U.S. patent application number 16/093766 was filed with the patent office on 2019-03-14 for process for producing a combustible product.
The applicant listed for this patent is INDUSTRIAL CHEMICALS GROUP LIMITED. Invention is credited to Darren Sharpe, Felix Sirovski.
Application Number | 20190078032 16/093766 |
Document ID | / |
Family ID | 55910734 |
Filed Date | 2019-03-14 |
United States Patent
Application |
20190078032 |
Kind Code |
A1 |
Sharpe; Darren ; et
al. |
March 14, 2019 |
PROCESS FOR PRODUCING A COMBUSTIBLE PRODUCT
Abstract
A process for producing combustible product from an organic or
biomass feedstock, the process comprising: mixing the feedstock
with an alkaline material to give an alkaline aqueous mixture;
heating the mixture by ohmic heating to a temperature in the range
of about 280.degree. C. to about 320.degree. C. and reacting the
mixture under subcritical conditions at the said temperature range
and a pressure of about 6.6 to about 11.6 MPa (65 bar gauge to
about 115 bar gauge); and removing at least some of the water to
leave a combustible product, which may be used to form an aqueous
slurry, suspension or emulsion and combusted in a suitable
engine.
Inventors: |
Sharpe; Darren; (Basildon,
GB) ; Sirovski; Felix; (Sturry, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INDUSTRIAL CHEMICALS GROUP LIMITED |
Grays, Essex |
|
GB |
|
|
Family ID: |
55910734 |
Appl. No.: |
16/093766 |
Filed: |
April 13, 2017 |
PCT Filed: |
April 13, 2017 |
PCT NO: |
PCT/EP2017/059012 |
371 Date: |
October 15, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C10L 1/328 20130101;
C10L 2290/544 20130101; C10L 1/1241 20130101; C10L 2290/52
20130101; C10L 1/326 20130101; C10L 2290/02 20130101; C10L
2200/0469 20130101; C10L 2290/28 20130101; C10L 2290/24 20130101;
C10L 9/086 20130101 |
International
Class: |
C10L 1/32 20060101
C10L001/32; C10L 1/12 20060101 C10L001/12; C10L 9/08 20060101
C10L009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 15, 2017 |
EP |
16165676.4 |
Claims
1. A process for producing a combustible product from an organic or
biomass feedstock, the process comprising: mixing the feedstock
with an alkaline material to give an alkaline aqueous mixture;
heating the mixture by ohmic heating to a temperature in the range
of about 280.degree. C. to about 320.degree. C. and reacting the
mixture under subcritical conditions at the said temperature range
and a pressure of about 6.6 to about 11.6 MPa (65 bar gauge to
about 115 bar gauge); and removing at least some of the water to
leave a combustible product.
2. The process as specified in claim 1, wherein the weight ratio of
solid:liquid in the mixture is in the range about 1:2 to about
1:25, preferably about 1:3 to about 1:7, particularly preferably
about 1:3 to about 1:5.
3. The process as specified in claim 1, wherein the reaction under
subcritical conditions is carried out over a period of at least
about 1 minute, preferably at least about 2 minutes, particularly
preferably at least about 5 minutes; and at most about 60 minutes,
preferably at most about 30 minutes.
4. The process as specified in claim 1, wherein the alkaline
material is added in an amount about from 0.1 to 10% (calculated on
100% base content), preferably about 1 to about 5% w/w.
5. The process as specified in claim 1, wherein the feedstock is an
organic feedstock selected from one or more of: sewage material,
sewage sludge, digested sewage sludge, digested human sewage
sludge, digested dewatered sewage sludge cake, farm slurry,
livestock slurry, pig slurry, cow slurry, compost, plant material,
food production wastes, food production slurry, anaerobic
digestate.
6. The process as specified in claim 1, wherein the feedstock is a
biomass feedstock selected from one or more of: wood chippings,
wood fines, saw dust, waste wood, waste treatment wastes, bio
crops, plant matter.
7. The process as specified in claim 1, wherein the alkaline
material is selected from: sodium hydroxide, potassium hydroxide,
sodium carbonate, potassium carbonate, sodium formate, potassium
formate, calcium hydroxide.
8. The process as specified in claim 1, wherein the reaction is
carried out at a pressure in the range of about 8.1 to about 11.1
MPa (80 to about 110 bar gauge), preferably about 9.1 to about 10.6
MPa (90 to about 105 bar gauge).
9. The process as specified in claim 1, carried out in a tubular
reactor or a plug flow reactor.
10. The process as specified in claim 1, the process further
comprising the step of combusting at least some of the combustible
product.
11. The process as specified in claim 1, further comprising milling
the combustible product to produce fine particles, preferably of
size less than about 10 .mu.m, and mixing the particles with water,
optionally with a surfactant and/or a stabiliser, to produce an
aqueous suspension suitable for use as a liquid fuel.
12. The process as specified in claim 1, further comprising
extracting a liquid hydrocarbon fraction from the combustible
material, and mixing the extracted hydrocarbon fraction with water,
optionally with a surfactant and/or a stabiliser, to produce an
emulsion suitable for use as a liquid fuel.
13. The process as specified in claim 1, wherein the feedstock is
waste wood fines.
14. The process as specified in claim 1, wherein the feedstock is
sewage sludge.
15. The process as specified in claim 13, further comprising mixing
the combustible product with water without milling the combustible
product, optionally with a surfactant and/or a stabiliser, to
produce an aqueous suspension suitable for use as a liquid
fuel.
16. A process for producing a liquid fuel from an organic or
biomass feedstock, the process comprising: mixing the feedstock
with an alkaline material to give an alkaline aqueous mixture;
heating the mixture by ohmic heating to a temperature in the range
of about 280.degree. C. to about 320.degree. C. and reacting the
mixture under subcritical conditions at the said temperature range
and a pressure of about 6.6 to about 11.6 MPa (65 bar gauge to
about 115 bar gauge); removing at least some of the water to leave
a combustible material; extracting a liquid hydrocarbon fraction
from the combustible material, and combining the extracted
hydrocarbon fraction with water to produce an emulsion suitable for
use as a liquid fuel.
17. A process for producing a combustible product from an organic
or biomass feedstock, the process comprising: mixing the feedstock
with an alkaline material to give an alkaline aqueous mixture;
heating the mixture by ohmic heating to a temperature in the range
of about 280.degree. C. to about 320.degree. C. and reacting the
mixture under subcritical conditions at the said temperature range
and a pressure of about 6.6 to about 11.6 MPa (65 bar gauge to
about 115 bar gauge); removing at least some of the water to leave
a solid combustible material; milling the solid combustible
material to produce particles of size less than about 10 .mu.m; and
mixing the particles with water and a surfactant to produce an
aqueous suspension suitable for use as a liquid fuel.
