THERMAL PROCESSES TO DESTROY ORGANIC MATTER

There are numerous thermal processes allowing to destroy the organic matter of wastewater treatment plant sludge, and so to reduce considerably the quantities to evacuate. Among the available technological solutions, we can distinguish the processes working under pressure and those operating in atmospheric pressure.

Pressurised Atmospheric pressure

- Wet oxidation - Co-incineration with other waste- Dedicated incineration

- Pressurised gasification - Gasification/Pyrolysis-Thermolysis

Our interest here is specifically the dedicated incineration process at atmospheric pressure.

PROCESSES AT ATMOSHERIC PRESSURE

Processes at atmospheric pressure are all based on destroying the links that form organic matter by increasing temperature and free oxygen partial pressure. When free oxygen partial pressure is about 6%, corresponding to about 40% excess air, the process is in the field of traditional incineration that can be dedicated (specific sludge conditions) or combined with other products (as in the co-incineration of household waste combined with special industrial waste), in cement plants or thermal power plants.If this oxygen partial pressure is virtually nil, the process is in the field of gasification (pyrolysis and thermolysis).

Specific incineration and thermal decomposition technologies have been known and applied by many industries for decades. They can be described according to the technology used in the main reactor (furnace).

Î Rotary furnace

This is a revolving drum equipped with refractory materials and generally followed by a vertical post-combustion chamber when it involves waste treatment. The most well-known furnace of this type is the cement plant furnace. It is also used in the production of lime. This simple principle has been extended to treat waste, particularly heterogeneous and/or special waste that needs to go through a post-combustion process to burn fully and reach the legally required temperature. It is rarely used for treating

wastewater treatment plant sludge due to the difficulty of controlling the excess air (which impacts the thermal balance) and the existence of a high-viscosity zone that causes highly disruptive agglomeration phenomena.

Î Grate furnace

This is usually restricted to the treatment of household waste. Note that sludge can be co-incinerated, either in pre-dried form or simply dewatered. Some suppliers, however, use this technology for dried sludge.

Î Multiple hearth furnace

This type of furnace, which has been used for over a century, was very popular in the United States in the 1960s for direct incineration of wastewater treatment plant sludge. Its use spread throughout Europe and France in particular but it then fell from popularity after a series of oil price hikes.

This technology is now resurfacing in the sludge field, no longer for direct incineration but in more elaborate pyro-gasification processes such as, for example, in a large plant close to Paris.

Degrémont has a tradition of sharing its employees’ passion for water treatment with the public.To supplement the Water Treatment

Handbooks, Degrémont has issued the «Handbook Factsheets» to promote a better

understanding of the different techniques available and discovery of the new products and major technological changes.

Degrémont Water Treatment Handbook Factsheets

Thermylis®

Biosolids Dewatered sludge Thermal processes

Biosolids

Degrémont Water Treatment Handbook Factsheets

Î Fluidised bed furnace

This type of furnace also goes back a long way. Once sludge incineration surfaced in Europe in the 1970s, it quickly became established and is today the most-used technique for direct incineration of dewatered sludge.This is the only technology described hereunder for dedicated incineration.

GENERAL OPERATING PRINCIPLES OF A FLUDISED BED FURNACE

Î Fluidisation

In a reactor containing solid particles through which a fluid circulates from bottom to top, each particle is subjected partly to the force of gravity and partly to friction due to the passage of fluid. A balance results defining a critical speed. If the upward speed is less than the critical speed, the particle decants, but if the upward speed is greater, the particle will be propelled to the top by the fluid.A fluidised bed does not behave like a set of free particles. On the contrary, as it consists of a set of particles interacting with each other, it behaves more like a fluid, which is why the process is called “fluidisation”.The expansion of the material, which is non-existent at first (fixed or “compact” bed), then increases with the speed of the fluid. In other words, “the fluidised bed expands”.Conversely the head loss starts by rising in the compacted bed and then remains constant between two characteristic speeds. When it exceeds a certain speed, the bed loses its cohesion and its material is gradually carried away by the upflow.In fact, fluidised beds are mostly used within the range of fluidisation speeds.

