHG, green house gas; SNG, synthetic natural gas; CNGCL, China Natural Gas Corporation Limitliquefied natural gas; CCPP, combined cycle power plant.⇑ Corresponding author. Tel./fax: +86 10 82544800.

E-mail address: csli@home.ipe.ac.cn (C. Li).

Rauf Razzaq, Chunshan Li ⇑, Suojiang ZhangBeijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences,Beijing 100190, PR China

a r t i c l e i n f o

Article history:Received 22 August 2012Received in revised form 20 May 2013Accepted 22 May 2013Available online 14 June 2013

Keywords:Coke oven gas (COG)COG reformingMethanol synthesisMethanation

a b s t r a c t

The global demand for energy is constantly on the rise because of population explosion, rapid urbaniza-tion, and industrial growth. Existing energy resources are struggling to cope with the current energyrequirements. Aside from exploring renewable energy alternatives, available energy resources must beutilized to their maximum potential. Coke oven gas (COG) is highly rated as a valuable by-product of coalcarbonization to produce coke in the steel industry. Typically, a single ton of coke generates approxi-mately 360 m3 COG. China annually produces 70 billion N m3 COG; however, only 20% of the gas pro-duced is utilized as fuel. Disposing COG without an effective recovery and efficient utilization is aserious waste of an energy resource and results in environmental pollution. COG is regarded as a poten-tial feedstock for hydrogen separation, methane enrichment, and syn-gas and methanol production. Itcan also be effectively utilized to produce electricity and liquefied natural gas. The availability, properties,purification, and utilization of COG are reviewed in the current study. COG utilization routes are summa-rized in detail, with focus on some major industrial projects in China and other countries that are basedon COG utilization technology.

� 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2882. COG properties and purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

2.1. COG properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2892.2. COG purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

2.2.1. NH3 removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2902.2.2. COG desulfurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

3. COG utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

3.1. COG combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

3.1.1. Direct COG combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2913.1.2. COG combustion for electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

3.2. Direct reduction iron (DRI) production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2913.3. Feedstock for hydrogen separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2923.4. Hydrogen and syn-gas production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

3.4.1. Partial oxidation (PO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2933.4.2. Dry (CO2) and steam reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

3.5. Methanol synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2943.6. COG tar utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

benzene,re swinge water–ed; LNG,

288 R. Razzaq et al. / Fuel 113 (2013) 287–299

3.7. COG methanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

4. Overview of COG utilization technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2955. COG investment and economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

5.1. World scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2975.2. China’s perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

Table 1Mass and energy flow of a typical coke-making plant[10].

Energy input (42.7 GJ/t coke)Coal 91.44%Electric power 0.37%Fuel gas (firing gas) 7.61%Steam 0.58%

Energy output (42.7 GJ/t coke)Coke 69.63%COG 17.92%Tar 2.77%BTX 0.98%Sulfur 0.05%Energy loss 8.65%

1. Introduction

The threat of the world’s energy supply by depletion of fossilfuel reserves, along with rapid industrialization and urban growth,has not only encouraged the search for alternative energy sources,but it has also pushed for an effective and efficient utilization ofavailable ones. Fossil fuels have predominantly been the major en-ergy source for industrial, transportation, and domestic use overthe centuries. Approximately 90% of the global energy require-ments are covered by these sources alone [1]. Global primary en-ergy usage rose by approximately 2.4% in 2007 and is likely toincrease further in the future, with developing Asian countries con-tinuously improving their standard of living. The energy demand inChina rose by 7.7%, followed by 6.8% and 1.6% in India and USrespectively [2]. For the past decade or so, China has enjoyed themost drastic economic growth, with a 10% increase in the grossdomestic product (GDP). This growth allowed China to recoverquickly from the 2007–2008 financial crisis. With such economicand population boom, the energy consumption in China is alsoon the rise and, more importantly, strongly affects the global en-ergy balance [3].

The energy structure of China has traditionally been dominatedby coal. Although other fuel options have also entered the energysetup in the past few decades, coal still plays a leading role in Chi-na’s energy scenario. Model forecasts predicted an annual increasein coal demand by almost 2% for 2010 [4]. With a coal reserve of anestimated 5570 billion tons, the third largest in the world, China isregarded as the world’s largest coal producer and consumer [5].

For the past few decades, steel has become an icon of modernurbanization and industrial development, with coal serving asthe backbone of the iron and steel industries. According to theWorld Coal Association, approximately 70% of the total global steelproduction relies on coal [6]. The steel industry has played animportant role in the economy of China because of its rich coal re-serves, with a rapid growth overtaking that of Japan to become thelargest steel producer in the world. Despite such achievements,the energy efficiency of China’s steel industry is the lowest amongthe major steel-producing countries around the globe. However,research and development are continuously improving the energyefficiency to achieve a sustainable development [7]. In 2004, theaverage net energy usage for the coking process in China was4.3 GJ/t, whereas the international average was 3.8 GJ/t [8].

Coke oven gas (COG), sometimes simply called ‘‘coke gas,’’ is aby-product of the coke-making process, where volatile coal matteris generated as COG, leaving carbon intensive coke behind. Coke isa very strong macro-porous carbonaceous material produced bythe carbonization of a specific coal grade or of different coal blendsat temperatures P1400 K. Approximately 90% of coke producedfrom blends of coking coals is used to maintain the iron productionprocess in a blast furnace (BF) [9]. Typically, 1.25–1.65 tonnes ofcoal produces a single tonne of coke, while generating approxi-mately 300–360 m3 of COG (6–8 GJ/t coke) [8]. Table 1 shows theenergy balance for a typical coke-making plant along with differentraw materials and product distribution [10]. In China, the annualcoke output in 2007 was about 335 million tons, or approximately

60% of the total global coke production. The annual COG produc-tion in China for 2007 was estimated at around 70 billion N m3.However, only 20% of the produced COG is utilized as fuel; mostof the gas is directly discharged into the atmosphere, with seriousenvironmental consequences and considerable energy waste.Developing new technologies to recover and utilize COG from thesteel industry is therefore urgently needed [11,12]. In China, thecoking enterprises located near coal mines only recover 24% ofthe COG by-product, losing a high percentage of potential energyas well as generating 25 Mt of carbon dioxide (CO2) [8]. Moreover,converting COG into more energy-valued products can signifi-cantly enhance the energy efficiency of the steel industry in China.

