Biomass gasification enters the premier league

Utilization of biomass resources to produce electricity is growing worldwide, but its gasification to produce biogas has stubbornly remained small-scale. Now an innovative project in Finland is about to change all that.

Dr Heather Johnstone, Chief Editor

As an alternative fuel to meet the world’s growing energy needs biomass continues to attract a lot of attention. The latest update report on the status of the global renewable energy sector, recently published by REN21, reinforces this.

According to the report, by the end of last year installed biomass power capacity worldwide totalled 62 GW, with the US continuing to lead with 10.4 GW. Significant advances were also seen in the European Union (EU), with its gross electricity production from biomass increasing by 10.2 per cent between 2008 and 2009, up from 79.3 TWh to 87.4 TWh.

Biomass’s future is also looking positive. The European Biomass Review, published by RISI, an information provider to the global forest products industry, forecasts a 44 per cent increase in demand for biomass by 2020, with the demand primarily coming from the energy sector and driven by European renewable energy policy. A similar picture is forecast in the US, where, the 2011 edition of the Energy Information Administration’s World Energy Outlook forecasts electricity from biomass will quadruple between 2009 and 2035, when it will account for 39 per cent of total non-hydroelectric renewable power.

Figure 1: An overview of the bio-gasification system planned by Vaskiluodon Voima Oy, showing its connection with an existing pulverized coal boiler Source: Metso
Figure 1: An overview of the bio-gasification system planned by Vaskiluodon Voima Oy, showing its connection with an existing pulverized coal boiler
Source: Metso

Biomass can offer economic advantages as well as carbon-neutral credentials. Work by the European Biomass Industry Association (EUBIA) comparing biomass with other renewables ” onshore and offshore wind, small and large hydro and solar photovoltaics ” found that biomass was the cheapest of the renewable resources when it came to electricity production. According to EUBIA, the needs of certain countries to reduce their dependency on imported energy and therefore increase security of supply is also driving up biomass’s popularity.

One of the most common uses of biomass in the electricity generation sector, particularly in Europe, is to directly co-fire it with coal ” seen as a relatively easy and cost-effective way of reducing carbon dioxide (CO2) emissions from coal fired power stations.

For example, Drax power station, the UK’s largest power plant, is undertaking the world’s largest biomass co-firing project. The company that operates the plant, Drax Power Limited, is also working with Siemens Project Ventures to jointly develop three dedicated biomass-fired renewable energy plants in the UK.

Only last month Drax Power launched the ‘Back Biomass’ campaign together with the Renewable Energy Association to urge the UK government to provide support to the country’s emerging biomass industry, ahead of an upcoming review of renewable energy subsidies.

Aside from the subsidies issue, however, several technical issues need to be addressed when directly co-firing with biomass, not least when retrofitting to an existing coal fired boiler. Guaranteeing a reliable supply of high-quality biomass can be a major challenge, as well as its reception, storage and handling.

Although the REN21 report says all biomass sources have enjoyed a rapid increase, interestingly, it is biogas that has experienced the most significant increase in the EU ” up a healthy 18 per cent.

To produce biogas from biomass involves a conversion process, primarily either anaerobic digestion or gasification. Both are currently used to produce ‘green’ electricity, also known as bio-electricity, but both remain small scale. The latter, however, is showing signs of potential larger-scale application.

Vaskiluodon's power generation objectives Source: Metso
Vaskiluodon’s power generation objectives Source: Metso

WHAT IS BIOMASS GASIFICATION?

According to EUBIA, gasification in essence is the conversion by partial oxidation (i.e. less oxidizing agent than for complete combustion) at an elevated temperature of a carbonaceous feedstock such as biomass into a gaseous energy carrier. The process has two main steps, which take place simultaneously in a circulating fluidized bed (CFB) gasifier.

The biomass is partially burned to form producer gas and charcoal. In the other stage, the CO2 and water produced in the first stage are chemically reduced by the charcoal, forming carbon monoxide (CO) and hydrogen (H2).

Gasification requires temperatures of around 800 à‚°C or more to minimize the residues of tars and high hydrocarbons in the product gas. This biogas contains H2 (18″20 per cent), CO (18″20 per cent), CO2 (8″10 per cent), methane (2″3 per cent), trace amounts of higher hydrocarbons such as ethane and ethene, water, nitrogen (if air is used as the oxidizing agent) and various contaminants such as small char particles, ash, tars and oils. The richness of the gas also depends on fuel properties, mainly moisture.

The partial oxidation can be carried out using air, oxygen, steam or a mixture of these, although the former is most commonly used. Air gasification produces a low heating value gas (4″7 MJ/Nm3 higher heating value) suitable for boiler, engine and turbine operation, but not for pipeline transportation because of its low energy density.

