PROCESS FOR THERMAL TREATMENT BY INDIRECTLY HEATED KILN

Description

Field of the Invention

[0001] The present invention relates to a process for the thermal treatment of a material by an indirectly heated kiln.

Background of the Invention

Problem to be solved

[0002] Directly fired kilns normally use fossil fuels, thereby resulting in high CO₂ emissions.

[0003] At the same time, off-gas treatment is challenging for gas fired kilns, as gaseous compounds deriving from the treated material add to combustion gas from the fuel. Thus, gases leaving the kiln comprise not only compounds evaporated from the thermally treated material, but also off-gas resulting from the combustion of the fuel.

[0004] Therefore, the invention aims to solve a problem to be solved is the reduction of the CO₂ footprint of thermal processes in a cost-efficient way, including lowering the costs for off-gas treatment.

Means for solving the Problem

[0005] It has been found that utilizing an indirectly heated rotary thermal reactor as the kiln can overcome the shortcomings of the prior art processes.

[0006] Subject of the invention is a process for the thermal treatment of a material in a kiln, wherein

i) the kiln is an indirectly-heated rotary thermal reactor, and

ii) electricity is used to heat the kiln,

wherein the electrical energy supply to the kiln fluctuates, with the maximum hourly average value thereof within a 24-hour period being greater than 150%, preferably greater than 200%, and more preferably greater than 400% of the minimum hourly average value within a 24-hour period.

[0007] Fluctuation of the electrical energy supply is due to the high cost of electricity in times of high demand and/or low supply. On the other hand, in times of low demand and/or high supply the cost for electricity can go down close to zero or even below zero (= negative cost).

[0008] As the supply of energy is highly dependent on weather conditions (sunshine and wind) and demand for electricity cannot follow these fluctuations, there sometimes exists a severe mismatching between supply of and demand for electricity. Therefore, the buffer function of a kiln using electricity to heat the kiln can function as a big relief for supporting a stable system of public electricity supply. And, on the other hand, this buffer function can deliver a big operating cost reduction that even overcomes the cost of higher nameplate capacity of the equipment.

[0009] In one embodiment of the invention the feed rate of material into the kiln fluctuates, with the maximum hourly average value within a 24-hour period being greater than 150%, preferably greater than 200%, and more preferably greater than 400% of the minimum hourly average value within a 24-hour period.

[0010] In another embodiment of the invention the speed of rotation of the kiln and/or the inclination angle of the kiln is varied in such a way that the rate of energy transfer into the material being treated is equivalent or nearly equivalent in periods of high, medium, and low energy supply. The criterium or criteria used for assessing this "equivalence" is the product quality or product parameters of the material leaving the kiln.

[0011] This means that during periods of low energy supply the other operating parameters of the system are modified in such way that the product parameters of the material leaving the kiln are akin to the product parameters of the material leaving the kiln in periods of high energy supply.

[0012] This can result in periods of time where the speed of rotation of the kiln is extremely low, up to zero or close to zero rotation of the kiln, until the energy supply increases and returns back to normal or high levels.

[0013] In another embodiment of the invention hot gas is fed into the kiln in addition to the indirect heating.

[0014] Preferably, the hot gas is generated using CO₂-neutral energy sources, e.g. CO₂-neutral electricity, CO₂-neutral H₂, biomass, or waste heat resulting from industrial processes.

[0015] Preferably, the indirectly-heated rotary thermal reactor has two or more temperature zones. And more preferably, the indirectly-heated rotary thermal reactor has three temperature zones.

[0016] Preferably the total material retention time in the temperature zones is at least 30 min and/or the total material retention time in the reactor is at least 45 min.

[0017] Preferably, the temperature in the first temperature zone is set to 850-1100 °C, and the temperature in the second temperature zone is set to 750-1000 °C.

[0018] The inventive process can be applied to thermal treatment of many materials, e.g.

a) materials comprising or consisting of gypsum, magnesium sulfate, iron sulfate, titanium dioxide, iron titanate or compositions comprising one or more of these materials,

b) materials comprising or consisting of dust or sludge or filtercake containing zinc or lead

c) materials comprising or consisting of clay, cementitious materials or materials exhibiting hydraulic properties after thermal treatment,

d) materials comprising or consisting of calcium-containing compounds, such as calcium oxide CaO,

e) materials comprising or consisting of phosphorous-containing compounds, preferably phosphorous-containing compositions, or

f) materials comprising or consisting of geopolymers or geopolymer cements.

