You have two options:
    1. Use angular-fontawesome library
Just follow the instructions on their github page.  
    2. Use fontawesome 5 directly
Make sure you have installed all the relevant npm packages.
For Pro packages check out this.
- Import relevant icons: - import {faChevronLeft, faChevronRight} from '@fortawesome/fontawesome-free-solid';
import fontawesome from '@fortawesome/fontawesome';
 
- Add the icons to - fontawesomelibrary in global scope (not inside the component's constructor):
 - fontawesome.library.add(faChevronLeft, faChevronRight);
 
- Use it in html: - <span class="fas" [class.fa-chevron-left]="direction==='left'" [class.fa-chevron-right]="direction==='right'"></span>
 
- Mind the prefixes in html: - 
- fasfor- fontawesome-free-solidicons (works also with- fa)
 - <span class="fas fa-chevron-left"></span>
 
- fabfor- fontawesome-free-brandsicons
 - <span class="fab fa-bitcoin"></span>
 
- farfor- fontawesome-free-regularicons
 - <span class="far fa-chevron-left"></span>
 
- falfor- fontawesome-free-lighticons (pro)
 - <span class="fal fa-chevron-left"></span>
 
 
Important note: 
It's fine to use variables to define fontawesome classes as soon as it is done only once (at initialization). However, if the variable changes its value it won't be reflected in html.
Consider this example:
<span class="fas fa-chevron-{{direction}}"></span>
This will put the right icon at the initialization time, but if the direction changes afterwards it won't be reflected.
The reason for this is that fontawesome 5 replaces the elements classed with fa ... with appropriate svg and once it is replaced no variable affects this.
If you want the above html to reflect runtime changes you have to change it like this:
<span *ngIf="direction==='right'"><span class="fas fa-chevron-right"></span></span>
<span *ngIf="direction==='left'"><span class="fas fa-chevron-left"></span></span>
The outer span is necessary as the inner span is replaced with svg so you can't put *ngIf on it.
Further reading: