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Carbonization technology converts biosolids to an economical,
renewable fuel
K. M. Bolin*, B. Dooley*, R. J. Kearney**
*EnerTech Environmental, Inc., 657 Seminole Ave, Suite 207, Atlanta, GA 30307 USA
(email: [email protected]; [email protected])
**EnerTech Environmental, Inc., 13006 Warren Avenue, Los Angeles, CA 90066 USA (email:
[email protected])
Abstract: Most biosolids in the USA are recycled via agricultural land application. In several states this practice is
considered unacceptable by some people. Many agencies are seeking technologies, like thermal drying, to produce a
more acceptable Class A product. While drying does produce a fine marketable product that can be used as fertilizer
or fuel, it is expensive to evaporate water from biosolids containing 20%-25% total solids. EnerTech Environmental,
Inc.’s patented SlurryCarb™ process chemically converts biosolids into a renewable solid fuel, providing an
environmentally and economically sound solution for biosolids management. The process heats biosolids under
pressure to significantly improve dewaterability. Biosolids are mechanically dewatered to 50% total solids and fed to
the dryer. Thus, the amount of thermal energy needed to dry the biosolids is reduced by almost two-thirds and the cost
by about one-third. Construction of a 614 metric tons (675 wet tons) per day biosolids processing facility has started.
The dried product from this facility will be used as a renewable fuel in lieu of coal in a cement kiln. This beneficial
use lowers greenhouse gas emissions, reduces ore consumption, provides permanent recycling of the biosolids, and
eliminates the health and environmental concerns associated with agricultural use.
Keywords: Biosolids; carbonization; heat treatment; renewable fuel; SlurryCarb
INTRODUCTION
About two-thirds of all biosolids in the USA are recycled via agricultural land application (U.S. EPA, 1999).
However, in the past decade many rural areas that receive biosolids from urban areas have questioned this
practice and enacted ordinances either to ban or restrict it severely (NACWA, 2006; Gillette, 2007). In response
to these concerns, wastewater agencies have developed Environmental Management Systems to improve their
management of biosolids and implemented expensive methods (such as thermophilic digestion, composting,
or thermal drying) to improve the perceived quality of their biosolids from Class B to Class A (Baroldi et al, 2006;
Gillette, 2007).
Despite these efforts, the future of agricultural land application of biosolids is uncertain. In June 2006, 83
percent of voters in Kern County, California approved an initiative to ban agricultural land application of all
products containing biosolids in the unincorporated areas of the county (Barmann, 2006; Kern County, 2006).
The City of Los Angeles challenged the initiative in federal court and won a preliminary injunction staying the
January 22, 2007 effective date pending the outcome of the trial.
As an alternative to land application, EnerTech Environmental, Inc.’s (EnerTech) patented SlurryCarb™ process
chemically converts biosolids into a renewable solid fuel. The process takes advantage of the energy value
inherent within the biosolids and provides an environmentally friendly and economically sound solution for
biosolids management. EnerTech is currently developing a regional biosolids processing facility in Southern
California incorporating the SlurryCarb™ process. Five Southern California wastewater agencies have committed
621 wet MT/day (683 WTPD) of biosolids to this facility as part of their biosolids management portfolios.
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The renewable solid fuel produced from this project will be purchased by local cement kilns and ash produced
from the fuel’s incineration will be incorporated into the clinker produced by the kilns. This ash will include the
iron, silica, calcium, and alumina that are present in biosolids, resulting in the permanent recycling of the
original biosolids. Furthermore, due to the renewable nature of the solid fuel produced by the SlurryCarb™
process, incineration of the fuel creates essentially zero net greenhouse gas emissions. Unlike burning fossil
fuels, which were once contained within the earth, burning renewable fuel does not release any new carbon
dioxide into the atmosphere; it recycles carbon dioxide that was recently circulating through the ecosystem.
The utilization of a beneficial reuse technology like the SlurryCarb™ process – that minimizes waste byproducts
and environmental impact – can assist wastewater agencies with developing sustainable biosolids management
portfolios and the alleviating health and environmental concerns associated with agricultural use.
THE SLURRYCARB™ PROCESS
The SlurryCarb™ process uses heat and pressure to carbonize the organic matter in biosolids and lyse cell
walls to release bound water. The resulting slurry dewaters to 50% total solids by centrifugation and is then
dried using 60% less energy. The final dried product has a heating value of about 15.120 kJ/kg or 4.190 Kwhr/MT (6.500 Btu/lb) and is an excellent replacement for coal and other fossil fuels. Figure 1 presents a
detailed flow diagram of the SlurryCarb™ process (Bolin and Kearney 2007).
Figure 1. Flow Diagram of the SlurryCarb™ Process
Step One - Slurry Preparation: Biosolids are received from the wastewater treatment plant at 20% to 30% total
solids. The feedstock is macerated until all particles are less than ½ inch.
