Geothermal heat sources around the world are utilised for green power production but can also deliver commercially attractive carbon dioxide (CO2). However, the valuable gas is typically polluted by sulfur and other VOCs (volatile organic compounds) that prevent its industrial use and can also cause unpleasant odour.

Haldor Topsoe’s new selective oxidation catalyst SMC™ removes sulfur from the off-gas, eliminates smell, and produces commercial grade CO2. The CO2 quality of the end-product is sufficient for greenhouses within piping distance, or can be used as feed for a CO2 liquefaction plant – enabling the production of food-grade CO2 in pressurised tanks.

Icelandic geothermal power company HS Orka hf has signed a contract with Topsoe for a demonstration plant that transforms excess CO2 into a commercial product. The plant is the first to use the SMC™ catalyst to remove sulfur compounds from CO2, which is an attractive commercial opportunity for many geothermal and other gas plants around the world.

“It is our aim and responsibility to fully utilise the environmental and commercial possibilities from the geothermal sources we manage. The agreement with Topsoe should allow us, as we have done with other streams of resources, to exploit the CO2 sources to its full extent. As an added benefit, the CO2 will be made available in the local market,” said Kristín Vala Matthíasdóttir, VP Resource Park, HS Orka hf.

CO2 for local greenhouses 

The SMC™ catalyst will be at the core of a new demonstration plant that will be placed next to HS Orka’s existing geothermal power plant at Svartsengi, in Iceland. The plant will process 10% of the total off-gas stream to produce commercial grade CO2. HS Orka will market the CO2 to local industries, including greenhouses and algae producers, who can grow products for export and local consumption even in the dark Icelandic winter, aided by high CO2 levels in the greenhouses and electric lighting and heat, all from green geothermal power.

A typical greenhouse CO2 level is 1,000-1,300 ppm (parts-per-million) depending on the density of crops. The greenhouses in Iceland each cover an area of between 20 and 100 ha (hectare) and consume CO2 at a rate of 45-90 kg/ha/hour. A 50 ha greenhouse will consume around 70kg CO2/ha/h or more than 28,000 metric tons of CO2 per year.

The contract is for an operational lease agreement where Topsoe delivers an integrated solution with engineering and process design, equipment and complete services, including remote monitoring and catalyst replacement. HS Orka pays Topsoe per metric ton of CO2 that meets the purity requirements. The containerised plant can produce more than 7,000 metric tons of CO2 per year.

greenhouse grade co2

Low-temperature oxidation cuts cost

At the core of Haldor Topsoe’s catalytic H2S (hydrogen sulfide) oxidation technology is the SMC™ catalyst which selectively oxidises H2S to SO2 (sulfur dioxide) at low temperatures.

When the off-gas to be treated is a lean gas with combustible products, a low catalytic temperature is very beneficial because it requires less supplementary fuel and electrical power, while providing high selectivity towards SO2 rather than SO3 and its associated acid mist. If required, the SO2 content in the treated gas can be removed using a caustic scrubber. For heat recovery, several options exist to optimise the total investment cost and operating cost in accordance with local conditions.

The catalytic H2S oxidation can in practice often be operated completely without supplementary fuel even when the gas has a very low share of combustibles. This is the principle behind Haldor Topsoe’s catalytic H2S oxidation technology that is based on several decades of experience from large scale catalytic oxidation plants for a variety of components such as hydrogen sulfide, hydrocarbons, hydrogen and carbon monoxide, providing a variety of process options, catalysts and equipment designs. 

Haldor Topsoe’s expertise in accurately calculating and measuring the acid dew point for sulfur-containing gases secures efficient operation, while minimising the risk of acid corrosion in the plant equipment.

Integration of catalytic H2S converter

Source: Haldor Topsoe

Performance packed in a container

The Topsoe catalytic H2S removal plant is easy to operate. The entire plant fits in a standard 40ft container and can be delivered ready to connect at the customer site. The container holds all equipment needed to remove H2S from a CO2 stream, including oxidation air blower, feed-effluent heat exchanger, control system and oxidation reactor with the SMC™ catalyst installed.

The catalyst does not require any special treatment during start-up or operation. The catalyst is monolithic, which dramatically decreases any risk of plugging. The plant operates at moderate temperature levels and does not contain any sensitive high-temperature equipment. The typical expected emission performance for a CO2 stream with sulfur compounds such as H2S, CS2 and COS (carbonyl sulfide) is more than 99.9% conversion and an H2S slip < 3 ppm. The slip can easily be removed in an activated carbon bed. The final product from the container will be CO2 with nitrogen and trace amounts of oxygen ready for use at greenhouses within piping distance or as feed to a final purification and compression plant producing liquefied food-grade CO2.

Customer pays per tonne

Stranded and minor gas projects demand an appropriate business model to be mutually beneficial and sustainable. The Svartsengi contract defines a model, where HS Orka pays per metric ton of CO2 that meets the purity requirement and Haldor Topsoe holds the technical risk within the battery limits.

Haldor Topsoe offers the SMC™ gas-conditioning solutions technology as a full scope turnkey NCG purification plant. In this business model, payments comprise a mobilisation fee and an engineering services and catalyst replacement fee, depending on plant throughput and performance. This arrangement has several advantages for the customer, such as minimised CAPEX and upfront payment, minimised technology risk, payment based on plant performance, and only when product is in spec.

Haldor Topsoe provides continuous remote monitoring of plant performance and handles supply, replacement and discharge of catalysts and sorbents, as well as operator training and continuous support.  This business model has also been used with great success for containerised ultra-pure CO generators, commonly known as the eCOs technology, and is currently considered for biogas and landfill gas upgrading. The bearing principle in all containerised operational lease gas treating projecting is to enable the customer to unlock a hidden revenue stream in a mutually beneficial business model with low technical and financial risk.