Retrofitting fossil fuel-fired boilers with biomass gasifiers

The delay of the Emissions Trading Scheme (ETS) until 2013 certainly slows investment in the renewable energy sector and favours only highly profitable/low CapEx projects. Despite the ETS delay, there are two incentives remaining for energy consumers to cut fossil fuel consumption: rising fuel prices and the generation of Renewable Energy Certificates (RECs).

The reduction of fossil fuel consumption requires new equipment, for example, more efficient turbines, heat recovery or biomass boilers. All options are capital intensive and that’s why ERK suggests retrofitting biomass gasifiers to existing fossil fuel-fired systems (Figures 2 and 3). According to the company, this is a low-cost approach as it is possible to use existing equipment, such as the boiler and turbine.

Suitable plant types

Many plants are suitable to retrofit gasifiers. These can be 5 MWth industrial plants up to 1000 MWth power stations. Depending on the biomass feedstock available, ERK suggests retrofitting gasifiers with firing capacities of up to 80 MW, as the calorific value (usually 10-18 MJ/kg) is comparatively low. The larger the biomass plant, the more fuel is required, and this could result in higher transportation costs associated with the fuel supply. However, if a large-scale fuel supplier is adjacent to the plant, for example, saw or sugar mill, larger retrofits are worth considering.

Possible types of biomass

Gasifiers are fuel flexible and accept any species of wood. The system also accepts corn, bark, bagasse and biomass mixtures. Research into using refuse derived fuel is promising.

The gasifier technology offers the advantage of using other solid fuels directly in the adjacent torsional combustion chamber. These include: sunflowers, cotton, peanuts, barley; soybean and chestnut husks; sawdust, snuff and sander dusts; grape and olive marcs; flax and corn straw; as well as coffee bean by-products.

How does the gasification technology work?

The technology is an updraft, counter-current, fixed bed gasifier connected to TCC with >100 references worldwide (1-70 MWth). Updraft gasifiers have been used for decades and are well understood.

Inside the reactor four process stages occur:

• In the first (topmost) stage, the fuel is dried by the hot ascending gas flow.
• As the fuel moves down, its volatile compounds become vaporised in the second stage.
• In the third stage, the carbon dioxide in the gas is reduced to carbon monoxide by reaction with parts of the fuels fixed carbon.
• Finally, in the bottom stage, the remaining fixed carbon burns in fresh air, thus providing the carbon dioxide for the third stage.

Figure 2: General arrangement of a supplementary biomass firing.

Since some of the reactions in the gasifier are exothermic, its walls are water cooled and integrated into the boiler’s circulation. Therefore, all the thermal energy is carried into the boiler either by the hot gases or the water cycle. The gasifier’s water walls also reduce the need of refractory materials, allowing reduced maintenance.

Due to the construction principle of the reactor, most fly ash particles remain inside the gasifier. This reduces dust-induced soiling and erosion of the equipment.

The main technical benefits of a gasifier solution are that solids fuels are usable in liquid/gaseous fuel-fired boilers and that high-temperature corrosion/ash fusion issues are significantly reduced. They are reduced because the gasifiers operate at low temperature, which keeps chlorine compounds and other substances in the gasifier’s ash. This is particularly important for high-temperature steam applications, eg, coal-fired utility boiler.

Leaving the gasifiers, the syngas enters the TCC, which basically is an improved cyclonic furnace. The syngas enters the TCC at the gasifier’s side and combustion air is injected tangentially through nozzles. The generated field of flow provides a long residence time, leading to an efficient combustion process with little excess air and an excellent burnout. Thereby very low emissions of carbon monoxide, volatile hydrocarbons and particles are achieved.

The combustion temperature reaches up to 1200°C. Therefore, the TCC’s walls are water cooled. To raise efficiencies, the TCC’s water circulation is integrated into the boiler’s water circulation too.

System integration

The TCC (E-5) attached to the gasifiers (E-4) acts as a supplementary firing, reducing the load of the fossil fuel furnace (E-2)/fossil fuel (P-1) consumption.

The biomass storage (E-3) supplies the gasifiers equally with fuel (P-2). After gasification, the syngas is burnt in the TCC. Entering the boiler, biomass and fossil fuel flue gases mix and consequently pass through the following heating surfaces for temperature reduction (P-3).

Adding the gasifiers and TCC only requires the TCC connection to the boiler’s water-steam cycle. The system is redundant as the fossil fuel furnace can take over fully in case of a biomass shortage or during maintenance.

Retrofitting advantages

The implementation of additional equipment always needs a justification before setting up a project. In case of biomass retrofitting the justification comes down to the following benefits:

• REC generation and carbon cost reduction (carbon price and fuel input),
• Short lead times/small investment due to low system complexity and cost-intensive components already available,
• Simple integration and possible use in liquid and gaseous fired applications,
• Significantly reduced high-temperature corrosion and ash fusion issues,
• Gasifier and TCC are well proven with >100 references; and,
• Extending lifetime of older power/industrial plants.

A retrofitting example

The following is an example of a 20 MWth gasifier system connected to a 61 MWth gas/oil-fired boiler (see Figure 3).

Figure 3: Retrofitting proposal of a 20 MWth gasifier to a 61 MWth gas/oil fired boiler.

Assuming industrial steam parameters of 480°C and 65 bar, a steam turbine would generate ~5 MWel. Using this energy in a utility boiler (steam parameters 600°C and 290 bar) the output increases to ~7 MWel.

To achieve 20 MWth over 8000 h/year, approximately 50,000 t of wood is required (calorific value 11 MJ/kg). Table 1 shows the fossil fuel and carbon dioxide reduction potential resulting therefrom.

A general assessment on the economics is quite complicated, as retrofitting costs vary significantly due to building and safety requirements etc. However, assuming a 20 MWth biomass gasification system, a natural gas price of AU$7/GJ and a biomass price of AU$40/t, the amortisation period is around 3.3 years considering fuel costs and RECs. Taking future CO₂ costs of AU$20/t into account, the amortisation period will be around 2.9 years.

According to ERK, retrofitting biomass gasifiers is promising to industrial as well as utility applications due to the low investment, fossil fuel substitution and CO₂ reduction potential. Furthermore, communities benefit from such an integrated approach as the biomass supply is usually operating locally.

*About Eckrohrkessel

ERK Eckrohrkessel is an engineering company offering licences to manufacture Eckrohr boilers as well as engineering services relating to industrial boilers/thermal equipment. Currently, ERK has 30 licensees worldwide and >5700 industrial boiler/heater references. Of these, more than 1000 use alternative fuels. www.eckrohrkessel.com.