Intuser Energy
Newsletter
 Subscribe
 Unsubscribe
Renewable Energy Sources  
 

7.2.4.4 Emissions from direct biomass combustion


The great variety of both biomass fuel types and boiler designs, and the large range of plant size (10kW to 50 MW), all make impossible any precise comparison between procedures, products and plant, when considering not only technical and economic performance, but also environmental impact. Table 7.5 gives a general guideline about airborne emissions.

Plant size
Flue gas O2 content [Vol. %]
CO[ g/m3 }
Emission limits
Dust [mg/m3]
OrganicC [mg/m3]
NOx [mg/m3]
15 kW - 100kW
13
4
150
-
-
100kW - 5MW
11
0,25
150
50
500
5MW - 50MW
11
0,25
50
50
500
Table 7.3 Harmful emissions - Plants burning straw and stalks [ref 7.1]

7.2.4.5 Fluidised Beds


For larger plants, a more complex option is fluidised bed combustion, in which mixing of the fuel and air is assisted by being blown through a bed of inert dry particles such as sand. This technique allows either complete combustion or gasification of the fuel (see below). Commercial developments are categorised as either bubbling fluidised bed (BFB) combustion using low air velocities, or circulating fluidised bed (CFB) combustion in which higher velocities result in some sand being blown out of the top and recirculated (figure 7.14). CFB tends to be more expensive, but reduces NOx emissions due to lower operating temperatures.

Figure 7.14 Fluidised bed boilers Source: ref 7.7

 

Fluidised bed combustors are more technically intricate with associated higher costs of design, construction and operation. As a general rule, there is a threshold in the region of 8 MWth above which they begin to gain economic advantage over fixed bed combustors. On the other hand, fluidised bed has the following advantages over fixed bed combustion:


· The high thermal inertia of the bed provides conditions for stable ignition, despite variability of fuel quality. Hence it is more tolerant of a wider range of fuel characteristics.
· Control of bed temperature allows a range of fuels with varying ash properties to be burnt while avoiding ash softening conditions in the bed.
· Relatively low combustion temperatures mean that NOx emissions are low.
· If limestone is added to the bed material then in-situ capture of SO2 is possible, though this is not necessary for biomass fuels since they are low in sulphur.


BFB units are commercially offered up to 100 MWe, and CFB units up to 400-600 MWe. CFB boilers have proven feasibility to burn about 70 different fuels alone or in co-combustion mode. BFB boilers have proved their feasibility for biomass and waste fuels with similar characteristics especially in lower capacities, starting from 5 MWth with well-processed fuel.