Classical gasification turned upside down

In a conventional biomass gasification process the intention is usually to convert all the incoming material to product gas, typically by pyrolysing the biomass to produce char and gasifying the char using air at high temperature and pressure.

In contrast, in DM2's Blue Tower process part of the incoming biomass material (about 20% by mass) is pyrolysed (at 550°C) to form char, which is taken from the pyrolysis reactor and combusted to provide process heat (for the pyrolysis/gasification itself and the subsequent reforming stage). The remaining 80%, the volatile fraction (pyrogas), moves up the tower to the reformer. Here the organic species of the pyrogas are cracked and reformed, mainly to H2 and CO using steam. The steam is generated by passing the hot product gas through a waste heat recovery boiler before it passes through a quench and cleaning system. The steam reforming is a simple, homogeneous, purely gas phase, reaction. Also it is spatially separated from the pyrolysis, hence the term “staged reforming.”

Unlike air gasification, the product gas quality can be optimised as it is unaffected by flue gas from the char combustion, which is carried out separately. A further key feature of the Blue Tower technology is use of a circulating bulk material, eg ceramic balls, as the heat carrying medium. This means that heat exchange surfaces are always clean and it becomes possible to use input biomass materials that would be prone to causing slagging, fouling and corrosion in conventional systems.

Heat is transferred from the char combustion flue gas to the inert bulk heat carrier inside a simple moving bed reactor. This heat carrier preheater is located at the top of the tower.

On its way through the process, the heat carrier gives off its heat first in the reformer and then in the pyrolyser.

The pyrolysis and reforming occur in separate gravity driven moving bed reactors free of internal obstructions and without any equipment mounted inside. The heat-carrying medium is conveyed by a combination of mechanical drive and gravity.

The three main vessels, pyrolyser at the bottom, reformer in the middle and heat carrier pre-heater at the top constitute the Blue Tower. The vessels are connected by sluices, with pneumatically operated valves, through which the heat carrier medium flows. The vessel walls of the pyrolyser and pre-heater contain a ceramic fibre insulation layer through which nitrogen is passed to prevent hydrogen diffusion. The reformer vessel wall contains bricks.

Essentially, any carbon based matter with sufficient organics can be converted into a hydrogen rich fuel gas by staged reforming, provided, most of it can be released as volatile matter into the gas phase during the pyrolysis stage. Possibilities include: wood and wood wastes; green cuttings; hay and straw; chicken waste; domestic refuse; energy crops; sugar cane and bagasse; and a host of other waste materials.

A key advantage of the process is the high hydrogen content of the product gas, estimated to be 30-40% higher than with air gasification. The plant also promises to be cheaper to run, the gasification cycles faster and the reaction vessels smaller.

Capital costs are estimated at 8-10 million euro for a 10MW plant once serial production is established. The content of the product gas from the Blue Tower pilot plant typically consists of (vol %): hydrogen, 43.6% moist, 56.7% dry; carbon dioxide, 18.1% moist, 23.5% dry; carbon monoxide, 14.1% moist, 18.32% dry; CH4, 1.15% moist, 1.5% dry; H2O, 23% moist, 0% dry. The calorific value of the gas is around 12 MJ/Nm3.

A version of the technology has also been developed for dealing with liquids, Blue Tower Liquid (BTL) as opposed to Blue Tower Solid (BTS)), with vapourisation in the pyrolysis stage and CaO as the heat carrier. With this process, essentially all organic liquids and pastes can be converted to a product gas rich in hydrogen, even halogen and sulphur containing waste. Examples include: waste oils; solvents; used refrigerants; PCB/FC hydrocarbons; grease and fat; industrial emulsions; and animal and vegetable oils.

In the liquid case the reforming results in a product gas of calorific value 11 MJ/Nm3, with 75 vol % hydrogen or better, plus small amounts of carbon dioxide and carbon monoxide, and negligible methane.

DM2 hopes to have a BTL prototype built and operated within a joint hydrogen technology R&D project in 2005.

Blue Tower projects

Following the founding of DM2, based at Zukunftszentrum Herten (ZZH), in 1999, the Blue Tower pilot plant, also at Herten, started up in 2001 and since then has been tested on a variety of biomass types.

German energy legislation and impending waste dumping restrictions are tending to favour technologies of the Blue Tower type, and plans are under consideration for Blue Tower installations in about 25 locations in Germany. But there is also interest from other countries. For example, 2002 also saw the granting of licences to two Japanese companies and a Mexican company, Vimaza Energia, while in 2003 a licence was granted to EdRB do Brasil. There is also a licensee in Turkey.

Among key projects to date are the following:

Herten pilot plant

A number of trials have been conducted at the Herten pilot plant, using municipal "green cuttings", wood, straw and hay as input. The conversion of biomass into a valuable product gas rich in hydrogen has been successfully demonstrated.

Presently, the process is being optimised further and in the near future, gasification of other input materials is going to be tested in detail.

The product gas quality achieved so far is suitable both for CHP production using gas engines and further processing with a view to using the technology as a way of producing hydrogen or synthesis gas from renewable sources, ie biofuels. A particular emphasis will use of the product gas for powering fuel cells.

The Herten pilot plant has run relatively well, the main problem being ceramic balls getting caught in the sluices (solved by making the vessel profiles smoother) and adjustments to the screw systems that transport the coke and heat carrier out of the pyrolysis vessel.

H2Herten project - 10MWt demo plant

A Blue Tower with a capacity of 10 MWt is also envisaged as the nucleus of a new "hydrogen competence centre" planned for Herten, at the disused Ewald coal mine, to the south of the town. Start of construction is scheduled for this summer.

One of the major objectives of the centre will be production of hydrogen from renewable sources such as biomass and other wastes.

Detailed design, manufacturing and operation of this 10 MWt Blue Tower will be a joint undertaking of ThyssenKrupp Plant Service, Bottrop and H2Herten GmbH.

As well as the demonstration of hydrogen production, the handling of hydrogen and its use in fuel cells will be areas to be developed by the competence centre.

Funding will come from the EU and North Rhine Westphalia, which offers attractive subsidies to renewable projects and provided 45% of the funding (some 3 million euro) for the first Blue Tower. The town of Herten provided the site free of charge.

Emsland project

Emsland, the biggest German Lander, sited in the far north west, is mostly agrarian and aspires to a 100% renewable power supply, in which biomass will be crucial. While technology for the organic digestion of liquid biomass is already established there, staged reforming has been chosen as the preferred conversion process for solid matter, since this opens up the possibility of participating in development of a new technology. Staged reforming also offers great flexibility in terms of input biomass and this is important because the supply of input material – hay, straw, hemp amongst others – is very variable depending on the season.

Planning for a 2.5 MWt/750 kWe Blue Tower gasifier demonstration plant at a site in Emsland has been underway since October 2002. The design phase is being funded by the German ministry of agriculture and nutrition which indicated it might fund construction as well. Project co-ordination is being done by AVD, a subsidiary of the local farmers' association. As investor and future operator, a big regional electric power producer has been engaged as partner.

Another German Lander, Hessen, is also planning a 2.5 MWt Blue Tower.

Acapulco and Tepic projects

Meanwhile in Mexico, Vimaza EnergÌa is looking at building up to four Blue Towers (total capacity about 140 MWt) in Acapulco and in Tepic as a way of disposing of the city's domestic waste in future. A model of the first proposed facility has been presented to the public and Vimaza EnergÌa is presently planning further Blue Tower plants elsewhere in Mexico.