1.1 Setting the Stage: The Environmental Challenge of Fossil Fuels

Welcome. Today, we will explore the critical field of fossil fuel cleaning, a set of technologies designed to mitigate the environmental impact of our primary energy sources. The central problem we face is the emission of pollutants, particularly sulfur oxides, during combustion. To grasp the scale of this issue, consider that in the United States alone, more than thirty million tons of sulfur dioxide are discharged into the atmosphere annually. A staggering 75% of these emissions result directly from the burning of fossil fuels. While we can capture these pollutants after combustion from flue gases, a more proactive and often more energy-efficient strategy is to treat the fuels prior to combustion. This lecture will focus on these pre-combustion cleaning processes, examining the principles, technologies, and economic realities of removing sulfur and other contaminants from oil, natural gas, and coal.

1.2 A Global Snapshot of Fossil Fuel Resources and Consumption (circa 1980s-2000s)

To understand the necessity of these cleaning processes, we must first appreciate the global landscape of fossil fuel supply and demand during the late 20th century.

Oil and Natural Gas

In 1980, the world’s production of oil stood at 66 million barrels per day (MBPD), with projections for the year 2000 rising to 77 MBPD. This relatively modest anticipated increase reflects the growing impact of energy conservation efforts and the application of alternative fuel sources. For context, the USSR’s output was around 14 MBPD, compared to a combined 12 MBPD for the United States and Canada.

In the realm of natural gas, worldwide reserves were estimated at approximately 2,500 trillion cubic feet (TCF). The United States held about 200 TCF of these reserves, with an annual consumption rate of 20 TCF. Geographically, most of the world’s oil and natural gas reserves are concentrated in a crescent-shaped area that extends from Northern Algeria northward to West Siberia.

Coal

Coal remains a cornerstone of energy production, particularly for electricity generation. The US utility industry alone consumed coal at a rate of 600 million tons annually. However, a significant environmental challenge lies in the composition of this coal, particularly from the major production regions in the Eastern United States.

Table 1: Sulfur Content of Representative U.S. Coals (Dry Basis)

As the data in Table 1 clearly illustrates, of the four major Eastern coals listed, only one—the Pocahontas seam—has a sulfur content below the 1% threshold often considered low-sulfur. The other representative Eastern coals all contain sulfur levels exceeding 3.3%, underscoring the regional imperative for effective coal cleaning technologies to meet environmental standards.

Globally, the world’s recoverable coal reserves were estimated at 780 billion tons. A commanding two-thirds of these reserves were held by three nations: the United States, the USSR, and China. Coal’s strategic importance to the United States cannot be overstated, as it accounts for 90% of the nation’s proven energy reserves.

Energy Consumption Metrics and Projections

To quantify energy on a national scale, we use the term “quads,” which stands for quadrillion BTUs. In 1980, US consumption for electric generation was 13 quads, while non-electric industrial use accounted for 16 quads. The total US fossil fuel consumption for that year was approximately 76 quads. Looking forward, the World Energy Council predicted that worldwide energy consumption would double over the subsequent 25 years.

Projections for the year 2015 illustrated the continued dominance of fossil fuels in electric generation, with anticipated usage in billions of kilowatt-hours (kWh) as follows:

• Coal: 2,000
• Natural Gas: 1,000
• Nuclear: 400
• Petroleum: < 100
• Renewables: 400

To put these large numbers in perspective, we can convert them back to quads. This is done by dividing the kWh value by a conversion factor that accounts for power plant efficiency—typically around 100 for a conventional 34% efficient Rankine cycle plant, or as high as 170 for more advanced combined-cycle plants.

1.3 The Regulatory Drivers for Desulfurization

The development of fuel cleaning technologies has been largely driven by legislative action. Governments have set firm limits on the allowable sulfur content of fuels and on the SO₂ emission rates from new sources. For example:

• In California, regulations limited fuel oil use to those containing 0.5% or less sulfur.
• In New York City, a strict limit of 0.3% sulfur in oil was in effect as early as 1968.
• In Massachusetts, a 1% sulfur limit on coal was established in 1980.

These regulations create significant economic trade-offs. For instance, chemically or physically reducing a 3% sulfur coal to a 1% sulfur coal could add approximately 10% to its cost. However, this premium can be offset by savings in transportation (as less non-combustible material is shipped) and, critically, by reducing or eliminating the need for expensive post-combustion flue gas desulfurization systems. This brings us to the core dilemma faced by the energy sector: the public’s dual requirement for both increased quantities of fuel to meet growing demand and the cleaner preparation of that fuel to protect our environment. The technologies we will now discuss are the engineering solutions to this fundamental challenge.