Leading Edge of Coal Utilization Technologies for Gasification and Cokemaking

Abstract

Coal is a very important resource for power generation and cokemaking. Moreover, coal is a very useful resource for producing city gas and chemical materials by gasification technology. Low grade coals are suitable for the coal gasification resources because they are easily decomposed and converted to generate gas in the gasifier. On the other hand, high quality coals such as good quality bituminous coals are required for producing metallurgical coke. Recently, the amount of high quality coals has been decreasing. The expansion of raw coal brands for producing metallurgical coke is very important. In this paper, the development of high energy efficiency gasification technology, ECORO^®, and new cokemaking technologies such as the DAPS and the SCOPE21 enabling the expansion of coal resources are introduced. These technologies are contributed to the expansion of coal resources and energy savings.

1. Introduction

Coal is a very important resource for power generation and cokemaking. Moreover, coal is a very useful resource for producing city gas and chemical materials by gasification technology. Low grade coal is suitable as coal gasification resources because they are easily decomposed and converted to generate gas in the gasifier.

On the other hand, high quality coals such as good quality bituminous coals are required for producing metallurgical coke. Fig. 1 shows the coal band and the expansion target of coal resources for producing metallurgical coke (Kato, 2008). In the conventional cokemaking process, only high quality bituminous coals can be used as raw coals. Recently, the amount of high quality coals has been decreasing. The expansion of raw coal brands for producing metallurgical coke is very important. In Japan, improvement of coke quality is strongly demanded to smoothly operate large inner volume blast furnaces. Therefore, the effective coal utilization technologies involving the dry coal charging process and the new cokemaking process were developed.

Fig. 1

Coal band.

In this paper, the trend of gasification technologies and new cokemaking processes enabling the expansion of coal resources are introduced.

2. Coal gasification technology

2.1 Trend of coal gasification technology

Coal gasification is the technology to convert coal into gas such as hydrogen (H₂), carbon monoxide (CO) and methane (CH₄) at high temperature using air or oxygen and steam as the gasifying agent. The main coal gasification reactions are shown in equation (1) to (8). The generated gas is used as the clean gas for raw materials to produce city gas, chemical products etc.

• 1) Pyrolysis;

  Coal to gas component, tar, heavy oil, char(C)

• 2) Reaction with oxygen;

    

  $$\text{C}+{\text{O}}_2={\text{CO}}_2+393\hspace{0.17em}\text{kJ}/\text{mol}$$(1)
    
  $$\text{C}+1/2{\text{O}}_2=\text{CO}+111\hspace{0.17em}\text{kJ}/\text{mol}$$(2)
    
  $$\text{C}+{\text{CO}}_2=2\text{CO}-171\hspace{0.17em}\text{kJ}/\text{mol}$$(3)

• 3) Reaction with steam

    

  $$\text{C}+{\text{H}}_2\text{O}={\text{CO}}_2+{\text{H}}_2-131\hspace{0.17em}\text{kJ}/\text{mol}$$(4)
    
  $$\text{C}+2{\text{H}}_2\text{O}={\text{CO}}_2+2{\text{H}}_2-76\hspace{0.17em}\text{kJ}/\text{mol}$$(5)
    
  $$\text{CO}+{\text{H}}_2\text{O}={\text{CO}}_2+{\text{H}}_2+41\hspace{0.17em}\text{kJ}/\text{mol}$$(6)

• 4) Reaction with hydrogen

    

  $$\text{C}+2{\text{H}}_2={\text{CH}}_4+75\hspace{0.17em}\text{kJ}/\text{mol}$$(7)
    
  $$\text{CO}+3{\text{H}}_2={\text{CH}}_4+{\text{H}}_2\text{O}+206\hspace{0.17em}\text{kJ}/\text{mol}$$(8)

Fig. 2 shows the history of the coal gasification technology (Gräbner M., 2014). The history of coal gasification is divided into three generations. The first generation of industrial gasification arose from the idea of supplying a chemical synthesis produced from coal. A typical example is the Winkler fluidized bed gasifier, which found its first commercial application in 1926 in Germany. Among the first generation of gasification technologies, the Lurgi fixed bed dry bottom technology was developed.

