Closing the carbon cycle

The energy stored in almost all fuels on earth comes from the sun. In the distant past, organic materials formed what we now call fossil fuels. When we burn these fuels such as coals, natural gas or oil today, a lot of carbon is released that has been saved for billions of years. In contrast, bioenergy is extracted directly or using (bio)chemical techniques from recently formed biomass. This biomass consists of organic materials. HoSt, based in Enschede, is a specialist in bioenergy systems that help close the carbon cycle. Closing the cycle means that balance is created between the emission of CO2 during the digestion or burning of organic material and the absorption of CO2 during its formation.

HoSt from Enschede in Twente is one of the most prominent suppliers of bioenergy installations. They focus on the technological development of the processing of biomass and waste streams for sustainable energy generation. Their state-of-the-art installations for anaerobic fermentation are used for upgrading biogas and for fermenting sludge. In the fermentation process, a gas is created that can be used to produce electricity, or, after upgrading to natural gas quality, as green gas. This green gas can be injected in the natural gas grid and used to fuel (transport) vehicles.

HoSt offers biogas installations for industrial and agricultural applications. Using anaerobic digestion, industrial or agricultural waste and manure can be converted into biogas. They offer three types of biogas plants: microferm manure digestion, farm-scale biogas plants and industrial scale biogas plants. The resulting gas can be burned in a cogeneration installation to generate heat or electricity, or it can be upgraded to green gas. HoSt designed dozens of biogas plants within and outside of Europe. The plants offer high efficiency thanks to their high quality and application of the latest technologies. 

Wood, straw, chaff or other organic waste is available in large quantities worldwide and is more environmentally friendly and cheaper than natural gas and oil. HoSt supplies biomass and waste-fired combined heat and power plants from 1 MWt to 50 MWt boiler capacity and up to 12 MWe.

HoSt can process and add value to biomass and waste streams through their extensive expertise. Such streams in the food industry include (contaminated) organic waste, supermarket waste, offal and sugar beet pulp. In the agricultural sector manure, corn, straw, (roadside) grass and sludge are included. Other organic waste such as wood (including leaves) and chaff can be used as well, resulting in sustainable heat and green electricity. High electrical is efficiency obtained by a high-pressure water tube steam boiler in combination with an efficient multi-stage steam turbine. Their net electricity yield is 20% to 30% higher than a comparable installation. This is achieved by a low own electricity consumption and a high boiler efficiency.

The HoSt installations offer high availability in excess of 8200 running hours a year, equivalent to an availability of 94%. During the summer the electrical efficiencies are extra high due to a lower temperature of the heating water, allowing the steam in the turbine to expand further and therefore generate more electricity. The HoSt combustion technology is highly flexible in applicable fuels: such as moisture contents ranging from 10% to 55% and fuels with low ash melting points like straw, chaff, manure, olive pulp and chicken manure. HoSt can supply flue gas cleaning to minimise NOx, sulphur, chlorine, dust and other pollutants to meet required or desired emission standards.

A flexible process and optimal combustion are realised through an intelligent configuration of the furnace and the temperature control system in various combustion zones. This is achieved by injecting primary, secondary and tertiary air and recirculating flue gasses. In this way, the combustion temperature can raise to 1000 °C so that complete combustion occurs with very low CxHy, CO and NOx emissions. HoSt has realised multiple biomass and waste combined heat and power plants, including at biomass recycling firm Bruins & Kwast in Goor, Twente. For this particular job, a new steam turbine was integrated into an existing waste combined heat and power plant.

Aside from biomass to bioenergy plants, HoSt also offers sludge to bioenergy digestion plants. Municipal wastewater treatment plant (WWTP) sludge treatment is often an energy-consuming waste management headache. Large amounts of sludge are present in ditches, streams, lakes and rivers, and must be removed and treated. This is a costly process. With the sewage sludge treatment systems of HoSt, WWTPs have the potential to become energy neutral, or even to turn the plant into an energy provider in the local electricity grid.

To reduce the communal sludge amount and to increase the energy output, HoSt uses a combination of two technologies: a thermophilic digester and a mesophilic after-digester. Furthermore, Thermal Pressure Hydrolysis can be implemented as a pre-step. This leads to a further increased biogas production by 15%, a reduction of sludge, and an improved dewaterability. By using Thermal Pressure Hydrolysis, structures that are usually hard to convert by bacteria, such as lignin in plant material and cells in sludge can be converted. This breakdown step makes the organic matter more available for the bacteria, improving the speed of energy conversion. At the same time, it leads to fewer discharge costs at a WWTP. The sludge at the WWTP contains valuable raw materials that can be recovered and re-used. One of these materials is Struvite, which can be used in agriculture as a fertilizer or as a sustainable source of phosphate for example.

By combining a thermophilic digester (55 °C) with a mesophilic digester (40 °C), the biogas yield is improved by more than 25%. Thermophilic digestion means digestion at a higher temperature compared to a conventional, mesophilic digestion process. More heat must be used to heat the sludge, but this leads to increased biogas production. A patented heat recovery system designed by HoSt ensures that there is a positive energy balance. HoSt designs their advanced bioenergy systems after which the necessary parts are constructed at subcontractors throughout Twente, including Stork, Powerspex, Moekotte and Jotem. The systems are finally installed at the customers’ location. This way of working ensures control of the complete process, especially since worldwide sales and installation at such locations are included. HoSt collaborates with Bright Biomethane from Enschede regarding installations that upgrade biogas to natural gas. Bright Biomethane distributes such systems.