[0019] The invention also provides an apparatus for the thermal treatment of materials by means of the process of the invention, wherein the apparatus takes the in form of a kiln, wherein

i) the kiln is an indirectly-heated rotary thermal reactor, and

ii) electricity is used to heat the kiln,

[0020] The invention is associated with a number of advantageous effects. These include, but are not limited to the following:

1. Low CO₂ footprint.

2. Efficient and low-cost process.

3. Buffer system for fluctuating supply of electricity.

4. Efficient and low-cost off-gas treatment.

Detailed Description

[0021] Embodiments according to the present invention will now be described in more detail.

[0022] The objectives of the present invention are achieved by the implementation of an indirectly-heated rotary thermal reactor for the low CO₂ footprint thermal treatment of a material in a kiln, wherein

i) the kiln is an indirectly-heated rotary thermal reactor, and

ii) electricity is used to heat the kiln,

wherein the electrical energy supply to the kiln fluctuates, with the maximum hourly average value thereof within a 24-hour period being greater than 150%, preferably greater than 200%, and more preferably greater than 400% of the minimum hourly average value within a 24-hour period.

[0023] In one embodiment, the disassembled unit employed in the process according to the invention can be made mobile as the parts fit into a series of shipping containers. This has the advantage that the unit is easily transported to/from a specific job site.

[0024] In another embodiment according to the invention, the unit employed in the process (and/or the process) allows for high-volume continuous feed processing.

Indirectly-heated rotary thermal reactor

[0025] In indirectly-heated rotary thermal reactors, heat is applied to the outside of a drum containing the material to be treated. In the process of the invention the heat source is an electrical furnace. This has the advantage of improving the mobility of the unit and lowers the CO₂ footprint of the process.

[0026] Advantages of the reactor being indirectly-heated and rotary include avoiding direct contact between the heat source and the material, providing a better control of the temperatures and allowing the internal processing atmosphere to be controlled.

[0027] Electricity as a heat source opens up the possibility of using CO₂-neutral energy for the thermal treatment, thereby leading to an even better CO₂ footprint of the products.

[0028] In a preferred embodiment a significant overcapacity of equipment is installed and said equipment is operated merely part-time. Thus, consumption of electricity can be shifted to the most favourable times.

[0029] For example, the electrically heated reactor may be designed with 200% of nameplate (or nominal) capacity and operated 50% of the time only. This allows the supply and demand of electricity to be balanced in such a way that high consumption of electricity is activated in times of oversupply (sunny and windy periods and low demand of electricity) and low consumption of electricity is established in times of shortage of electricity (dark and windless periods and high demand of electricity).

[0030] The advantages of lower cost for electricity generally can overcome the higher investment for the higher nameplate capacity.

[0031] In a further embodiment the feed material enters the system at ambient temperature.

[0032] In an embodiment, the reactor can be heated up to 1200 °C. Preferably the reactor is heated to temperatures of from 600 °C to 1000 °C. The temperatures are defined as the temperatures of the outside reactor wall in the middle of a temperature zone.

[0033] The term temperature zone refers to a section of the reactor in which the energy input can be controlled independently of adjacent sections and wherein the amount of thermal energy supplied per unit surface area of the reactor is kept constant throughout the temperature zone (variation of less than 10%, preferably less than 5%).

[0034] It is possible for there to exist sections of the reactor that are not temperature zones, e.g. non-heated sections.

[0035] In a preferred embodiment the total retention time of the material in the temperature zones of the reactor is at least 30 minutes.

[0036] In an embodiment according to the invention the reactor can be adjusted to manage dwell time (retention time) by adjusting the inclination angle of the reactor and its speed of rotation. Adjusting the retention time has the advantage that the removal of all volatile compounds from the material is ensured.

[0037] Furthermore, the ability to adjust the retention time has the advantage that there is a large flexibility in the types of materials that can be processed and the level of thermal treatment achieved. For example, if it is only required to remove a certain amount of water, the retention time can be decreased.

[0038] In a preferred embodiment the reactor has a sealed feed assembly.

[0039] In a preferred embodiment the reactor has a sealed discharge assembly.

