Advantages of Gasification
Gasification has the potential for highly efficient power generation. While a conventional subcritical pulverized coal (PC) power plant has a typical plant efficiency of about 35% (please see below for an introduction to how efficiencies are calculated), an integrated gasification combined cycle (IGCC) power plant can have a plant efficiency from 38 to 41% depending on the gasification and heat recovery technologies employed and the degree of plant integration with other processes, like air separation, for example. When coupled with other advanced technologies under development, like hydrogen turbines and solid oxide fuel cells, a gasification power plant can have efficiencies as high as 60%—a very substantial gain over conventional technologies.
High Efficiency: Comparison of Gasification and PC Plants - Comparison of gasification based power production and PC power production.
A typical plant efficiency value reveals how much of the energy contained in a fuel (coal or natural gas, for example) is output as a useful product, often electricity. The efficiency value also tells of a plant's inefficiencies, like wasted heat or energy intensive subsystems (gas cleaning, for example).
Efficiency is limited by thermodynamics. Heat loss and friction typically account for most of a gasification system’s inefficiencies. Heat exchangers are used to try to reclaim and reuse heat and/or pressure lost during temperature and phase changes, but there are always losses to the environment—and any heat that can’t be reclaimed lowers the system efficiency.
Improved integration of systems like the gasifier and the turbine generators in IGCC can increase efficiency. Reducing the energy requirements of any subsystem (i.e., the gas cleaning system) increases the plant efficiency as a whole. More examples of ways to increase efficiency can be found below.
Energy Conversion Processes Efficiency
Efficiency, which is sometimes represented by the lower case Greek letter η (eta), is generically expressed mathematically: , or in other words, what percentage of the energy put in is converted to product (output) energy.
The energy input by a fuel is typically measured in two different ways: a higher-heating value (HHV) or a lower-heating value (LHV).
- The HHV—also known as the gross calorific value of a fuel —is the amount of heat released following total combustion and after the products have returned to the starting reference temperature (25ºC).
- The LHV—also known as the net calorific value of a fuel —is the amount of heat released upon total combustion (starting at 25ºC) and the products temperature has cooled to 150ºC.
- The difference in temperature at the end of the measurement means the LHV does not take into account heat recovered from the condensation of water (which returns to liquid state below 100ºC), while HHV does.
The heat rate measures how efficiently a power plant or other energy conversion process uses heat and is a common metric describing the efficiency of power plants. A plant that uses 450,000 lbs/hour of bituminous coal to produce a net 630 MWe would have a heat rate of 8,300 Btu/kWh (on an HHV basis). Heat rate is a like an inverted efficiency, it tells how much heat is needed (energy in) for power generated (energy out). Efficiency, in general, is the opposite of this. The Comparison of Gasification and Pulverized Coal Power Plants section compares the heat rates of gasification and other energy conversion plants.
Effect of Carbon Capture and Sequestration
While there is currently no government regulation on carbon dioxide (CO2) emissions, carbon capture and storage (CCS) is expected to play a large role in future greenhouse gas emission mitigation and environmental compliance. The energy required to separate, compress, transport and, ultimately, store CO2, however, imparts a significant efficiency reduction on the plant. Gasification based power plants (IGCC), however, offer several advantages over traditional power plants that increase plant efficiency when carbon capture technologies are employed. One main advantage is that gasification allows CO2 to be separated pre-combustion, when it is much more concentrated and, hence, easier to separate than the diluted CO2 in a post-combustion flue stream.
One of the greatest advantages of gasification, is its ability to integrate with other technologies—technologies that can each increase efficiency of the plant as a whole. For example, gasification with oxygen (O2) rather than air (in which nitrogen [N2] accounts for 78% of air by volume), is more efficient and makes CO2 separation easier (no N2 to dilute the CO2). However, obtaining pure O2 is difficult and expensive with large cryogenic distillation units required. New air separation technologies are making gasification with O2 a more viable option, however, as the cost decreases and efficiency increases.
In general, as related technologies become more efficient, gasification power production becomes more efficient.
Some other technologies that have promising potential for efficiency gains include:
- Fuel cells - With no moving parts and friction and heat losses minimized, fuel cells are a very efficient means of power generation. Using a fuel cell rather than a turbine generator could dramatically increase plant efficiencies, but further research and development is needed.
- Warm gas cleaning - Allowing the synthesis gas (syngas) temperature to remain high throughout the cleaning process reduces heat losses from temperature changes.
- Turbines (hydrogen turbines) - Turbines designed for higher operating temperatures (for syngas combustion) optimize the efficiency of power generation.
- New separation methods - Similar to gas cleaning, increasing the efficiency of valuable byproducts, like sulfur, increases plant efficiency. New air separation methods could also offer dramatic efficiency gains as the current technology, cryogenic distillation, is a large energy consumer.
- System integration - Minimizing the amount of temperature and pressure changes between system parts increases the efficiency of the plant.