Method of preventing blockage of circulating bed material in a circulating fluidized bed reactor arrangement
A method of preventing blockage of circulating bed material in a circulating fluidized bed reactor includes collecting a continuously flowing bed of solid particles in a gas lock in a return leg of a reactor, measuring gas lock bed pressure values within the bed of the particles, generating a gas lock bed height indication signal on the basis of measured gas lock bed pressure values. A definition stage includes defining and storing to a control system a range of normal gas lock bed height indication signals, formed in normal circulation flow conditions, as a function of the reactor load, and defining and storing to the digital control system a reactor load dependent alarm criterion. The method includes comparing a current gas lock bed height indication signal with the reactor load dependent alarm criterion, and decreasing the reactor load if the current indication signal fulfils the reactor load dependent alarm criterion.
1 . A method of preventing blockage of circulating bed material in a circulating fluidized bed reactor arrangement having a furnace, the method comprising steps of:
defining a reactor load at a digital control system of the circulating fluidized bed reactor arrangement;
feeding fuel and combustion gas at a predefined rate based on the reactor load into the furnace of the circulating fluidized bed reactor arrangement;
combusting the fuel with the combustion gas in the furnace and causing flue gases and solid particles to emanate from the furnace into a flue gas channel;
separating solid particles from the flue gases in a particle separator arranged in the flue gas channel and circulating a flow of solid particles from the particle separator via a return leg back into the furnace;
collecting a continuously flowing bed of the solid particles in a gas lock in the return leg;
measuring gas lock bed pressure values within the continuously flowing bed of the solid particles;
generating a gas lock bed height indication signal on the basis of measured gas lock bed pressure values;
executing a definition stage comprising the steps of:
changing the reactor load defined at the digital control system sequentially to multiple load values;
defining and storing to the digital control system a range of normal gas lock bed height indication signals, formed in normal solid particles circulation flow conditions, as a function of the reactor load; and
defining and storing to the digital control system a reactor load dependent alarm criterion, the fulfillment of which comprises that a current gas lock bed height indication signal is outside the range of normal gas lock bed height indication signals for the prevailing reactor load, for indicating deviation in the flow of solid particles being circulated;
comparing, at predetermined intervals, a current gas lock bed height indication signal with the reactor load dependent alarm criterion; and
decreasing the reactor load in case the current gas lock bed height indication signal fulfills the reactor load dependent alarm criterion, for preventing blockage of the gas lock.
2 . The method according to claim 1 , wherein the gas lock bed height indication signal is formed by averaging measured gas lock pressure values over a predefined measuring time.
3 . The method according to claim 2 , wherein the predefined measuring time is at least ten seconds.
4 . The method according to claim 3 , wherein the predefined measuring time is at least thirty seconds.
5 . The method according to claim 1 , wherein the definition stage of the method further comprises defining and storing in the digital control system a reactor load dependent upper limit for the gas lock bed height indication signal, and the fulfillment of the reactor load dependent alarm criterion comprises that a current gas lock bed height indication signal is above the upper limit for the prevailing reactor load.
6 . The method according to claim 1 , wherein the fulfillment of the reactor load dependent alarm criterion comprises that a current gas lock bed height indication signal is at least three times within a predefined time above a predefined upper limit.
7 . The method according to claim 1 , further comprising a step of measuring a furnace bed pressure value within a bed of particles in the furnace, wherein the fulfillment of the reactor load dependent alarm criterion comprises that the furnace bed pressure value executes a decreasing trend at the same time as the gas lock bed height indication signal executes an increasing trend.
8 . The method according to claim 7 , wherein the criterion for a decreasing trend of the furnace bed pressure value includes that, by at least five percent monotonously decreasing values are observed in at least three consecutive, averaged furnace pressure values, and the criterion for increasing trend of the gas lock bed height indication signal includes that, by at least five percent monotonously increasing values are observed in at least three consecutive the gas lock bed height indication signals.
9 . The method according to claim 1 , further comprising, in case the reactor load dependent alarm criterion is fulfilled, a step of starting to feed inert bed material, or increasing the rate of feeding of inert bed material, into the furnace.
10 . The method according to claim 1 , further comprising, in case the reactor load dependent alarm criterion is fulfilled, a step of starting to discharge bottom ash, or increasing the rate of discharging bottom ash, from the furnace, so as to enhance bed material change.
11 . The method according to claim 1 , further comprising, in case the reactor load dependent alarm criterion is fulfilled, a step of changing type of the fuel fed to the furnace.
12 . The method according to claim 1 , further comprising, in case the reactor load dependent alarm criterion is fulfilled, a step of starting to feed limestone, or increasing feeding of limestone, into the furnace.
13 . The method according to claim 1 , further comprising, in case the reactor load dependent alarm criterion is fulfilled, a step of starting to feed, or increasing feeding of, a reagent suitable to diminish forming of alkaline compounds that melt or become sticky at the temperature prevailing in the furnace.
14 . The method according to claim 1 , further comprising, in case the reactor load dependent alarm criterion is fulfilled, a step of giving an alarm in the digital control system for potential blockage.
15 . The method according to claim 1 , further comprising, in case the fulfilment of the reactor load dependent alarm criterion is ceased, a further step of increasing the reactor load.