[0040] In a preferred embodiment the reactor has sealed feed and discharge assemblies.

[0041] Preferably, the indirectly-heated rotary thermal reactor is an indirectly-heated rotary multi-zone thermal reactor with at least two temperature zones.

[0042] Heating the material in two, optionally three or more, different temperature zones ensures more efficient processing. The temperature zones can be arranged so as to continually increase the temperature of the material passing through the reactor, i.e. from lowest to highest temperature. However, the temperature zones can also be arranged so as not to continually increase the temperature of the material passing through the reactor.

[0043] In a preferred embodiment, the material is heated by the shell and lifters configured along the internal walls of the reactor to ensure the most efficient heat transfer.

[0044] In another preferred embodiment the processed material discharges into a container through a double airlock.

[0045] In a preferred embodiment the processed material discharges into a cooling screw through a double airlock.

[0046] In an embodiment the material is discharged into a cooling screw and stockpiled.

[0047] In an embodiment the flue gas exiting the reactor is treated in a vapour recovery unit comprising a scrubber unit and is not processed by a thermal oxidiser before entering the scrubber unit.

[0048] In a preferred embodiment the total material retention time in the temperature zones is at least 30 min.

[0049] In a preferred embodiment the total material retention time in the reactor is at least 45 min.

[0050] In a preferred embodiment the reactor can process material at up to 1200 °C.

[0051] In a preferred embodiment the flue gas exiting the reactor is treated in a vapour recovery unit comprising a scrubber unit and the exhaust gas exiting the scrubber of the vapour recovery unit passes through one or more further scrubbers.

[0052] The purification of the exhaust gas via a system including scrubbers provides a more environmentally friendly thermal treatment.

[0053] Preferably the off-gas from the thermal treatment is fed first into water without alkaline agents and subsequently is fed into an alkaline solution for capturing acidic compounds (SO₂, SO₃).

Claims

1. Process for the thermal treatment of a material in a kiln, wherein

i) the kiln is an indirectly-heated rotary thermal reactor, and

ii) electricity is used to heat the kiln,

characterized by the fact that the electrical energy supply to the kiln fluctuates, with the maximum hourly average value thereof within a 24-hour period being greater than 150%, preferably greater than 200%, and more preferably greater than 400% of the minimum hourly average value within a 24-hour period.

2. Process according to claim 1, wherein the feed rate of material into the kiln fluctuates, with the maximum hourly average value within a 24-hour period being greater than 150%, preferably greater than 200%, and more preferably greater than 400% of the minimum hourly average value within a 24-hour period.

3. Process according to claims 1 or 2, wherein the feed rate of material into the kiln, the speed of rotation of the kiln, the inclination angle of the kiln, and the energy supply to the kiln are changed simultaneously in such a way that the rate of energy transfer to the material being treated is "more or less constant".

4. Process according to any preceding claim, wherein hot gas is fed into the kiln in addition to the indirect heating.

5. Process according to claim 4, wherein the hot gas is generated using CO₂-neutral energy sources, e.g. CO₂-neutral electricity, CO₂-neutral H₂, biomass, or waste heat resulting from industrial processes.

6. Process according to any preceding claim, wherein the indirectly-heated rotary thermal reactor has two or more temperature zones.

7. Process according to any preceding claim, wherein the indirectly-heated rotary thermal reactor has three temperature zones.

8. Process according to any preceding claim, wherein the total material retention time in the temperature zones is at least 30 min and/or the total material retention time in the reactor is at least 45 min.

9. Process according to any preceding claim, wherein the material to be treated

a) comprises or consists of gypsum, magnesium sulfate, iron sulfate, titanium dioxide, iron titanate or compositions comprising one or more of these materials,

b) comprises or consists of dust or sludge or filtercake containing zinc or lead,

c) comprises or consists of clay, cementitious materials or materials exhibiting hydraulic properties after thermal treatment,

d) comprises or consists of calcium-containing compounds, such as calcium oxide CaO,

e) comprises or consists of phosphorous-containing compounds, preferably phosphorous-containing compositions, or

f) comprises or consists of geopolymers or geopolymer cements.

10. Apparatus for the thermal treatment of materials by means of the process of any of the preceding claims, wherein the apparatus takes the form of a kiln and

i) the kiln is an indirectly-heated rotary thermal reactor, and

ii) electricity is used to heat the kiln.