Recognizing that to produce more than 100 billion gallons of biofuels the United States would need to build hundreds of biorefineries, it is important to understand the local impacts that these facilities will have. The three types of environmental impact that are of most potential concern in the production of biofuels are air, water, and waste. Of course these facilities need to be sited carefully to avoid land, use and habitat impacts, but these facilities do not use a particularly large amount of land nor is there any reason that they need to be sited in sensitive landscapes. After careful review of the literature and consultation with experts, NRDC and UCS have concluded that there is no reason that biorefineries using biological, thermochemical, or combined biological and
thermochemical processes should have unacceptable pollution impacts if appropriate regulations and control technologies are adopted.
As discussed earlier, we primarily address air impacts on a life cycle basis in our comparative assessment of our eight different product packages. In this section we will
focus only on the air pollutants emitted from biorefineries that are of greatest local impact—nitrogen oxides (NOx), volatile organic compounds (VOC), carbon monoxide
(CO), and particulates. For all of these pollutants, biorefinery emissions should be either inherently low or controllable.
Biological processing to make cellulosic ethanol can produce significant quantities of VOC and particulates. The particulates are primarily fine dust that results from feedstock handling and are difficult but not impossible to control by doing more handling inside and using water to keep the dust down. The evaporative emissions are largely caused by the mixing of ethanol with gasoline, which is required by law to make the ethanol undrinkable, and again can be controlled by doing the mixing where the VOCs can be collected and treated.
By comparison, corn ethanol plants face a much larger air pollution challenge because they rely primarily on the on-site combustion of fossil fuels—most often coal—for energy to drive the ethanol processing. This results in significant emissions of SOx, NOx,
CO, mercury, particulates and CO2. It is worth noting that some early corn ethanol plants
had severe VOC air pollution problems, but these have largely been resolved through proper sizing of pollution control devices.
In contrast, biological processing draws its process energy from the thermochemical conversion of the non-carbohydrate portion of the biomass. During the early stages of development, it is likely that this non-carbohydrate portion will simply be burned with the energy captured through Rakine cycle steam boilers and turbines. This direct combustion can result in significant quantities of NOx, CO, and particulates. However,
traditional power plant emissions control technology should be able to reduce these emissions to acceptable levels.
Over time, it is likely that biological processing will be paired with gasification. The air pollution impacts of gasification come almost entirely from the combustion of the syngas. The local air impacts of syngas combustion for power are very similar to those from the combustion of natural gas. Sulfur and hazardous air pollutants are harmful to the advance turbines used with gasifiers, so these are removed from the syngas before combustion. The local air pollutants formed during syngas combustion are NOx, CO, and some VOC.
Again, these can be reduced substantially through pollution controls, and given their generally lower starting point, the resulting emissions can be extremely low. Production of Fischer Tropsch fuels or DME is also gasification-based, and there are minimal local air impacts from these processes.
Water and waste impacts should also be very low with proper regulations and control devices. Biological processing results in significant levels of soluble organics that, if released with wastewater without being properly treated can put a significant oxygen demand on waterways. Fortunately standard waste water treatment technologies can virtually eliminate this problem.55 In the context of our mature processes, all of these
materials are treated first in an anaerobic digester to capture methane gas that is then fed into the gasification process. The anaerobic digestion has the added advantage of
enabling much higher water recycling within the facility by removing compounds that would otherwise prohibit water reuse. About 95 percent of the treated water is recycled,
and the rest (about 280 gallons per minute for a 5,000 dry ton per day plant) is treated again before being released. This two-step process with high levels of water recycling is consistent with current practice in recently constructed corn processing plants and allows the processes we have analyzed to produce no untreated wastewater. Proper regulations will be necessary to ensure proper water treatment, but in our analysis the water treatment was done primarily for the energy value of the methane captured. In other words, the economics encourage good environmental practices.
The high level of water recycling also allows us to minimize the total amount of fresh water used. Approximately 2 kg water per kg dry biomass feedstock—about 1,700
gallons per minute—are required as make-up water to account for the treated discharge as well as water consumed during hydrolysis or lost to evaporation. Petroleum refineries, by comparison, typically use 1.8 to 2.5 kg process water per kg crude feedstock—4,400 to 6,200 gallons per minute for a 100,000 barrel per day refinery—and discharge between 1.7 and 3.1 times as much water.56
The only water pollutant of concern from the thermochemical process is waste heat. Traditionally boilers and other power plants located near bodies of water have used a once-through cooling system, drawing cool water and returning heated water. The water intake can damage fish and the heated water can destroy habitat. The alternatives are known as wet or dry cooling systems. Wet system use water evaporation to remove excess heat. Dry systems primarily use air.
In addition to the biosolids resulting from waste water treatment, the only solid waste that our combined biological and thermochemical processes will need to dispose of is the ash content in the cellulosic biomass. This material, which makes up about 4 percent of the weight of dry feedstock, will not break down in either processes.57 While the ash and biosolids can be disposed of with little anticipated difficulty, we suspect that uses for these products would be found in a mature, large-scale biorefining industry.
In the petroleum industry, for example, only a minor fraction of crude oil was utilized by early refineries with the remainder being treated as waste. However, modern refineries convert nearly 100 percent of the mass of petroleum taken in by the plant into salable products. When asphalt first was produced in oil refining, for example, there was little demand for it. Today, we use it as a road surface. Similarly, we think it likely that biosolids resulting from the biorefineries we envision could be used as a soil additive, and ash might be incorporated into concrete aggregate or other products. If protein is not recovered and sold as we have discussed, it should be possible to recover a high fraction of the feedstock nitrogen as ammonia fertilizer, as is currently done by coal refineries in South Africa. Recycling ammonia to the fields where bioenergy feedstocks are grown offers substantial benefits in terms of both cost and life cycle energy inputs in light of the energy-intensive nature of ammonia manufacture. In general, we see these and other integration strategies as natural outgrowths of the evolution of a mature biomass refining industry. Thus, while we believe appropriate regulations are essential to ensure careful management of “waste” flows and other environmental aspects associated with biomass refining, it is appropriate to recognize that such refining offers opportunities for multiple environmental benefits at many levels and that many of these benefits will be driven by the economics of the processes.