How Are PTFE Exchangers Used in the Cooling of Hot, Corrosive Syngas from a Biomass Gasifier?

They are a means to carbon-neutral power, a bubbling, fluidised bed of wood chips, gasified into a hot, flammable "syngas." But the syngas that leaves the gasifier is a raw, nasty, caustic, sticky combination of hydrogen, carbon monoxide, acidic tars and fine ash. But before the energy-rich petrol can be burnt in an engine, it needs to be cooled and cleaned. The heat exchanger that initially cools the raw syngas has to endure a violent assault from corrosion, erosion and fouling. Only a few pieces of equipment, for instance a PTFE shell and tube heat exchanger, are able to successfully withstand such hellish front-line service.

The Challenge of Cooling Raw Syngas during Biomass Gasification
Biomass Gasification is the process of converting solid organic matter (wood chips, agriculture wastes, or municipal trash) into a combustible gas mixture called syngas (synthesis gas). The gasifier works at high temperatures, usually from 700°C to 1,000°C. The raw syngas exiting the reactor consists of energy-rich gases (H₂, CO, CH₄) as well as a variety of hazardous contaminants:

Acidic condensates: If the syngas is cooled below its dew point, chemicals such as acetic acid, formic acid and hydrochloric acid will condense. These acids are aggressive to normal carbon steel and a lot of stainless steels.

Heavy tars: High molecular weight organic chemicals (polycyclic aromatic hydrocarbons) condense down to form a thick, viscous black tar-like substance which sticks tightly to cold surfaces.

Fine ash and particulates: Erosion is caused by entrained char and mineral ash particles which can also accumulate.

A traditional metal shell-and-tube heat exchanger in this application would be damaged in weeks, or even days. The acidic condensate would erode the inner side of tube walls. The tar deposits would gradually clog the gas channels, which would lead to an increase in pressure drop and eventually choke the flow. Cleaning a metal exchanger fouled with tar is tough and usually requires harsh chemicals or mechanical scraping which destroys the tube surfaces.

How a PTFE Exchanger Provides Dependable Syngas Cooling for Biomass Gasifiers
Polytetrafluoroethylene's unique mix of chemical inertness, non-stick surface characteristics and thermal stability at moderate temperatures benefit the PTFE exchanger biomass gasifier syngas cooling application. In a typical set-up the hot raw syngas passes through the tube side of a PTFE shell and tube heat exchanger. A cooling media, usually water or thermal oil, flows on the shell side of the PTFE tubes.

Corrosion resistance: resistant to acidic condensates
PTFE is chemically inert to practically all acids, including the acetic, formic and hydrochloric acids present in raw syngas condensate. No passive oxide layer is needed and there is no pitting, crevice corrosion or stress corrosion cracking. Even in the case of syngas cooling below the acid dew point, with liquid acid coatings forming on the tube surfaces, the PTFE tubes are unharmed for the entire service life of the exchanger. This immunity reduces the requirement for costly high alloy materials such as Hastelloy or Inconel.

Fouling resistance: Non-stick surface prevents tar build-up
The PTFE tubes have smooth low surface energy walls that resist adhesion of the thick viscous tars that would rapidly fill a metal or ceramic exchanger. Tar droplets and condensates prefer to bead up and run down under gravity, rather than forming a sticky baked-on film. This self-cleaning action greatly reduces the time between mandatory maintenance shutdowns.

Thermal Performance
One important technological constraint should be noted: PTFE is limited to a maximum continuous operating temperature of about 110°C (230°F) for the wetted tube wall. Therefore, raw syngas passing through the PTFE exchanger must be quenched or precooled from its gasifier exit temperature (700–1,000°C) to below 110°C. This is generally accomplished with a separate upstream quench system, frequently a direct water spray or radiant cooler, that cools the syngas to roughly 100-105 °C in a very short time. The final cooling to the target temperature (usually 40-60°C) required for downstream scrubbing and engine feeding is subsequently done in the PTFE exchanger.

Heat Recovery for Greater Efficiency
The recovered waste heat from the syngas cooling process is not wasted. In a well built biomass gasification plant the hot water or the thermal oil which is taking heat away from the PTFE exchanger is used to preheat the gasifier combustion air. Preheating of combustion air can improve the total thermal efficiency of the plant by 10–20% [ ]. This immediately decreases the biomass fuel consumption for the same power production. The PTFE cooler is a durable, chemically blind lung that breathes in the hot, unclean and acidic breath of the gasifier and serenely passes its fire to the clean energy cycle.

Process Note: Control of Tar Fouling Through Continuous Spray Wash
While PTFE is non-stick, heavy tar can eventually build up, especially if the syngas has unusually high quantities of high molecular weight polycyclic aromatic hydrocarbons. Thus, the industrial PTFE exchangers intended for the cooling of raw syngas are generally provided with a spray wash system incorporated in their design. Before tar condensates may build up, they are gently washed off the tube surfaces (or into the gas inlet stream) using a constant spray of water or solvent.

The spray wash is either:

Condensate recycle: Clean water or mild alkaline solution from downstream scrubbing, sprayed at low pressure.

Solvent injection: A bio-derived solvent (e.g. biodiesel or vegetable oil) which dissolves heavy tars without damaging PTFE.

The spray wash must be carefully controlled. If the flow is too high, it will flood the gas stream, and if the flow is too low, it will foul. The spray nozzles are positioned to ensure complete coverage of the tube bundle. The wash liquid, now containing entrained tar, leaves the exchanger and is directed to a separator or treatment unit. This active cleaning method allows the PTFE exchanger to run for months or years between intensive human cleanings.

Mechanical Design Aspects of PTFE Syngas Coolers
There are some significant differences between a PTFE heat exchanger for cooling biomass syngas and a regular PTFE immersion heater or small heat exchanger:

PTFE tubes are normally made with a wall thickness of 1-2 mm, which gives them mechanical strength and resistance to abrasion by the ash particles carried along.

Tube supports: The tube bundle needs closer support spacing to prevent vibration and sagging, as PTFE has a lower modulus of elasticity than metal. Perforated PTFE or polypropylene support plates can be employed.

Shell material: The shell side (cooling water circuit) can be made of carbon steel or stainless steel as it is not exposed to the corrosive syngas. PTFE lined tube or solid PTFE tubes (tube side alone).

Gasketing and seals: All flanged connections utilise fluoropolymer gaskets (PTFE or FEP) to prevent leaking of syngas.

SUMMARY AND CONCLUSIONS: Unlocking Efficient Renewable Energy from 'Dirty' Gas Streams
PTFE heat exchangers offer a durable, corrosion proof and fouling resistant solution to the harsh syngas cooling duty in a biomass gasification facility, providing a more efficient and dependable renewable energy production. The PTFE unit runs continually with little maintenance, as it resists acidic condensates that degrade metals and prevents tar adhesion that jams other exchanger kinds. The recovered heat preheats the combustion air and improves the overall efficiency of the plant, therefore increasing the economics of biomass to power projects.

Some of the dirtiest, most caustic gas streams contain the greenest energy. The chemically indifferent heart of the PTFE exchanger makes that process quietly, year after year-cooling the hot breath of the gasifier without complaint.