COG has been highly investigated as an important source ofhydrogen (H2), with Japan making some early developments inestablishing a sustainable H2 production technology from COG[12]. Purwanto and Akiyama [13] proposed a simple method ofH2 production from COG. Onozaki et al. [14] performed the partialoxidation and steam reforming of tar from hot COG to produce H2

at low cost and high efficiency without using any catalyst. H2

enrichment in COG can also be achieved through catalytic methaneCH4 reforming via catalytic partial oxidation [15]. The use of mem-brane technology can also lead to synthesis gas (syn-gas) produc-tion through the partial oxidation of COG [12]. The syn-gas(CO + H2) produced from COG via various routes, such as partialoxidation, steam reforming, or dry reforming, can be utilized toproduce important organic products, such as methanol [16]. Cata-lytic co-methanation of CO and CO2 in COG can also be used forCH4 enrichment. The choice and nature of a catalyst can signifi-cantly affect both the activity and selectivity in CH4 production.The use of both transition and noble active metal supported cata-lysts with different oxide supports have been previously reportedfor CO and CO2 hydrogenation to produce CH4 [17–21].

In the current review, we discuss the availability of COG withrespect to the steel industry and coke gas production, focusingon China’s perspective. The purification and utilization of COG islargely discussed, and current utilization routes and future techno-logical research and developments in the said field are considered.COG utilization facilities in China are highlighted, with key

Table 3Heating value of some typical gaseous fuels [25,26].

Fuel type Heating value

R. Razzaq et al. / Fuel 113 (2013) 287–299 289

investment groups and companies at the forefront of a constantlydeveloping technology. Finally, some concluding remarks are alsopresented.

(MJ/m3) (MJ/kg)

Natural gas 40.6 56.6Coke oven gas 19.9 41.6Water gas 18.9 21.9Blast furnace gas 3.9 2.7Producer gas 5.8 5.2

2. COG properties and purification

2.1. COG properties

Before the advent of natural gas, COG was used to meet thedomestic energy demand for industrial cities such as Sheffieldand Birmingham in England. However, COG was soon replacedby CH4. COG is essentially a mixture of combustible gases suchas CO, H2, and CH4, and non-combustible ones, including CO2 andN2 [22]. During the coking process, the composition of the releasedgas may vary depending on the nature of the utilized coal (Table 2).

Although COG is regarded as a non-standard gaseous fuel, it stillhas a reasonable energy content and calorific value (CV), which de-pend on the nature of coal and the type of carbonization used. Witha high H2 content, COG has a CV of 19.9 MJ/m3, which is five timeshigher than that of blast furnace gas (BFG) (3.9 MJ/m3) [8]. Table 3provides the heating values of natural gas and other synthetic gas-eous fuels for comparison.

As previously mentioned, the type of carbonization process sig-nificantly affects the production of COG and its properties. A low-temperature carbonization process performed at 700 �C producesa semi-coke, resulting in lower COG and ammonia yields and hightar. COG released under this process has a high CV with low H2

content. High-temperature carbonization produces a high COGyield with less tar and high NH3. Moreover, the CV of the generatedCOG is lower, with a high H2 content [27].

2.2. COG purification

The optimization of the industrial COG purification process hasbecome highly significant in achieving an efficient, economical,and eco-friendly purification. The potential for improvement ishuge, especially in the adsorption and desorption units of a COGpurification plant [28]. COG is a complex mixture of various com-ponents; aside from those listed in Table 2, COG also contains otherminor constituents, such as NH3, hydrogen cyanide, ammoniumchloride, tar components (tar acid gases such as phenolic acid,and tar base gases such as pyridine), and carbon disulfide. Of these,H2, CH4, CO, and paraffinic and unsaturated gases are useful in thefinal clean gas composition, whereas small amounts of CO2, N2, andO2 are retained in the final gas. The by-product plant should re-move as much of the remaining constituents as possible, both prac-tically and economically [29]. Traditionally, a coking plant isconnected to a network of integrated steel- and ironworks. A BFGwith low CV is used to heat the coke ovens, and CV is increasedby the addition of COG. In most energy-efficient facilities, excessCOG is utilized in small energy-intensive processes within theplant, such as ignition furnace heating, rolling mills, and power

Table 2Composition (vol%) of COG produced during thecoke-making process in the steel industry [23,24].

COG constituents vol%

H2 55–60CH4 23–27CO 5–8CO2 1–2N2 3–6C2H4 1–1.5C2H6 0.5–0.8C3H6 60.07H2S 63.2E�5

generation. Compared with the coking facilities around the globe,coke plants in Germany have acquired the highest technically pos-sible standards today. However, engineers and researchers are con-tinuously working to increase the overall plant efficiency andenvironmental standards [10].

The basic COG treatment process in the steel industry has notsignificantly changed in several years. Typically, hot, raw COG fromcoke oven batteries undergoes the following processes prior to NH3

and acid gas removal [30,31]:

1. Hot COG is pre-cooled from a large volume of coal tar mixturevia direct contact with a weak aqueous ammoniacal solutiondirectly sprayed into the collection system. The hot gases arecooled to a temperature between 70 and 100 �C using pri-mary coolers. During this process, approximately 30% of theinitial amount of NH3 and most of the tar components areremoved. After utilization, the discharged liquid, also knownas the ‘‘flushing liquor,’’ is recycled for further use.

2. The gas is further cooled to a temperature ranging from28 �C to 30 �C using either direct or indirect secondary cool-ers. In direct coolers, COG is cooled via direct contact with acounter-current stream of the same ammoniacal solution.The hot spent solution escaping from the unit is cooledusing water-cooled coils and again recycled for furtheruse. For indirect coolers, shell-and-tube heat exchangersare deployed, with cooling water running inside the tubesand hot COG flowing outside.

3. Finally, the cooled COG is passed through an electrostaticprecipitator to remove very fine tar droplets. Some facilitiesare also equipped with an on-site tar distillation unit.

4. Naphthalene is recovered in a separator unit, followed bythe removal of light oil. COG is fractionated to recover ben-zene, toluene, and xylene (BTX).

During the coke-handling, quenching, and screening processes,‘‘coke breeze’’ is produced, which can be used either on-site in thesinter plant or sold as a usable by-product.

A COG cooling system with a closed water circulation cycle pre-vents the escape of pollutants into the atmosphere, which resultsfrom using a cooling tower. The system has low installation costsand increased efficiency through the use of highly efficient, self-cleaning helical heat exchangers, rather than conventional ones.Moreover, because COG has no direct contact with the coolingwater, recycled water can be directly utilized [32].

