Air-flow (or pneumatic) separation

1452

Air-flow separators (or air classifiers, AC) are typically present in RDF/SRF production lines

1453

of MBT plants. Air classifiers have long been established in industrial applications, such as

1454

agriculture and minerals processing, where they are used to separate components from dry

1455

mixtures^{61, 63, 80}. In solid waste management (SWM) they were applied as a key part of

1456

conventional RDF production plants, operated initially on MSW and later commercial or

1457

source-separated waste⁶². Expectations for AC performance were initially high but a phase of

1458

scepticism followed in the 1990s. This can be attributed to off-the-shelf applications of ACs

1459

proven in other industrial operations, but not adapted or optimised to waste, combined with

1460

unrealistic expectations (e.g. separation of organic from inorganic items, despite their similar

1461

densimetric properties)61, 63, 153

. Currently the confidence in the effectiveness of ACs has

1462

been re-established in practice⁶⁵.

1463

Within MBT plants, ACs are mainly used for concentrating the high CV combustible

1464

fraction in their low-gravity product⁶⁵. Other specialised uses include the separation of a

1465

high-plastic film and paper fraction for subsequent material recovery, and for the removal of

1466

plastic from waste intended for landfill disposal in Germany, where legislative upper limits

1467

apply on the CV of landfilled material⁶⁵. Application of AC for compost product refinement,

1468

with emphasis on the removal of plastics, has recently been considered, with limited

1469

success¹⁵⁴. Timmel⁶⁵reported a typical throughput rate of ACs after the preceding

1470

classification at less than 15 Mg h^-1.

1471

Shapiro and Galperin⁸⁰provided a thorough overview of modern classification

1472

applications, including operation principles, features and performance parameters. However,

1473

their emphasis was not on waste separation, but on particle size separation applications.

1474

Timmel⁶⁵focused on residual and commercial waste treatment and an older RDF-production

1475

related overview can be found in Hasselriis⁶¹TABLE 8 provides relevant data from Timmel⁶⁵

1476

and other publications.

1477

1478

<<Table 8>>

1479

1480 1481

In typical configurations, separation is based on the differences in inertial (such as

1482

density) and aerodynamic properties (such as size and shape, i.e. measured as granulometric

1483

properties) of the in-feed particles. Air flows through the in-feed waste mixture causing

1484

high-gravity waste particles (constituting the reject) to either fall freely or to be deflected

1485

towards different chutes or conveyors. The low-gravity particles (being the extract) are either

1486

carried away with the off-gasses, to be concentrated downstream in cyclones or fabric filters,

1487

or are deposited on spacious settling chambers. Up to 70% of the classifying air can be

re-1488

circulated, in cross-flow designs⁴⁸. Within mining processing, separation occurs according to

1489

particle size⁸⁰, however, in waste treatment the density-dominant separation is more

1490

appropriate and efficient117, 122, 155

. Other sophisticated types of ACs have been developed

1491

that incorporate additional material properties, such as elastic behaviour⁶⁵. In residual and/or

1492

commercial waste separation, only gravity separators are used, and so far, centrifugal

1493

separators have not been introduced. Cross-flow separators prevail, in which the classifying

1494

air flows perpendicular to the waste and deflects the particles at various distances⁶⁵(FIGURE

1495

14).

1496

1497

<<Figure 14>>

1498

1499

The performance of ACs depends on the particular design, the mode of operation and the

1500

desirable^{65, 80}: (1) sufficiently narrow particle size ranges in the in-feed; (2) constant, and if

1502

possible, isolated feed of the individual particles; (3) well-defined and stable air-flow and

1503

reduced turbulence; (4) pneumatic conveying through pipelines applied to the low-gravity

1504

material; (5) separation of the low-gravity material from the classifying air; and (6) repeated

1505

cleaning of all fractions.

1506

Hasselriis⁶¹and Everett and Peirce¹¹⁷summarised the research that preceded the

1507

development of pulsed air classification. Bartlett¹⁵⁶showed that the performance of a zig-zag

1508

air classifier is compromised at high moisture content of the input, and the amount of

1509

adsorbent materials present in the input was identified as an important parameter. The main

1510

effect was on paper density and agglomeration, although plastics were also affected and

1511

reported to the low-gravity product. The composition of the feed, such as the paper-glass

1512

ratio, is also important¹⁵⁷.

1513

Both first principles and empirical modelling of the performance of air classifiers has

1514

been attempted, particularly outside waste management. For example, Wang et al.¹²⁰used

1515

computational fluid dynamics (CFD) simulation of cross-flow AC performance for size

1516

classification and Klumpar¹¹⁴examined performance optimisation of air classification in

1517

closed circuits with grinding. There is little research that is directly relevant to waste sorting.

1518

However, the principles for density-dominant separation through pulsed air classification are

1519

discussed in Vesilind¹²²and Everett and Peirce¹¹⁷. Validation of the air classifier unit

1520

operation of the GRAB^{99, 100}computer model using data from UK RDF plants showed

1521

adequate results for the raw mixed waste at that time, but different coefficients would be

1522

necessary for pulverised waste¹⁰¹. Parameters used were air flow, particle size and density,

1523

shape, and coefficient of variation. He et al.¹⁵⁵showed that non-waste simulation of airflow

1524

patterns within passive pulsing air classifiers can raise total effectiveness by 6-8% compared

1525

with conventional ACs. Biddulph and Connor¹⁵⁸used effective diffusivity to model and

1526

evaluate the performance of low-gravity and high-gravity products for different duct designs

1527

of ACs, operated at high values of air/solid ratio, reporting better performance for lower

1528

values.

1529

The exact performance of air-separators has to be evaluated by pilot tests, as accurate

1530

design calculations are thought to be impossible because of the problems associated with the

1531

granulometric description of waste particles⁶⁵. The selection criteria for the appropriate

air-1532

separation equipment include waste composition, particle size of waste stream to be sorted,

1533

required throughput rate and required performance⁶⁵.

1534

Rotter et al.⁴⁹presented a large scale comparative study on configurations of separation

1535

and classification equipment for SRF production for residual waste. This study provided

1536

insights into the material flow management performance of ACs. AC unit performance was

1537

among the top performing ballistic separation processes, which include air knife and

1538

crosswise. They achieved high enrichment in lower heating value (LHV) because of the high

1539

plastics percentage. However, this led to a high Cl content. Additionally, failure to

1540

incorporate the wet components into the SRF caused a high enrichment of cadmium (Cd).

1541

These results indicate that for the purpose of mechanical post-treatment of biodried output,

1542

air-classification may perform closer to ballistic separation both in terms of yield and Cl

1543

content, as it would be less difficult to incorporate the paper, card and textile fractions.

1544

TABLE 9 reviews results on air classification performance.

1545

1546

<<Table 9>>

1547

1548

In document Production and quality assurance of solid recovered fuels using Mechanical- Biological Treatment (MBT) of waste: a comprehensive assessment (Page 63-68)