Coal fuel gas cleaning by non-thermal pulsed corona discharge plasma and “reach” regulation compatibility assessemnt for trace elements extraction from gasification ash
Dissertação de mestrado, Inovação Quimica e Regulamentação, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2016Atmospheric small-scaled fixed-bed gasifiers fed with cheap low rank sub-bituminous coal produces
syngas (CO and H2) with high tar content, which is one of the impurities produced along the main
syngas from coal gasifications. This organic impurity with high molecular weight hydrocarbons is of
interest as they polymerize or condense to more complex structucres throughout the involved process
pipers or heat exchangers, leading to fouling and attrition problems, which eventually leads to lose of
overall plant efficiency and increased operation costs. To avoid such event, either expensive non-tar
forming coal (semi-Anthracite or Anthracite) must be used or an effective tar removal unit integration
in the overall process should be made.
Plasma is the fourth state of matter and it contains free radical, ions and excited molecules and they
create a highly reactive atmosphere as these reactive species carry enough energy to initiate tar
decomposition reactions. Non-thermal plasmas are already successfully utilized in air pollution control
for the VOC removal. Within the non-thermal pulsed corona discharge plasma scope, Technical
university of Eindhoven (TU/e) studied biomass tar reforming (naphthalene as the tar model) and
various syngas compositions were tested to study their impact on tar removal process. Furthermore,
non-thermal pulsed corona discharge plasma is found to be effective in tar reforming and is created by
supplying electricity and nitrogen gas to the plasma reactor. Created plasma dissociates the CO2
components in the syngas into CO and O radicals, which the unstable reactive O radicals oxidize tars
into light hydrocarbons (CH4). 50% nitrogen content in the syngas due to plasma requirement limits
its usage only as fuel gas for heating or electricity generation.
After determining utilizing of plasma together with atmospheric fixed-bed gasifier is technologically
possible, the demand for it in fuel gas application to generate heat is researched. The research involved
carefully looking at energy policy of that chosen particular country and their main source of energies.
According to the International Energy Agency’s 2015 statistics, China and India are placed largest
coal consumers in the non-OECD countries ranking. It was estimated that China currently needs over
8000 fixed-bed gasifier (8000 plasmas) to meet the industrial heat demand. Assuming a similar
development in India, in total 2000 fixed-bed-gasifiers will be needed in the next years. In the
researched countries, current alternative method to generate heat instead of Natural gas or LPG is fuel
gas via coal gasification. Non-tar forming quality coal are gasified, but they are either expensive due
to the high demand and are not widely available. Syngas from this case is cleaned through electrostatic
precipitator light tar collectors (if present) before utilizing it. These fuel gas-cleaning methods are to
remove very small amount of light tars (if present) and dusts. It is a common practice in developing
countries to produce fuel gas via coal gasification for the puspose of heat and electricity generation. It
was found that this method is cost effective than using natural gas or LPG. Furthermore, it was found
that fuel gas generation via plasma-involved case were even more cost effective than the current state
of art case by at least 10%. The fuel gas production cost via plasma involved proposing configuration
is competitive over the fuel gas production cost from the current state of art. In addition to cost
benefits, plasma cleaned fuel gas production approach allows utilizing of low rank coal and does not
utilize water, hence fresh water consumption and pollution is prevented.
Abundantly available coal ashes are potential untapped resource for trace elements (TE). In 2014, the
European union member states (EU-28) had consumed 285 million tones of hard coal and based on the
world trace elements average in world coal, the available TE for extraction exceeds 1 tonne per year.
Therefore, TE extraction from available coal ashes in EU-28 is subject to REACH regulation.
However, there is no entry on ECHA database for such process. The entries at ECHA database
regarding coal ash are only for the utilization for construction materials purpose. Lack of
commercially available extraction technology optimized for coal ash, limited understanding of trace
elements modes of occurrence, origin, and toxicological data relating to all possible chemical
contaminants rising from extraction process are not well understood and are not presently available.
More research and development effort must be done in order to obtain these missing information and
to perform full chemical characterization of the coal ash to optimize trace elements extraction process
for that particular coal and to identify all possible waste streams. Such that, needed toxicological data
according to REACH regulation is obtained