Fig. 2

Trend of coal gasification technologies.

Besides minor industrial factors, the oil crises relaunched interest in coal gasification again leading to the development of a second generation of coal gasification processes from 1970s until the early 1990s.

For fluidized bed and entrained bed processes, the gasification pressure should be raised from atmospheric to 2–6 MPa. For fluidized bed and fixed bed processes, the performance was enhanced in the points of high carbon conversion rate and high energy efficiency. Another focus was on the integration of heat recovery from syngas by steam generation if gasification is employed for power generation.

However, after a decade of relative silence surrounding coal gasification, beginning around 2000s, the following general trends have renewed interest in the technology.

• (1) Substitution of crude oil by other energy carriers, such as biomass or coal targeting supply security and local energy price stabilization
• (2) Increasing interest in the use of low grade coals with high ash or moisture contents in emerging nations

The Lurgi process uses the pressurized fixed bed gasifier. The Winkler process uses the fluidized bed type gasifier developed in German, and the Koppers-Totzek process uses the entrained bed gasifier developed in Germany in 1952.

After the first oil crisis of the 1970s, coal was reviewed as the industrial raw materials. Various types of gasification process were developed and the energy efficiency was improved. The feature of the Texaco and the GE process is a coal-water slurry type, and the Shell furnace is a dry process.

One of the coal gasification technologies developed in Japan is the ECOPRO^®. This technology has the feature of two stage coal gasification.

2.2 ECOPRO^® gasification technology

ECOPRO^® is the gasification technology with an entrained bed type gasifier with two stages (Kosuge et al., 2014). Generally, an entrained bed type gasifier can take high carbon conversion and high energy efficiency. Furthermore, the energy efficiency of the ECOPRO^® process is higher than that of a conventional entrained bed gasifier because it has two stage reactors. Fig. 3 shows the outline of the ECOPRO^® process. First, coal is crushed and dried to prepare pulverized coal for gasification. Pulverized coal is introduced into both of the chambers of the gasifier with the career gas, and quickly converted to the syngas. In the lower chamber, coal is reacted with oxygen and steam. Partial oxidation reaction occurs, and syngas is generated from 1300 °C to 1400 °C. The generated syngas is introduced into the upper part of the reactor. In the upper chamber, the coal pyrolysis reaction is occurred by the sensible heat of the high temperature syngas introduced from the lower chamber. Syngas including methane and char is generated at 1100 °C in the upper chamber.

Fig. 3

Process flow of the ECOPRO^®.

Fig. 4 shows the unique feature of the ECOPRO^® process. In the ECOPRO^® process, the sensible heat generated during coal partial combustion at the lower chamber can be used for coal pyrolysis at the upper chamber. Therefore, total energy efficiency of the process increases by 5 % compared with other conventional gasification processes.

Fig. 4

Comparison of energy efficiency involving the ECOPRO^® and conventional gasification process.

In other gasification processes, the sensible heat from the high temperature syngas obtained by coal gasification is recovered as steam by the boiler. Therefore, the maximum energy efficiency of these processes is 80 % (DOE/NETL, 2011). In the ECOPRO^® process, part of the sensible heat from the high temperature syngas generated at the lower chamber is used for coal pyrolysis in the upper part of gasifier. As a result, the ECOPRO^® provides 85 % energy efficiency (Kosuge et al., 2014).

Fig. 5 shows the experimental apparatuses of ECOPRO^®. Since the early 1990s, Nippon Steel & Sumikin Engineering has been developing coal gasification technologies and has developed the ECOPRO^® process. Based on the basic research with 1 kg/d-scale experimental apparatus (1992–1996) and bench scale tests with 1 t/d scale unit, pilot plant scale tests with 20 t/d started in 2003 (Fig. 6) (Kosuge et al., 2014).

Fig. 5

Experimental apparatuses of the ECOPRO^®.

Fig. 6

Development schedule of the ECOPRO® process.