Description
BACKGROUND
a. Field of the Invention
[0001] The invention relates to a process for producing a
combustible product from an organic or biomass feedstock, and to
methods of using the product. Other aspects of the invention
provide a liquid suspension or emulsion derived from the
combustible product and suitable for use as a liquid fuel.
b. Related Art
[0002] There have been many attempts at producing fuels and biogas
from various forms of biomass and organic materials. There has been
investigation into advanced conversion, using pyrolysis and
gasification, of organic, plant and human waste to produce fuels
and biogas. The biomass and organic feedstock is subjected to
advanced conversion at temperatures of, for example, from 250 to
800.degree. C.
[0003] Other processes include the generation of gaseous fuels,
comprising methane from various forms of biomass organic feedstock
using techniques such as anaerobic digestion.
[0004] One form of organic feedstock is sewage sludge in digested
or raw form from the treatment of human faecal waste. Water
treatment companies have the task of treating and disposing of
large amounts of sewage sludge each year. Traditionally, a large
proportion of the sewage sludge has been spread onto farmland as a
fertiliser and soil improver. This process has its disadvantages.
Firstly, the sewage sludge contains a large percentage of water,
usually around 70%, and therefore is expensive to transport from
the treatment site to the agricultural land to be treated. Also,
farmers are increasingly turning away from using such material as
fertiliser for various reasons including changes in legislation. It
is therefore becoming increasingly difficult and expensive to treat
and dispose of the large amount of sewage sludge produced each
year.
[0005] Biomass has many forms occurring both naturally and from the
waste industry. One such biomass material which is often treated as
waste is wood fines, which is produced particularly from the
recycling of wood materials. Wood fines are generally small pieces
of wood in various different sizes with a low or zero commercial
value. The particles of wood can vary from very small dust-like
particles up to chunks or splinters of wood, or even longer pieces
of material. They are often irregular in size and a batch of wood
fines is quite heterogeneous in shape and dimensions. The
legislation surrounding waste materials makes the disposal and
reuse of recycled wood fines especially difficult due to the
completely random and changing composition of the material.
[0006] US 2010/0162619 describes production of material or fuel
from biomass by treating a solid--liquid mixture in the presence of
an acid, at a temperature of above 100.degree. C. and at a pressure
of above 0.5 MPa (5 bar) for a treatment duration for at least one
hour.
[0007] US 2012/0073189 describes a method for treating an organic
waste, in which the organic waste is pressurised and continuously
supplied to a high temperature and pressure treatment apparatus to
produce a slurried material by blowing steam into the organic waste
to cause a reaction while heating, pressurizing and agitating.
However, blowing steam into a medium under high pressure involves
various engineering issues and is inherently unsafe. The method
cannot provide a uniform heating of the reaction mixture and leads
to unnecessary dilution of finished product with water, increasing
drying costs.
[0008] US 2013/0011327 describes a laminar stream reactor for the
production of hydrochar of a solid-fluid mixture of water and
carbon-containing component, wherein the solid-liquid mixture is
treated at a temperature of 100-300.degree. C., and a pressure of
0.5-7 MPa (5-70 bar). The reactor consists of tubular reactor units
of largely vertical holding sections and direction-changing
diverters. The holding sections are flown slower by the
solid-liquid mixture than the remaining tube distances, as they
have larger diameters. The total residence time is more than two
hours.
[0009] Having laminar flow in the vertical pipe would result in the
sedimentation of solid particles of the slurry as their density is
higher than that of water and possible clogging of the pipework,
especially at long residence times.
[0010] David Nelson et al. in "Application of direct thermal
liquefaction for the conversion of cellulosic biomass". Industrial
and Engineering Chemistry Research, v. 23, No. 3, September 1984,
pp. 471-475, DOI 10.1021/i300015a029 teaches the conversion of
primary undigested, dewatered sewage sludge and pure cellulose by
hydrothermal liquefaction. The conversion was performed in the
presence of sodium carbonate. Oil and char were obtained.
[0011] A paper by Jude. A. Onwudili et al. is entitled "Enhanced
methane and hydrogen yields from catalytic supercritical water
gasification of pine wood sawdust via pre-processing in subcritical
water". RSC Advances: An International Journal to Further the
Chemical Sciences, v.3, No. 30, January 2013, p.12432, DOI
10.1039/c3ra41362d. This paper describes the gasification of
sawdust to methane and hydrogen in supercritical water. Prior to
this, sawdust is pre-processed in subcritical water in the presence
of sodium carbonate or niobium (V) oxide. The described process
resulted in a relatively small increase in higher heating value
(HHV) of the product relative to the feedstock. It is desirable to
avoid the use of niobium (V) oxide catalyst, and to provide a
process which gives increased HHV.
[0012] There remains a need to treat organic and biomass materials
in a cost effective and sustainable way.
SUMMARY OF THE INVENTION
[0013] Aspects of the invention are specified in the independent
claims. Preferred features are specified in the dependent
claims.
[0014] The invention provides a way to treat organic and biomass
feedstock materials to produce a combustible product with high net
calorific value. The process takes no more than 30 minutes. Longer
reaction times may be employed but result in a significant
reduction in particle size of the end product, which can make
filtration difficult.
[0015] Advantageously, the feedstock materials do not need to have
a low moisture content or be dried before processing.
[0016] Among many applications, the process is suitable for
converting waste wood, including wood waste fines, into a useful
combustible product. This is particularly advantageous because at
present there is no viable long term sustainable solution to the
problem of disposal of wood waste fines that does not involve
landfill.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will now be further described, by way
of example, with reference to the following figures, in which:
[0018] FIG. 1 is a flow chart showing the preparation of organic
and biomass feedstock;
[0019] FIG. 2 is a flow chart showing the conversion of organic and
biomass feedstock into combustible products;
[0020] FIG. 3 shows three flow charts depicting the options for the
processing and use of the combustible product of the
treatment/conversion process;
[0021] FIGS. 4-6 are graphs of, respectively: corrected
conductivity for a wood fines/water mix; change in dielectric
constant of water as a function of temperature; and density of
water v temperature;
[0022] FIG. 7 shows apparatus for use in a method according to an
aspect of the present invention; and
[0023] FIG. 8 is a graph showing particle size of materials
obtained by conventional heating and by heating in accordance with
an embodiment of the present invention.
DEFINITIONS
[0024] As used herein, the term "feedstock", as in "organic
feedstock" and "biomass feedstock" as described above, is intended
to mean material that contains organic and biomass components that
have come from the remains of organisms, such as plants and
animals, and their waste products. The material may have been
treated in some way or be classed as waste before being used in the
process of the invention.
[0025] As used herein, the term "combustible", as in "combustible
product", is intended to mean material that is capable of being
burnt and sustain combustion on its own.
[0026] As used herein, the term "alkaline", as in "alkaline
material", is a substance that increases the pH of an aqueous
organic feedstock material.
[0027] As used herein, the term "subcritical" refers to an aqueous
mixture that is at a pressure and temperature below the critical
point, and so the water is in exclusively liquid form, and not in a
supercritical form or as steam.
DETAILED DESCRIPTION
[0028] The feedstock materials contain a large amount of organic
material which can be converted into a useful combustible product.
The process of the present invention is concerned with a process of
converting the feedstock, such as sewage sludge and wood, into a
combustible product. The combustible product is typically in the
form of a coal-like material and it may optionally be further
processed to provide a combustible product in liquid form.
