A series of LCAs was conducted on biomass, coal, and natural gas systems to quantify the environmental benefits and drawbacks of each. All those evaluations were conducted in a cradle‐to‐grave manner to cover all processes necessary for the operation of the power plant including raw material extraction, feed preparation, transportation, waste disposal, and recycling. We summarize data on energy balance, GWP, air emissions, and resource consumption for each system (NREL 2011).
The generation of electricity and the consumption of energy in general result in consequences to the environment. Using renewable resources and incorporating advanced technologies such as integrated gasification combined cycle (IGCC) may result in less environmental damage, but to what degree, and with what trade‐offs? LCA studies have been conducted on various power generating options in order to better understand the environmental benefits and drawbacks of each technology.
Material and energy balances were used to quantify the emissions, energy use, and resource consumption of each process required for the power plant to operate. These include feedstock procurement (mining coal, extracting natural gas, growing dedicated biomass, collecting residue biomass), transportation, manufacture of equipment and intermediate materials (e.g. fertilizers, limestone), construction of the power plant, decommissioning, and any necessary waste disposal.
The following systems were studied:
- A biomass‐fired IGCC system using a biomass energy crop (hybrid poplar)
- A pulverized coal (pc) boiler with steam cycle, representing the average for coal‐fired power plants in the United States today
- A system cofiring biomass residue with coal (15% by heat input will be presented here)
- A direct‐fired biomass power plant using biomass residue (urban, primarily)
- A natural gas combined cycle (NGCC) power plant.
Each study was conducted independently and can therefore stand alone, giving a complete picture of each power generation technology. However, the resulting emissions, resource consumption, and energy requirements of each system can ultimately be compared, revealing the environmental benefits and drawbacks of the renewable and fossil‐based systems.
Results
System Energy Balance
The total energy consumed by each system includes the fuel energy consumed plus the energy contained in raw and intermediate materials that are consumed by the systems. Examples of the first type of energy use are the fuel spent in transportation and fossil fuels consumed by the fossil‐based power plants. The second type of energy is the sum of the energy that would be released during combustion of the material (if it is a fuel) and the total energy that is consumed in delivering the material to its point of use. Examples of this type of energy consumption are the use of natural gas in the manufacture of fertilizers and the use of limestone in flue‐gas desulfurization (FGD). The combustion energy calculation is applied where nonrenewable fuels are used, reflecting the fact that the fuel has a potential energy that is being consumed by the system. The combustion energy of renewable resources, those replenished at a rate equal to or greater than the rate of consumption, is not subtracted from the net energy of the system. This is because, on a life cycle basis, the resource is not being consumed. To determine the net energy balance of each system, the energy used in each process block is subtracted from the energy produced by the power plant. The total system energy consumption by each system is shown in Table 6.13.
To examine the process operations that consume the largest quantities of energy within each system, two energy measurement parameters were defined. First, the energy delivered to the grid divided by the total fossil‐derived energy consumed by each system was calculated. This measure, known as the net energy ratio, is useful for assessing how much energy is generated for each unit of fossil fuel consumed. The other measure, the external energy ratio, is defined to be the energy delivered to the grid divided by the total non‐feedstock energy to the power plant. That is, the energy contained in the coal and natural gas used at the fossil‐based power plants is excluded. The external energy ratio assesses how much energy is generated for each unit of upstream energy consumed.
Table 6.13 Total system energy consumption.
Source: From NREL (2011).
| System | Total energy consumed (kJ/kWh) |
| Average coal | 12 575 |
| Natural gas IGCC | 8 377 |
| Biomass/coal cofiring (15% by heat input) | 10 118 |
| Biomass‐fired IGCC using hybrid poplar | 231 |
| Direct‐fired biomass power plant using biomass residue | 125 |
Because the energy in the biomass is considered to be both generated and consumed within the boundaries of the system, the net energy ratio and external energy ratio will be the same for the biomass‐only cases (biomass‐fired IGCC and direct‐fired biomass). In calculating the external energy ratio, we are essentially treating the coal and natural gas fed to the fossil power plants as renewable fuels, so that upstream energy consumption can be compared. The energy results for each case studied are shown in Figure 6.13.
As expected, the biomass‐only plants consume less energy overall, since the consumption of nonrenewable coal and natural gas at the fossil plants results in net energy balances of less than one. The direct‐fired biomass residue case delivers the most amount of electricity per unit of energy consumed. This is because the energy used to provide a usable residue biomass to the plant is fairly low. Despite its higher plant efficiency, the biomass IGCC plant has a lower net energy balance than the direct‐fired plant because of the energy required to grow the biomass as a dedicated crop. Residue resource limitations, however, may necessitate the use of energy crops in the future. Cofiring biomass with coal slightly increases the energy ratios over those for the coal‐only case, even though the plant efficiency was derated by 0.9 percentage points.
In calculating the external energy ratios, the feedstocks to the power plants were excluded, essentially treating all feedstocks as renewable. Because of the perception that biomass fuels are of lower quality than fossil fuels, the external energy ratios for the fossil‐based systems were expected to be substantially higher than those of the biomass‐based systems. The opposite is true, however, due to the large amount of energy that is consumed in upstream operations in the fossil‐based systems. The total non‐feedstock energy consumed by the systems is shown in Table 6.14. In the case of coal, 35% of this energy is consumed in operations relating to flue‐gas cleanup, including limestone procurement. Mining the coal consumes 25% of this energy, while transporting the coal is responsible for 32%. Greater than 97% of the upstream energy consumption related to the natural gas IGCC system is due to natural gas extraction and pipeline transport steps, including fugitive losses. Although upstream processes in the biomass systems also consume energy, shorter transportation distances and the fact that FGD is not required reduce the total energy burden.
Table 6.14 Non‐feedstock energy consumption.
Source: From NREL (2011).
| System | Total energy consumed (kJ/kWh) |
| Average coal | 702 |
| Natural gas IGCC | 1718 |
| Biomass/coal cofiring (15% by heat input) | 614 |
| Biomass‐fired IGCC using hybrid poplar | 231 |
| Direct‐fired biomass power plant using biomass residue | 125 |
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