The biomass alternative

Industry is being challenged to reduce carbon emissions and its reliance on non-renewable fuel sources. Spurring this on has been volatile oil prices as well as the likely introduction of schemes such as the Carbon Pollution Reduction Scheme (CPRS). In this article, Juergen Peterseim* and Prof Dr-Ing Udo Hellwig** from ERK Eckrohrkessel GmbH describe technologies that are available and can make use of biomass as an alternative fuel source for industry.

Biomass occurs in a wide variety of places, all with different qualities. Forestry waste is one source, others include biogenic by-products from industrial processes, agriculture and food processing. Biomass fuels are derived from products such as: wood waste, straw, bark, olive pits, nut shells, rice husk, bagasse, etc.

Considered a renewable energy source, biomass absorbs carbon dioxide during growth and releases it when being burnt. Depending on the type of biomass, this carbon cycle can be as little as a month using products such as straw, or up to several years using products such as wood.

Many manufacturing plants currently dispose of wood and other biomass as waste and then use non-renewable fossil fuels to produce their process heat and power. Identifying biomass sources that can be used to produce the renewable fuel is the first step to cutting carbon emissions.

The amount of biomass available is not important, even small quantities allow for the substitution of fossil fuels. Using small amounts of biomass is worth considering, as technologies exist to co-fire such fuels in existing boilers. This reduces the investment cost significantly, as all the capital-intensive components may already be available.

Processing

Size and quality of biomass varies significantly, which means it is often not directly usable as a fuel. For a feedstock to be used in a boiler, producing hot water or steam, it needs to be processed to achieve the following characteristics:

• No contraries (ie, contaminants such as stones, ferrous and non-ferrous metals);
• Size less than 80 mm; and
• Consistent moisture content/calorific value.

After extracting the biomass from the industrial process (after delivery), the fuel needs to be stored at a designated location. Contraries have to be separated from the material through screening, air separation, magnetic and non-ferrous separators. Valuable by-products such as metal can be sold to create extra revenue. Grinders are required to reduce the feedstock size to <80 mm.

After grinding, the fuel will be of a consistent size and contrary free; however, the calorific value might be still quite different, due to the difference in the moisture content of the waste material used. For example, construction waste will differ from green waste.

Two ways exist of solving this issue. The simple way is homogenising the fuel through mixing. The alternative is drying the fuel. Different technologies are available for drying, but they all require external heat. In situations where free energy is available, such as waste heat, drying is recommended. However, burning fossil fuel to dry biomass is environmentally questionable.

Power generation possibilities

Two options exist to generate energy from biomass: combustion and gasification. Combustion is better known and many plants operate worldwide; however, gasification is very promising for a variety of technical reasons, and there are industrial-sized plants in operation too.

Depending on the incineration system used, the design of the boiler system - which converts the fuel into thermal energy - has to be adapted to the fuel type. This is essential for finding the perfect balance between highest performance and lowest maintenance costs.

Figure 1: 25 MW biomass fired ERK boiler (combustions).

The recommended technology for strongly heterogeneous biomass is a boiler with a firing grate (see Figure 1). These systems accept any species of biomass and cope very well with different fuel qualities and sizes. They are also predominantly used in energy-from-waste plants. However, due to their technical characteristics, efficiencies are limited and very high steam temperatures are usually not recommended.

The second combustion technology is a fluidised bed firing system. Such systems achieve higher efficiencies than grate systems but their disadvantage is the complex fuel pre-treatment and high investment. Fluidised bed systems require a very specific fuel size and quality. For these reasons, the technology is rarely found in plants using heterogeneous fuels, such as waste materials.

Figure 2: 70 MW biomass fired ERK boiler (gasification).

Gasification offers the highest performance, as problematic substances, such as chlorine, remain in the gasifier’s bed and get extracted with the ash. Not having these substances in the flue gas allows long-term boiler operation at very high steam parameters. Gasifiers accept fuels of different sizes, similar to grate systems, and are a proven technology for biomass (see Figure 2). One very interesting aspect of gasification is its retrofitting potential to exiting fossil fuel boilers, thus providing a low investment cost.

Alternative fuels are a proven fuel source for process heat and power generation. Many plants operate throughout the world and have realised environmental and commercial benefits. Biomass is particularly promising to Australia as significant fuel quantities are available, plants are simple to build and additional income could be generated through RECs and a future CPRS.

Case study example:

An industrial area producing 50,000 tonnes of wood waste per year (calorific value ~12 MJ/kg) could operate a biomass power plant with an electric output of 5 MW and up to 10 MW of process heat (operation: 8000 h/a).

Producing this amount of electricity with coal (calorific value 22 MJ/kg) would require 28,000 tonnes of coal/pa, causing CO₂ emissions of around 88,000 tonnes. Assuming a mid-term carbon price of AU$20/t CO₂, this amounts to AU$1.76 million in carbon costs alone. These costs can be avoided using biomass as it is carbon neutral.

Generating electricity using biomass can also create Renewable Energy Certificates (RECs) that may be traded in 2011 at around AU$60/MWh. Assuming the aforementioned output, this amounts to AU$2.4 million a year. Selling the electricity to the grid will generate an extra income of AU$2.8 million/pa (assuming AU$70/MWh). In total, CO₂ savings and electricity income could amount to almost AU$7 million/pa. Reduced fossil fuel costs and avoiding waste disposal costs come on top of that. Building costs for such a plant are, depending on location, fuel etc, between AU$18-25 million.

* Juergen Peterseim, ERK’s Australian representative based in Sydney, Australia, is responsible for the local licensee support as well as the development of new boiler/heater applications. Particular interests are new cogeneration applications, multi-fuel systems and solar thermal energy.

** Prof Dr-Ing Udo Hellwig is the director of ERK Eckrohrkessel GmbH and teaches process engineering at the University of Applied Sciences Wildau, Germany.