[0029] Due to the difference in moisture content of organic and
biomass feedstock the process prepares the materials in different
methods to achieve the optimal feedstock conditions required.
[0030] FIG. 1 shows flow charts of the processes 20 for the
preparation of feedstock.
[0031] Organic feedstock 20 is fed into the feed hopper 21 via
mechanical loading machine and onto a conveyor belt. It passes a
metal separation stage 22 to remove any unwanted metallic objects
via an over conveyor magnet and or drum magnet. The material is
then deposited into the high pressure pump feed hopper 24 where the
alkaline material 23 is added. From here the feedstock is
introduced into the core process.
[0032] Biomass feedstock 25 is fed into the feed hopper 26 via
mechanical loading machine and onto a conveyor belt. It passes a
metal separation stage 27 to remove any unwanted metallic objects
via a drum magnet and eddy current separator and/or optical metal
separator. The conveyor then feeds the biomass into a mill/grinder
28 where the particle size is reduced to less than 5 mm. The
mill/grinder 28 deposits the material onto a conveyor belt which in
turn feeds into the biomass/water mixing vessel 29. Water 30 is
added into the biomass/water mixing vessel 29, the water to wood
ratio being anywhere from 10:1 to 2:1. The alkaline material 31 is
then added and the 3 components are mixed together. Alkaline
material could be added to water prior to mixing with wood fines.
Once mixed the material is then pumped into the high pressure pump
feed hopper 24.
[0033] From here the feedstock is introduced into the core
process.
[0034] It is significant that the feedstock does not need to be
dried or otherwise treated before use in the present invention.
This provides advantages in terms of cost savings and time
savings.
[0035] The process is preferably carried out in a continuous
manner, for example, by pumping the feedstock reaction mixture
through a sufficiently long heated reaction vessel or a reaction
vessel preceded by a heater. Alternatively, it can be carried out
as a batch process.
[0036] The process according to embodiments of the invention is
carried out at sub-critical conditions. In other words, the
temperature and pressures used in the process of the invention
result in the water in the reaction mixture being liquid rather
than being a gas or as a super-critical fluid. Subcritical water is
less polar than water under normal conditions and solubility of
organic materials is significantly increased. Pressures of 6.6-11.6
MPa (65-115 bar gauge) correspond to saturated steam pressure at
any given temperature. We found that the lowest practical
temperature for the process is about 280.degree. C. At lower
temperatures virtually no reaction occurs. The upper temperature of
320.degree. C. is preferred for practical reasons. A higher
temperature entails complicated reactor arrangement owing to overly
high pressure that does not bring any significant gains in terms of
product yield or reaction time. The preferred solid:water ratio is
about 1:2.5 to about 1:25. At solid ratios higher than about 1:2.5
the yield drastically drops; at ratios below 1:25 product isolation
becomes complicated.
[0037] FIG. 2 shows flow charts of the processes 40 for the core
treatment/conversion technology.
[0038] The prepared feedstock mixture present in the high pressure
pump feed hopper 40 (24 in FIG. 1) is fed into the high pressure
feed pump 41 which in turn feeds the feedstock heater 42 at the
desired pressure, which may be from 8.1 to 11.1 MPa (80 to about
110 bar gauge) as previously described. The feedstock heater 42
raises the temperature of the mixture to the desired range, which
may be from 280.degree. C. to 320.degree. C. as discussed
previously. A preferred way of feedstock heating is to use ohmic
heating to efficiently and rapidly raise the temperature of the
mixture before the reaction is performed. Ohmic heating works by
using the conductivity of the feedstock and passing an electric
current through the material, which heats the material itself
throughout its bulk. Not only is this efficient, it also minimises
problems with traditional heating which can cause fouling around
the walls of a reactor or heat exchanger, and also inefficient
heating and temperature drop away from the heating element. Other
forms of heating appropriate to the process that provide uniform
feed heating can also be considered, such as induction heating or
microwave heating.
[0039] As the temperature of the reaction mixture rises in the
feedstock heater 42 to the desired temperature, for example from
280.degree. C. to 320.degree. C. the pressure also increases to the
desired pressure, for example from 6.1 to 11.6 NPa (60 to 115 bar
gauge). Under these conditions the aqueous reaction mixture is
sub-critical. In other words, the water in the mixture is present
in a liquid phase, rather than as a gas, or as a super-critical
fluid. It is believed that the properties of the sub-critical water
at these temperatures and pressures facilitate the conversion of
the organic material into the desired combustible product.
[0040] After the temperature and pressure of the reaction mixture
has been raised to the desired levels in the feedstock heater 42,
the material then passes into a high pressure reactor 43.
Preferably the reaction mixture is maintained at the two ranges of
temperature and pressure within the high pressure reactor 43 for
around 2 to 30 minutes, which is sufficiently long to convert the
material to the desired combustible product. Longer reaction times
do not necessarily increase the quality or yield of the product
significantly, but instead appear to change the size of the
particles in the final product. Other traditional forms of heating
may be used to maintain the temperature of the mixture as it passes
through the high pressure reactor 43.
[0041] After being retained at the desired temperature and pressure
for a sufficiently long period of time (for example from 2 to 30
minutes, as discussed above) the mixture leaves the high pressure
reactor 43 in a purely aqueous state and then passes on for further
processing. In FIG. 2, the material is shown passing through
strainers 44 to remove unwanted heavy particles or contaminates
such as glass, sand and stones. Heat is then recovered from the
reaction mixture using one or more high pressure heat exchangers
45, this rapidly cools the reaction mixture and improves the
overall efficiency of the process.
[0042] Pressure regulation 46 equipment maintains the desired
pressure range, as described above, within all the high pressure
equipment whilst at the same time allowing the finished reaction
product to pass into atmospheric pressure via a flash tank 47. At
this point the finished reaction product is an aqueous mixture
containing water and a combustible product, amongst other
components. Small amounts of CO2 are sent from the flash tank 47 to
a gas scrubber 48 before being passed to atmosphere. From the flash
tank the finished reaction product is pumped to the combustible
product buffer/holding tank 49.
[0043] The process flow rate is controlled via the high pressure
pump 41 at the start of the process and the pressure regulation 46
at the end of the process, this enables full control of the
feedstock flow and resonance time of the feedstock in the high
pressure reactor 43 to allow for the reaction/conversion to occur
and produce the desired combustible product.
[0044] The final stage of the core technology is to dewater the
finished reaction product using hydro cyclones and a dewatering
screen 51. The finished reaction product is pumped from the
combustible product buffer/holding tank 49 to the hydro cyclones
and dewatering screen 51, here the finished reaction product is
subjected to centrifugal forces to separate solids from liquid, the
final stage dewatering screen produces the combustible product with
a moisture content in the range of 25%-40%. Other forms of
dewatering such as filtration, pressing or centrifugation can also
be used.
[0045] Process water 50 recovered from the hydro cyclones and
dewatering screen 51 has two applications depending on the type of
feedstock used in the process.
