Turndown Limits of a Biomass / Straw-Fired Superheated
Steam Boiler operating in Combined Heat & Power Mode
• Biotricity intend to build a biomass /straw-burning CHP plant at Rhode, Co. Offaly to produce 15 MW of electricity for supply to the national grid under the European
Renewable Energy Feed In Tariff Scheme. Heat from the process will be supplied to an adjoining biomass fuel production facility for the evaporation of water from various forms of biomass which will be supplied to the energy market following processing.
Biomass & Straw present additional challenges for use as fuel in high-efficiency steam boilers owing to the presence of a significant proportion of alkali, alkali earth metals and silica in their composition. These give rise to a form of ash that is prone to forming a corrosive and very adhesive slag where it comes in contact with the radiant super heater surfaces in the furnace.
Operating experience gained from several operating plants has informed the boiler designer with respect to the reformation and adhesion of volatised components of the combustion gasses when they come in contact with the radiant super heater surfaces. Extensive efforts have been made to remove these deposits with various forms of soot blowing but only to result in further corrosion and blockage of the super heaters.

Turndown Limits of a Biomass / Straw-Fired Superheated
Steam Boiler operating in Combined Heat & Power Mode
• In order to overcome the challenges presented by biomass /straw as a fuel, the boiler plant must operate with slag-melting conditions in the upper furnace, and especially at the radiant superheater, in order to keep the furnace clean and to provide long-term reliability. In order to achieve satisfactory slag-melting conditions at the radiant superheater, the boiler cannot operate at loads below
70% of MCR otherwise the super heater will have insufficient gas temperature to reach the slag tap temperature required to keep the super heater clean and as a result blockage will occur.

• Biomass / Straw from arable crops is, at first sight, a very attractive biomass fuel. It is usually fairly dry, at average
14% moisture content (wet basis); and it is readily available, as the market for straw in Ireland is comparatively immature. However, straw has a relatively high ash content compared to woody biomass, and the ash content contains a high proportion of silica and alkali metals (potassium, K) and alkali earth metals (calcium, Ca), as well as significant amounts of chlorine (Cl), magnesium
(Mg), phosphorous (P) and sulphur (S). These give rise to an ash that has a low melting point and is volatile – it evaporates in the furnace; then condenses on cooler surfaces, e.g. the superheater, to form a slag layer.

• The actual amount of each ash component in the straw depends on the soil in which the crop was grown; as a result, it varies from country to country. See below Table 1: Straw Chemical Composition from Teagasc analyses of straws available for energy use on the Irish market.

Straw
Barley
Triticale
Rape
Wheat
C
47.5
43.9
47.1
45.6
H
5.8
5.9
5.9
5.8
O
41.4
43.8
40.0
42.4
N
0.46
0.42
0.84
0.48
K
1.38
1.05
0.79
1.01
Ca
0.49
0.31
1.70
0.31
Mg
0.07
0.05
0.22
0.10
P
0.21
0.08
0.13
0.10
S
0.089
0.056
0.270
0.082
Cl
0.40
0.27
0.47
0.19
The ash arising from all species of straw is partly vaporised in the furnace flame and condenses on cooler surfaces such as the furnace walls and the superheater.
Following the current best practice for straw combustion, it is desirable that the vaporised portion of the ash should all condense on surfaces within the furnace. While the boiler turndown is maintained above 70% load the furnace temperature can be kept high enough to allow the slag to re-liquefy and volatise and so run off the surfaces without the need for extensive and very aggressive sootblowing.

Picture 1: Radiant superheater in a biomass boiler, showing typical slag build-up

• The slag in the Picture 1 above has been molten, and normally runs down the superheater panels to drip off at the bottom. What appears in this picture is the additional build up of slag that occurred during periods of operating the boiler at below 70% load. After a period of a few hours the slag will join between the superheater tube platens and block the gas path. Further development of the superheater design has led to an increased pitch spacing of up to 600 mm.
However even with this increased pitch the slag will still join across the superheater platens if the firing rate goes below 70% boiler load.

• Figure 2 above shows a typical temperature profile through a biomass /straw-fired steam boiler of the type proposed for Biotricity (the temperature scale is in absolute degrees, Kelvin, not degrees Celsius). As can be seen, the temperature in the upper part of the furnace is about 1300K (about 1030°C), but within the pendant superheater section, the only superheater section in the radiant zone of the furnace, the gas temperature quickly drops to about 1100-1200K (about 830-
930°C). These temperatures are just above the melting temperature of the ash; any lower gas temperature and the ash will not be liquid enough to run off the pendant superheater elements or the upper walls of the furnace.
• In the bottom of the furnace the flame temperatures are high enough to keep the ash molten and so prevent deposits building up on the lower furnace walls.

• The preferred method to control the slag on the superheater and on the furnace walls is to encourage it to deposit on a radiant part of the superheater.
When the ash layer builds to a point that it partly insulates the superheater the outer surface of the ash is raised above the so called melting temperature so that the slag drips off the panels back down into the furnace. However, the necessary temperature cannot be achieved at firing rates below 70% of MCR.
Therefore, the boiler must be kept at 70% firing rate or more for all normal continuous firing. The furnace must be heated as quickly as possible (within normal permitted start-up/shut-down rate of change of temperature limits) during start-up to above 70% firing rate; likewise, during shut-down the furnace should be shut down as quickly as possible once the firing rate is reduced below 70% MCR.
• Because of the need to keep ash on the furnace walls and on the superheater molten, the boiler cannot be operated at firing rates less than about 70%
MCR. Lower firing rates may only be used during start-up and shut-down.