In general, to operate properly, a fluidised reactor needs:• good fluid distribution at the base of the reactor;• a contact material that is homogeneous and abrasion-resistant.The thorough mixing of the bed of particles in the fluid to be treated maximizes exchanges between the material and this fluid.

Î Fluidised bed furnace

A fluidised bed furnace is based on the principle of suspending dewatered sludge in a current of hot air preheated containing precalibrated sand particles.The fluidised sand bed brought up to incineration temperature is an extremely turbulent environment in which heat exchanges reach very high transfer coefficients. The dewatered sludge feeding this bed is very quickly disintegrated by the turbulence of the sand, with evaporation being produced instantaneously along with the combustion of the organic matter with the fluidisation air as comburent.

DEGRÉMONT’S HTFB* FURNACE

Î Construction principle

The furnace is in the shape of a vertical steel vessel, slightly flared, with refractory elements. It has six sections that stack from bottom to top as a self-stabilised and self-supporting system.

• Wind box

Cylindrical in section, its wide radial inlet accepts the inlet duct for the fluidisation air from the fan. If the pre-heated air that arrives is below 450°C, it is called a cold wind box; above that temperature it is called a hot wind box. Most fluidised bed furnaces that treat sludge are designed as hot wind boxes.

This wind box has silico-aluminous type refractory elements. The upper part of the wind box supports the fluidisation dome. The wind box is equipped with a burner (potentially retractable) which is used for preheating during the startup procedure.

*HTFB = High Temperature Fluidised Bed

Thermylis®

Degrémont Water Treatment Handbook Factsheets

• Fluidisation domeThis is the essential part of the furnace as it is through this dome that the fluidised air is distributed. In the hot wind box version, the dome consists of a set of rings made of refractory bricks. These rings tighten against each other to form a self-supporting vault. Each brick has a vertical hole that holds the fluidisation nozzles.

The fluidisation nozzles are usually shaped like a mushroom with a circular head. The speed of the air in the nozzle is of the order of 40 m·s–1. This nozzle, made of heat-resistant cast iron sealed in the hole of the refractory brick support, is a key factor in the

construction of the furnace. Any loss of nozzle creates a serious disruption because the fluidisation sand will pass directly in the wind box and the fluidisation will be consequently disrupted.• Fluidisation zone properThis corresponds to the height of the expansed sand bed that is between 1.30 and 1.50 m in most cases. This zone also has silico-aluminous refractory materials. It is into this zone that sludge is fed and fuel make-up injected (fuel oil, gas, biogas).• Expansion zoneIt is flared to take into account the increase in volume of gas products due to temperature as well as the combustion completion. This flared shape also helps to progressively reduce the drive speed so as to defluidise the finer sand particles. This expansion zone also serves as a post-combustion area. The combustion products are held there for a long time (at least 5 seconds) which, under any circumstances, ensures that waste incineration standards are met. Fluidisation sand make-up is also added here to offset losses through attrition.• Upper vaultThis is made of rings of refractory bricks to create a self-supporting vault. It rests on the external envelope of the expansion zone. The center of this vault is hollow and forms the flue mouth of the outlet for combustion products. This vault also contains water injectors for temperature control (safety).• Connecting flueThis is located between the furnace outlet and the inlet of the recovery exchanger. The flue is also heat resistant and must contain an expansion joint.

Î Operating principle

The sand bed is thus suspended by the preheated fluidisation air coming from the wind box; the fluidisation sand behaves just like a boiling liquid.The turbulence in this bed allows virtually total combustion of the organic matter of the sludge. In fact, the sludge entering the sand bed is immediately exploded into fine particles, thus creating close contact between the organic matter and the oxygen supplied by the fluidisation air.The combustion of “residual” organic matter is achieved in the post-combustion zone where the flue-gas is raised to a temperature of about 850°C for about 6 to 7 seconds (including at least 2 seconds of contact time at a temperature greater than or equal to 850°C in compliance with European regulations).The injection of the sludge can be accompanied by an injection of additional fuel (gas or liquid) depending on whether the sludge is autothermal or not.Sludge is aid to be autothermal if the calories that it brings are sufficient to maintain incineration at a temperature of 850°C in the post-combustion zone.

