Wrap each line with a span tag
You can split your text 

{% set text = block.heroTitle|nl2br %}
{% set splited = text|split('&lt;br /&gt;')%}
{% for part in splited %}
    &lt;span&gt;{{ part }}&lt;/span&gt;
{% endfor %}


If you need you can use the trim filter as well

&lt;span&gt;{{ part|trim }}&lt;/span&gt;