[0046] Process water 50 derived from biomass feedstock is recovered
and reused within the biomass/water mixing vessel 29 as described
in FIG. 1. Process water derived from organic feedstock, depending
upon the characteristics of the initial feed, may contain materials
that are commercially beneficial to recover. Materials such as
ammonium sulphate for fertiliser, the recovery of phosphorous for
conversion into phosphoric acid and/or other organic nutrients and
elements can also be recovered.
[0047] Combustible product produced from the hydro cyclones and a
dewatering screen 51 can be utilised for fuel in a number of ways
to generate electricity and/or heat. FIG. 2 shows the three
combustible product options 52 for use of the combustible product
as a liquid or dry solid fuel, FIG. 3 describes these combustible
product options 52 in more detail.
[0048] We shall now discuss the various options for the processing
and use of the combustible product. FIG. 3 shows flow charts with
three options for the processing and use of the combustible
product.
[0049] Firstly we shall refer to the FIG. 3 option 1 flow chart of
the processes 60 which shows the combustible product being produced
into a hydrocarbon water slurry that can be used as fuel for
engines to produce electricity and/or as fuel for boilers to
produce heat. This option is especially prudent for the types of
combustible product produced with a low inorganic content. From the
hydro cyclones and dewatering screen 60 (51 in FIG. 2) the
combustible product is fed into a high shear wet milling machine or
emulsifier machine 61 where the particle size is reduced below 10
microns, if required additional water can be added so that the
final finished hydrocarbon water slurry is made to a ratio of
between 60%-65% combustible material and 35%-40% water. The
hydrocarbon water slurry can then be directly combusted 62 in
boilers 63 or engines 64 to produce energy as described above.
[0050] Secondly we shall refer to the FIG. 3 option 2 flow chart of
the processes 60 which shows the combustible product being
processed to remove inorganic materials and then produced into a
hydrocarbon water slurry that can be used as fuel for engines to
produce electricity and/or as fuel for boilers to produce heat.
This option is especially prudent for the types of combustible
product produced with a high inorganic content.
[0051] From the hydro cyclones and dewatering screen 60 (51 in FIG.
2) the wet combustible product is fed into an inorganic and organic
chloroform separation system 65 that utilises chloroform to remove
the combustible organic product from any inorganic material. In
this system the combustible product is mixed with chloroform so
that the hydrocarbons become miscible with the chloroform. This
mixture is then separated from the residual water via gravity and
the chloroform evaporated which results in the combustible product,
now separated from all inorganic material, being recovered and
produced to a high quality. The chloroform is recycled to the front
of the separation process. Depending on the type of organic
feedstock used the inorganic material recovered by the process may
have commercial value in applications such as a soil improver,
fertiliser and/or pest control product.
[0052] Once recovered from the chloroform separation system 65 the
liquid combustible product is fed into a high shear wet milling
machine/emulsifier 61 where the droplet size is reduced below 10
microns, if required additional water can be added so that the
final finished hydrocarbon water emulsion is made to a ratio of
between 60%-85% combustible material and 15%-40% water. The
hydrocarbon water emulsion can then be directly combusted 62 in
boilers 63 or engines 64 to produce energy as described above.
[0053] Finally, we shall refer to the FIG. 3 option 3 flow chart of
the processes 80 which shows the combustible product being
processed to produce a dry solid product that can be commercially
marketed as biofuel briquettes and/or used as fuel in various types
of burner/boilers in order to raise high pressure steam that is
utilised in a steam turbine to produce electricity and heat.
[0054] From the hydro cyclones and dewatering screen 80 (51 in
FIGS. 2 and 60 in FIG. 3 option 1 and FIG. 3 option 2) the
combustible product is fed into a filter press 81 for further
dewatering. The combustible product is then sent to a low
temperature dryer 82, on exiting the dryer 82 the product is now in
a dry solid form and can be utilised in a number of ways.
[0055] Firstly, the solid material is sent from the dryer 82 to a
pulveriser/grinder 83 so that the solid material is reduced to a
small uniform particle size or dust that is utilised in a dust
burner/boiler 84 to raise high pressure steam 85, which in turn is
provided to a steam turbine 86 for the production of electricity
and heat.
[0056] Secondly, the solid material can be sent from the dryer 82
directly to a fluidised bed burner/boiler 87 to raise high pressure
steam 85, which in turn is provided to a steam turbine 86 for the
production of electricity and heat.
[0057] The final option is to send the solid material produced from
the dryer 82 to briquette production 88. Here the solid material is
manufactured into compacted bricks/blocks of fuel which can be
marketed via hydrocarbon sales 89 or the briquettes utilised within
a moving grate burner/boiler 90 to raise high pressure steam 85,
which in turn is provided to a steam turbine 86 for the production
of electricity and heat.
[0058] It will be appreciated that any suitable dewatering
techniques may be employed in the process; for example,
centrifuging, dewatering screw, cyclone, or filter press.
Similarly, any suitable mixing means may be used for mixing
feedstock such as wood fines with water prior to conversion.
Suitable dewatering apparatus and mixing apparatus will be well
known per se to those skilled in the art.
[0059] The invention is illustrated by the following examples.
EXPERIMENTAL
Feedstock: Digested Sewage Sludge Cake
[0060] Digested sewage sludge cake from Anglian Water Whitlingham
site was used at the beginning of this study as the primary raw
material. The sludge cake is dewatered residue left after full
sludge processing and anaerobic digestion. The dewatered sludge
cake is black in colour and a spongy sticky material with an
organic odour. It has non-Newtonian rheology properties (i.e.
shear-dependent) and has a tendency to bridge. The dewatered sludge
cake has moisture content of 70% (as determined by Anglian Water
and confirmed by ICL West Thurrock lab). According to the data
obtained from Anglian Water the solid fraction of the cake has the
following composition:
Organic material 61.5%;
Ash 39.5%.
[0061] The following data obtained from Anglian Water has been
confirmed by ICL West Thurrock lab. In summary the ash
determination is 11.99% and dry Solids are 40% in wet sludge cake.
This approximately corresponds to the Anglian Water data.
[0062] Sewage sludge cake statistics table from Anglian Water are
shown in Table 1.