Biomass gasification is far from a new technology and actually dates back to the 18th century, but its commercial implementation has been slow and, as mentioned above, existing bio-gasification plants are relatively small scale ” the largest between 30″40 MWf. But an innovative new project in Finland looks set to change all that, with a biomass gasification plant that sports a whopping fuel capacity of 140 MWf.

BIO-GASIFICATION ENTERS THE BIG LEAGUE

In June of this year, Vaskiluodon Voima Oy, a Finnish heat and power production company that operates two thermal facilities (one in the city of Vaasa and the other in Seinajoki), announced plans to build a 140 MW bio-gasification plant adjoining its existing 565 MW coal fired power plant in Vaasa.

The primary aim of the project is to co-combust the biogas (also known as producer gas) with the coal to significantly cut the carbon emissions coming from the coal plant. Like many other EU countries, Finland is also looking to boost renewable energy’s share of its generation mix and has set a target of 38 per cent by 2020, up from the current 25 per cent.

The total project investment by Vaskiluodon Voima is said to be approximately €40 million, with the bio-gasification plant slated for operation in December 2012.

Sustainable technology and services provider Metso was awarded the turnkey contract, the scope of which includes: equipment for the reception, resizing and pre-treatment of the biomass; a large-scale belt dryer; the biomass gasifier or reactor; modifications to the existing coal fired boiler; and the supply of all related electrics and automation. Figure 1 provides an overview of the bio-gasification system and illustrates how it connects with the existing pulverized coal boiler.

Wittgenstein cogeneration station and pellet plant, an example of Europe's growing number of biomass installation Source: RWE Innogy
Wittgenstein cogeneration station and pellet plant, an example of Europe’s growing number of biomass installation Source: RWE Innogy

The modifications to the boiler house in actual fact are relatively minor. According to Juhani Isaksson, product manager at Metso, the company will primarily add new burners for the product gas, locating them in the bottom section of the boiler. This means that all of the existing coal burners will remain ” that is, the existing coal firing capacity is untouched ” allowing them to be used at any time if required. To accommodate this modification, space will also have to be freed up in the boiler building, so Metso will reorganize some auxiliary equipment, such as fans, pipe connections and cable lines. Metso, working with Vaskiluodon Voima, will also conduct the fine-tuning of the boiler to ensure it accommodates the new biogas input, maintaining its performance.

Mauri Blomberg, managing director of Vaskiluodon Voima, explains that the investment allows a relatively low-cost and rapid deployment of bioenergy: “Metso proved to be a partner that could supply a comprehensive bio-gasification technology solution, which combines the power plant process, biomass drying, modifications to the coal fired boiler and automation into one competitive and functioning unit”.

Metso has developed its own type of reactor or gasifier, over the last 30 years, which utilizes circulating fluidized bed (CFB) technology ” well-suited for large-scale biomass conversion, as well as waste applications.

According to Isaksson, the CFB reactor is not dissimilar to a CFB boiler and essentially contains the same principal components, such as the cyclone with bed material recycle loop and the air distribution grid area.

The gasifier at Vaasa will primarily use forest residues sources from the surrounding provinces, with the resulting biogas co-combusted with the coal in the power plant’s boiler. Thus enabling renewable energy to that replace between 25″40 per cent of the coal, with an estimated reduction in CO2 emissions of 230 000 tonnes annually. The percentage of coal displacement is impressive when you compare with direct biomass co-firing, which rarely can account for more than 10 per cent.

An interesting feature of Metso’s gasification plant is its integrated drying using a large KUVO dryer. Drying the biomass improves the efficiency of the gasification process. An additional benefit is that Vaskiluodon Voima can use any excess heat from the plant, making the drying process highly economical.

Although the bio-gasification plant at Vaasa will be primarily fuelled by forest residue, the system’s integrated drying also enables diverse biofuels to be used in the combustion. This diverse fuel capability provides Vaskiluodon Voima with flexibility in what type of biomass it purchases. Furthermore, in case there were temporary problems in the biomass availability, the system still enables the operator to use the full coal capacity.

Metso sees another advantage of its gasification solution in the fuel range diversity, because drying biomass such as humid wood chips is part of the plant’s operation, and any excess plant heat can be utilized in the drying process. The automation scope of the project includes automation and information management systems for the gasifier and fuel handling, which are connected to the existing Metso DNA system, and control changes in the old coal boiler.

The sheer scale of the implementation at Vaasa is undoubtedly unique. When you consider that the majority of the world’s energy is still produced in big coal boilers, gasification technology at this scale clearly offers a new cost-efficient alternative to the operators of large coal power plants ” increasing the share of biomasses, while simultaneously reducing coal consumption and emissions.

Isaksson could not agree more, and is upbeat about the technology’s future. “We believe that bio-gasification has a lot of potential in the power generation market. We have worked hard to raise its profile and are now seeing real interest in this type of application, particularly in Europe and North America.”

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