Biomass combustion power generation technologies
(Report number 95029)

Other relevant reports: 95100, 96092

Summary

At present power generation from biomass is mostly done by means of direct combustion. This report aims at making an inventory of the state of the art of biomass combustion technologies and to compare efficiencies, investment costs and emissions. The study focuses at power plants larger than 10 MWe.

This study is a background report within the Joule II+ project "Energy from biomass : an assessment of two promising systems for energy production". The objective of this overall Joule project is to provide support for the design and operation of biomass energy conversion units based on existing plans in the Netherlands and Ireland. In the Netherlands these plans are to construct a 30 MWe biomass gasifier with a combined cycle, fuelled by biomass residues and organic waste material. In Ireland the plans are to assess the feasibility to substitute peat with biomass in the form of short rotation coppice for energy production.

This general report on biomass combustion is meant as a support for a more specific study on retrofit options of Irish peat plants in order to enable biomass firing.

The inventory which has been undertaken consists of a general boiler technology description on the basis of qualitative criteria and a comparison of most recently built and planned power plants on the basis of quantitative criteria.

Important methodological considerations have been made on the field of recalculating CHP output to single power output, calculations on overall-electric, boiler and turbine efficiencies and the conversion of different monetary data into one common basis. Final comparison of efficiencies has been done on the basis of lower heating values, which is the most fair basis to do so.

If biomass as a fuel is compared with different kinds of fossil fuels, the most important differences can be found in the variability of fuel characteristics, higher moisture contents and low nitrogen and sulphur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies. NOx formation in biomass combustion can be kept low, partly because of low nitrogen contents in most biomass fuels and partly with the help of control techniques like combustion at low temperatures and staged combustion.

Qualitative analysis

A qualitative analysis has been made of existing boiler technologies to combust biomass. Boiler technologies which have been dealt with are : pile burners, stoker fired boilers, suspension fired boilers and fluidized bed boilers.

• In pile burners the biomass is dumped on piles in a furnace and burned with the help of combustion air coming from under and above the pile. Advantages of this technology are the fuel flexibility and the simple design. An important disadvantage is the generally low boiler efficiency and relatively poor combustion control.
• Within the group of stoker fired boilers, one can distinguish boilers with a stationary sloping grate, a travelling grate and a vibrating grate. Common to these types is a fuel feeding system which puts a layer of fuel on the grate which is relatively small and more evenly distributed than in the case of pile burners. In a stationary sloping grate boiler the grate does not move, but the fuel burns as it slides down the slope. Disadvantages of this type of boiler are the difficult control of the combustion process and the risk of avalanching of the fuel. In a travelling grate boiler the fuel is fed on one side of the grate and has to be burned when the grate has transported it to the ash dumping site of the furnace. Combustion control is improved here. Because of the small layer of fuel on the grate, carbon burnout efficiency is also better in comparison with the stationary sloping grate boiler. With a vibrating grate boiler the fuel is fed evenly on the whole grate. The grate has a kind of shaking movement which spreads the fuel evenly. This type of grate has less moving parts than a travelling grate and therefore has lower maintenance requirements. Carbon burnout efficiency is further improved and in the case of water cooled grates it is possible to increase the amount of over-fire air, which has the advantage of less thermal NOx creation.
• In suspension fired boilers the fuel is fired as small particles which combust while they are fed into the boiler. This system can be compared with the pulverised coal firing technology. A disadvantage of this system is that a lot of pretreatment of the fuel is required. The main advantage is the high boiler efficiency.
• In fluidized bed systems the combustion air from below the boiler has such a high speed that the fuel becomes a seething mass of particles and bubbles. At commercial scale one can distinguish between bubbling and circulating fluidized beds. A general feature of fluidized bed systems is that they are flexible in the kind of fuel which is fired, which makes them quite suitable for cofiring different kind of fuels. Carbon burnout efficiency is very high in fluidized bed systems. Another important advantage is the possibility to control NOx creation by low combustion temperatures and to minimize SOx creation in the case of fossil fuel cofiring. A disadvantage of fluidized bed systems is their high fan capacity requirement to provide for the fluidizing air. An advantage of bubbling fluidized beds over circulating fluidized beds is their lower capital requirement at modest scales (smaller than 20 MWe). Circulating fluidized bed system have better carbon burnout efficiencies and are better in absorbing acid gases.

The main results of the qualitative analysis can be found in the following table. All criteria have been formulated here in a positive way, so that a 'plus'-sign means that this is a positive score on one of the criteria used.

Table 1 Representation of main results of qualitatieve analysis

Quantitative analysis

In a more quantitative way, individual power plants, with boiler technologies as described above, have been analyzed on their efficiency, investment costs and emissions. From the range of recently built and near term planned power plants, a selection has been made of nine plants. One of these has a kind of pile boiler technology, three are stoker fired plant, another three have fluidized bed boilers and one is a pressurized combustion system in combination with a gas turbine. Some of the main results of this inventory can be seen in the following table.

Table 2. Main results of inventory of biomass combustion power plants

All boiler technologies which have been discussed above are still built nowadays. The maximum steam temperature which has been used is about 540 oC. Efficiencies of recently built plants are all in the neighbourhood of 30 % (LHV). The travelling grate boiler gave the lowest efficiencies, but has also relatively low capital requirements. The vibrating grate plant in the inventory, which is under construction at this moment has a projected efficiency of 33 % (LHV). The highest efficiency for a near term application, 37 % (LHV), was found with the cofiring of pulverised demolition wood with a pulverised coal fired boiler. Investment costs of such an option would have to be increased with a part of the residual value of the existing plant, to make them comparable with the other ones.

Efficiencies for longer term application, like the whole tree energy concept, which is still in experimental phase, and the Danish cofiring plans, range between 38 and 44 % (LHV). The whole tree energy concept also has the advantage of low investment costs. It has become clear that efficiencies in the range of 38 - 44 % are only obtainable with biomass combustion systems if large scale plants are used (ranging from 100 MWe up to 250 MWe).

With clean fuels like forest residues investment costs of biomass combustion plants normally do not exceed 2000 $/kW. Higher scales will also lower investment costs. Some plants in the inventory showed considerably higher investment costs, but these were mainly caused by additional investments needed for municipal solid waste firing, combined heat and power production or straw firing systems.