TABLE-US-00001 TABLE 1 Sewage Sludge Cake Statistical Analysis
Number of Name Units samples Mean SD MIN MAX pH Sludge pH units 67
8.56 0.18 8.21 8.91 Copper Total in Dry Solids Mg/kg Cu 7 441 83
276 554 Zinc Total in Dry Solids Mg/kg Zn 7 705 114 475 807 Cadmium
Total in Dry Solids Mg/kg Cd 7 0.83 0.14 0.66 1.09 Mercury Total in
Dry Solids Mg/kg Hg 7 1.19 0.22 0.95 1.6 Lead Total in Dry Solids
Mg/kg Pb 7 114 39 68 172 Chromium Total in Dry Solids Mg/kg Cr 7
32.3 5.4 23.3 38.7 Selenium Total in Dry Solids Mg/kg Se 7 4.22
0.95 2.63 4.98 Molybdenum Total in Dry Solids Mg/kg Mo 7 7.1 1.2
4.8 8.7 Nickel Total in Dry Solids Mg/kg Ni 7 27.6 5.2 21 36.6
Arsenic Total in Dry Solids Mg/kg As 7 5.1 0.8 3.6 5.8 Sulphur
Total in Dry Solids mg/kg S 7 17657 2936 14200 23200 Total Organic
& Volatile Solids (50.degree. C.) % DS 7 61.5 0.9 60.2 62.6
Total Dry Solids at 105.degree. C. % 67 27 2.1 20.6 33.6 Nitrogen
Kjeldahl in Dry Solids % N 7 4.75 0.42 3.94 5.12 Ammonia Total in
Dry Solids % N 7 0.82 0.09 0.73 0.97 Phosphorus Total in Dry Solids
% P2O5 7 7.45 1.08 5.11 8.5 Magnesium Total in Dry Solids Mg/kg Mg
7 5413 1859 2070 7080 Potassium Total in Dry Solids % K2O 7 0.24
0.07 0.14 0.3 Fluoride Total in Dry Solids mg/kg F 7 207 45 134
264
Digested Sewage Sludge Cake Basildon Site
[0063] Digested sewage sludge cake from Anglian Water Basildon site
had the same overall appearance as the sludge from Whithingham
site. However, the solids and ash content was slightly different
from that of Whithingham. The average water content was 79.7%, the
ash content in total sludge cake was 6.7%, making ash content in
solids 33%.
Digested Sewage Sludge Cake Thames Water Beckton Site
[0064] The overall appearance of dewatered sewage sludge cake from
Thames Water Beckton site was completely different from both sludge
samples obtained from Anglian Water. The analysis revealed water
content of 81% and ash content 2.6% making ash content in solids
13.7%.
Feedstock: Anaerobic Digestion Liquor
[0065] Another feedstock tested was liquor from an anaerobic
digestion plant obtained from Strutt and Parker Farms.
[0066] The analysis of the liquor is shown below.
TABLE-US-00002 Solid material content in AD liquor 5.40% Nitrogen
content in AD liquor 0.39% Ash content in solid material 20.15%
Feedstock: Compost from RIO Soils Ltd
[0067] Compost obtained from RIO Soils was also tested as potential
biomass feedstock. It had the lowest water content among all
feedstocks: 49.7% and the highest total ash content: 34.8% making
ash content in solids 69.2%.
Bench Scale Process: Rapid Subcritical Hydrothermal
Carbonification
[0068] Rapid Subcritical Hydrothermal Carbonification is the
process of treatment various organic and biomass wastes with water
at increased temperature and pressure at subcritical or critical
parameters. The result is a solid/liquid combustible product with
calorific value significantly higher than that of the original
feedstock.
[0069] Bench scale carbonification was carried out on a batch basis
using a pressurised stainless steel bomb heated in an oven. The
bomb was charged with 300-500 g of sewage sludge cake and 2-4 g of
a catalyst dissolved in 40-60 ml of water. The following compounds
were tested as the catalysts (solid sodium hydroxide, sodium
carbonate and sodium formate). Caustic solution (50%) was also
tested as the catalyst.
[0070] In case of compost the load was .about.300 g of compost and
.about.200 g of water as intrinsic water content in compost was
much lower than that in the sewage sludge cake. Caustic solution
(50%) and solid sodium carbonate were tested as catalysts.
[0071] In case of AD liquor the load was 450-700 g of AD liquor.
Only sodium carbonate was tested as a catalyst.
[0072] After testing several catalysts and running the process in
the absence of catalysts it was decided to run all further
experiments in the presence of sodium carbonate or 50% caustic
solution. The weight loss in their presence was minimal and the
product after carbonification had the most pungent hydrocarbon
odour.
[0073] Once the bench scale parameters had been established
repeatable reactions and results were able to be obtained. Once the
bomb was charged with feedstock and placed in an electric oven the
following parameters were used;
1. Electric oven heated to 300-320.degree. C. 2. Duration 2.5-3.5
hours. Some of the experiments were run up to 72 hours.
[0074] After the reaction the bomb was immediately cooled by tap
water to 60-70.degree. C. and plugs opened. A gas with a sharp
hydrocarbon odour was allowed to escape. The main contents of the
bomb a dark coloured liquid material is transferred into a sampling
flask. From here the material is weighed and then separated into
solid and liquid fractions using a lab centrifuge at 3000 rpm for
30 min-1 hour. To enable gas analysis the bomb was equipped with a
needle valve and gas was collected into a five litre gas sampling
bag. Gas was sampled. However, due to Intertek lab requirements
finally gas was analysed using headspace from a solid product
sample.
[0075] Bench scale reaction analysis is given in Table 2:
TABLE-US-00003 TABLE 2 Time Temperature, hour Catalyst .degree. C.
Reaction 2.5 1 250 0 2.5 0 280 0 6 0 280 1 2.5 0 300 0 0.5 0 320 0
6 0 300 1 6 1 320 1 5 1 300 1 3 1 300 1 2.5 1 300 1 Catalyst: 1 =
presence of the catalyst, 0 = absence of the catalyst Reaction: 1 =
reaction proceeds, 0 = no reaction occurs
[0076] The above data demonstrates that using the bomb reactor the
best conditions are 300.degree. C., 2.5 hours heating time and 0.7%
catalyst (calculated on a sludge cake weight). Sodium carbonate was
used as the catalyst.
[0077] However, the actual reaction time is significantly shorter
due to the long time required for the reaction mixture within the
bomb to reach the necessary temperature.
[0078] The products obtained from digested sewage sludge were
characterised and analysed using PSA, XRF, IR, ICP, wet chemistry
methods and calorimetry.
Bench Scale Process: Outputs and Analytical Results
Whitlington Site Sludge
[0079] The products produced through the reaction are gas,
combustible solid material, oil (absorbed on the solids) and water
(yields quoted are calculated in relation to solids in the sludge).
Surprisingly, no pure liquid hydrocarbon products were directly
obtained from the reaction.
[0080] Gas from the reaction was sampled using a needle valve and
gas sampling bags. Gas yield were estimated from the mass balance
of charged and obtained material. On average the gas yield does not
exceed 10% and is estimated as being between 1 and 5%.
Combustible Solid Material
[0081] The reaction mixture upon discharge from the reactor was
separated using a lab centrifuge at 3000 rpm into a solid fraction
and wastewater. The solid fraction is a coal-like black material
with a strong characteristic hydrocarbon odour with a moisture
content after centrifugation of 53%. As a test the obtained solid
material was self ignited when placing it into a hot oven
confirming its combustibility. The self-ignition temperature is
around 600-650.degree. C.
[0082] The solid material obtained after centrifugation was further
dried in an oven at 105.degree. C. to remove the remaining residual
water and once dried analysed for ash content. The result of this
analysis was an ash content of 46.32%; this percentage is what was
expected and confirms the elevated ash content in the raw
feedstock. The ash content in the crude sludge cake is 11.9% which
is equivalent to 40% of ash content as dry solids in the crude
sludge cake. This also corresponds to the content of organic matter
as dry solids in crude sludge cake (61.5%). The ash remains
insoluble in water and is incorporated into the produced
hydrocarbons. The ash composition is circa 15-25% iron (in the form
of iron III oxide and iron III phosphate), 5.03 alumina
(Al.sub.2O.sub.3), circa 5% phosphorus (in the form of iron
phosphate), 10-20% calcium with the remainder being silica.
