TECHNICALLY SPEAKING
biosolids, of which almost half can be electricity,
or close to 1 kwh/t of solids (if digestion and
electricity generation are optimised).
WHB
Thermal Conversion Options
While biogas generation is a common method
Recovered
for energy recovery, incineration has also long
Heat for
Primary Hex Secondary Hex
Pre-heating
been used for volume minimisation, and
Fluidizing Air
produces excess heat that can be converted to
electricity through the use of steam turbines.
Other thermal conversions are more recent
Energy Recovery
11.4 Gj.h
options to recover energy from biosolids,
or 0.8 MW-h/h
producing gases that can be converted to energy
using modified combustion processes. Their
Feed Solids
process characteristics depend on the oxidising 45 tonnes/d
or reducing media, temperature, and pressure, 26% TS
Fluidized Bed
Auxiliary
and are commonly classified as:
80% VS
Incinerator
Fuel
35 Gj/h
0 mmBtu/h
■ Thermal decomposition in a primarily non-
reactive environment (commonly called
pyrolysis)
Syngas Energy Values and Potential Uses
■ Decomposition in a chemically reactive
environment (usually called gasification if the
Type Energy Value
products are primarily fuel gases)
(Heating Value) MJ/m
3
Use
In the most established thermal process, Low 3,5-10 Gas turbine fuel, boiler fuel
incineration, biosolids are burnt in a combustion
chamber with excess air (oxygen) to form mainly
Hydrogen production, fuel cell feed,
CO
Medium 10-20 chemical and fuel synthesis, requires
2
and H2O. For autogenous combustion, that
is, combustion without supplemental fuel, the
methanation to produce SNG
biosolids need to be dewatered to a minimum
Similar uses as medium heating value
of 28% dry solids (DS). This dry solids content is
High 20-35 gas; requires less upgrading and
generally achieved by use of centrifuges. Once
methanation to produce SNG
the biosolids have been combusted in the
fluidized bed, the gases then pass through the
Substitute natural Directly substitute for natural gas with
> 35
heat recovery system. In larger facilities this can
gas (SNG) no additional treatment
provide enough steam to power a steam turbine
for power generation. An energy balance for a 45 Gasification power generation, hydrogen, liquid fuel, or
tonne/day incineration example is presented in Gasification involves the reaction of carbon in the chemical production. Four types of syngas can
Figure 2. The energy balance only includes wastewater solids with air, oxygen, steam, carbon be produced, depending on the gasification
energy requirements for the incineration process dioxide, or a mixture of these gases at elevated configuration, operating conditions, and feed. The
itself and does not include ancillary processes temperatures (500-1400°F). The products of the syngas types, which are classified based on
such as dewatering. Once the internal energy process include heat, which can be used to heating value, and their uses are described in
needs are covered, incineration would have a generate power, or fuel materials. Typically, the Table 3. Note that syngas types of higher energy
net energy production of 6,1 GJ/tonne of dry majority of the energy is in the form of CO. CH4 values are also suitable for any uses listed in the
solids, or similar to anaerobic digestion. can be produced through the addition of table for lower energy gases.
After heat recovery, the air is treated in a series hydrogen (H2) in a hydrogasification process or While gasification has long been used in the
of cleaning devices to minimise the presence of through specialised catalytic gasification. The coal industry, these systems have much greater
unwanted by-products. Emissions from the gasification process also produces CO
2
, and water capacities than required for the biosolids industry.
incineration process can be managed through a (H2O). In contrast to combustion processes Currently, there are three commercial gasification
combination of prevention (improvement of the (incineration) that work with excess air, technologies suitable to biosolids: KOPF
burning unit and feed) and pollutant capture gasification processes operate at oxygen-starved Gasification Technology (Germany), EBARA fluid
(flue gas and residue cleaning and treatment). conditions, with only enough oxygen added to bed gasification technology (Japan), and
Two of the most demanding challenges are generate heat to drive the chemical reactions. MaxWest Environmental Systems (North
heavy metals and dioxins/furans. The chemical composition of the end America). An energy balance of the gasification
Heavy metals accumulate in the ash, while products and heating value of the product gas process for a 45 dry tonne/day system is shown
mercury can be captured in the flue gas. are affected by the gasification agent (air, oxygen, on Figure 3. As the upfront drying process needs
Dioxins’ and furans’ synthesis are dependant or steam), the gasifier operating temperature and a large amount of the generated heat,
upon numerous factors (temperature, pressure, and feed characteristics (type, dry gasification example would have a net
chlorine content, and surface reactions) and solids, volatile solids). The gas generated energy production of 1,7 GJ/tonne dry solids,
are generally well below the standards and through the gasification process, also known as or less than 30 % of the more conventional
not usually an issue. “syngas”, may require cleaning prior to its use for options above. ■■■
September 2009 Water & Wastewater Treatment 39