MILENA is an "indirect" gasification concept, combining CFB (circulating fluidised bed) pyrolysis and BFB (bubbling fluidised bed) combustion in the same vessel, while OLGA addresses what has been seen as the Achilles heel of waste gasification, namely tar. The combined technologies mean that high quality syngas can be produced from waste, and promise a considerable increase in the efficiency of waste-to-energy (WtE) plants, enabling them to employ IGCC technology.
Staff report
To demonstrate its gasification- based waste-to-energy offering, Royal Dahlman of the Netherlands is working with the plant owners to modify an existing chicken manure gasification system in Portugal to operate as a WtE facility.
Initially the OLGA tar removal system will be used with the existing (conventional CFB) gasifier, with the intention of replacing this, in due course, with a MILENA gasification unit. This would then become the first WtE demonstration of this combination of technologies at industrial scale, namely 4 MWt.
The Portuguese gasification plant is in Tondela, which is inland, roughly mid- way between Porto (11⁄2 hour drive by car) and Lisbon (21⁄2 hour drive by car). The site has been operated since 2009 by the Portuguese company Iberfer to demonstrate the gasification of chicken manure (4 MWt). The syngas produced by the gasifier is used to run a 1 MWe Caterpillar gas engine (type 3516A+).
To condition the gas before utilisation in the reciprocating engine, Iberfer selected OLGA technology to remove the tars. Analysis campaigns by a third party showed tar removal efficiencies of over 99.9%, with final phenol and naphthalene concentrations below the analytical detection limits.
Royal Dahlman subsequently developed and financed a modification, test and demonstration programme at the site to develop and demonstrate the use of refuse derived fuel (RDF) as feedstock for gasification. For this purpose the first half of 2014 saw the Tondela plant re-engineered, modified and upgraded.
In the summer of 2014, gasification using the existing gasifier started at the site. A long duration test successfully proved that RDF was a suitable fuel for the gasifier.
Following this first proving test, OLGA tar removal technology was employed in conjunction with the RDF gasification, and successful results were achieved, as reported at the end of November 2014 (see Figure 1), with a tar dew point of < 10°C.
The final set of tests will involve processing the gas from the OLGA system through the water scrubber and into the engine. These tests will be carried out during the first quarter of 2015.
Following these tests, there is a plan to install, during 2015, a MILENA gasifier at the site, which promises to increase the overall efficiency of the plant by 10%.
With its substantial investments in the Portuguese site, Royal Dahlman says it is taking a leap forward in the world of WtE, claiming that the MILENA-OLGA combination offers the highest efficiency route for producing syngas from waste.
The syngas produced is also of high quality and can be used as a source for carbon based chemical production, and gaseous and liquid transport fuels, including substitute natural gas (SNG) and synthetic diesel using the Fischer Tropsch process, as well as fuel for gas turbine generators.
Royal Dahlman says its technology is particularly suitable for countries and regions where landfilling is common practice and which lack a strong incineration infrastructure, citing "interesting markets" in the UK, Italy and some southern and eastern European countries, plus potential worldwide to licence the technology.
The technology has been developed in close co-operation with its inventor, the Energy Research Centre of the Netherlands (ECN), with Royal Dahlman acting as industrial partner since 2001.
The starting point was the development of the OLGA tar removal technology, as tar is seen as the Achilles heel for successful implementation of biomass and waste gasification.
Royal Dahlman also worked with ECN on the development of the MILENA gasifier, which, says Dahlman, is distinguished by its high conversion efficiency and product gas quality relative to competing technologies.
Royal Dahlman says it is becoming increasingly active as a total plant integrator in the WtE area and has developed designs for complete WtE plants in the size range 7-50 MWe.
The technology was one of three selected by the UK’s Energy Technologies Institute (ETI) for further consideration in its programme aimed at arriving at a design for "the most cost-effective, economically viable and efficient waste to energy plant possible", the other two being proposals from Broadcrown and Advanced Plasma Power (APP).
