THE New technologies Demonstrator programme Mechanical heat treatment facility by OX93Mu



* University of Southampton. °Orchid Environmental Ltd. ªFairport Engineering Ltd.

SUMMARY : The New Technologies Demonstrator Programme supported the
development of waste treatment technologies that are under-utilised in the UK,
including gasification, anaerobic digestion, composting, and mechanical heat
treatment (MHT). A Demonstrator plant using MHT technology was built and
operated in Huyton, Merseyside. The plant took in municipal solid waste (MSW)
which was shredded, thermally processed, and then sorted using a range of
technologies to recover solid recovered fuels (SRFs) and recyclables (including glass,
plastics, ferrous and non-ferrous metals). Most of the SRF recovered by the plant was
used by a cement kiln, but other samples of fuel were produced e.g. for a combined
heat and power plant (CHP) and for gasification. Mass and energy balances were
constructed for the plant supplying fuel to the cement kiln operator and the CHP
plant. The energy inputs and outputs are compared and the relative benefits of each
option are discussed.


The UK Department for Environment, Food and Rural Affairs (Defra) has
implemented a Waste Strategy (Defra, 2007) and a number of initiatives to drive
forward the recovery of resources and meet the targets for biodegradable waste
diversion from landfill imposed by the Landfill Directive (1991/31/EC). One of these
initiatives, the New Technologies Demonstrator Programme (Brooks and Powrie,
2007;Defra 2010), was introduced to promote the development of large scale waste
treatment plants using technologies that are under-utilised in the UK, including
gasification, anaerobic digestion, composting, and mechanical heat treatment (MHT).

A demonstration plant using MHT technology was built and operated in Huyton,
Merseyside with support from Defra and the Merseyside Waste Disposal Authority.
The plant (designed and built by Fairport Engineering Ltd and operated by Orchid
Environmental Ltd – both part of the Orchid BioEnergy Group ) commenced working
in May 2008 and operated under the auspices of the Demonstrator Programme until
March 2009. The plant accepted mixed municipal solid waste (MSW) which was
then shredded, thermally processed, and sorted to recover recyclables (including
glass, plastics, ferrous and non-ferrous metals) and a range of solid recovered fuels
(SRFs). The remaining material was sent to landfill.
SRFs have the potential to substitute for fossil fuels used in electricity and heat
generation and contribute to the reduction of greenhouse gas emissions and to the
increased production of energy from renewables as required by the EU Renewable
Energy Directive, 2009 (2009/28/EC). Most of the fuel produced by the MHT plant
was used by a cement kiln operator but samples of other types of SRF were produced.

A monitoring and evaluation programme was established to obtain a full
understanding of the MHT process and its potential role in the management of
biodegradable MSW and resource and energy recovery. A mass balance was
constructed for the plant and the energy inputs and outputs from the plant were
determined. An evaluation of the efficiency of energy use and recovery was made
under two different scenarios: the production of SRF for a cement kiln operator and
the production of fuel for a combined heat and power (CHP) plant.


2.1   Design

The MHT process at the Huyton plant has three main stages (Figure 1): waste
reception, thermal processing and materials separation. Incoming waste is inspected
and any large items such as gas canisters are removed. A trommel then separates the
waste into two size fractions around a nominal size of 120 - 150 mm. The oversize
material is shredded and combined with undersize material to be conveyed to a
“homogenous” stockpile. The mixed and shredded waste is next conveyed to two
process lines, each with its own wet preparation feed hopper, wet preparation drum
and thermal processor drum. In the wet preparation drum a series of lifters and knives
break up the waste and mix it thoroughly. Water can be added at this stage to increase
the waste moisture content, but this was found to be unnecessary for the great
majority of waste material. The waste is then pushed into the thermal processor drum.
The waste material is moved along the drum by tilted and flat lifters while hot air is
blown along the drum counter-current to the waste to dry the waste, break down
putrescible matter and to clean and sanitise other materials. Typically, the air
temperature ranges from 80oC at the material inlet of the thermal drum to
approximately 270oC at the material exit and the waste remains in the drum for about
40 minutes. The internal pressure of the drum is atmospheric. The rotational speed
(normally 6 rpm) and internal temperature can be altered to achieve the optimum
moisture content for material exiting the drum.

