(225d) Modelling of Integrated Gasification Combined Cycle (IGCC) Systems with Environmental Concerns

Integrated Gasification Combined Cycle (IGCC) systems are an alternative to Pulverized Coal (PC) combustion systems. They are believed to replace the aging PC combustion plants and out increasingly expensive natural gas plants. IGCC systems have the potential to realize higher efficiency and better environmental performance for power generation. They also offer greater fuel flexibility and can offer multiple products. The process offers options to eliminate greenhouse gases, produce Hydrogen and/or liquid fuels. The potential for CO2 sequestration makes IGCC technology even more appealing technology and environmentally friendly.

Due to the emission of different pollutants, specially Green House Gases, from the widely existing coal combustion plants for power generation, environmental regulations are driving the development of new coal based electric power generation technologies.

In comparison with modern coal combustion plants, IGCC systems are characterized by lower SO_x and NO_x emissions, comparable VOC and mercury emissions, 20% less CO2 emissions, use of 20-40% less water, operating at higher efficiencies thus requiring less fuel and producing less emission. Current efficiency is 42% ( with high turbine efficiency), with efficiencies as high as 60% expected in the near future using high efficiency turbines and some other process improvements.

These days, energy conversion systems, like IGCC, pay attention to improve their conversion efficiency, leading to more potential use of the coal resource. Moreover, the decrease in coal consumption also contributes to the reduction of green house gases and other pollutants which are released due to the coal utilization.

2. Process Description and Modeling

The coal, (Illinois #6 for the case considered), is crushed and mixed with water to produce a slurry that is 35.5% by weight water. This slurry is pumped into the gasifier along with oxygen. The gasifier operates in a pressurized, down flow, entrained design and gasification takes place rapidly at temperatures in excess of 1250 ºC. The raw fuel gas produced is mainly composed of H2, CO, CO₂, and H₂O. The coal's sulfur is primarily converted to H2S and a smaller quantity of COS. This raw fuel gas leaves the gasifier at 1250 ? 1550 ºC along with molten ash and a small quantity of unburned carbon. No hydrocarbon liquids are generated. This gas/molten solids stream enters either a radiant syngas cooler (RSC) and convective syngas cooler (CSC) sections.

IGCC Base Case has been developed for a Texaco Gasifier  with Radiant/Convective cooling system. In this design, the mix of gas/solids from the gasifier enters a radiant syngas cooling (RSC)system, where cooling to approximately 815 ºC is accomplished by generating high-pressure steam. A convective syngas cooling(CSC) /gas scrubbing system cools the raw fuel stream to about 150 ^ºC (27.5 bars) by generating additional steam. It uses a gas scrubber and a low temperature gas cooling/heat recovery section to reduce the raw fuel gas stream to 40 ^oC prior to entering a CGCU section for sulfur removal. 3. Methodology

Simulation was controlled by several FORTRAN routines and design specifications. Since this is a large and complicated simulation with many nested loops, it has been recognized that the simulation is very sensitive towards the loop's break points and their initial conditions. After detailed analysis, a specific computational sequence was set up for each model, and the ranges of initial conditions were established.

The GT net power output is  fixed to be 272MW, then the coal flow rate is adjusted accordingly. The oxygen is controlled so that no external energy is required for complete gasification (i.e. until the net duty of the gasifier is zero).

  4. Results and Discussions 4.1 Effects of gasification temperature

The sensitivity of the process for the gasification temperature is done under practical range of temperatures where gasification can take place with slagging of the ash. The range taken is between 1250ºC and 1550ºC.

As the gasification temperature increases the thermal efficiency decreases due to a decrease in the cold gas efficiency . This decrease in cold gas efficiency is due to an increase in the O₂:C ratio  in order to combust more carbon to attain high temperature level. The Air:Syngas ratio for the combustor also decreases due to a decrease in the heating value of the syngas as a significant amount of the carbon is combusted to CO₂. With the increase in gasification temperature, the total net power increases  because of an increase in the steam turbine power output due to an increase of the slurry input for the same quantity of gas turbine output; but the net power output per ton of coal consumed has a decreasing trend because of a decrease in thermal and cold gas efficiencies.
Inline with the decrease in thermal efficiency, the CO₂ and SO_x emissions per unit of power output increase due to the increase in the coal consumption for the same level of GT power output.  But the NO_x emission per unit of power output decreases due to  a decrease in the Air: clean syngas ratio thereby lessening the thermal NO_x formation.
4.2 Effects of gas turbine inlet  temperature (syngas combustion temperature)

The sensitivity of the process for syngas combustion temperature is performed for the range of temperatures between 1250 ºC and 1550 ºC where the base case is at 1416 ºC. For this analysis (in fact for the rest of the analyses) the gasification temperature is taken to be 1250ºC because it is the value with best thermal efficiency obtained from the previous analysis.

As the combustion temperature increases, the thermal efficiency increases because of a decrease in coal consumption for a fixed amount of gas turbine output. This is evident from. that as the combustion temperature increases the power output per ton of coal input also increases. Decreasing of the coal (slurry) consumption implies a decrease in steam turbine output; therefore, the total power output has a decreasing trend.
Along with an increase in thermal efficiency the CO₂ and SO_x emissions per unit power output also decrease. This is again due to the decrease in the level of coal consumption for the same GT power output. But, the NOx emission increases because of an increase in thermal NOx formation at higher temperatures.
4.3 Effects of level of N₂ injection

For this analysis, all being the same to the original base case, the gasification temperature is taken to be 1250ºC and the syngas combustion temperature is taken to be 1550ºC.

As the fraction of N₂ injection to the GT combustor increases:

1.      The thermal efficiency increases due to a decrease in the slurry (coal) requirement as more N₂ is used to drive the turbine.

2.      Net power output decreases due to a decrease in the steam turbine power output which is intern due to a lower level of available energy to be recovered by the heat recovery steam generator (HRSG) due to a decrease in coal flow.

3.      Net power output per ton of coal input increases because of the decrease in coal requirement for the same level of GT output.

4.      CO₂, SO_x and NO_x emissions decrease also due to the decrease in the coal consumption and the diluting effect of the N₂ so that inhibiting thermal NOx formation.
Effects of solid concentration in coal slurry

A sensitivity analysis was done for a solid concentration range of 60% to 80%.

As the solids concentration increases, the O2:coal ratio decreases because the required energy to vaporize and superheat the water decreases. Therefore the syngas heating value increases because less call is being used to supply energy for the gasification. Due to this, the thermal efficiency, the net power output and also the net power output per ton of coal input increase. The CO₂ and SO_X emissions have a decreasing trend because of the use of lesser amount of coal for the same amount of GT output. But, the NO_X emission increases due to an increase in the air : syngas ratio required by the GT combustor.

At the end of these analyses, the maximum thermal efficiency /(LHV) that is attained is 41.2%. This result corresponds to a gasification temperature of 1250ºC, a combustion temperature of 1550ºC, 98% of N2 injection to the GT combustor and a slurry solid concentration of 80%. For practical application of this improvement, among other considerations like capacity of the equipments, the flowability of the slurry at this level of solids has to be studied.

Further improvements in the efficiency could be considered with  the gas turbine, compressor and steam turbine parameters like isentropic efficiency, mechanical efficiency and pressure ratio. But, improvements related with the turbines has to consider the current level of technology with these equipments. For example, for an isentropic efficiency of the GT air compressor equal to 92%, the thermal efficiency will be 42.5% and with further improvements, higher efficiencies could be attainable.