## M&W ASKETEKNIK

Sampling and process control systems for coal/biomass fired power plants

Sampling and process control systems for coal/biomass fired power plants

Representative samples prepared by the sampling system are used for:

- Combustion optimization based on “as fired” coal/biomass
- Adjusting the flow to the mills
- Calculating the amount of by-products from each batch
- Calculating the emissions from each batch of coal/biomass
- Payment adjustment in case of coal/biomass received from external suppliers

The basic purpose of sampling is to collect a manageable mass of material which is representative of the total mass of material from which it was collected. This manageable mass of material, called a “sample”, is subject to certain preparation procedures, which render it suitable for either physical testing, or lab. analysis.

The method by which samples are collected, the frequency of collecting samples and the accuracy of the samples collected, that is, how closely they represent the true characteristics of the total mass. A totally homogeneous material will require the collection of only a single sample in order to determine its characteristics accurately, whereas a lumpy heterogeneous material will require the collection of many small samples, or increments, which, when combined, will represent the total mass, or lot, with an acceptable degree of accuracy. These increments should be collected from all parts of the lot, with the number required to be collected being dependent on the variability of the material constituting the lot.

Since it is required to collect increments from all parts of the lot, it is necessary that the total lot is accessible. In other words, it is of fundamental importance that all particles in the lot have the same probability of being in the final sample. To achieve this requirement, it is desirable to sample the lot whilst it is in a dynamic state.

Knowledge of the raw coal/biomass properties:

- Heat value
- Moisture content
- Hardness (Hard Grove Index)
- N content
- S content
- Ash percentage

is essential when optimizing the combustion process at coal or biomass fired power plants.

The properties are identified by taking representative samples from each coal/biomass batch used at the power plant.

The Representative Sampling System is an integrated part of the control system available to the power plant operators.

The Representative Sampling System provides the operator with the means for executing the extraction and preparation of representative samples of the coal/biomass.

The Representative Sampling System operates in accordance with approved international standards.

In order to verify the coal properties for a given coal lot it is necessary to take out a number *u *of sampling units. This number depends on the mass of the lot and on the desired overall sampling precision. Minimum requirements are listed in Table 1.

Each sampling unit is taken by the sampling equipment as *n *increments, where *n *should be 10 or more.

The properties of the coal lot can be determined with an overall precision *P _{L} *using equation 1 where:

- V
is variance of sample preparation and test. If no data_{PT} - available: Assume 0.2
- V
is the variance of primary increment. If no data available: Assume 20_{I}

The overall precision is a function of the number of sampling units *u*, the variance of sample preparation and test *V _{PT}*, the number of increments

If a given precision *P _{L} *is required the number of increments

The values *n *and *u *are adjusted upwards and recalculated to a

convenient combination respecting the required minimum values.

** **

**Equation 1:**

**Equation 2:**

**Equation 3:**

Mass of Lot tonnes | No. of sampling units |
---|---|

1 – 5.000 | 1 |

5.001 – 20.000 | 2 |

20.001 – 45.000 | 3 |

45.001 – 80.000 | 4 |

80.001 – 125.000 | 5 |

125.001 – 180.000 | 6 |

180.001 – 245.000 | 7 |

The mass of each primary increment can be calculated using

Equation 4 where:

- C is flow rate [t/h] on conveyor belt.
- A is cutting aperture [mm].

Should be ≥ 3 times the nominal top size of coal.

- S is cutter speed [m/s].

If *S *is constant the incremental mass is a function of the flow rate *C*, the nominal top size of the coal lumps and cutting aperture *A*.

As a guideline for lay-out of sampling systems the reference increment mass from Table 2 should be used as minimum values.

The total mass of the sample (sampling unit) is at least *n *times the increment mass where *n *is at least 10 as previously discussed.

**Sample division**

To obtain convenient sample masses, the sample is divided into a number of statistically identical but smaller samples and a rest which is returned to the coal conveyor.

From the preceding section is seen, that reducing the mass needs a reduction in particle size in order to preserve the representative nature of the sample. For this reason a system for extraction of representative samples of coal consists of multistage extraction, particle reduction and division equipment.

**Equation 4:**

Nominal top size of coal [mm] | Reference incr. mass [kg] |
---|---|

300 | 100 |

200 | 25 |

150 | 15 |

125 | 10 |

90 | 5 |

63 | 3 |

45 | 2 |

31,5 | 1 |

22,4 | 0,75 |

16 | 0,50 |

11,2 | 0,25 |

8 | 0,15 |

**Lot:**

A quantity of material for which the overall quality to a particular precision needs to be determined.

**Sub-lot:
**Equally sized parts of a lot, which properties are to be determined.

**Norminal top size:**

The smallest sieve on which not more than 5% of the sample is retained.

**Primary Increment:
**The amount of material which the primary sampling tool takes out of the material stream in one cut.

**Cut:**

An increment taken by a sampler.

**Gross Sample:
**A material sample consisting of all the primary Increments taken from a sub-lot.

**Division:
**A sample preparation process, in which the sample is divided in a number of smaller portions.

**Sample:
**The quantity of material consisting of all the increments taken from a lot/sub-lot.

**Final sample:
**A sample prepared from the sample of a sub-lot.

**Precision:
**Expressed in % and within 95% confidence level.

**Bias:
**A systematic error that creates results that on regular basis are higher or lower than the true value.

Main Conveyor Belt (1), Cross Belt Bucket Sampler (2), Dosing Conveyor (3), Twin Roll Crusher (4), Rotary Tube Divider (5), Bias Valve (6), Buffer Container (7), Dosing Conveyor (8), Rotary Tube Divider (9), Sample Containers (10) & Reject Conveyor (11).

We are comprised of 4 different companies. Today we are 50 employees and have 3800m² under the roof with design, construction department and mechanical and electric workshops.

We handle all processes from drawing to delivery and are specialized in different fields. The fields covers: machinery for representative sampling; process monitoring and sampling for power plants, development engineering and sub-supplier assignments.

M&W

Oldenvej 5

3490 Kvistgaard

Denmark

Phone: +45 4913 9822

E-mail: m-w@m-w.dk