The thermal processed materials are separated by size into two categories, nominally
>50 mm and <50 mm. The >50 mm materials are passed over a ballistic classifier to
be separated into three fractions: heavy, light and fine material. The buoyant or light
material, such as plastics and paper, is granulated and conveyed to a blending bin for
use as a fuel product. The fine fraction is remixed with the <50 mm material. The
heavy material, consisting of items such as tins, bottles, wood and stones, passes
beneath an overband magnet which separates off the ferrous metal. Remaining
material passes to an eddy current separator. Residual ferrous metal is separated to a
container and non-ferrous metal is conveyed to a bulk container. An optical sorter
separates the remaining materials coming from the eddy current separator using an
infra-red detector and compressed air. Plastics are deflected onto a reversible
conveyor which either feeds a baler or may be conveyed to a granulator and from
there to a blending bin to be used in the fuel products. Any residual material is
transported via a conveyor to a compactor where it leaves the plant to be landfilled.
Fine material from the ballistic classifier is mixed with the <50 mm waste stream
from the sizing screen. Ferrous and non-ferrous metals are separated from this stream
using an overband magnet and eddy current separator. A twin deck screen separates
the remaining material into three nominal size fractions: >16 to <50 mm, >6 to <16
mm, and <6 mm. From the twin deck screen, the various materials are further refined
using a patented biomass density separator system (comprising pneumatic conveying
pipework, rotary valves, cyclones, air filters, vibratory hoppers and feed conveyors)
which separates heavy material such as glass and rubble from the lighter fuel
elements. The various fuel products recovered by the plant can then be blended in the
outloading section of the plant.

Figure 1: The Orchid Mechanical Heat Treatment Process

2.2   Operation

Over the eleven month demonstration period the operation of the plant was
continually reviewed and improvements made to the process. During the early stages
of operation, it became apparent that the proportion of textiles and plastics in the
waste was around twice (c. 7% each) what was expected from the predicted
composition of the waste prior to commissioning. The textiles wrapped around
rotating machinery and also caused blockages. A second shredder was added to cut up
the textiles; this reduced the number of stoppages due to blockages/equipment failure.
The air flow system in the thermal processor drums and in the air filtration system
was modified to improve the efficiency of drying the waste.
The plant was built to process 50,000 tonnes of incoming waste per annum (although
capable of processing up to 75,000 tpa, the Defra contract and planning permission
limited it to 50,000 tpa). However, only 20,500 tonnes of MSW were processed
because of problems which occurred during the early stages of operation, and because
operations at the plant were halted for three months (November 2008 to January
2009) after a fire at the facility. After repairs to parts of the affected equipment and
building, the plant reopened and continued working for a further two months until the
planned end of the demonstration period, 31st March 2009.

As a result of the improvements at the plant there was a steady rise in the amount of
MSW treated. 200 tonnes per week were processed initially but this rose to a
maximum of 954 tonnes per week (Figure 2). As problems were resolved there was a
reduction in the amount of material sent to landfill from around 65% of the incoming
waste to a minimum of 9%. At the same time there was an increase in the amount of
recyclables recovered reaching approximately 15% by the time of the fire closure
(from week 26 to 37). The amount of material diverted to fuel products rose from
10% to a maximum of 76% at the end of the demonstration period (Figure 3).

                  1200                                                                                      80
                                 Waste to landfill                                                                       Biomass: % of incoming waste
                                 Incoming waste                                                             70           Recyclables: % of incoming waste

                                                                                      % of incoming waste

Tonnes per week


                  600                                                                                       40



                    0                                                                                       0
                         0   5           10          15     20   25   40   45   50                               0   5         10        15       20        25   40   45   50

                                                          Week                                                                                  Week

Figure 2: Incoming waste and outgoing                                                Figure 3: Recovered recyclables and fuel
landfilled material                                                                  products as a percentage of incoming waste

3.                       METHOD

Data were collected from an automated monitoring system operating at the MHT
plant. This gave information on the mass of incoming waste, the mass of waste
passing through the plant at different points along the process, and the amounts of gas
(m3) and electricity (kWh) used. Additional data on the mass of recyclables, fuel
products, and outgoing waste to landfill were obtained from the plant’s weighbridge
system. Samples of the incoming waste, outgoing fuel and recyclables were taken for
physical and chemical characterisation. Compositional analysis of the incoming waste
(7 samples) was carried out by hand-sorting into the following categories: paper and
card, putrescibles, textiles, fines, glass, miscellaneous combustible, miscellaneous
non-combustible, metals (ferrous and non-ferrous), plastics (hard or film), household
hazardous waste and waste electrical equipment. The fuel products were examined to
determine the biomass content (by manual sorting and dissolution methods (CEN/TS
15440:2006), calorific value (CEN/TS 15400:2006), moisture content and ash
content. Particle size analysis and density measurements were also carried out on the
fuel samples.


4.1   Incoming waste

The composition of the incoming waste is given in Table 1; there was greater
variation in the content of the paper and organic material than in the other waste
fractions. The average gross calorific value (GCV) of the incoming waste was
estimated to be 11.8 MJ/kg, based on the percentage of each fraction and its calorific
value (Defra, 2006a). The moisture content of one waste sample was determined to
be 40% (mass of water to total mass). The biodegradability of the waste was
estimated to be 61% based on the percentage biodegradability of each fraction as
defined in the Landfill Allowance and Trading Scheme (LATS) (England)
Regulations (Defra, 2006b).

Table 1: Characteristics of the incoming waste (% by wet mass)

                                             Average*           St. deviation
           Paper/Card                            25.9                     6.3
           Plastic Film                           8.8                     3.0
           Dense Plastic                         10.0                     3.3
           Textiles                               7.1                     3.2
           Combustibles                           3.7                     1.9
           Non Combustibles                       1.5                     1.5
           Glass                                  4.0                     1.8
           Organic                               21.4                    13.6
           Ferrous Metal                          3.4                     2.2
           Non Ferrous Metal                      1.7                     0.9
           Fine Material <10mm                    6.6                     2.1
           Waste Electrical and Electronic        1.7                     1.7
           Specific Hazardous household           0.4                     0.7
           *from 7 samples

4.2   Characteristics of the fuel products

The majority of the fuel produced at the plant during the demonstration period was
supplied to a cement kiln operator. The preferred characteristics for fuel of this type
are NCV of 10-15 MJ/kg, a maximum of 20% moisture and ash content, a particle
size of less than 30 mm, and mean chlorine content 0.5-1% w/w (van Turbergen et al,
2005; Eckhart and Albers, 2003). The characteristics of the cement kiln (CK) fuel are
shown in Table 2: the average moisture content, ash content and calorific value of 36
samples taken during operation of the plant is shown together with particle size
analysis, bulk density and biomass content of one of the samples. The results show
that these samples of fuel are achieving the required characteristics, although the
maximum particle size may have exceeded 30 mm. The measured sulphur (<0.2%),
chlorine (<0.25%) and mercury contents (<0.1%) of this fuel were low; these
parameters, taken with the calorific value, show that the fuel has the potential to be
classified as a solid recovered fuel in accordance with European standards being
drawn up by the CEN Technical Committee (prCEN/TS 15359).

Table 2: Characteristics of samples of recovered fuel.