Step Two - Slurry Pressurization: The feed material is pressurized above its saturated steam pressure to prevent
boiling. Under this pressure, the slurry remains a liquid throughout processing, and thermal energy inputs for
the evaporation of water are minimized.
Step Three - Slurry Heating: Heat exchangers raise the temperature of the pumpable slurry to approximately
230°C (450oF).
Step Four – Reaction: The cellular structure of the biological sludge is ruptured (lysis) and the carboxyl groups
(-C-O-O) of the organic molecules are broken off and released as carbon dioxide gas, a step called
“carbonization.” This reaction significantly reduces the size and improves the uniformity of the solid and
polymer-like waste molecules, which also lose their affinity for water. This reaction is stopped short of pyrolysis.
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Step Five – Dewatering: After partial cooling through the same heat exchangers in Step Three and
depressurization, the carbonized slurry is dewatered using centrifuges to at least 50% total solids. The
dewatered slurry can be used as a fuel at 50% total solids or dried to 90+% total solids and pelletized into a
renewable fuel. The choice as to the dryness of the fuel depends on the energy recovery capabilities of the site,
i.e., if an energy recovery facility that can utilize the slurried fuel is available on site, a separate drying step
may not be needed.
Step Six - Filtrate Recycle: Trace contaminants are removed from the centrate using membrane filters followed
by anaerobic digestion and aerobic treatment and recycled, if appropriate.
Step Seven – Combustion: The renewable fuel, called E-fuel is combusted using less than 20% excess air for
effective carbon burnout. E-Fuel is an excellent fuel for cement kilns, oil boilers, gasifiers, etc.
Simple Mass Balance
Assuming a facility receives 110 wet metric tons (100 wet tons) of biosolids at 80% moisture in Step 1 of the
process, the carbonization reaction will create reaction gas which is 90% CO2. This gas will reduce the solids
content by 10% so that only 19.8 MT (18 tons) of the original 22 MT (20 tons) are centrifuge dewatered to 50%
total solids. See Figure 2. The water coming off the centrifuge has been separated from the original biosolids
without evaporation. This accounts for 78% of the original water in the received biosolids. The remaining
reacted cake is then dried and pelletized to form the final product fuel.
Simple Energy Balance
The equation below shows the theoretical energy consumed to evaporate the 88 MT (80 tons) of water in the
example above using conventional drying:
(2.326 kJ/kg)(88 MT of water)(1.000 kg/MT) = 204.688.000 kJ
(1.000 Btus/lb)(80 tons of water)(2.000 lbs/ton) = 160.000.000 Btus
The SlurryCarb™ process, by observing the saturated steam curve and heat recovery, only consumes 407 kJ/kg
(175 Btu/lb) to reach its reaction temperature through Step Four, as follows:
(407 kJ/kg)(88 MT on water)(1.000 kg/MT) = 35.820.400 kJ
(175 Btu/lb)(80 tons of water)(2.000 lbs/ton) = 28.000.000 Btus
The dryer following SlurryCarb™ then evaporates the 19.8 MT (18 tons) of water in the centrifuged cake:
(2.326 kJ/kg)(19.8 MT of water)(1 000 kg.MT) = 46.054 800 kJ
(1.000 Btus/lb)(18 tons of water)(2 000 lbs/ton) = 36.000 000 Btus
Therefore, the SlurryCarb™ process consumes a total of 81.875.200 kJ (64 MM Btus) or approximately (16064)/160 = 60% less energy than a typical dryer, while using about the same electrical load. Figure 2 illustrates
the related simple mass balance.
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Figure 2. Simple Mass Balance
TESTS OF SOUTHERN CALIFORNIA BIOSOLIDS
In preparation for the design of EnerTech’s forthcoming regional biosolids processing facility, samples of Los
Angeles area biosolids were shipped to Atlanta, Georgia and processed through the Process Development Unit
(PDU), a 54.5 l/hr (14.4gal/hr) pilot facility, to verify performance of the PDU using Los Angeles area biosolids
and to get design data. These tests confirm that about two-thirds of the biosolids ash consists of iron, silica,
calcium, and alumina, the four main ingredients used in making cement.
Testing with vendors has helped determine the best types of equipment and design criteria for such equipment
as heat exchangers, reactor, centrifuge, dryer, and membrane technologies. As a result, about 60% of the
facility design is complete.
RIALTO REGIONAL BIOSOLIDS PROCESSING FACILITY
The facility being developed will be located in Rialto, California, about 60 miles east of the Los Angeles area
(Figure 3 is an aerial view of the site location for this facility). The cities of Rialto, San Bernardino, and
Riverside, as well as the County Sanitation Districts of Los Angeles County and the Orange County Sanitation
District have all signed biosolids supply agreements and are contracted to provide a total of 621 wet metric
tons (683 short tons) per day of biosolids to the project.
Figure 3. Rialto Site
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The facility will have an initial nominal design capacity to process 614 MT/day (675 WTPD) of biosolids and will
produce approximately 127.3 MT/day (140 tons) of E-Fuel. The City of Rialto will be a partner in the project and
will share in the immediate and long-term benefits (both environmental and financial) of the EnerTech project.
Rialto Net Energy Production
Using the list of all connected horsepower from the latest design, the Rialto project is expected to consume 96.1
Kw-hr/MT (0.30 MM Btu/ton) of electrical power as shown in Table 1 below. The natural gas demand is 506 Kwhr/MT (1.57 MM Btu/ton), for a total energy demand of 602 Kw-hr/MT (1.87 MM Btu/ton). Since the facility will
process a total of 614 MT/day (675 WTPD), the total daily load is 369 689 Kw-hr/day (1 262 MM Btu/day).
Table 1. Energy Consumption
The latest mass balance shows that 614 MT/day at 22% total solids (TS) will produce 127.3 MT/day (140
tons/day) at 90% TS of renewable E-Fuel. Since the E-Fuel has a heating value of 4.190 Kw-hr/MT (6.500
Btu/lb), the E-Fuel provides 533.369 Kw-hr/day (1820 MM Btu/day) of energy as shown in Table 2 below.
Table 2. Energy Production
Therefore, using the energy consumption and production figures from Tables 1 and 2, the SlurryCarb™ process
produces 44% ((533.369 – 369.689)/369.689) more energy than it consumes to make the E-Fuel and is a net
energy producer.
Equipment Modifications
Many changes have been made in the selection of equipment for the process in the last year as a result of PDU
and vendor testing. The changes affect pumps, heat exchangers, the reactor, and the dryer. The original design
specified a pair of high pressure positive displacement pumps prior to the pre-heaters and prior to the heaters.
The design now incorporates one pair of positive displacement pumps prior to the first two pre-heaters, and
one pair each of progressive cavity pumps prior to the second two pre-heaters and the reactor.
The original heat exchangers were typical tube and shell design with 1.9 cm (¾-in) diameter tubes varying
between 9.1-15.2 m (30-50 ft) long. Although this is a tried and true design that utilized the minimum footprint,
the small tube diameters were a concern from a clogging and cleaning perspective. A different, newer design
was tested which incorporates 3-inch by 6-inch tubes arranged in a serpentine horizontal manner with multiple
vertical layers. The tube clusters are arranged in a way that provides easy access to inspect and clean the
tubes. This arrangement eliminates the need for a gantry crane, which would be required to remove the tube
bundles from the tube and shell design for inspection and maintenance. However, because of the larger size
tubes (with smaller surface area per cubic meter), more heat exchangers of this type are needed to provide the
same heat transfer surface. See Figure 4 for a diagram of the new heat exchanger design.
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Figure 4. Rialto Heat Exchanger Design
Costs
Agricultural land application has been the most popular form of biosolids management in California for 15
years because it has been relatively inexpensive. Costs in Southern California currently range from
approximately $110 to $250 per dry MT ($100 to $225 per dry ton) depending primarily on cake solids
concentration. This is considerably cheaper than the approximately $520 to $650 per dry MT ($470 to $590 per
dry ton) for conventional thermal drying technologies. In 2004, the Orange County Sanitation District received
two proposals for heat drying biosolids. Table 3 below shows the costs for these two proposals (OCSD, 2005).
Table 3. Biosolids Drying Costs
The Rialto project will provide an energy management option for about $250 to $385 per dry MT ($225 to $350
per dry ton), which is similar to what some California agencies are now paying for agricultural land application
and considerably less than other drying options. Additionally, the Rialto project is being privately financed, so
none of the agencies needs to raise any capital or incur any financial risk.
Rialto Project Schedule
All permits necessary to begin final design and construction have been obtained. Construction will begin in
April 2007. Startup testing will begin in April 2008 and plant commissioning will be complete in July 2008 (Bolin
et al., 2006).
CONCLUSIONS
Many California wastewater agencies are re-evaluating their biosolids management options, particularly
agricultural land application, because of local regulatory requirements, the uncertainty of future changes in
regulatory requirements, and growing local opposition (Baroldi et al, 2006). Activists are concerned that
biosolids may contain pathogens, endocrine disrupting chemicals, pharmaceuticals, and other toxic substances
that could threaten the environment and public health (Reilly, 2001), and are fighting to ban or severely restrict
the practice until science can prove it is safe (NRC, 2002). A California State Senator has introduced a bill in
the state legislature that would require a publicly owned treatment works to indemnify parties receiving
biosolids for any liability for remediation costs associated with the disposal or processing of sewage sludge
(Florez, 2007).
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Some California agencies have decided that drying all or a portion of their biosolids prior to land application
is a way to overcome opposition because of pathogens, odors, and other nuisance concerns, despite the
increased cost (OCSD, 2005). The SlurryCarb™ process is a new, innovative way of achieving this goal at
significantly less cost. In addition, it can eliminate impacts on public health and the environment from land
application by producing a fuel for cement kilns.
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Biosolids Applied to Land: Advancing Standards and Practices (2002). National Research Council (NRC),
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