A conventional process for the COG treatment plant is shown inFig. 1. Water and tar contents are transferred into a crude tar recov-ery unit, and COG is then cooled at around 27 �C. NH3 and hydrogensulfide (H2S) are scrubbed in a scrubbing unit, and benzole is re-moved and recovered for further utilization. The water used inthe scrubbing unit is recycled before being re-pumped into thescrubber. Finally, NH3 is cracked using a catalyst at high tempera-ture and atmospheric pressure into N2 and H2 (2NH3 M N2 + 3H2)while H2S is obtained as elemental sulfur through ‘‘Claus process’’.A biological effluent treatment unit continuously recovers anddecomposes different hydrocarbons (HCs) and nitrogenous com-pounds [10].

Fig. 1. Schematic diagram of a typical COG treatment process [10].

290 R. Razzaq et al. / Fuel 113 (2013) 287–299

2.2.1. NH3 removalConventionally, NH3 in COG is removed while in contact with

the gas containing a solution of sulfuric acid to form ammoniumsulfate, which is then recovered, crystallized, and dried before itis sold as fertilizer. More advanced modern processes of NH3 re-moval from COG include the water wash process [33]. After COGis cooled to �27 �C in the secondary cooler, the gas is introducedinto the NH3 removal unit equipped with a stripping section. Thegas then enters the adsorption section of the vessel, while a portionof adsorbed solution is cooled and recycled. Water from the freeNH3 stripper is introduced at the top of the absorber. Water fromthe stripping section is continuously cooled and recycled, while ex-cess water is treated in the fixed NH3 stripper before discharge.Lime or caustic soda is used to react with non-volatile acids to re-lease the fixed NH3, allowing it to be steam-stripped from the solu-tion. Vapors escaping from both stripping units are passed througha partial condenser to recover and dispose NH3 and other acidgases. A typical water wash NH3 removal process reduces COG-NH3 content from 200–500 g/100 scf to 2–7 g/100 scf [30].

2.2.2. COG desulfurizationThe removal of hydrogen sulfide from COG using NH3 liquor is

well-established in the coke-making industry and regarded as asufficiently developed separation process. According to Europeanstandards, H2S in COG must be removed to the acceptable residual

value of 60.5 g/m3. Therefore, the use of NH3 to remove H2S fromCOG has gained considerable attention. The process includes thecapture of H2S using liquid NH3, followed by treatment with acidgas under the ‘‘Claus process’’ (Fig. 1) to obtain elemental sulfurwhich can be sold as a commercial product. China has installed fivesuch units based on this sulfur removal technology from COG. Cer-tain advantages associated with this process include zero toxicemissions and the generation of a useful by-product (sulfur) [34].

During COG desulfurization, one-third of the H2S content is firstconsumed under partial oxidation according to the followingreaction:

3H2Sþ 3=2O2 ! 2H2Sþ SO2 þH2OðgÞ ð1Þ

After the oxidation reaction, H2S undergoes a Claus reaction, asfollows:

2H2Sþ SO2 ! 2H2OðgÞ þ 3=nSn ð2Þ

The reaction is performed between 230 and 250 �C over an Al2O3

catalyst.One disadvantage of the above process is that the high NH3 con-

tent in acid gas can trigger a secondary route of oxygen consump-tion to produce water, N2, and NOx, resulting in low elementalsulfur content. Another major drawback includes the poisoningof the catalyst by H2S. To overcome this problem, a high-tempera-ture (>1100 �C) catalytic oxidation of H2S is required. The joint

R. Razzaq et al. / Fuel 113 (2013) 287–299 291

removal of NH3 and H2S from COG using the Ama-sulf technologywas thus introduced. This approach involves the partial oxidationof H2S and the simultaneous decomposition of NH3 over a Ni cata-lyst at 1100 �C to 1200 �C [34,35].

Another challenge to this process is the imperfect H2S separa-tion from aqueous ammoniacal solution, in which approximately2% H2S remaining in the stripping solution causes environmentalproblems due to SOx emissions [36]. Park et al. [36] studied theselective removal of H2S from COG in the presence of water vaporand NH3 over V2O5/SiO2 and Fe2O3/SiO2 catalysts. The catalystsshowed high selectivity toward H2S removal, with low SO2 forma-tion. The results showed a complete conversion of H2S to a mixtureof elemental sulfur and ammonium salt.

3. COG utilization

For both commercial and environmental reasons, particularimportance is given to the utilization of coke-making by-products,including COG. COG contains approximately 30 wt% tar as heavyHCs and 70 wt% in the form of light gases, including bulk H2 andCH4. Conversion of the entire COG (including tar) into lighter fuelconstituents can in term of figures meet approximately 4.1% ofthe global demand for electric power generation [37]. To date, dif-ferent on-site and off-site COG utilization routes have been pro-posed, including energy generation and syn-gas, H2, methanol,and CH4 production (Fig. 2). COG at the high temperature of800 �C has continuously attracted increasing attention as one ofthe most promising sources of H2. Through catalytic reformingand water–gas shift reaction (WGSR), the amount of H2 producedfrom COG will be several times higher than that of the originalH2 in the feed COG [38]. Over the years, hot COG has been cooledand cleaned to obtain a number of important chemicals, includingtoluene, benzene, and other HCs; this process results in valuablethermal and chemical energy loss. Therefore, the concept of hotCOG utilization should be highly considered to address both eco-nomic and environmental issues [39].

3.1. COG combustion

As one of the key discharge products from a coke-making plant,hot COG at a high temperature (�800 �C) carries 20–30% valuablethermal energy. The first step toward COG utilization should al-ways be the use of such thermal energy and the reduction of

Fig. 2. Potential routes to

thermal heat loss via reinforced sealing and the thermal insulationof coke oven batteries [40]. Raw COG can be burned on-site for usein BFs and in coke batteries in the coking process. Otherwise, thegas can be used to generate steam for power and electricity.

3.1.1. Direct COG combustionAfter the subsequent removal of heavy HCs, COG with a heating

value of approximately 18.6 MJ/m3, can be effectively combustedin small combustion units such as process heaters and boilers.COG combustion results in the generation of very low levels of haz-ardous air pollutants, similar to those generated by the natural gascombusting unit. COG shares similar combustion properties withnatural gas (e.g., flame temperature), indicating that both gaseswould result in the efficient destruction of combustible organiccompounds under optimal combustion conditions [41,42].

In China, some indigenous coke ovens are heated via the directcombustion of COG in a coal carbonization chamber. In the case ofmachinery ovens, COG is combusted inside a coal carbonizationchamber, and the coke oven is heated via heat transfer betweenthe chamber walls. Some of the coking coal may be burned duringthe coke-making process because of direct COG combustion in theoven [43].