The pilot plant operation with three different types of coal samples of brown coals and sub-bituminous coals had been conducted for 3101 h in total. Fig. 7 shows the coal characteristics used for the pilot plant operation. It was clarified that the ECOPRO^® process is suitable for a wide range of coal resources such as sub-bituminous coals and brown coals.

Fig. 7

Coal map suitable for gasification technology.

3. Cokemaking technology

3.1 Conventional cokemaking technology

Coke is mainly used for producing pig iron using the blast furnace method. More than 90 % of coke produced in Japan is used for blast furnace (Fig. 8). The role of coke is mainly as follows, iron ore reducing agent, heating material and permeability maintaining spacer that sustains the flow passes in the blast furnace (Fig. 9). Coke is very important because no alternative of coke is available in the blast furnace process.

Fig. 8

Schematic diagram of ironmaking process flow.

Fig. 9

Role of coke on blast furnace operation.

Fig. 10 shows the conventional cokemaking process flow. First, several brands of raw coals are blended and crushed by a coal crusher. Blended raw coals are charged into coke ovens for producing metallurgical coke. Fig. 11 shows the appearance of a coke oven battery. The carbonization time is from 19 to 24 h and the raw coals are carbonized in the coke oven. The carbonization temperature is around 1,000 °C.

Fig. 10

Conventional cokemaking process flow.

Fig. 11

Appearance of coke oven battery.

3.2 Dry coal charging process for improving coke quality

Dry coal charging processes such as coal moisture control (CMC) process and dry-cleaned and agglomerated precompaction system (DAPS) were successfully developed by Nippon Steel & Sumitomo Metal Corp (Kato, 2004; Kato et al., 2006).

The first CMC process using indirect heating in a rotary dryer was operated in 1983. Coal moisture of raw coal charged into a coke oven is reduced from 10 mass% to 5–6 mass% with the CMC process. The use of the CMC process has been spreading because this technology saves energy, permits the increased use of non- or slightly-caking coals, stabilizes the operation of cokemaking process by keeping the moisture content of coal charges constant.

To increase the blending ratio of non- or slightly-caking coals in coal charges, the new pretreating technology for the coal charge, DAPS was developed and came on stream at Nippon Steel & Sumitomo Metal Oita works in 1992 (Kato, 2004).

At first, to evaluate the dust occurrence during the coal transportation from a coal dryer to coke oven, the relationship between the coal moisture and dust occurrence was investigated using a dust occurrence tester. Fig. 12 schematically illustrates the experimental apparatus (Kato, 2004).

Fig. 12

Experimental apparatus for dust occurrence measurement. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

Sample coal 1 kg in weight was put into the experimental apparatus from the top; the coal particles floating inside the tube were sucked by a blower until the tube inside became visually clear of the particles, and the quantity of the particles collected was measured.

Fig. 13 shows the results (Kato, 2004). The dust occurrence increased as the moisture of coal decreased. Fig. 14 shows the photomicrographs of coal grains with different moisture contents (Kato, 2004). With the high moisture content, fine particles either adhere to coarse grains or cohere with each other to form pseudo-particles with water serving as a binder and the dust occurrence is low. On the other hand, when the coal is dried for pretreatment, the pseudo-particles disintegrate into fine particles and the dust occurrence increases.

Fig. 13

Relationship between dust occurrence index and coal moisture. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

Fig. 14

Coal particles in charging coal (SEM). Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

Fig. 15 shows the relation between the fractions of fines that are 74 μm or less in size in the feedstock coal and dust occurrence (Kato, 2004). As a result, it was presumed that the coal particles that are 74 μm or less in size were mainly responsible for the dust occurrence.

Fig. 15

Relation between dust occurrence and the content of under 74 μm of coal charge. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

From the above, it was clarified that agglomeration of fine coals was important to reduce the dust occurrence and stabilize the dry coal charging process.

The application of the fluidized bed method for drying and classification of fine coal in the cokemaking process was studied as the first case in the world. Moreover, a fluidized bed coal dryer was developed that is capable of efficiently drying and classifying roughly 6,800 t/d of coal (Fig. 16) (Kato, 2004).

