Abstract
Ash-free coal (AFC), along with residual coal (RC), was produced from low rank coal by solvent extraction method using a single column type solvent extractor (SinExtTM), which is a simple, efficient, and easy tool to remove ashes from coals.
The produced AFC is clean (< 200 ppm ash) and highly calorific (> 8000 kcal/kg), and thus potentially applicable in the following fields
- Complementary/alternative fuel for coal-fired power generation, next-gen power generation process combined with gasification.
- Distributed power generation system with direct-fired micro gas turbine.
- Alternative fuel for heavy oil, fuel for direct carbon fuel cell (DCFC).
- Additive for coking coal, reducing agent, and various functional carbon materials.
Introduction
In the past, the clean coal technology (CCT) focused on handling the environmental pollutants generated when coal was combusted. Recently, the technology has extended its use to reduce the greenhouse gas (carbon dioxide) and attracted much interest.
Three targets for the development of the clean coal technology are as follows:
- To increase the energy efficiency compared to the previous coal-fired power plant; USC, IGCC, IGFC, EFCC, DCFC
- To remove ashes and impurities in coal; UCC, HPC
- To upgrade low rank coal that is cheap and rich; drying and stabilization technology
Ash removal can be carried out by physical, chemical, and biological refining method. The chemical refining method is the most popular, such as namely ultra clean coal (UCC) (Steel and Patrick, 2001) and hyper-coal (HPC) (Okuyama et al., 2004) process.
In the UCC process that has been developed by CSIRO in Australia and NEDO in Japan since 1995, ash in coal is removed by acid and/or alkaline washing. Energy technology research institute at AIST in Japan has developed the HPC process since 1999, where soluble carbon component in coal is extracted thermally by organic solvents.
In Korea, the basic research on the ash removal was initiated by Korea Institute of Geoscience and Mineral Resources (KIGAM) from the mid of 1990s. The KIGAM removes ashes using a superacid (for example hydrofluoric acid), which is basically similar to Cenfuel performed in CarbonX in USA(KIER-A92414, 2010).
The KIER has performed small-scale AFC production based on the organic solvent extraction process from 2006 to 2010. SinExtTM that is an improved version of the previous extraction system consisted of extraction, settling, and filtration part, was developed and patented in 2011. Since last year, a 1 t/d SinExtTM plant is under development supported by the government, and expected to operate in 2015.
Single column type solvent extractor (SinExtTM)
SinExtTM integrated extraction, separation, and filtration process within a single reactor and therefore the construction and maintenance is simple and easy.
Continuous process of AFC production combines the reactor with a slurry mixer, dryer and solvent recovery machine. In the slurry mixer, coal and organic solvent are mixed at 1:4~1:10 ratio, which is then preheated at 130 ⁰C in order to remove moisture in coal. The slurry injected from the slurry mixer to the reactor is heated at 350 ⁰C, dissolved, settled, and filtered. The filtered solution flowing out of the top of the reactor is transferred to a dryer where AFC is obtained. RC is attained by drying non-extracted slurry at the bottom of the SinExtTM.
Figure 1. Continuous manufacturing process of ash-free coal (SinExtTM).
Properties of ash-free coal and upgraded residual coal
Table 1 shows the proximate analysis and calorific value of AFC and RC. Those of a parent coal (PC, low rank coal) are also given.
A moisture and ash content of AFC are decreased to almost 0 and the calorific value is increased by 94% compared to PC. Ash content in RC is higher than PC. However, most of the moisture is removed after the extraction process. So the calorific value of RC is increased by 69% compared to PC.
Table 1. Proximate analysis and calorific value
Applications
- Complementary/alternative fuel for coal-fired power generation, next-gen power generation process combined with gasification
- Distributed power generation system with direct-fired micro gas turbine
- Alternative fuel for heavy oil, fuel for direct carbon fuel cell (DCFC)
- Additive for coking coal, reducing agent, and various functional carbon materials
Figure 2. Highly efficient power generation system adopting AFC production process.
Future Prospects
- At coal-fired power plant, increase of generation efficiency and reduction of ash and CO2
- At gasification, configuration of compact gasifier by excluding ash fusion phenomena
- At combined cycle gas turbine (CCGT) system, increases in efficiency (~50%) and life span
- Alternative for expensive coke-making materials, various additives & functional carbons
- Low price of low rank coal to ease economic burden of power generation
The 10 largest coal producers and exporters in Indonesia:
- Bumi Resouces
- Adaro Energy
- Indo Tambangraya Megah
- Bukit Asam
- Baramulti Sukses Sarana
- Harum Energy
- Mitrabara Adiperdana
- Samindo Resources
- United Tractors
- Berau Coal