3.1.2. COG combustion for electricityIn the steel industry, different surplus combustible gases can

serve as potential feed stocks to generate heat and electricity forcombined heat and power plants (CHP). A BFG with a low heatingvalue can be mixed with COG to generate sufficient energy forpower production [44]. The first COG-based CHP system in Chinahas been in operation since 2006 at Shandong Jinneng Coal Gasifi-cation Co., Ltd. (Jinneng, China). The system consumes 9700 m3/hCOG at a power generating capacity of approximately 1.60 kW h/m3, with 3.09 kg simultaneous steam production [45]. Fig. 3 showsa simplified process in a COG-based CHP plant.

3.2. Direct reduction iron (DRI) production

Conventional BF iron making is popular throughout the worldbecause of the readily available coke and continuous improve-ments in BF technology. BF processes account for almost 90% ofthe global total iron production. Although the process is regardedas highly efficient, it also has some key disadvantages, includingthe availability of high metallurgical-grade coke and iron ore as

COG utilization [10].

Fig. 4. A schematic diagram of an integrated COG-based DRI plant in the steel industry [44].

Fig. 3. Simplified process in a COG-based CHP plant [45].

292 R. Razzaq et al. / Fuel 113 (2013) 287–299

potential feed stocks, high running costs, and the generation ofgaseous pollutants such as CO2 and sulfur oxide (SOx). The directreduction process can serve as a complementary alternative to ironmaking in the steel industry. The process is regarded as environ-ment friendly, with less dependence on high-grade metallurgicalcoke. During this process, the reduction of iron oxides is conductedin a solid state below the fusion temperature of pure Fe. The oxy-gen is extracted from the iron ores (Fe2O3/Fe3O4) leaving behindgangue constituents (un-valuable minerals) in the sponge iron,which must be separated in the electric arc furnace (EAF). Differentreducing gases are used for this purpose, including CO, CH4, andH2; other carbon-bearing materials may also be utilized [46–48].An increase in the use of DRI processes has recently been observeddue to less investment costs as compared to a BF technology[49,50]. Although DRI produces less amount of CO2 as comparedto BF technology, the amounts off-gas emissions are fairly highcalling for a subsequent gas-recycling during the process [51].CH4 is mainly utilized as a reducing gas for DRI production and ispopular among countries with rich natural gas reserves. China re-lies on coke-making and BF processes because of its vast coal re-serves. Natural-gas-based DRI production technology includesMidrex and Hyl processes, and the Finmet process, which uses afluidized bed [46].

COG from an existing coking facility can be used as an alterna-tive reducing agent to natural gas in the DRI process for steel pro-duction [44]. Ahrendt and Beggs [52] patented a methodestablishing the use of high sulfur-containing COG for the directreduction of iron oxides. The process is based on an in-situ desul-furization of the reducing gas (COG) via the reaction of sulfur in the

process fuel with hot DRI, before the process fuel is admitted intothe reformer. Alternatively, purified COG can be converted into areformed gas under steam reforming, and the resulting gas canproduce DRI (Fig. 4). A mixture of recycled gas from the directreduction plant and COG is heated in a reducing gas heater andintroduced as the reducing gas to the reduction zone of the DRIreactor. The process is conducted at a counter flow, with an intro-duction of O2 and hot tar gas inducing partial oxidation to produceDRI. CH4 from COG is converted to H2 and CO at the bottom of thereduction zone. Gas leaving the DRI reactor is cleaned via CO2 re-moval to produce tail gas. The resulting DRI may be used in theBF, in the converter, or in the EAF [10,53].

3.3. Feedstock for hydrogen separation

Efficient, high-performance, and low-cost technologies for H2

production are urgently needed to boost H2 consumption, whichis regarded as a future clean energy source. At present, H2 can beproduced from an extensive range of source materials, includingfossil fuels, alcohol, biomass, and some industrial chemical by-products [54,55]. COG containing 50–60% H2 is a high-potentialsource of H2, especially in countries with high coke productionand utilization [56]. Joseck et al. [57] estimated the net amountof H2 that can be produced annually from COG is approximately370,000 t/yr. The production is based on the following ratios: coke:coal (0.7 t/t), COG: coke (506 m3/t), and H2:COG (0.043 kg/m3). Atpresent, some on-site coke plants in the steel industry use pressureswing adsorption (PSA) technology to obtain H2 from COG. Theprocess is carried out in a cyclic adsorption-desorption operation

Fig. 5. Schematic diagram of H2 separation from COG using the PSA system [57].

R. Razzaq et al. / Fuel 113 (2013) 287–299 293

using different adsorbant materials such as alumina oxides or zeo-lites [24]. Fig. 5 illustrates a typical PSA unit for H2 separation fromCOG. Other important H2 separation techniques include cryogenicdistillation and membrane separation. PSA and cryogenic distilla-tion are the two commercially available processes for H2 purifica-tion but they are considered to be highly energy intensive.Membrane separation technique provides an attractive alternativeto obtain high purity hydrogen using dense metallic membranes.The process consumes less energy and provides a possibility of amore continuous operation [58]. Although large scale industrialapplication of H2 separation through membrane technology stillneeds to be addressed and more economical and environmentallyfriendly ways to recover H2 from COG would be required in the fu-ture [59].

3.4. Hydrogen and syn-gas production

H2 rich syn-gas is an important raw material in many industrialprocesses for the synthesis of different organic chemicals and fuels[60–64]. At present, bulk of syn-gas production comes from steamreforming of natural gas and oil through a catalytic reaction [65].COG reforming provides an attractive alternative to a less energyintensive and clean syn-gas production [12,44,66–68].

