(203w) Development of a New Polygeneration Plant Based On Integrated Coal Gasification, Natural Gas Reforming and Carbonless Energy

Development
of a new polygeneration plant based on integrated coal gasification, natural
gas reforming and carbonless energy

Yaser
Khojasteh Salkuyeh, Thomas A. Adams II

Department of Chemical Engineering,
McMaster University, 1280 Main St W, Hamilton, Ontario, L8S 4L7, Canada.

Polygeneration
is a processing technique that utilizes multiple feedstocks such as coal, natural
gas and biomass to produce multiple energy products such as diesel, gasoline,
methanol, hydrogen, dimethyl ether (DME), and electricity. It is a promising
approach in terms of energy efficiency, profitability, and the ability to
decrease dependency on limited and unsecure sources of crude oil. In addition, by
producing a mixture of products, one can take advantage of synergies between each
process to achieve both higher profits and better energy efficiency than
standalone processes.

In
this work, a novel polygeneration plant is proposed that uses coal, natural gas
and also optionally high-temperature helium from a Gas Turbine Modular Helium
Reactor (GT-MHR, a 4th generation nuclear power plant), and produces
electricity, methanol and DME.  In traditional processes which convert natural
gas to liquids, a reformer unit is used to convert the natural gas to syngas;
however, this reaction is endothermic and traditionally a large amount of extra
natural gas has to be burned to provide this heat.  The aim of integrating the
GT-MHR unit into the polygeneration plant is to offer a novel strategy that
supplies the heat requirement of the natural gas reformer in order to avoid
this natural gas burning and reduce fossil fuel consumption.

As
shown in Figure 1, two different types of gasifiers are used for liquid fuel
and hydrogen production. The operating conditions and syngas composition of
each gasifier are suitable for either liquid fuel synthesis or hydrogen
production. Furthermore, incorporation of iron-oxide chemical looping
gasification (CLG) is studied such that there is no direct carbon emission from
this process. In this approach, the generated syngas is oxidized with oxygen
carrier (Fe₂O₃ here) to get reduced solid (Fe) and high
temperature CO₂ and H₂O gas phase stream. This CO₂
stream can be easily purified by condensation of steam, achieving 100% CO₂
capture. Then, the reduced metal will be oxidized in another reactor using steam
and produces pure hydrogen and metal-oxide intermediate. For the final step of
CO₂ capturing, hydrate formation of CO₂ is considered as
a novel strategy that requires operating pressure less than typical
liquefaction technologies and crystallization temperature above the freezing
point of CO₂. By using this, CO₂ hydrate trucks can be
used instead of CO₂ transportation pipelines.   

Fig.
1.
Simplified block diagram of polygeneration plant.

To
determine the operating conditions of each unit of this complex process, steady
state simulations in Aspen Plus process simulator was performed. A sensitivity
analysis for a wide range of coal to natural gas input ratios and power output
to liquid fuel ratios is considered to determine the thermal efficiency of
plant based on market demand. In addition, nickel-oxide chemical looping
combustion (CLC) is modeled and compared with a conventional hydrogen
combustion turbine for a range inlet temperatures. The results indicate that
the optimal operating conditions and efficiencies vary widely depending on the
desired product portfolios.  For example, the results show that maximum net
power generation can be achieved with temperatures between 1250 and 1325 °C for
the NiO loop, but around 1400 °C is optimal for the combustion turbine. The
results indicate the highest achievable efficiency is about 33% (HHV) for coal
only and power generation only, which is greater than all other traditional coal-to-electricity
processes with 100% carbon capture. Furthermore, by using the natural gas
reformer with the GT-MHR unit, this efficiency can be increased to about 37.5%
when just 40% of fossil fuel input is natural gas. In this case, about 7% of
thermal input comes from GT-MHR plant. In addition, after adding DME and
methanol to the product portfolio, the thermal efficiency can be increased up
to 49.5% (HHV), while 50% of marketable output is electricity and 100% of CO₂
wastes are captured. Overall, the efficiencies range from 33.3% to 53.3% (HHV) depending
on the desired mixture of fuels and products.