TerraNova Energy

Hydrothermal Carbonization (HTC) is a chemical process that converts biomass such as green waste, biowaste or sewage sludge into a biocoal. The term hydrothermal carbonization means ‘carbonization in an aqueous environment at elevated temperature’. The process takes place at temperatures around 180 °C and at pressures around 25 bar.

Hydrothermal processes (HTP) take place in an aqueous environment and with the addition of heat and are used to refine biomass or to utilize biogenic residues. HTP is an all-encompassing term for HTC, THP (thermal hydrolysis process) and HTL (hydrothermal liquefaction), among others.

THP is operated at 160-170 °C. In this process, the organic structures of the feedstock are broken down by the exposure to temperature and pressure, which improves their availability in the subsequent digestion process and increases the digester gas yield.

In HTL, a temperature level of about 250-400 °C is required. In this process, the biomass is almost completely liquefied and can then be further processed into various fuels (kerosene, gasoline, diesel, etc.), among others.

In contrast to hydrothermal processes, pyrolysis takes place in a dry environment. Under exclusion of air and at temperatures of approx. 400-800 °C, carbon-containing input materials such as forestry, agricultural or food residues, but also fabric, paint, household or industrial wastes are converted into various pyrolysates, which are used, among other things, as soil improvers, feed additives or building material additives.

In nature, biomass has been converted to coal over periods of thousands to millions of years. The carbonization of biomass can be accomplished with HTC within a few hours. In 1913, the process was first described by the german chemist Friedrich Bergius. Since about 2006, HTC has been intensively explored again and transferred into commercial, technical processes with different objectives.

Biomass is converted in an HTC plant into a coal slurry, which is then separated into a solid coal and a liquid process water.

The coal produced can be used as a CO2 neutral fuel to replace fossil coal in the cement industry, power plants and waste incinerators, but cannot be used as barbecue coal. The coal can also be used as a soil conditioner, thus also enabling CO2 sequestration (Carbon Dioxide Capture and Storage – CCS) due to its stabilized carbon structure.

The recovered process water contains high concentrations of C, N and P and can therefore be used as a liquid fertilizer, if permitted, or can increase the digester gas yield by up to 15% when recycled to a digester. Generally, the process water can not directly be discharged into the sewer network and must be treated.

Since during carbonization the sewage sludge remains in a pressure reactor for several hours at 180 °C, all microorganisms and other biogenic pathogens, such as viruses, are destroyed or inactivated. HTC products are therefore sanitized.

HTC products from sewage sludge generally do not reach the end of waste (EoW) according to Section 5 (1) of the German Waste Management Act (KrWG).

Biogenic residues such as sludge, biowaste, or green waste can be processed in a TerraNova Ultra® system as long as the input material is pumpable, has a water content greater than 70 %, and an organic dry matter content (oTS) greater than 30 %. Since lignin is practically not broken down during HTC, biomass with a high lignin content, such as wood, is hardly carbonized or not carbonized at all.

For good pumpability, depending on the structure of the biogenic residue, it may be helpful to mechanically pulp the input material before transfer to the input hopper.

The input material first enters the input hopper. From there, it is conveyed under pressure by the input pump through the input heat exchanger into the reactor. In the input heat exchanger, the biomass is preheated, then acid is added and finally converted into a coal slurry in the reactor within a few hours. The slurry leaves the reactor, is cooled in the output heat exchanger, and is expanded back to ambient pressure. The slurry is collected in the slurry tank and separated in batches by means of a chamber filter press into a solid phase, the coal, and a liquid phase, the process water.

The area between the feed pump and the slurry outlet is the pressure area of the plant. All other components are outside the pressure range.

A TerraNova Ultra® system operates continuously and fully automatically. Among other things, the reactor level, the reactor temperature, the pH value and the volume flow are automatically kept constant within the desired range. The collected slurry is separated batchwise into carbon and process water in a chamber filter press. The automatic system alerts the operator, among other things, that feedstock or auxiliary materials need to be refilled or that the chamber filter press needs to be started.

In a TerraNova Ultra® plant, bio waste materials are upgraded with the aim of saving disposal costs, refining the waste materials, or recovering nutrients.

For example, sewage sludge can be dewatered just mechanically after HTC treatment to a dry matter content of about 65 %, which leads to energy savings of 80 % compared to conventional drying. In addition, if the process water is recycled, the digester gas yield can be increased by up to 15 %, which further improves energy efficiency. In addition, 60 – 70% of the phosphorus contained in the sewage sludge can be recovered. The recovered HTC coal can replace fossil coal or be used as a soil conditioner or for CO2 sequestration.

The coal slurry recovered after HTC of sewage sludge is first acidified, which transfers the phosphorus from the solid to the liquid phase. Then, the now P-rich process water is separated from the coal particles. The dissolved phosphorus is then precipitated, and the phosphorus-containing particles are separated from the process water.

Phosphorus recovery can be optionally integrated into existing TerraNova Ultra® systems.

The economic efficiency mainly depends on the saved disposal costs. Thus, as a rule of thumb, the use of an industrial TerraNova Ultra® system is economically viable as soon as the disposal costs for the biomass exceed approximately 50 €/t.

For industrial use, modules with input capacities of 1, 2 or 3 t/h are offered (approx. 7,500 / 15,000 or 23,000 t/a). For processing larger quantities, several such modules are operated in parallel.

Possible revenues that could result from the use of the products obtained from HTC – for example, for calorific value in thermal use, for use as fertilizer or as a soil conditioner substrate, or for CO2 sequestration – are not taken into account here.

For R&D purposes, in the context of a pilot operation or for smaller input quantities (up to about 3,800 t/a), a TerraNova Ultra® container plant with an input capacity of about 0.5 t/h is offered, in which, however, an economical operation cannot usually be achieved. The container plant is also designed for stationary operation.

For the operation of a TerraNova Ultra® plant, about 18 kWh of electrical energy and about 130 kWh of thermal energy per ton of input have to be expended to produce HTC coal with about 65 % dry matter (DM). With final drying of the HTC coal to over 90 % DM, approx. 24 kWh of electrical energy and approx. 180 kWh of thermal energy per ton of input must be expended.

By using the separated process water, the digester gas production can additionally be increased by up to 15 %.

The necessary process heat can be provided by a suitable waste heat source, such as a CHP plant.

A TerraNova Ultra® system is designed in accordance with Directive 2014/68/EU on pressure equipment (Pressure Equipment Directive – PED) or the Boiler & Pressure Vessel Code of the American Society of Mechanical Engineers (ASME).

The space requirement for an industrial plant is approx. 12 m x 30 m floor space per module with a free height of approx. 12 m.

Once the order has been placed, it takes about twelve months until the system is handed over to the customer.

The annual availability is at least 7,680 h, during which the plant runs continuously. The time required for maintenance work is about 1-2 weeks twice a year. The system is designed for a total service life of at least 15 years.

A TerraNova Ultra® system runs in continuous operation. Since no steam is used in the HTC system, it can run in single-shift operation without continuous supervision. An integrated chamber filter press should be operated under supervision, but this can be done during single-shift operation of the HTC system.