3.4.1. Partial oxidation (PO)PO (oxy-reforming) has been highly investigated as a potential

route to H2 and syn-gas production due to its mild-exothermic nat-ure making the process more economical and less energy-intensivecompared with stream reforming [69]. CH4 oxy-reforming for H2

enrichment in COG is attracting considerable attention from boththe academia and the industry. Moreover, the process can also pro-duce syn-gas (H2/CO ratio �2) considered suitable for methanoland fischer tropsch synthesis (FTS). Although non-catalytic PO toproduce syn-gas is a well established process, the use of catalystcan significantly reduce the operating conditions i.e. temperature,pressure and make process more economical. However, additionalproblems associated with catalyst deactivation through carbondeposition and metal sintering should be addressed [70]. Anotherproblem is the high cost associated with pure oxygen supply. Toaddress this problem, a new reformer with mixed ionic and elec-tronic conducting oxygen-permeable ceramics is being investi-gated and developed. Most recently, ceramic membranetechnology has proven quite effective in air separation and naturalgas conversion. This technology provides a combination of oxygenseparation from air and partial oxidation of CH4 in a single unit.The application of this combined technology significantly reduces

the energy and cost input associated with H2 production [71]. Todevelop a commercial process for H2 production from COG usinga membrane reactor, high-temperature modules with gas-tightseals operating at 850 �C should be fabricated. The large membranearea requirement brings additional problems, including sealingand the high pressure drop. Therefore, tubular membranes havebeen developed to minimize such engineering obstacles, includinghigh-temperature seals [72]. Zhang et al. [72] used a high-temper-ature membrane reactor system to assess the oxygen permeationflux and the conversion of CH4 in COG. The closed-one-endedmembrane tube on the stainless steel support was sealed using asilver-based reactive-air-brazing alloy with 8 mol% CuO. COG, witha typical commercial composition of 57.09% H2, 28.18% CH4, 7.06%CO, 3.16% CO2, and 4.51% N2, was fed into the membrane reactorthat has its annulus region packed with a commercial Ni-based cat-alyst. Air was blown into a smaller tube inside the membrane tube.The membrane reactor was operated at 875 �C. CH4 conversion, H2

and CO selectivity, and oxygen permeation fluxes were deter-mined. A CH4 conversion of approximately 95% was achieved,along with H2 and CO selectivity of approximately 91% and 99%,respectively.

Yang et al. [12] also reported the partial oxidation of CH4 in COGto syn-gas using a Ba1.0Co0.7Fe0.2Nb0.1O3�d membrane in a mem-brane reactor. The reaction was performed using NiO/MgO as thecatalyst. The reforming process was successfully performed at875 �C with 95% CH4 conversion, 80% H2 selectivity, and 106% COselectivity. Cheng et al. [39] achieved H2 enrichment from stimu-lated hot COG using a combined medium of a membrane reactorequipped with a Ru–Ni/Mg(Al)O catalyst. The technique, utilizingthe partial oxidation of HCs under atmospheric pressure, resultedin twice the amount of net H2 present in the feed gas. The aidingcatalyst used in the process exhibited high catalytic activity and in-creased the carbon resistance. Corbo et al. [69] studied H2 produc-tion through the catalytic partial oxidation (CPO) of CH4 andpropane using Ni/Al2O3 and Pt/CeO2 catalysts; both reactions re-sulted in high H2 yields.

3.4.2. Dry (CO2) and steam reformingCH4 reforming of COG for H2 and syn-gas production can also be

achieved through dry and steam reforming reactions [73,74]. Boththe reactions are considered to be energy intensive due to theirendothermic nature. Dry reforming produces syn-gas with lowH2/CO ratio which is more suitable for the production of higherHCs. The process contributes towards fixation of two importantGHGs i.e. CO2 and CH4. However, catalytic deactivation throughcarbon deposition is a major problem for such a reaction [70,75].

294 R. Razzaq et al. / Fuel 113 (2013) 287–299

Coal char proved to be an active catalyst for CH4 reforming, withlow carbon formation and high selectivity. Zhang et al. [73] re-cently investigated the CO2 reforming of CH4 in COG over coal char(acting as a catalyst) under the following reactions:

CH4 ! Cþ 2H2 ð3Þ

CH4 þ CO2 ! 2COþ 2H2 ð4Þ

The reaction operates at 700 �C to 1200 �C, and nearly 100% CO2

conversion was attained at approximately 1065 �C. Fidalgo et al.[76] studied dry CH4 reforming in a microwave-assisted reactorover an activated carbon catalyst. Such heating mechanism provedsuperior in CO2 methane reforming compared with conventionalmethods. Results showed that 100% CH4 and CO2 conversion canbe achieved for a long period at an optimum temperature rangeof 700–800 �C. Certain thermodynamic factors play an importantrole in the COG CH4 conversion to syn-gas via steam, includingthe H2O:CH4 ratio, the conversion temperature, and the reactiontime. Experimental results have shown that a H2O:CH4 ratio be-tween 1.1 and 1.3 and a temperature range of 1223–1273 K wereoptimum for maximum CH4 conversion. A CH4 conversion ofP95% can be achieved over a H2O:CH4 ratio of 1.2 and at a temper-ature of �1223 K [77].

Catalytic steam reforming of CH4 is currently the most studiedand applied process for H2 enrichment and syn-gas production.The reaction does not need any gas purification system as in caseof PO and CO2 reforming. It is also suitable for high pressure appli-cations and provides easy separation of products. Moreover, steamreforming can produce syn-gas with high H2/CO ratio, which issuitable for the synthesis of different chemicals in many industrialprocesses [70,74,78]. COG steam reforming process occurs throughthe following main reactions [74]:

CH4 þH2O $ COþ 3H2 ð5Þ

CH4 þ CO2 $ 2COþ 2H2 ð6Þ

COþH2O $ CO2 þH2 ð7Þ

The product selectivity and final composition may be influenced bysome side reactions including dry reforming (6) and WGSR and re-verse water–gas shift reaction (RWGSR) (7). During steam reform-ing, carbon deposition can be successfully addressed by injectingmore steam than required stoichiometrically to carry out thereforming reaction (H2O/CH4 > 1) [24]. Yang et al. [74] studiedsteam reforming of COG using NiO/MgO catalyst. A high CH4 con-version (�97%) under low steam/carbon (S/C) ratio was achievedat 875 �C with less carbon deposition and enhanced thermal stabil-ity. It was revealed that CH4 and CO2 conversions strongly dependupon S/C molar ratio and reaction temperature. Cheng et al. [79] re-ported H2 enrichment through catalytic reforming of hot COG usingNi/Mg(Al)O catalysts. The process can successfully utilize the sensi-ble heat that may otherwise be wasted resulting in considerable en-ergy loss. Results showed that the amount of H2 in the resultantmixture increased 4-fold in comparison to the starting mixture withS/C molar ratio of 1.7.

3.5. Methanol synthesis

Methanol is an important chemical feedstock that can be usedto produce different chemicals, including formaldehyde, methyltertiary butyl ether, and acetic acid. Methanol is also used as akey solvent in many industrial processes, as fuel cells in automo-biles, and for power generation. The production capacity of meth-anol in China was expected to reach 25 million tons by the end of2010, with nearly 80% of the production relying on coal-basedtechnologies. The global demand for methanol is also expected to

surpass 32 million tons each year, with high growth rates nearlymatching the GDP. COG with high H2 contents is considered idealfor a sustainable methanol production. In China, the COG-basedmethanol production capacity of Shanxi reached 2.06 million tonsin 2006 [80].