Fig. 16

Outline of fluidized bed dryer. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

Fig. 17 shows the process flow of DAPS (Kato, 2004). In the process, the coal is dried in the fluidized bed dryer and fine coal is separated from coarser grains by the gas flow, collected by a cyclone separator, and formed into agglomerated by a roll compactor. The ratio of fine coal is about 30 mass% of the coal charge. The agglomerated fine coal is added to the coarse coal and charged into coke ovens.

Fig. 17

Process flow of DAPS. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

The relation between the bulk density of coal charge and total dilatation coefficient of fine coal was investigated. The result is shown in Fig. 18 (Kato, 2004). The dilatation of fine coal increases when the bulk density is increased. From the result, it is apparent that the agglomeration of fine coal not only suppresses the generation of dust but also improves the dilatation of fine coal.

Fig. 18

Relation between bulk density and total dilatation coefficient of fine coal. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

The coke strength from the DAPS process was compared the one from the CMC process under the same coal blending conditions. As a result, the coke quality in the DAPS was improved. The coke quality is thought to be improved in the DAPS process owing to the increase in the bulk density of coal charge, which is caused by a decrease in the moisture of coal charge (Nomura et al., 2004), and the improvement in coal dilatation, which is caused by the agglomeration of fine coal.

Fig. 19 shows the comparison of non- or slightly-caking coal ratio in each process without deteriorating coke strength (Kato, 2004). By the application of the DAPS process to the cokemaking plant, it was found that the ratio of non- or slightly-caking coal in coal charge is increased by 20 mass% compared with the CMC process without deteriorating coke strength.

Fig. 19

Comparison of non- or slightly caking coal ratio in coal charge. Reprinted with permission from Ref. (Kato, 2004). Copyright: (2004) The Iron and Steel Institute of Japan.

As a result, it was found that coke strength was improved in the DAPS process owing to increase in the bulk density of coal charge, due to decrease in the moisture of coal charge. So, it was clarified that the DAPS process was suitable for expansion of coal resources.

3.3 SCOPE21 process

Research and development of a new cokemaking process —super coke oven for productivity and environmental enhancement toward the 21^st century (SCOPE21)—was conducted in Japan from 1994 to 2003 by the Japan Iron and Steel Federation (JISF) (Nishioka et al., 2004).

Fig. 20 shows the SCOPE21 process flow (Kato, 2010). The SCOPE21 process mainly consists of three units. First is coal rapid preheating unit, second is coal carbonization unit and third is coke quality upgrading and quenching unit by coke dry quenching equipment. The aim for dividing the whole process into three is to make full use of the function of each process in order to maximize the total process efficiency.

Fig. 20

Schematic diagram of the SCOPE21 process flow. Reprinted with permission from Ref. (Kato, 2010). Copyright: (2010) The Iron and Steel Institute of Japan.

Preheating the charging coal heated up from 330 °C–400 °C in the coal pretreatment facility has the effect of reducing carbonization time (Fig. 21) (Kato, 2010).

Fig. 21

Comparison of carbonization time between SCOPE21 and conventional process. Reprinted with permission from Ref. (Kato, 2010). Copyright: (2010) The Iron and Steel Institute of Japan.

The pilot plant had a coal pretreatment facility, which was the scale-up version of the bench scale plant and a coke oven (Fig. 22) (Kato, 2010). The coal pretreatment facility was designed to have a 6 t/h coal throughput, and the basic specifications were determined from the bench scale plant data. One coke oven was constructed. The coke oven chamber was 8 m in length, which was almost half of the length of the commercial plant, 7.5 m in height, and 450 mm in width.

Fig. 22

Process flow of SCOPE21 pilot plant. Reprinted with permission from Ref. (Kato, 2010). Copyright: (2010) The Iron and Steel Institute of Japan.

