Electrify Asphalt Plant Heat Efficiency
BY Rick Jay
With the costs of doing business on the rise and energy in flux, an asphalt producer should consider any measure he can take to improve the bottom line. The method and efficiency of a plant’s heat—for asphalt and hot oil—are areas that often are overlooked. But energy costs contribute significantly to the operating costs of an HMA plant, and fuel choices can impact producers in more ways than just cost.
Operators often choose a method of heat based on current energy prices, but the cost of natural gas, LP gas, fuel oils and electricity have all fluctuated over the years. Just seven years ago, electricity was far cheaper to use as an energy source for heat than other fossil fuels. Today, natural gas is less expensive.
Given the volatility of these variables, how can a producer make the best energy choice for heating asphalt cement (AC) and hot oil?
When choosing an AC heating method, it’s important to look at the entire picture. Cost-per-unit of energy to heat AC and emulsions is certainly part of the puzzle, but it is not the only cost. Also important is the heater’s thermal efficiency. According to Heatec Inc.’s Technical Paper T-140, “Heating and Storing Asphalt at HMA Plants,” a small difference in efficiency can make a big difference in how much fuel a heater burns over its lifetime.
Most fossil-fuel-fired burners operate at 80 to 85 percent efficiency when new. This means 15 to 20 percent of the burner’s heat is wasted as exhaust. With age, the burner’s efficiency drops. Also, fuel-fired asphalt heaters are maintenance-intensive. If the proper maintenance and tuning are not conducted to keep the efficiency up, then the efficiency drops even more dramatically. In fact, by the time the burner is seven to eight years old, this efficiency can easily be at only 50 to 60 percent, meaning that up to 50 percent of the heat is wasted as exhaust.
An electric-powered asphalt tank or hot oil heater, in comparison, operates at 100 percent efficiency with no heat or emissions exhausted into the air. Over the lifetime of the heater, that efficiency never drops.
True Cost Comparison
At first glance, it appears that natural gas and No. 6 oil are currently the cheapest forms of heat energy. But that is only if you assume that all heater types operate at 100 percent efficiency. The far right column in the table shows the cost of various energy forms with dropping efficiencies, which take place over time with aging burners and poor maintenance practices. Keeping in mind that electric heaters operate at 100 percent efficiency throughout their lifetimes, you can see that the energy costs begin to level out.
Test Electric Energy Costs
Several years ago, Process Heating Company (PHCo) performed tests over a three-week period on a well-insulated 20,000-gallon AC storage tank holding product at 340˚ F at a 500-TPH drum mix plant. The electric tank heater consumed 192 actual kilowatt hours (kWh) per day while the plant was running. Using current industrial electric cost of $0.0673 per kW (see Table 1), the cost per day for maintaining heat in the tank over 192 kWh is $12.92.
At the same plant, an electrically heated hot oil system used 422 actual kWh per day as the plant was running. Using the current industrial electric cost of $0.0673 per kW, the cost per day to heat the plant’s hot oil is $0.0673 x 422 kWh, or $28.40. The cost per ton of output is $28.40/2,400 tons produced per day, or $0.012 (1.2 cents per ton).
This plant has two identical AC storage tanks, as well as the hot oil system. To figure total electricity costs per day using electric heaters, take $12.92 x 2 = $25.84 + $28.40 for a total of $54.24. Total cost per ton of output is $54.24/2,400 tons produced per day, or $0.0226 (2.3 cents per ton).
Factors that must be taken into consideration with this scenario are that the plant where testing took place was in excellent condition, and the tanks and hot oil system were well-insulated. Also, to achieve optimal electric rates, the equipment was interfaced with other large electric-using equipment to avoid adding to demand charges. Additionally, electric heaters used time clocks for operating during off-peak hours, to take advantage of lowest possible rates without impacting performance or operation of the plant.
With energy costs fluctuating, it also can pay to install both electric and fuel-fired heat sources for hot oil and AC, to take advantage of the lowest energy costs available. That is exactly what one asphalt producer in South Carolina did in the summer of 2008, when fuel prices rose dramatically to above $4 a gallon.
The company, which produced between 500,000 and 800,000 tons of HMA annually during that period, chose to add a PHCo electric hot oil heater at one facility that produced about 200,000 tons of asphalt per year. By adding an electric-powered heater to its hot oil tanks during a time when fuel prices were high, the company cut its asphalt-heating costs considerably. The company had previously used diesel-fuel-fired hot oil heaters exclusively.
An average drum mix asphalt plant using a PHCo heater will require a 140 kW heater, which is adequate for two 30,000-gallon asphalt tanks and associated jacketed pumps, meters and asphalt lines. The electricity cost at that time for the plant was about $0.11 per kW, so with a 140kW heater running around 4 hours a day in the summer and 6 hours a day in the winter, the cost to heat asphalt with electricity was approximately $62 per day in the summer and $92 per day in the winter. Alternately, if the plant used almost 20 gallons of diesel fuel per day in the summer and about 27 gallons per day in the winter (based on the 80 percent efficiency seen in Table 1), with diesel prices at $3.70, as they were during the summer of 2008, the asphalt heating costs became $70 per day in the summer and $100 per day in the winter.
This case study may also be used to extrapolate results using current fuel prices. With diesel fuel costs of $2.43 per gallon as of deadline, the per-day asphalt heating cost becomes the following: 20 gallons of diesel fuel (summer) at about $49 per day and 27 gallons (winter) at about $66 per day. This is compared to current industrial electricity costs of $0.0673 per kW, with a 140 kW heater running four hours per day (summer) for $38 in daily costs, and six hours per day (winter) for $57 in daily costs.
This particular plant chose to keep its diesel-fired heater, as well as its electric heater, so that it may switch between the two, depending on which system is providing the most cost-effective heat.
Reduce All Costs
Electric-powered heaters require little maintenance, which serves to reduce operating costs. Because they typically last more than 30 years, electric-powered heaters can also reduce capital costs for the plant.
Another aspect to consider is the benefit of electric heat produced by low watt density heaters, which dissipate controlled heat as low as 3 watts per square inch on the heater’s sheath to prevent coking or damaging of temperature-sensitive material. In hot oil systems, this reduces the need to change out expensive heat transfer oil.
Increasing emission regulations are one more factor to consider. As shown in Table 1, different types of fuel produce different levels of emissions and add different types of expenses for producers when it comes to emissions control. A plant operator in California recently disclosed that adding a single stack at his plant would increase his annual permit cost by $5,000 to $6,000. This operator chose to heat his asphalt with electricity because it creates zero emissions; therefore eliminating associated stacks, permits and expenses.