Syn-gas produced from COG partial oxidation, dry reforming,and/or steam reforming is considered highly useful in methanolsynthesis. Although steam reforming of COG has been more inten-sively studied compared to dry reforming, the latter has someadvantages over the former, including energy savings resultingfrom CO2 utilization/recycling, as shown in Fig. 6. Moreover, COGdry reforming produces syn-gas with a H2:CO ratio of approxi-mately 2, which is considered ideal for methanol synthesis [16].

During syn-gas production, H2 in the feed COG can cause twomajor problems: (a) a shift in the reaction equilibrium to the reac-tants, resulting in low CH4 and CO2 conversion to syn-gas; and (b)the induction of the undesired RWGSR. Both effects result in a de-crease in H2 content in the final syn-gas, which in turn results in alow H2:CO ratio, which is unsuitable for methanol synthesis. How-ever, results suggested that RWGSR has a more significant effect onthe process compared with the shift in the reaction equilibrium. Tolimit H2 consumption under RWGSR, the reaction should be con-ducted at higher temperatures (P1000 �C) [16]. Maruoka andAkiyama [81] proposed a new method for thermal energy recoveryfrom hot flue gases generated by the steel-making convertor. Theprocess includes the use of both latent and endothermic heat fromthe reaction. The stored heat after recovery is supplied to COG toinduce steam reforming of CH4 and obtain syn-gas, which is thenutilized for methanol synthesis. Results indicated the process tobe quite promising for large scale methanol production.

3.6. COG tar utilization

Hot COG released at a very high temperature (750–850 �C) fromcoking facilities contains �100 g/m3 tar (benzene, toluene, naph-thalene, and sulfide). Tar separation from hot COG not only resultsin valuable heat loss, but also has serious environmental conse-quences, with the additional challenge of recovering tar from thestripping liquid. Hot COG tar can be converted into small, lighterHCs via catalytic hydro-cracking or reforming reactions using thesensible heat and chemical energy of hot COG [82,83]. Ni-basedcatalysts have been largely investigated for the hydro-cracking ofcomplex higher HCs because of their low cost and high activity.However, such catalysts are also prone to sulfur poisoning, carbondeposition, and sintering, especially when operated at high reac-tion temperatures, which result in serious catalyst deactivation.A key advantage of using hot COG is its 10–15% water content,which drives the steam reforming reaction to counter coke forma-tion during COG tar hydro-cracking [82]. Fei et al. [83] used Ni/Ce–ZrO2/Al2O3 catalysts with varying Ni-loading and Ce–Zr contents tostudy the hydro-cracking of a model tar (toluene, naphthalene)from a hot COG. The reaction was conducted at 800 �C under atmo-spheric pressure. The catalyst exhibited high activity, long-termstability, and some sulfur tolerance. The tar constituents were allconverted into lighter gaseous fuels even at a low steam-to-car-bon-mole ratio (steam: C = 0.44). Coke formation was negligibleover a 7 h operating period, and the catalyst showed excellent per-formance in the direct removal of tar compounds in hot COG andconverting them into useful light HCs. Ni/MgO/Al2O3 catalysts[82] also exhibited excellent activity, stability, and carbon resis-tance for the catalytic hydro-cracking of tar in hot COG. The resultsshowed complete conversions of tar compounds into light fuelgases, whereas the presence of H2S in the feed gas actually inhib-ited carbon deposition, resulting in an enhanced catalytic activityand long-term stability. Bimetallic catalysts have also been inves-tigated for the catalytic reforming of tar compounds from hot

Fig. 6. Dry CO2 reforming of COG for methanol synthesis [16].

R. Razzaq et al. / Fuel 113 (2013) 287–299 295

COG. Catalysts promoted the complete conversion of toluene in hotCOG tar into light CH4, CO, and CO2 gas molecules between 600 and750 �C at atmospheric pressure. Therefore, a high H2 content and alow reaction temperature promote CH4 formation [84,85].

3.7. COG methanation

The catalytic co-methanation of CO and CO2 (COx) in COG forCH4 enrichment can be regarded as a simple and highly efficientway of producing gas with high heating value and extensive indus-trial and commercial use. COG methanation can occur without theaddition of other reagents while CH4 can be separated as a valuableand clean fuel. The reaction has been extensively used in the re-moval of carbon oxides from gas mixtures in NH3 plants, as wellas in H2 purification in refineries and ethylene plants [86]. Sabatierand Senderens [87] first studied the methanation reaction at thebeginning of the 20th century. They found that Ni and other metals(Ru, Rh, Pt, Fe, and Co) catalyze the reaction of CO with H2 to pro-duce CH4 and water [88]. Many reactions take place during CO andCO2 hydrogenation, and CH4 is formed as a result of the competi-tion between such reactions. CO reacts with H2, provided thatthe stoichiometric ratio of the reactants (H2/CO) is at least 3:1,according to Eq. (R-1). However, for a feed gas with a low H2/COratio, CO may be hydrogenated via the reaction in Eq. (R-2). CO2

methanation is carried out under a higher H2/CO2 ratio (Eq. (R-3)). Undesired WGS reaction occurs in accordance with Eq. (R-4)which may significantly affect CH4 selectivity. Another reaction(Eq. (R-5)) may also occur in case of low H2/CO ratio, resulting incarbon formation and catalyst deactivation [88,89].

COþ 3H2 $ CH4 þH2OðgÞ DH0289K ¼ �206:2 kJ=mol ðR-1Þ

2COþ 2H2 $ CH4 þ CO2 DH0289K ¼ �247:3 kJ=mol ðR-2Þ

CO2 þ 4H2 $ CH4 þ 2H2OðgÞ DH0289K ¼ �164:9 kJ=mol ðR-3Þ

COþH2O $ CO2 þH2 DH0289K ¼ �41:2 kJ=mol ðR-4Þ

2CO $ Cþ CO2 DH0289K ¼ �172:54 kJ=mol ðR-5Þ

The methanation reaction for CO and CO2 is usually performedbetween 150 and 400 �C in a catalytic reactor. The choice and prep-aration of the catalyst is the most crucial stage in CH4 synthesisusing COG. Numerous studies have revealed the importance of

the catalyst both for the reactivity and selectivity. The methanationof CO2 is more rapid and more selective compared with that of COover Ni, Fe, Rh, and Ru catalysts [90]. Ni is the most studied andconsidered the most suitable catalyst for methanation of carbonoxides because of its high selectivity for CH4 and its relativelylow price [91–94]. Different support materials have been used forthe dispersion of Ni and other active metals, including SiO2, Al2O3,TiO2, ZrO2, CeO2, and zeolite [95–99]. The activity and selectivity ofthe supported metal catalysts are strongly affected by the amountof metal loading, the size of the dispersed metal particles, the inter-actions between the support and the active species, and the com-position of the support material [100]. Since methanation is anexothermic reaction, the reaction heat results in severe metal sin-tering and poor catalytic stability. Therefore, it is imperative to de-velop highly active low temperature methanation catalysts toensure long-term thermal stability and also minimize operatingcosts for large scale industrial applications. Table 4 lists the differ-ent catalytic systems used for COx methanation reaction.