 Sample                                         Cement Kiln fuel      Cement Kiln           CHP
                                                  (average of 36       fuel Sample           fuel
 Moisture content%                                          20.5                               30
 Ash%                                                       15.5                            24.6
 Bulk density (kg/m3)                                                            99          272
 Gross CV (MJ/kg)                                            15.1                           10.2
 Net CV (MJ/kg)                                              13.6                             8.5
 % biomass content by calorific value                                            76            87
 % biomass (paper/card/textiles/food/wood)                                       73            86
 % plastic (hard and film)                                                       19           7.6
 % glass, metal and stones                                                      7.4           6.9
 Particle size analysis (% by mass)
 <6.3 mm                                                                       23.7         51.5
 >6.3 mm to 10 mm                                                               7.3         29.4
 >10 mm to 50 mm                                                               69.0           19
 >50 mm                                                                           0            0

During the demonstration period steps were taken to improve the fuel supplied to the
cement kiln operator. The MHT process was adjusted to granulate the 16-50 mm fuel
fraction and measures were taken to promote additional drying of the waste by
reducing the rotational speed of the thermal drum. The proportion of 0 to 6mm
fraction in the fuel mixture was also reduced because this fraction has a higher inert
content than the larger fractions of fuel, whereas the proportion of plastics in the fuel
was increased to raise the calorific value. These modifications enabled the plant to
produce a fuel with a calorific value of 16-18 MJ/kg, 15% ash content, 18% moisture
content and less than 30 mm particle size (Defra, 2010).

Samples of fuel were also produced for trials by a CHP plant operator. Table 2 shows
results from one sample of this fuel. The biomass content is close to 90% by mass and
by calorific value and therefore nearly achieves the requirement to be termed
‘biomass’ in accordance with the definition given in the Renewables Obligation
Orders, i.e. at least 90% of the energy content of the fuel must be derived from animal
or plant origin (Renewable Obligation Orders, 2009). In a later sample produced for
the same client, greater than 90% biomass by weight was achieved (Orchid
Environmental, 2010). The removal of plastic from the CHP fuel resulted in a lower
calorific value than the CK fuel. The fuel is taken principally from the 0 to 6 mm
fraction isolated by the biomass density separator. Particle size analysis shows some
material greater than 10 mm, but again, further refinement of the process enabled the
specification to be met.
4.3   Mass Balance

A mass balance was established from data from the period after the fire when the
plant was operating optimally (09/02/2009 to 16/03/2009). Data were available for
energy used, mass of incoming waste, outgoing recyclables, landfilled waste, cement
kiln fuel, and water supply and discharge. The mass of evaporated water was
estimated by subtracting the mass of fuel products, recyclables and residual waste
from the mass of incoming waste to be 17.5% for the operational period in question.
The mass balance for this period is shown in Table 3.

Only a small sample of fuel was produced for the CHP plant, so the mass balance for
the process could not be determined in the same way. However, the fractions of
biomass, plastic and inert material in the CHP fuel sample were known (Table 2) and
assuming the same separation efficiency of biomass was achieved as occurred in the
production of CK fuel and the same percentage recovery of glass and metals, the mass
balance for recovery of CHP fuel can be approximated. A larger fraction of plastics
was recovered as a recyclable material; a small amount of additional inert material
was assumed to be landfilled (Table 3).

Table 3: Outgoing materials as a percentage of the incoming waste from the MHT

                                              Average % by mass
                                       Cement kiln      CHP fuel sample
                                             sample           (deduced)
              Ferrous metals                    3.8                  3.8
              Non-ferrous metals                0.7                  0.7
              Plastics                          3.3                  9.2
              Glass                             5.3                  5.3
              Fuel products                    46.8                39.7
              Landfilled waste                 22.8                23.3
              Evaporated water       17.5 (deduced)                   18
              Total                           100.2                 100

4.4 Energy Inputs and Outputs
Energy balances were constructed for the production of CK and CHP fuels, with
recovery of recyclables. In both scenarios it was assumed that the energy input to the
plant was the same, although there may have been some differences in the way the
plant equipment was operated with consequent impacts on energy use. For example,
to produce sufficient mass of fine material (<6 mm) for the CHP fuel, an additional
shredder may be required. The average rate of energy used per tonne of incoming
waste was based on data collected when the plant was running optimally (during the
same period as that used to determine the mass balance). Gas was used to heat the air
in the thermal drums at an average rate of 22.8 m3/tonne incoming waste, equivalent
to 900 MJth/tonne (assuming 39.6 MJ/m3 for the calorific value of natural gas).
Electricity was used at an average rate of 100 kWh/tonne. This value was converted to
a thermal equivalent of 690 MJth/tonne, assuming electricity generation from
combined cycle gas turbine at an efficiency of 52% (DECC, 2009a). Approximately 2
litres of diesel per tonne of incoming waste was used to move materials at the site —
equivalent to 78 MJ/tonne. Thus, the total energy used for the plant operating
optimally was 1,670 MJth/tonne, or 464 kWhth/tonne. Energy used in transporting the
waste, fuel and recyclables to and from the plant were not considered in the energy