[0083] The hydrocarbons formed as a result of sludge
carbonification are in two components; a solid hydrocarbon and a
tar hydrocarbon. For research purposes the solid sample after
centrifugation was washed with acetone and the acetone evaporated.
The evaporation produced a tar-like black viscous hydrocarbon
liquid with approximately 9% yield (calculated on the total solids
weight). That liquid supports combustion when ignited.
[0084] Even though as part of these bench scale trials the
hydrocarbon tar has been separated and analysed, in reality both
these fractions are combined as the tar is absorbed into the solids
making a solid hydrocarbon type fuel similar to fine coal or
coke.
[0085] From the reaction the total hydrocarbon yield is about
85-90%, based upon the organic content in the dry sludge cake.
[0086] The solid portion of hydrocarbons was analysed by
IR-spectroscopy. Its IR spectrum has bands characteristic to
aliphatic and olefin groups and is similar to that of the coal. The
IR spectrum of tar is similar to that of heavy fuel oil.
[0087] The total gross calorific value (CV) of product (including
43% ash) is 15.80 MJ/mol. That makes the CV of the combustible
fraction of the product (without ash) 36.7 MJ/kg. The calorific
value of the solid fraction without tars is 12.55 MJ/kg. Assuming
the liquid fraction of the product as 9% of the total, the CV of
liquid tars is (15.8-12.55)/0.09=36 MJ/kg.
[0088] The analytical results obtained on the biofuel product
clearly show an excellent energy content, thus proving the
viability of the process used with sewage sludge even with the high
ash content.
[0089] The results show a substantial increase in calorific value
compared with the feedstock as will be shown further in Table
7.
[0090] A solid sample (0.1 g), obtained from sewage sludge cake as
the feedstock, was dissolved in a mixture of toluene and heptane
(80:20 v/v, 10 mL) which was then filtered to remove the
undissolved sample. The chloroform extract was diluted 1 in 2 with
chloroform before both samples were examined by liquid injection
onto an Rxi-5 ms capillary column (30 m.times.0.25 mm.times.0.25
.mu.m) interfaced with a PE Glarus quadrupole mass spectrometer
operating in full scan mode, scanning from 33 to 620 Daltons.
Results are summarised in Table 3.
TABLE-US-00004 TABLE 3 Identifies assign to the ten most prominent
peaks Retention time (min) Compound CAS number 23.62
3,6-Diisopropylpiperazin-2,5- 5625-44-5 dione or similar 25.17
3,6-Bis(2-methylpropyl)-2,5- 1436-27-7 piperazinedione or similar
25.35 3-Isobutylhexahydropyrrolo[1,2- 5654-86-4
a]pyrazine-1,4-dione isomer or similar 25.41
3-Isobutylhexahydropyrrolo[1,2- 5654-86-4 a]pyrazine-1,4-dione
isomer or similar 25.60 Palmitic acid 57-10-3 28.68 Palmitamide
629-54-9 29.45 3-Benzyl-6-isopropyl-2,5- 14474-71-6 piperazinedione
30.76 Cyclo(leucyl-phenylalanyl) 7280-77-5 isomer of similar 31.08
Cyclo(leucyl-phenylalanyl) 7280-77-5 isomer or similar 39.28
Cholestanol 80-97-7
Compost
[0091] The process was carried out in on compost the same manner as
that of sewage sludge. The discharge of product was difficult due
to high amount of foreign inorganic impurities (stones, pebbles,
etc). The gross CV was 7.25 MJ/kg, ash content 70-75%, ash-free CV
.about.26 MJ/kg.
AD Liquor
[0092] Carrying out the process on AD liquor produced material with
gross CV 22.48 MJ/kg, ash content 20%. Ash-free CV.about.28
MJ/kg.
[0093] The data presented above clearly demonstrate that results of
the biomass conversion are highly consistent regardless of the
feedstock, Calorific value of the obtained combustible product on
an ash-free basis is in the range of 28-36 MJ/kg thus significantly
exceeding that of the feedstock.
Reaction in a Stirred Parr Reactor Under Controlled Conditions
[0094] After completion of the cycle of experiments in the
unstirred pressurised bomb it became clear that this set-up does
not allow determination of the required residence time as the exact
temperature and pressure inside the bomb are not known and could
only be estimated by the oven temperature. Also it was not known
how long it takes for the temperature inside the bomb to reach the
required value.
[0095] The experimental set-up included stainless steel stirred
reactor Parr Series 4520 Bench Top Reactor one litre volume
equipped with variable speed electric motor with magnetic drive,
electric heater, internal cooling coils, thermocouple, pressure
gauge and pressure transmitter and several valves attached to the
lid. For safety purposes the reactor was equipped with a bursting
disk. The set-up was controlled with 4870 Process Controller and
4875 Power Controller. The process control and data logging was
implemented using SpecView software. The cooling of the reaction
mixture was performed through internal cooling coils connected to
Endocal RTE-Series refrigerated bath.
[0096] The experimental programme comprised running experiments at
300.degree. C., 10.45-11.14 MPa (1500-1600 psig) and different
residence times. The following residence times were selected (in
minutes): 15, 30. 45, 60, 90, 120. The reactor was charged with 400
g of Basildon digested sewage sludge, 10-12 g of 50% caustic, then
it was sealed, heating was started and the mixture was stirred at
250-300 rpm. The moment the temperature in the reactor reached
297.5-298.degree. C. it was taken as the start of the reaction and
after the required residence time was reached, the heater was
switched off and removed and reactor contents were quickly cooled
by passing cold water through the inside coils. When the internal
temperature reached 28-32.degree. C., the stirring was stopped and
excess of formed gas was flashed through one of the valves on top
of the reactor. The reactor was opened and contents were
transferred into a 2-litre beaker and weighted. The weight loss in
all the cases did not exceed 5%.
[0097] Obtained reaction mixture was analysed for particle size
distribution and separated onto the solid fraction and the aqueous
fraction using centrifuge or vacuum filtration.
[0098] The digested sludge was completely converted into
combustible product even at short residence time (15 min). Increase
in residence times resulted in a small decrease in particle size
and an increase in residual gas pressure implying higher gas
yield.
[0099] We found that longer residence times result in finer
particle formation, confirming results obtained in the unstirred
bomb. However, the same or even small particle size is obtained in
significantly shorter time (15-30 min) instead of 3-5 hours.
[0100] Particle size distribution results are given in Table 4.