Catalytic CO and CO2 methanation have been mostly performedusing a fixed-bed catalytic reactor. However, the use of an efficientreactor designed using electrochemical techniques can provehighly cost-effective and environmentally friendly by providingmaximum yield with minimal waste production [106]. Bebeliset al. [107] studied the electrochemical CO2 methanation reactionover a Rh/YSZ catalyst and concluded that high CH4 productionrates can be achieved as a result of an increase in the Rh catalystpotential (electrophobic property) while operating at a tempera-ture range of 346–477 �C. An extensive experimental study onthe methanation reaction using a fluidized bed reactor operatedat 320 �C has also been conducted very recently. For all the exper-iments, a commercially synthesized Ni/Al2O3 catalyst with a largesurface area was used. The reactor achieved 100% CO conversionto CH4. The experimental results revealed that the methanationactivity inside a fluidized reactor is significantly affected both ther-modynamically and by the reaction boundary conditions. More-over, an additional benefit of a zero-carbon deposition was alsoachieved, providing the catalyst with high durability and enhancedlifetime [88].

4. Overview of COG utilization technology

COG processing and utilization will not only boost the energyefficiency of the steel industry but also prevent the emission of

Table 4Comparison between the different catalysts used for COx methanation reported in literature.

Catalyst Preparation method %XCO %XCO2 Highlights Ref.

Ni–Co/CeO2–ZrO2 Co-precipitation 100 95 – Feed gas with similar composition to COG [101]– High COx conversion– CH4 selectivity of �99%

Ni/ZrO2 Wet-impregnation 100 100 – Low temperature COx methanation [91]– Promising system for H2 purification

Co3O4 Precipitation 100 _ – CO methanation in COG [102]– Catalyst active at very low temperature (177 �C)

Ni/Zr–Sm Oxidation reductiontreatment

100 30 – Complete CO conversion at 200 �C [21]

– High conversion attained using a second reactor in series– Long-term stability

Ru/carbonnanofibers

Wet-impregnation 100 50 – Complete CO conversion at 340 �C for solo CO methanation – Poor methanation activity forCO/CO2 mixtures

[103]

– Undesired RWGSR can be inhibited by injecting 30% water (steam)Ru/Al2O3 Impregnation 100 20 – Complete selective CO methanation at �220 �C [104]

– Operating at high gas-hourly-space-velocity– Very low H2 consumption

Rh/Al2O3 Impregnation 99.9 30 – Higher CO conversion [86]– CO2 suppression caused by addition of 30% water content– 99% CH4 selectivity

Ru/TiO2 Wet-impregnation 100 _ – Complete and selective CO methanation at �230 �C [105]– Long-term catalyst stability

Table 5Technological evaluation of different COG utilization routes.

COG utilization Technology Advantages Disadvantages

COG combustion – Direct combustion – Easy on-site implementation – Problems of COG surplus– Steam and electricitygeneration

– Low operating cost – Need to remove heavy HCs

– Simple in operation – Combustion inefficienciesDRI production – COG as reducing gas – Easy to industrialize – COG purification and reforming requirements

– Low investment costs – Potential SOx emissions– Lower energy consumption – Greater amounts of off-gas emissions– Enhanced steel quality – Resource issues– Less CO2 emissions – Early stages of development

Hydrogen separation – PSA – Developed both industrially and commercially – Adsorbant recycling issues– Highly energy intensive– Requires purification of heavy and light HCs

– Membrane separation – High purity H2 – Less developed industrially– Consumes less energy – Separation required for heavy HCs– Low capital and operating costs– Simple and Continuous operation

– Cryogenic distillation – Commercially available process – High operating costs– High H2 purity – Needs further development

Hydrogen and syn-gasproduction

– Partial oxidation – Low Operating costs – Problems associated with pure oxygen supply

– Less energy intensive – Carbon deposition and metal sintering in case of acatalytic system

– High energy efficiency– Ideal syn-gas ratio for fine chemical synthesis

– Dry reforming – High product selectivity – Energy intensive process– Potential utilization of GHGs (CO2, CH4) – Low H2/CO ratio– Low H2/CO ratio suitable for higher HC synthesis – High carbon deposition

– Steam reforming – H2 enrichment – Energy intensive process– Low carbon deposition – High H2/CO ratio needs adjustment for methanol

and/or FTS– Suitable for high pressure applications– Ease of product separation

Methanol synthesis – Partial oxidation – CO2 utilization/recycling – Issues relating H2/CO ratio adjustment– Dry reforming – Ideal H2/CO ratio for methanol synthesis in case of

dry reforming– Technology under development

– Steam reforming – Industrially viable processCOG tar utilization – Hydro-cracking – Hot COG utilization – Carbon deposition and catalyst deactivation

– Tar reforming – Eliminates tar separation process – Technology under development– Recovery of valuable energy (sensible heat andchemical energy)

COG methanation – Methane enrichment – Simple in operation – Carbon deposition– High COx conversion – Metal sintering– High CH4 selectivity – Technology under development

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harmful green house gases (GHGs) including CO2 and CH4. COGwith high H2 contents carries huge industrial and commercial va-lue considering the future hydrogen economy. At present manysteel enterprises are trying to minimize their COG surplus whileutilizing the gas in different on-site process during steel making.Although extensive research and development has been carriedout to utilize COG surplus, substantial amount of COG is still beingwasted resulting in poor production efficiencies and GHG emis-sions. Table 5 provides a summary of different COG utilization pro-cesses with some key advantages and disadvantages.

5. COG investment and economy

COG with high energy content not only serves as an importantheat and power source for a coking facility, but is also an importantfeedstock from which a number of valuable chemicals and byprod-ucts can be recovered.