The projects in the running for ETI support are: a 7.6 MWe (net), 23 MWt, WtE plant using MILENA-OLGA technology for a site in Grimsby, UK; a 5 MWe facility using APP’s Gasplasma technology at Tyseley, Birmingham, UK; and Broadcrown’s 3 MWe demonstration facility in Wednesbury, UK. The outcome of the ETI’s deliberations is expected to become known in the Spring of 2015.
In the June 2012 proposal for Grimsby a net efficiency of about 30% for generation of electricity from RDF/SRF employing MILENA- OLGA was promised, but an "evidence based" estimate now increases this to 32% for a 7.6 MWe plant. The promised efficiency for a 17 MWe plant is now put at about 33%, rising to around 37% following optimisations and further innovations. A key task is to now to provide a robust demonstration of these figures based on real plant operation.
Figures 2 and 3 show, respectively, the MILENA-OLGA flow scheme and the layout for the proposed Grimsby, UK, facility.
Introducing MILENA…
Gasification can be defined as the thermal conversion of carbon rich fuels into a gas that can be used to produce heat, power, fuels and/ or chemicals. The gas produced by gasification is called syngas or product gas. Syngas is defined as containing hydrogen, carbon monoxide, carbon dioxide and water, while product gas is defined as containing syngas as well as methane and other hydrocarbons. The product gas produced by gasification will vary depending on how the gas is to be used.
When gasifying biomass and wastes, a key issue is the production of tars which can cause serious downstream processing issues and must be effectively dealt with before the product gas is used.
Biomass and wastes are normally gasified at temperatures in the range 700-900°C. The first stage of gasification is drying which happens as the feedstock temperature starts to rise. At about 350°C and upwards, the feedstock starts to devolatalise – a process called pyrolysis.
Pyrolysis is an endothermic process, it needs an energy source to maintain the temperature level. In an indirect gasifier, such as MILENA, a controlled amount of oxygen is fed in, which combusts part of the fuel, thereby supplying the energy for the gasification process. This is an efficient process, but it has some disadvantages:
• it does not achieve full carbon conversion, it is normally around 90-95%;
• the product gas is diluted with combustion flue gas (CO2, H2O);
• the product gas is typically diluted with nitrogen, if air is used as the oxygen source. The latter problem can be addressed by using an oxygen/steam mixture instead of air, but this will increase the plant costs and decrease efficiency.
Product gas from air blown gasification has an energy value of up to about 6 MJ/Nm3 (150 Btu/scf) after tar removal and water condensation, good enough for a gas engine but too low for a gas turbine.
MILENA, which has been demonstrated in ECN’s 0.8 MWt pilot facility, is, as already noted, an indirect gasifier, meaning that the pyrolysis and combustion reactions are separated (but only one vessel is used to accommodate both reactions). The main advantage of this is that MILENA has a separate combustion flue gas exhaust, so the product gas is not diluted with the nitrogen present in air, nor CO2 from the combustion – although the combustion flue gas needs to be treated to comply with local environmental legislation.
After gas cleaning and polishing, in particular to remove tars and water, the MILENA product gas has an energy value of up to 20 MJ/Nm3 (500 Btu/scf), well within specification for a gas turbine.
In the MILENA process (Figure 4), solid fuel (biomass or waste) enters the system in the central riser. A small amount of steam or air is also injected in the riser to fluidise the hot sand (catalytically active bed materials like olivine can also be used). Using the energy of the hot sand, biomass reacts to create three basic products: solids (char); condensibles (tars); and product gas (mainly H2, CO, CH4, C2H4, CO2, and H2O).
Because of the volatilisation, the product gas, sand and char flow upwards through the riser into the settling chamber. In this settling chamber the majority of the solids (sand, ash and char) are separated from the product gas. Raw product gas (containing gaseous tars) flows to the downstream equipment, eg OLGA, for tar removal.