The first scenario was the recovery of fuel for the cement kiln operator. The average
net calorific value of the fuel produced for this client was 13.6 MJ/kg, and when
operating optimally 46.8% of the incoming waste was recovered as fuel. The
available thermal energy from the fuel is therefore 6,365 MJ (1,768 kWhth) per tonne
of incoming waste. As well as fuel, the plant recovered recyclables. The energy saved
by recycling these materials instead of using virgin materials in the manufacture of
goods was calculated taking data from the Inventory of Carbon and Energy
(Hammond and Jones, 2008) to be 5,195 MJth/tonne incoming waste
(1,443 kWhth/tonne). The remaining material (22.8% of the incoming waste) was sent
to landfill. The biodegradation of waste in a landfill produces gas with an energy yield
of 2,340 MJth/tonne of wet MSW (Powrie and Dacombe, 2006). From a limited
number of samples of the outgoing waste from the plant, the biodegradability was
calculated to be about half that of MSW. Assuming 75% of the landfill gas is
collected (Golder Associates, 2005), the energy derived from the waste sent to landfill
was 200 MJth/tonne incoming waste (55.6 kWhth/tonne). The net total amount of
energy recoverable when the plant is recovering CK fuel was therefore
10,090 MJth/tonne of waste or 2,803 kWhth/tonne.

In the second scenario, the energy balance was constructed for the recovery of CHP
fuel at the plant. Because of the lower content of plastic the CHP fuel sample had a
lower net calorific value (8.5 MJ/kg) than the CK fuel product. The energy
recoverable from the CHP fuel was 3,375 MJth/tonne incoming waste. The
recoverable embodied energy of the recyclables was calculated to be 9,500 MJth/tonne
incoming waste. This is much greater than when the plant was recovering CK fuel,
because of the increased recovery of recyclable plastic. It is assumed that the same
amount of energy was generated from landfill gas recovered from the residual
material. In total the net recoverable energy in this scenario is 11,405 MJ/tonne (3,168
kWhth/tonne) incoming waste.

Considering the process as a whole, about 10% more energy is recoverable when the
CHP fuel is produced. In terms of the fuel recovered, the energy value of the CK fuel
is greater than that of the CHP fuel, but under this scenario more energy is recovered
from the savings in embodied energy of the recyclable plastic (Figure 4). This
assumes that all the plastic can be recycled and that the maximum embodied energy
savings from recycling plastic (c.60-70 MJ/kg) can be made. No account is made of
potential loss of material which may occur during processing into recycled plastic.
Similarly, downstream energy used involved in transporting the recycled material to
the processing plant was not considered. Recycled plastic must be clean and consist of
one type of plastic if used as a substitute for virgin materials. Since the material
exiting the MHT process comprises mixed plastics, further sorting and cleaning
would be required to recover the individual plastic types. Alternatively, the recovered
mixed plastic could be used in the production of lower grade mixed composition
plastics (e.g. plastic lumber, clothing). However, the production of goods from such
materials may be more energy intensive than if the original material is used, e.g.
production of plastic lumber may use more energy than the production of lumber from
wood (Astrup et al, 2009). If the mixed plastic cannot be processed to make it suitable
for high quality use, it may be preferable to recover its energy value as a fuel, either in
SRF as in the CK fuel or as a fuel in its own right (the net calorific value of mixed
plastic is estimated as 30-40 MJ/kg).