5.1. World scenario

In the past, US iron and steel industry relied heavily on by-prod-ucts such as COG for its electricity production. However, with theincrease in natural gas supply and reliance on electric arc furnaces,the on-site COG utilization for self-generated electricity has de-clined significantly [108]. Almost 40% of the COG produced in theUS is now being utilized as a fuel in BF to replace part of the naturalgas. US Steel Corp. has reported an annual savings of over 6 milliondollars with a payback period of less than a year while utilizingCOG as BF fuel. The company with a state-of-the art COG process-ing and cleaning facility at Clairton coke plant, processes the COGuntil its content is approximately 50–60% H2. Moreover, the sulfurcontent of the COG is significantly reduced during the cleaningprocessing, allowing it to be used efficiently in the BF [109].

COG with a low calorific value can be burnt in specially adaptedturbines with up to 33% efficiency. Mitsubishi Heavy Industries hassuccessfully developed such turbines which are installed in manysteel plants around the world such as Kawasaki Chiba Works (Ja-pan) and Corus Ijmuiden (Netherland) [110]. In Brazil, the Com-panhia Siderurgica Tubaro (CST) power project based on theRankine regenerative cycle, utilizes off-gasses such as COG andBFG with a total capacity of about 200 MW [22]. A Swedish basedSSAB Strip Products which is an integrated steel plant is consider-ing COG reforming for methanol production. The company esti-mates that approximately 400 GW h of COG would be availableper year for methanol production, yielding approximately300 GW h of methanol [111]. In Ukraine, The Alchevsk Coke Plantpower generating project with waste heat recovery is successfullyinstalled to displace the use of natural gas and grid electricity. Thesystem consists of a highly efficient boiler firing COG and BFG cou-pled with a 9 MW turbine generator with a net annual power gen-erating capacity of 54 GW h [112].

5.2. China’s perspective

Given its high COG throughput, China proves an ideal place forresearch and investment in sustainable COG utilization technolo-gies. Haldor Topsoe, a Danish-based company, has signed contractsto build two of the world’s largest COG plants in China to replacenatural gas (SNG) plants. The project is under contract with ChinaNatural Gas Corporation Limited (CNGCL), operating under the Pet-ro-China Group. The plant in Wuhai is set to convert waste COGinto clean and valuable liquefied natural gas (LNG). The productionfacility has a capacity of 650 million N m3 of LNG/yr and is ex-pected to be in operation by 2012. The facility will be a modelfor sustainable development for the energy sector in China [113].

Another running project by Camco-Eurofo in Shanxi Province cap-tures waste COG for power generation in a combined cycle powerplant (CCPP). The unit is expected to generate 864,667 MW h ofelectricity per year, with an annual 849,576 ton CO2 emissionreduction [114].

The COG-based methanol production project, one of the largestof its kind, began production in 2009 in Changzhi City, China. It is amulti-investor project run by the Tianji Coal Chemical IndustryGroup Co., Ltd. and the Shanxi Lu’an Environmental Energy Devel-opment Co., Ltd. The project is designed to recover 400 million m3

of raw COG to be converted into 300,000 tons of refined methanol,11,000 tons of crude benzene, and 10,000 tons of ammonium sul-fate each year. Both investors poured 1.36 billion CNY into the pro-ject, with expectations of further technological advancements inthe COG utilization field [115]. Recently, another methanol produc-tion plant of the Wuhai Shenhua Energy Company in the XilaifengIndustrial Zone has been placed in the production line. This COG-to-methanol production unit is based on a new energy-efficientand cost-effective patent technology developed by Sichuan TianyiScience and Technology. The plant is expected to reduce coal-basedmethanol production costs by 1100–1200 yuan/ton and gas-basedmethanol by 800–1000 yuan/ton, thereby enabling China to com-pete with the Middle East, which enjoys the lowest cost in naturalgas-based methanol production in the world [116].

For a typical COG plant, the determination of product costs,including purified COG, tar, benzene, and sulfate, helps in deter-mining the overall efficiency of the process. Cost analysis of COGin coke plants is conducted on the basis of planned profit levelsfor commercial purified COG minus the cost associated with the re-moval of NH3, aromatic HCs (tar), and H2S from raw gas. On suchbasis, the raw material (coking coal) should also be considered toachieve an improved cost distribution between raw materialsand products. Hence, the total net cost of individual productsshould be distributed with respect to the relative volumes in whichthey are produced [117].

6. Concluding remarks

The fast economic growth rate of China is pushing its energysector to the limit, with some serious environmental conse-quences. As the largest producer and consumer of coke, China re-lies heavily on its vast coal reserves to meet its energy demand.The coal chemical industry in China is expected to become animportant player in ensuring a sustainable development for agreen future.

COG is an important and valuable by-product produced duringthe coke-making process in the iron and steel industries. Chinaannually produces large amounts of COG, but most of the gas isnot recovered and used to its potential. COG contains valuablecombustible constituents, such as CH4 and H2, with a reasonableheating value. The first step toward COG utilization is its recoveryand purification from a coke battery. Hot COG can be used in a BFas make-up gas or in heating furnaces within a coking facility. RawCOG is sent to a treatment plant for NH3, H2S, and tar (BTX) re-moval. Different by-products obtained during COG purificationcan be marketed and individually sold.

COG after purification is considered suitable for H2, syn-gas, andmethanol production. The large amount of H2 in COG can be effi-ciently separated using the PSA technology. Installing a PSA unitto recover H2 from COG for onsite use or separate sale is highly rec-ommended in the steel industry. The high H2 content of COG alsomakes COG an ideal feedstock for syn-gas and methanol produc-tion. The installation of such COG utilization units within a cokingfacility can promote the self-sufficiency and environmental friend-liness of the steel industry.

298 R. Razzaq et al. / Fuel 113 (2013) 287–299

Enrichment of the CH4 content in COG can also be achievedthrough catalytic co-methanation of CO and CO2. Research isneeded in developing a highly active and stable methanation cata-lyst that could promote the complete COx conversion at a low tem-perature and, at the same time, maintain high CH4 selectivity.Although large-scale, cost-effective commercial COG methanationmay not be viable, a small pilot scale can be successfully appliedwithin a coking facility, depending on the COG availability andCH4 demand.

For the past few years, research and development in COG utili-zation technology have made it a highly commercial and profitableindustry in China, attracting both local and international investors.Aside from some installed COG recovery and utilization facilities, anumber of projects are in the pipeline to effectively harness COGfor power, LNG, and methanol production on a large commercialscale.

Acknowledgement

The authors gratefully acknowledge financial support from theNational Natural Science Foundation of China (No. 21006113 and51104140), the National High Technology Research and Develop-ment Program of China (No. 2011AA050606).

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