The solids collected in the settling chamber flow under the influence of gravity to the combustion chamber. In this combustion chamber the char fraction burns, heating up the sand after which the hot sand flows back to the riser completing the cycle.
…and OLGA
As already emphasised, the presence of tars in the product gas is a big problem for the commercial utilisation of gasification as a source of sustainable energy. Once the tar problem is solved, Royal Dahlman believes, gasification could play a more prominent role in energy supply.
ECN tried almost every available tar removal system, after which it was concluded that a new approach was necessary. From this research the OLGA technology was born. OLGA, an acronym for oil-gas scrubber, is a patented ECN invention in which an oil is used to clean the gas. The scrubbing oil is reused and tars are recycled back to the gasifier; energy is kept in the process and a tar waste stream is avoided.
Tars formed in gasifiers comprise a wide spectrum of organic compounds, generally consisting of several aromatic rings. In simple terms, tars can be categorised as heavy or light.
Heavy tars condense out as the gas temperature drops below 350-450°C and if uncontrolled cause major fouling, efficiency loss and unscheduled plant shutdowns. The tar dew point, ie, the temperature at which tars start to condense, is a critical factor.
Light tars like phenol or naphthalene have less influence on the initial tar dew point, but, if left untreated, are no less problematic. Light tars like phenol chemically pollute the bleed water from downstream condensers and aqueous scrubbers. Naphthalene is an issue as it is known to crystallise at the inlet of gas engines causing high maintenance costs.
The OLGA tar removal system employs a multi stage scrubber in which gas is cleaned using a specially designed scrubbing oil. See Figure 5. In the first OLGA loop the gas is gently cooled down by the scrubbing oil. Heavy tar particles condense and are collected, after which they are separated from the scrubbing oil and recycled back to the gasifier.
In the second loop lighter tars are absorbed by the scrubbing oil. In the absorber column the scrubbing oil is saturated by these light tars. This saturated oil is regenerated in a stripper. Hot air or steam is used to strip the tar out of the scrubbing oil. All heavy and light tars can be recycled to the gasifier where they are broken down, and contribute to increased energy efficiency, while a tar waste stream is avoided.
The basic principle of OLGA is dew point control (Figure 6). Before entering the OLGA tar removal system the syngas is cooled and the coarse particles are removed by a cyclone.
The first OLGA column accepts gas above the tar dew point (400-500°C) and cools it down to a temperature close to, but safely above, the water dew point. Heavy tars are removed by condensation. Fine particles and aerosols are removed by the column as well as the downstream electrostatic precipitator (ESP). This ESP cleans itself thanks to the tar/oil fraction it captures. The condensed tars and captured solid particles are circulated together with the scrubber oil. A recovery system in a bypass stream separates the heavy tars and the solids from this circulation loop. The heavy tar and dust mixture is recycled to the gasifier.
Downstream of the collector and the ESP the gas is free of heavy tars and solids. The light tars, with their key components of phenol and naphthalene, are removed in the absorber. Absorption is a limited process as the absorption oil is saturated with tars. Therefore, the saturated oil is sent to a stripper in which the tars are stripped off by heated air or steam. As this stripper uses the combustion/fluidisation air required by the gasifier these light tars are recycled and destroyed in the gasifier.
The syngas no longer suffers from tar related problems and can be dewatered, further cleaned if necessary and put to good use. Because all captured solids, and heavy and light tars are recycled to the gasifier, OLGA is in principle a waste free system.
It is also important to note that the captured tars contain a considerable amount of energy. This is not wasted but kept in the system and contributes to the energy output of the plant.
OLGA has been demonstrated on the 0.8 MWt ECN MILENA pilot facility, in France on a 4 MWt facility and, as already mentioned, in Portugal on the 4 MWt Tondela facility (Figure 1).
(Originally published in MPS January 2015)