In terms of size, sludge that is 70% Volatile Solids (VS) and 28% dry matter is considered to be autothermal.

Î The Thermylis® unit

Degrémont’s Thermylis® unit includes a storage area for the sludge to be incinerated, a fluidised bed furnace, a heat recovery system and a flue-gas treatment system.When it enters the incinerator, biological sludge often still contains 80% water. It therefore requires additional fuel to burn the sludge at 850°C.The heat recovery section is split into two parts:- the recovery exchanger that preheats to 650°C the fluidisation

air that feeds the wind box, by recovering the calories contained in the flue-gas leaving the furnace;

- the cooling exchanger that cools the flue-gas to a suitable temperature for the flue-gas treatment.

Optimizing energy consumption is a key point in the design of a dedicated incineration unit. The autothermal point is continually sought by looking for the optimal valuation of the sensitive heat of the combustion products. It is a compromise between minimizing additional fuel consumption and minimizing investment and operating costs.

Î The Thermylis® 2S unit

The Thermylis® 2S sludge incineration unit has two stages. Based on the principle of energy recovery, it combines the drier and incineration furnace to aim at energy autonomy.The flue-gas energy recovered in the exchanger is used to pre-heat the fluidisation air as well as to pre-dry the sludge. The purpose of this sludge “pre-drying” process is to bring the sludge to autothermal dryness. In the furnace, the organic matter in the sludge maintains the temperature of 850°C, reducing the need for additional fuel to zero.

This combination of pre-drying and heat recovery from the flue-gas significantly reduces fossil fuel-based CO2 emissions as well as the unit’s operating costs.

Contact Thermylis®: innovation.mailin@degremont.com

Degrémont Water Treatment Handbook Factsheets

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Î The Thermylis® 2R unit

When sludge is sufficiently autothermal or when the quantity of incinerated sludge is sufficient, the second heat exchanger can be used to heat a thermal fluid that can meet heating needs or

electricity production needs. Thermylis® thus becomes a real “green concept”.

A few DEGRÉMONT references40 Thermylis® units installed around the world

• Lakeview (Canada) - 4 reactors - 4 x 100 t. DS(1)/day• Duffin Creek (Canada) – 2 reactors - 2 x 105 t. DS(1)/day• Cleveland (United States) - 3 reactors - 3 x 91 t. DS(1)/day

• Le Havre (France) - 1 reactor - 33 t. DS(1)/day• Bilbao (Spain) - 1 reactor - 63t. DS(1)/day• Shenzhen (China) - 2 reactors - 2 x 80 t. DS(1)/day

Î

• Focus on the Mill Creek facility (Cincinnati, Ohio, United States) (Wastewater treatment plant)

- Reactors I, II, III : Ø 8.1m each, hot wind box- Capacity : 3 x 100 t. DS(1) per day, 22.5 MM kcal/kg- Sludge characteristics : 26% dryness, 70% VS(2), 5,800 kcal/kg VS(2)

- Energy recovery : primary and secondary heat exchangers- Flue gas treatment system : wet Venturi scrubber, GAC(3)

Î

• Focus on the Kielce facility (Gdansk, Poland) (Wastewwater treatment plant)

- Reactors I, II, III : Ø 5.2m each, hot wind box- Capacity : 20 t. DS(1) per day, 7.1 MM kcal/kg- Sludge characteristics : 35% dryness, 60% VS(2), 5,500 kcal/kg VS(2)

- Energy recovery : dryer, primary and secondary heat exchangers thermal oil loop

- Flue gas treatment system : dry cyclone, bagfilter, chemical injection

(1) Dry Solids - (2) Volatile Solids - (3) Granular Activated Carbon

Thermylis®

Thermylis® 2S