Determination of the maximum theoretical efficiency of model coal gasification/liquefaction processes by second law efficiency analysis

A thermodynamic model of coal gasification and indirect lique-faction was developed to examine the potential upper bounds on the energy efficiency of such processes. The idealized model, embodied in the GASIF computer code developed specifically for this study assumed chemical equilibrium in the raw gas from the gasifier, ideal acid gas removal unit and oxygen plant separative work consumptions, no heat or mass transfer resistance, reversible gas compression, and selective conversion to simple products such as fuel gas, methane, generic gasoline, methanol, and benzene. Available Energy (Exergy) analysis was employed to identify extent of unavoidable energy losses in the gasification and conversion process, determine plant utility coal requirements, and optimize heat recovery.

Parametric studies were conducted on the impact of gasifier temperature and pressure, steam and oxygen feed rates, and coal drying on gasifier and overall plant efficiency for a wet (30 percent moisture) Wyoming subbituminous coal and lower moisture Illinois No. 6 and Pittsburgh seam bituminous coals. Efficiencies are reported in terms of available energy ratio (Aout/Ain), cold-gas higher heating value ratio (HHVR), and cold-gas lower heating valve ratio (LHVR).

For the gasifier and its supporting utilities, it was found that an optimum steam to oxygen ratio may exist such that the coal is completely gasified at the gasifier temperature without the release of excess heat. This optimum feed also determines the maximum efficiency for the gasifier and utility system. Any departure from the optimum feed ratio results in a decrease in efficiency. A plot of efficiency versus temperature under optimum feed conditions yields curves with peak efficiencies around 90 percent. Pressure operation and coal drying were not found to be useful, except for high temperature gasification.

The raw synthesis gas is converted to a number of model products, a 10:1 H2:CO fuel gas, methanol, benzene, generic gasoline (-CH2-), and methane. A plot of overall plant efficiency for each product vs gasifier temperature is relatively flat and lacks the peak observed for the gasifier and utility efficiency. This is due largely to increased production of methane in the gasifier at the low temperatures where gasifier efficiency is impaired. The high methane content reduces the proportion of the synthesis gas converted to the main product and moderates overall plant losses. Maximum fuel gas overall efficiencies of 90 percent (HHVR) were predicted by the model. Methanol production yielded the highest liquefaction efficiency, 77 percent (HHVR), while all of the hydrocarbons achieved 70 percent (HHVR). Current gasification/liquefaction designs fall 10 to 20 percent below these levels, indicating that some improvement may be obtained with advanced designs.