TABLE-US-00005 TABLE 4 Reaction Size, micron time % 15 30 45 60 90
120 5 2.627 2.341 2.685 1.22 2.054 2.441 10 3.52 3.06 3.33 2.41
2.48 2.86 15 4.64 3.96 4.21 2.82 2.83 3.3 20 6.11 4.96 5.75 3.27
3.2 3.87 25 7.9 6.1 9.83 3.91 3.67 4.73 30 9.91 7.54 11.53 5.04
4.38 6.14 35 12.07 9.17 12.71 6.75 5.51 9.21 40 14.44 11.02 13.9
9.85 7.51 10.83 45 17.59 13.38 15.54 11.37 12.44 13.41 50 21.72
15.9 31.35 12.44 11.23 12.6 55 27.87 19.75 51.49 13.41 12.14 13.41
60 35.42 25.42 74.64 14.41 12.96 14.24 65 44.97 33.73 96.65 15.68
13.23 15.35 70 57.13 43.57 110.8 19.76 14.93 17.55 75 73.55 53.42
122.4 30.47 17.41 28.77 80 98.08 73.38 131.4 35.92 31.03 32.82 85
120.3 89.33 139.3 50.14 35.66 49.97 90 138.5 102.3 146.9 71.58
56.64 59.23 95 158.5 126.4 157.4 81.47 67.93 85.59 99 188.6 135.9
173.8 93.64 83.02 98.75
[0101] After the separation, the combustible product was dried for
24 hours at 60-80.degree. C. and was analysed for calorific value
and ash content. Ash was determined by burning a sample in a
porcelain crucible at 850.degree. C. for 7 hours. Results are given
in Table 5.
TABLE-US-00006 TABLE 5 Residence time, min 15 30 45 60 90 120 Ash,
% 47.65 53.68 50.21 51.38 47.41 47.56 CV gross, MJ/kg 16.32 16.22
16.92 16.42 17.46 16.92 CV (ash-free), MJ/kg 31.2 35.01 33.98 33.77
33.2 32.26
[0102] Several experiments were devoted to extracting oil from the
combustible product obtained using Basildon sludge as a feedstock.
The reaction mixture was stirred with 250-300 ml of chloroform for
4-5 at 50-55.degree. C., then the solids were filtered off and the
liquid part was separated into aqueous phase and chloroform. The
solids were extracted several times with chloroform, chloroform
extracts were combined and filtered, and chloroform was removed
under water-jet vacuum. Black viscous oil was obtained in
quantities ranging from 22.4 to 30 g. That approximately
corresponds to 63% yield of oil (based on organic content in the
sludge). Residual solids were dried at 50.degree. C. We obtained
solids with ash content ranging from 65 to 80%. The calorific value
of the oil averaged at 36.5 MJ/Kg again demonstrating a substantial
increase over that of the original feedstock.
[0103] It became evident that by a conservative estimate the
residence time between 15 and 30 min at 300.degree. C. and 11.14
MPa (1600 psig) is sufficient for the conversion of the sewage
sludge.
Conversion of Southern Water Sludges
[0104] We were supplied with five samples of sludge from different
sites of Southern Water. The properties of the sludges are given in
Table 6.
TABLE-US-00007 TABLE 6 Digested Dry Solids in the Cake Dry Solids
Product CV Ash Waste Water Treatment Works Liquid/Cake Cake, % CV,
MJ/kg Free, MJ/kg Anglian Water Whitlingham Site Cake 30.1 12-14
36.7 Anglian Water Basildon Site Cake 18 12-14 35.6 Southern Water
Ashford Site Liquid 4.7 -- 34.6 Southern Water Budds Farm Site
Liquid 4 -- 32.0 Southern Water Ford Site Liquid 3.7 -- 33.9
Southern Water Millford Site Cake 25 12-14 34.6 Southern Water
Peacehaven Site Cake 23 12-14 33.8
[0105] The samples were converted to combustible product using the
same method as previously (.about.300.degree. C., .about.10.1 MPa
(100 barg), 15 min at this temperature, then rapid cooling using
cooling coils of the reactor). Results are given in Table 7.
TABLE-US-00008 TABLE 7 Solid fuel properties Fuel CV yield on CV on
increase dry Gross ash-free relative solids, CV, basis, to
%.sup..dagger. MJ/kg Ash, % MJ/kg sludge, % 68.9 14.4 58.41 34.62
87.9 15.64 51.09 31.98 55.7 14.3 57.45 33.86 69.9 19.8 42.72 34.57
51.74 74.2 16.74 50.44 33.78 34.79
[0106] Samples prior to CV determination were dried at 105.degree.
C. overnight. In order to check whether if there is a loss of light
combustible components one sample was dried at room temperature
under vacuum. Its gross CV (Millford) was 17.52 MJ/kg, roughly the
same as of that dried at 105.degree. C., so no light components are
present. Analysis results are given in Table 8.
TABLE-US-00009 TABLE 8 Results Basis Method As As Dry Test
Reference Units Received * Analysed Dry * Ash Free * Analysis
Moisture CA2 % -- 1.8 -- -- Ash CA3 % 46.5 49.5 50.4 -- Total
Moisture SP19 & CA2 % 7.8 -- -- -- Volatile Matter CA6 % 38.0
40.5 41.2 83.2 Sulphur CA31 % 1.66 1.77 1.80 3.63 Chlorine CA36 %
0.02 0.02 0.02 0.04 Gross Calorific Value CA11 kJ/kg 16500 17574
17896 36105 Net Calorific Value * kJ/kg 15504 -- -- -- Carbon CA9 %
35.94 3828 38.98 78.64 Hydrogen CA9 % 3.78 4.03 4.10 8.27 Nitrogen
CA9 % 2.83 3.01 3.07 6.19 Fixed Carbon * % 7.7 8.2 8.4 16.8
[0107] The processing of sewage sludge under controlled conditions
in a stirred reactor confirmed the consistency of the properties of
the obtained combustible product regardless of the feedstock and
also confirmed the previous results obtained in the unstirred
bomb.
Wood Fines Conversion
Properties of Waste Wood Fines
[0108] Bulk weight 300 kg/m.sup.3 Moisture content .about.15-30%
Ash content 2.5-3.6% Gross calorific value 12-15 MJ/kg Wood fines
(60-90 g) were mixed in a Parr reactor with water (220-250 g) and
50% sodium hydroxide (3-5 g) and reacted using the same process
conditions as previously used for the sludge carbonification. The
experiment was repeated in excess of 20 times. Product yield was
circa 50%. Analysis results are given in Table 9.
TABLE-US-00010 TABLE 9 Results Basis Method As As Dry Test
Reference Units Received * Analysed Dry * Ash Free * Analysis
Moisture CA2 % -- 1.4 -- -- Ash CA3 % 18.1 18.9 19.2 -- Total
Moisture SP19 & CA2 % 5.5 -- -- -- Volatile Matter CA6 % 42.9
44.8 45.4 56.2 Sulphur CA31 % 0.32 0.33 0.34 0.42 Chlorine CA36 %
0.03 0.03 0.03 0.04 Gross Calorific Value CA11 kJ/kg 24813 25890
26257 32478 Net Calorific Value * kJ/kg 23614 -- -- -- Carbon CA9 %
61.20 63.86 64.76 80.10 Hydrogen CA9 % 4.98 5.20 5.27 6.52 Nitrogen
CA9 % 3.33 3.47 3.52 4.36 Fixed Carbon * % 33.5 34.9 35.4 43.8
Method Results Test Reference Units I.D. Softening Hemisphere Flow
Ash Fusion Temperatures ** deg C. 1050 1070 1090 1150 Reducing
Atmosphere
[0109] The results of waste wood conversion confirmed the
reproducibility of the results and consistency of the properties of
the obtained combustible product. The obtained combustible product
has gross calorific value nearly double that of the original waste
wood material.
[0110] The combustible product of the process, obtained from
organic or biomass feedstock, may be burnt like coal to produce
heat, or it may be further treated by milling and mixing with water
to produce a suspension suitable for use as a liquid fuel. Such a
liquid fuel may have advantages over other liquid fuels made from
fossil coal, because the solid particles from the process according
to the present invention are far less abrasive than fossil coal and
are therefore more suitable for use in engines such as compression
ignition engines, which suffer from excessive wear with
conventional fossil coal-based fuel suspension.
[0111] The combustible product with high ash content may also be
treated to extract a liquid hydrocarbon fraction which may be used
as a fuel or formed into a water-based emulsion for use as a liquid
fuel.
Ohmic Heating
[0112] One of the most important parameters is the electrical
conductivity of a feedstock to be processed. As stated (in the
context of pasteurisation of milk and other food materials) in "A
comprehensive review on applications of ohmic heating (OH)" by
Mohamed Sakr, Shuli Liu (Renewable and Sustainable Energy Reviews,
39 (2014) 262-269, DOI: 10.1016/j.rser.2014.07.061): "Electrical
conductivity of any sample is not constant and it is dependent on
the material temperature (normally linearly) and it is increased
with increase of the material temperature".
[0113] Referring now to FIG. 4, the dependence of electrical
conductivity of wood fines--water mix (1:2.5) vs. temperature is
shown. The primary data was corrected taking into account water
expansion with temperature. We found that dependence of the
feedstock (i.e. waste wood fines--water mix) on temperature is far
from being linear. The graph of FIG. 4 is plotted using our process
results. Dependence of electrical conductivity on temperature is
more or less linear up to 200.degree. C., and then it increases
quite sharply. The trend lines on the graph show that the straight
line (i.e. linear dependence) is a very poor approximation of the
actual experimental curve, its regression coefficient being 0.9563
as compared with 0.99 for the non-linear polynomial approximation.
That means that known in the prior art mathematic models of ohmic
heating using linear approximation of electrical conductivity of
feedstock vs. temperature could not be applied in the development
of our process and were virtually useless if not totally
misleading.
[0114] Previously sewage sludge was heated using ohmic heaters only
to water boiling point (Murphy A B, Powell K J, Morrow R. Thermal
treatment of sewage sludge by ohmic heating. IEE Proc: Sci. Meas.
Technol., 1991; 138: 242-8.) The aim of the work was sterilisation
of sewage sludge and destruction of possible pathogens present in
it. This is again a low temperature and low pressure
application.
[0115] Ohmic heating is used commercially at low temperatures and
pressures for the pasteurisation of foodstuffs and cooking of raw
food; however these processes do not require extreme pressures and
temperatures. The development of our conversion process using ohmic
heating required encroaching into completely unchartered territory,
because properties of water at these temperatures and pressures
differs significantly from those under moderate temperatures.
[0116] Dielectric constant (electric permittivity) of water is also
an important parameter that will influence the conductivity of the
media to be converted. As shown in FIG. 5, at relatively moderate
temperatures (200.degree. C.) the polar compounds in types II &
III organic matter are extracted by water, which has a dielectric
constant of 35, equivalent to somewhere between acetonitrile and
methanol; at relatively high temperatures (300.degree. C.) the
nonpolar saturated hydrocarbons in types I, II and III organic
matter are extracted, when the water has a dielectric constant of
20, close to that of acetone. ("Subcritical water extraction of
organic matter from sedimentary rocks". Duy Luong, Mark A. Sephton,
Jonathan S. Watson. Analytica Chimica Acta 879 (2015) 48-57, DOI:
10.1016/j.aca.2015.04.027). This means that at 300.degree. C. the
dielectric constant of water is close to that of acetone rather
than water at lower temperatures and pressures, i.e. four-fold
lower. The prediction of the behaviour of electrical conductivity
of the feedstock under these conditions is not possible.
[0117] As can be seen from FIG. 6, sourced from:
http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html
the density of water with temperature is also non-linear. It is
seen from both FIGS. 5 and 6 that properties of water at high
temperatures and pressures differ sharply from those at ambient
temperatures and thus the behaviour of water during ohmic heating
could not be predicted from data obtained at lower
temperatures.
[0118] The conversion of dewatered sewage sludge filter cake and
waste wood fines was carried out in the set-up shown in FIG. 7. The
bespoke ohmic heater rig was used to heat the media up to
300.degree. C. (>100 bar) and hold at that temperature for 15-20
minutes.
[0119] Pressure & temperature transducers were fitted into the
0V flange to record internal parameters.
[0120] The heater (15 l volume) was charged with 7 l of waste
wood--water mix (1:2.5), 1% of 50% caustic solution was added and
heating was started. After the required temperature of 300.degree.
C. was reached the rig was held at this temperature for a further
15 min, and then the voltage was switched off. The rig was allowed
to cool naturally. Then it was opened and the material
unloaded.
[0121] The most surprising and unexpected result of the ohmic
heating experiments was that the obtained combustible product was
significantly finer than the same material obtained by conventional
(through the wall) heating.
[0122] Table 10 and FIG. 8 show the particles size distribution of
the combustible product obtained by the conversion of waste wood
fines and sewage sludge filter cake by ohmic heating and by
conventional heating (through the wall).
Waste Wood Fines and Sewage Sludge Filter Cake Conversion
(300.degree. C., 10.1 MPa (100 Barg), 15 Min Residence Time)
TABLE-US-00011 [0123] TABLE 10 Waste wood fines Sludge Ohmic
Conventional Ohmic Conventional heating heating heating heating %
Size, micron D10 2.594 9.088 2.732 3.52 D25 3.307 23 4.315 7.9 D50
5.065 48.53 9.538 21.72 D75 8.165 88.44 25.75 73.55 D90 10.06 129.8
63.21 138.5 D99 12.07 180.4 113.7 188.6
[0124] It is clearly seen that the conversion process using ohmic
heating yields much finer combustible product as compared with the
same product obtained by conventional heating. The combustible
product obtained by the conversion of waste wood fines has average
particle size of 5 microns. That means that this combustible
product would not require further milling for the preparation of
aqueous slurry to power a compression ignition engine. So the use
of ohmic heating for the conversion of waste food fines into
combustible product allows us to eliminate any high
energy-consuming milling processes due to the reduced particle size
of the combustible product obtained by ohmic heating.
[0125] The combustible product obtained by the conversion of sewage
sludge using ohmic heating is also significantly finer. Obtaining
finer particles of this combustible product allows us to accelerate
extraction of hydrocarbon oil from this material and increase the
yield of oil due to more complete extraction.
* * * * *
References