In the coal rapid preheating test, the coal was heated slowly to 300 °C in a fluidized bed dryer, and then heated rapidly to 380 °C in a pneumatic preheater, and carbonized in the coke oven (Matsuura et al, 2004; Kato et al., 2004, Matsuura et al., 2005). The quality of the obtained coke was measured by the JIS drum strength index (DI¹⁵⁰₁₅) ( DI: Drum Index). (Kubota et al., 2004). Non- or slightly-caking coal was blended 50 mass% in the coal charge. As a result, coke strength (DI¹⁵⁰₁₅) became about 2.5points higher than the conventional level by virtue of the rapid preheating effect and the increased bulk density (Fig. 23) (Kato, 2010). Pilot plant scale test of the SCOPE21 process was conducted successfully and targets of the project were confirmed by the pilot plant test (Sugiyama et al., 2005).

Fig. 23

Technologies for improving coke quality. Reprinted with permission from Ref. (Kato, 2010). Copyright: (2010) The Iron and Steel Institute of Japan.

The SCOPE21-type new coke oven battery was constructed at Nippon Steel & Sumitomo Metal Oita works from 2006 to 2008 and the operation of the new coke plant was started in 2008. The coke production capacity is 1 million ton per year. Fig. 24 shows the process flow of the new coke plant and Table 1 shows the main specification of Oita No. 5 coke oven battery (Kato, 2010). The coal is dried in a fluidized bed dryer and fine coal is separated, and then agglomerated by an agglomerater. Next, the agglomerated fine coal is added to the coarse coal, and charged into coke ovens. The coal is pre-heated rapidly to 350 °C in a pneumatic pre-heater, and carbonized in the coke oven.

Fig. 24

Process flow of Oita No. 5 coke oven battery. Reprinted with permission from Ref. (Kato, 2010). Copyright: (2010) The Iron and Steel Institute of Japan.

Table 1 Specification of Oita No. 5 coke oven battery. Reprinted with permission from Ref. (Kato, 2010). Copyright: (2010) The Iron and Steel Institute of Japan.

Equipment		Specification (Capacity)
Coal pre-treating	Fluidized bed dryer	155 dry-t/h
	Pneumatic pre-heater	106 dry-t/h
	Agglomerater	34 dry-t/h ~2
Coke oven CDQ	Coke oven chamber	64 ovens, 6.7 mH * 0.45 mW * 16.6 mL
		120 t/h

The 2^nd SCOPE21-type new coke plant was constructed at Nippon Steel & Sumitomo Metal Nagoya works and the operation of the plant started in 2013 (Kato et al., 2013).

These two plants have been operated very smoothly with high productivity and high non- or slightly-caking coal ratio in coal charge.

4. Conclusion

Coal is a very important resource for power generation, cokemaking, gasification, etc. R & D of effective coal utilization technologies has been conducted. In this paper, the trend of the coal gasification technology and new coal gasification technology, ECOPRO^®, are introduced. Furthermore, new cokemaking technologies are discussed. They are summarized as follows.

• (1) New gasification technology the ECOPRO^® has been developing. ECOPRO^® is a suitable process for low rank coals gasification with high energy efficiency.
• (2) To expand the coal resources for metallurgical coke making process, dry coal charging process DAPS was developed. Coke strength was improved in the DAPS process owing to increase in the bulk density of coal charge, due to decrease in the moisture of coal charge. So, it was clarified that the DAPS process was suitable for expansion of coal resources.
• (3) Furthermore, new cokemaking technology SCOPE21 for improving coke quality was developed. New coke plants of the SCOPE21-type were constructed at Nippon Steel & Sumitomo Metal Oita and Nagoya works. The new coke plants have been operated very smoothly.

Author’s short biography

Kenji Kato

Kenji Kato is a general manager of R & D institute, Nippon Steel & Sumikin Engineering Co. Ltd. since 2013. He graduated Chiba University and entered Nippon Steel Corporation in 1981. He was a general manger (2009–2012) of Nippon Steel Corporation, a general manager (2012–2013) of Nippon Steel & Sumitomo Metal Corporation. He received his doctoral degree from Tohoku University (Environmental Science) in 2005. His research area ranges over cokemaking, low rank coal utilization technologies and environmental technologies.

Keiji Matsueda

Keiji Matsueda is a general manager of head office, Nippon Steel & Sumitomo Metal Corporation. He received his master degree from Nagaoka Universiyty of Technology and entered Nippon Steel Corporation in 1991. His research covers metallurgical cokemaking technology.

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