The MHT facility has demonstrated the potential to recover high quality fuel products
and reclaim recyclables from MSW. A number of modifications were required to
enable the process to deal with larger than expected quantities of difficult wastes like
textiles and further adjustments were made to the thermal processor to promote more
rapid and even drying of the waste. As the process was improved, there was a
significant increase in the amount of fuel and recyclables recovered while the tonnage
of waste sent to landfill decreased. The fuels produced were of high quality, the CK
fuel meeting the classifications of a solid recovered fuel, and the CHP fuel
demonstrating the potential of this technology to produce high biomass materials
suitable for ROCs accreditation. When operating optimally and producing CK fuel,
about half of the incoming waste (by mass) was recovered as fuel. An estimated mass
balance for the production of CHP fuel suggested that slightly less fuel would be
recovered due to the diversion of plastic to the recyclables. The energy balance
demonstrated that the potential net energy recovered or saved per tonne of incoming
waste was greater overall when producing fuel for the CHP plant, so long as the
plastic is recycled to substitute for high quality plastics. If this is not possible then the
use of plastics to produce higher calorific value fuels may be preferable

                    MJ/tonne incoming waste (gross)    7000







                                                                  Scenario 1:           Scenario 2:
                                                              1       2
                                                                  Cement kiln
                                                                                3   4   5      6        7
                                                                                         Combined heat and

Figure 4: Energy derived from materials recovered when producing two types of fuel product.


The authors wish to thank Defra, Fairport Engineering Ltd and Orchid Environmental
Ltd for supporting the MHT research and evaluation project.


Astrup, T., Fruergaard, T. and Christensen, T.H., 2009. Recycling of plastic: accounting
  of greenhouse gases and global warming contributions. Waste Management and
  Research, 27, 763-772.
Brooks, D and Powrie, W. Briefing: Defra’s New Technologies Demonstrator
  Programme. Proc. ICE, Waste and Resource Management, 2007, 160, 1, 5-10.
Council for the European Communities. Directive 1999/31/EC (1999) on the landfill of
  waste. Official Journal of the European Communities, 1999, L182 (16/7/99), 1.19.
CEN/TS15400: 2006. Solid Recovered Fuels – Methods for the determination of
  calorific value. European Committee for Standardisation, Management Centre, Rue
  de Stassart, 36, Brussels.
CEN/TS 15440: 2006.Solid Recovered Fuels – Method for the determination of
   biomass content. European Committee for Standardisation, Management Centre,
   Rue de Stassart, 36, Brussels.
Defra, 2006a. Carbon balances and energy impacts of the management of UK wastes.
   Defra R&D Project WRT 237.
Defra, 2006b. Guidance on the Landfill Allowance Schemes, Municipal Waste. Defra,
   Local Authority Waste Performance Unit, Ashdown House, London, SW1E 6DE
Defra, 2007. Waste strategy for England 2007.
DECC, 2009. UK Energy in brief.
Defra, 2010. Orchid Mechanical Heat Treatment Process Demonstration Final Report.
   Defra New Technologies Demonstrator Programme
Eckardt, S., Albers, H., 2003. Specifying criteria for the utilisation of refuse derived
   fuels (RDF) in industrial combustion plants. Proc. Sardinia, Ninth International
   Waste Management and Landfill Symposium, Cagliari, Italy.
Golder Associates, 2005. UK Landfill methane emissions: evaluation and appraisal of
   waste policies and projections to 2050.
Hammond, G. and Jones, C., 2008. Inventory of carbon and energy. University of Bath.
   Available from
Powrie, W. and Dacombe, P., 2006. Sustainable waste management – what and how?
   Proceedings of the ICE, Waste and Resource Management, 159,3,101-116.
prCEN/TS 15359. Solid recovered fuels – Specifications and classes. European
   Committee for Standardisation, Management Centre, Rue de Stassart, 36, Brussels.
The Renewables Obligation Orders, 2009. Statutory Instrument, No.785, 2009. The
   Stationery Office Limited, UK. ISBN 0580485350
Van Turbergen, J., Glorius, T., Waeyenbergh, E., 2005. Classification of solid
recovered fuels. European Recovered Fuel Organisation.

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